Citation
Sinkhole activity in West-Central Florida

Material Information

Title:
Sinkhole activity in West-Central Florida
Series Title:
Sinkhole activity in West-Central Florida
Creator:
Commins, Kathleen
Publisher:
Kathleen Commins
Publication Date:
Language:
English

Subjects

Subjects / Keywords:
Aquifers ( jstor )
Carbonates ( jstor )
Counties ( jstor )
Databases ( jstor )
Geology ( jstor )
Limestones ( jstor )
Maps ( jstor )
Population density ( jstor )
Sediments ( jstor )
Sinkholes ( jstor )
Town of Suwannee ( local )

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Resource Identifier:
003087097 ( alephbibnum )
774148534 ( OCLC )

Downloads

This item has the following downloads:


Full Text










SINKHOLE ACTIVITY IN WEST-CENTRAL FLORIDA:
A GEOGRAPHIC INFORMATION SYSTEMS ANALYSIS

















By

KATHLEEN COMMINS


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2004















ACKNOWLEDGMENTS

Although there are many to whom I owe thanks, it is Dr. Randazzo to whom I give

special thanks. It was his outstanding patience and guidance that kept me on the right

track. I thank Dr. Smith for being a committee member. Also, during my tenure in

Geological Sciences at the University of Florida, both Dr. Randazzo and Dr. Smith have

always shown enthusiasm for geology and far exceeded their call of duty. I would also

like to thank Dr. Meert for being such a conscientious committee member, and due to his

input, the final product is a better one! Special thanks go to GeoHazards, Inc, for

allowing me to access proprietary materials for this database, and for gainful employment

while working on it.

I would like to acknowledge all those at the Southwest Florida Water Management

District for their patience and for supplying some of the data within this thesis; and to

Sam Palmer for his assistance, and for sharing his ArcView expertise with me. Without

this selfless cooperation I would still be trudging on!
















TABLE OF CONTENTS

page

A CK N OW LED G M EN TS ................................................................................................ ii

LIST O F TA BLES ...............................................................................................................v

LIST O F FIGU RES ..................................................................................................... vi

A B STRA CT..................................................................................................................... viii

CHAPTER

1 IN TRO D U CTIO N ................................................................................................... 1

B ackground........................................................................................................... ..... 1
Sinkhole A architecture ................................................ ........................................... .2
Purpose .................................................................................................................. 5

2 GEOMORPHOLOGY OF THE STUDY AREA............................................. ............ 7

Structural and D epositional H history ..................................................... ...............7
G eom orphology.................................................................................................... 9
H ydrogeology ............................................................................................................. 11
Clim ate.............................................................................................................14

3 M ETH OD O LO G Y .....................................................................................................17

M ethodology of Sinkhole Investigation ..................................... ............................ 17
Previous Studies..........................................................................................................18
Criteria for Sinkhole Recognition...................................................... ... ...........18
Soil Testing..........................................................................................................19
G eophysics ....................................................................................................21
D ata.............................................................................................................................22
A acquisition of D ata..............................................................................................22
Processing of D ata ...............................................................................................25

4 RESU LTS ..................................................................................................................28

H ydrogeology .............................................................................................................28
Overburden Thickness to the top of the Floridan Aquifer System...........................28









Physiographic Provinces .......................................................................................34
Recharge/Discharge in the Floridan Aquifer System...................................... ...40
Potentiometric Surface..........................................................................................48
G eology................................................................................................................. 51
Major Wells in the Study Area ..................................................................... 54
Population Density................................................................................................58

5 A N A LY SES ......................................................................................................... 64

6 CONCLUSIONS ..................................................................................................77

APPENDIX

A GLOSSARY OF ARCMAP TECHNICAL TERMS ...................................... ...83

B HISTORICAL RAINFALL FOR STUDY AREA.......................................... ...85

REFERENCES CITED................................................................................................86

BIOGRAPHICAL SKETCH ....................................................................................... 92
















LIST OF TABLES


Table page

2.1 Relationship of regional hydrogeologic units to measure stratigraphic units..........16

3.1 Parameters of the counties of the study area ....................................................27

4.1 Thickness of the overburden to the Floridan aquifer system, compared to various
sinkhole param eters..................................................................................... ..........29

4.2 Sinkhole parameters of the Physiographic Provinces ............................................36

4.3 Area of the discharge and recharge zones of the Floridan aquifer system in relation
to various sinkhole param eters................................... ..........................................40

4.4 Subsurface geology of the study area in relation to various sinkhole parameters ...52

4.5 Highest annual water pumpage (gallons) for top wells in each county of study area
for selected years................................................................................................55

4.6 Population density in relation to various sinkhole parameters...............................61

5.1 Results from GIS analyses of sinkhole distribution and significant themes ............67
















LIST OF FIGURES


Figure page

1.1 Human-induced sinkhole development in the study area, Pasco and Hernando
counties (after Tihansky 1999)........................................ ..................................3

1.2 Types of sinkholes (from Tihansky 1999) ............................................ .............4

1.3 Map of the five-county study area (from Southwest Florida Water Management
D district 1990)........................................................................................................ 6

2.1 Structural features of the Florida peninsula (from Scott 1997)..............................8

2.2 Location of sinkholes in physiographic provinces of the study area .....................12

3.1 Counties of the study area showing locations of sinkhole activity of both the old
database and new ................................................................................................27

4.1 Distribution of sinkholes and overburden thickness to the Floridan aquifer
system ................................................................................................................. 30

4.2 Sinkhole density in relation to thickness of overburden ........................................31

4.3 Relative frequency of sinkholes in relation to thickness of overburden to the
Floridan .............................................................................................................. 32

4.4 Confining units of the Floridan aquifer system (after U.S. Geological Survey
"Ground Water Atlas of the United States, modified from Miller 1990) ................33

4.5 Sinkhole density in each of the Physiographic Provinces of the study area ............36

4.6 Relative frequency of sinkholes in each physiographic provinces of the study
area ..................................................................................................................... 37

4.7 Physiographic provinces versus sinkhole relative frequency and percentage of
total area ............................................................................................................. 38

4.8 Distribution of sinkholes and the Subsurface geology of the study area, with
overlay of rivers ................................................................................................. 39

4.9 Counties map with sinkhole locations, major roads and major rivers....................43









4.10 Recharge/Discharge zones measured in inches/year and sinkhole distribution.......44

4.11 Recharge/Discharge zones of the study area, (a) Sinkhole density (b) Relative
frequency of sinkholes .......................................................................................45

4.12 Overlay of the Thickness of the Overburden theme and the Recharge/Discharge
theme with the sinkhole distribution theme .....................................................47

4.13 The potentiometric surface values with sinkhole locations ...................................49

4.14 Sinkhole density and lithostratigraphic units of study area....................................53

4.15 Relative frequency of sinkholes in lithostratigraphic units of study area ...............54

4.16 Sinkhole distribution on the geology theme, with the 30 most highly pumped
wells in each county for the study area ................................................................57

4.17 Distribution of sinkholes and the US Census Bureau's Block Groups showing
population density for 2000..............................................................................60

4.18 Population density (km2) versus sinkhole density using US Census Bureau block
groups 2000 ........................................................................................................ 62

4.19 Sinkhole relative frequency for population density 2000 (km2) ............................63

5.1 GIS overlay of all the areas of highest sinkhole relative frequency for all digital
parameters used in the analyses.......................................................................... 68

5.2 All digital parameters with the greatest sinkhole density for the study area...........70

5.3 Recharge and Dishcarge zones with sinkhole distribution............................ ...72

5.4 Recharge/Discharge theme overlaid with the Depth to the Floridan Aquifer
System them e ..................................................................................................... 73

5.5 GIS intersection of all the parameters that displayed the highest sinkhole relative
frequency ............................................................................................................... 76

6.1 Proximity to the nearest sinkhole, indicated by one, two, and three kilometer
buffer zones ........................................................................................................ 81

6.2 Proximity to the nearest sinkhole, indicated by 0.5, 1.0, and 1.5 kilometer buffer
zones........................................................... ....................................................... 82















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

SINKHOLE ACTIVITY IN WEST-CENTRAL FLORIDA:
A GEOGRAPHIC INFORMATION SYSTEMS ANALYSIS
By

Kathleen H. Commins

May 2004

Chair: Anthony F. Randazzo
Major Department: Geological Sciences

Geographic Information Systems (GIS), involving spatial analysis, was used to

examine correlations between locations of sinkhole activity and the factors influencing

sinkhole distribution in West Central Florida. For the analyses, new data were collected

from private sources and combined with the existing database of the Florida Geological

Survey. The new data include 648 sites that were investigated from 1990-2001. Factors

that were suspected to be influencing sinkhole distribution were overlain and examined

for correlations. These factors were compared using sinkhole density and sinkhole

relative frequency and included the following themes: thickness of the overburden

overlying the Floridan aquifer system, physiographic provinces, discharge/recharge of the

Floridan aquifer system, potentiometric surface data, Florida's subsurface geology, the

major wells in each county, and the U.S. Census Bureau's population density block

groups. Analyses showed some strong and some weak correlations.









Certain factors appear to be of significance. Within the study area, the greatest

sinkhole activity is where the thickness of the overburden to the Floridan aquifer system

is 75 feet or less, where the near surface geology consists of the Hawthorn Group, the

Quaternary undifferentiated sediments, or the Suwannee Limestone, and within the Gulf

Coastal Lowlands and Coastal Swamps province.

The new database was used to construct maps showing locations of sinkhole

activity, and the various themes that could affect sinkhole frequency and distribution. In

an effort to address prediction of areas of future sinkhole activity, a GIS application of

nearest neighbor occurrences of sinkholes was conducted. It produced a map showing

areas where future sinkhole activity can be expected. These include most of Northern

Pinellas, southwest Hernando, the western coastline of Pasco, and northwestern

Hillsborough counties.














CHAPTER 1
INTRODUCTION

Background

The most commonly recognized feature of karst topography is the sinkhole.

Sinkholes are a significant geologic hazard and lead to a loss of many millions of dollars

each year. Sinkhole-related land subsidence problems are global and occur within 45

states in the United States, affecting over 17,000 square miles (Galloway et al. 1999).

A better understanding of sinkhole development in Florida is of importance to

planners at all levels. These include homeowners; architects; and engineers who design

both private and commercial structures; land developers and realtors; insurance agencies;

bankers; and local governments that issue permits for construction, waste disposal, and

water use. Awareness of sinkhole development and areas most sensitive to its occurrence

can be turned into a tool to be used to help avoid damage and financial loss caused by this

geologic feature.

Florida's rain water has a pH of 4.77 on average (Upchurch and Randazzo 1997),

and is an integral participant in the dissolution of limestone. As rain percolates through

the soil, it obtains CO2 from the metabolization of humus (from organic soils), lowering

the pH of the water entering the aquifer and dissolving Florida's Eocene to Miocene

carbonate bedrock, particularly during sea-level lowstands. Dissolution has given the

carbonate bedrock its high permeability or "swiss-cheese characteristic." One direct

benefit of this is the Floridan aquifer system (FAS), but a negative effect is the sinkhole









activity and the traveling of overburden into the voids left in the bedrock by the ongoing

dissolution process.

Many sinkholes are anthropogenically induced and are mainly caused by either

water withdrawal from wells or construction activities (Lane 1986). It is these activities

that can trigger instability, leading to sinkhole formation. For example, Sinclair (1982)

documented that in Tampa, Florida over 30 small sinkholes occurred north of a municipal

well-field within 1 year of the massive pumping from this well. One well field increased

from 5 mgal/d to 14 mgal/d, and 64 new sinkholes were reported within 1 month. Figure

1.1 shows some anthropogenically induced sinkholes.

Sinkhole Architecture

Sinkholes form by two processes (Sinclair et al. 1985): traveling, which is

downward erosion of unconsolidated sediments into preexisting cavities; and collapse, a

process where weak acidic solutions dissolve carbonate bedrock, creating cavities that

can collapse, or in which unconsolidated overburden sediments can migrate. Lithology

and thickness of the overburden determine sinkhole morphology (Sinclair 1985; Schmidt

and Scott 1984). According to Schmidt and Scott (1984) the frequency of sinkhole

occurrences in Florida, and their sizes are associated with the thickness of the overburden

(i.e., depth below land surface) to the FAS. In other words, both frequency and karst

feature size are directly related to stratigraphy. The three classic sinkhole types in

Florida are Dissolution, Cover-collapse, and Cover-subsidence (Figure 1.2).

Dissolution sinkholes are formed by weak acidic solutions infiltrating areas where

carbonate is exposed or only a thin layer of overburden is present (Tihansky 1999). The

acidic solution dissolves the underlying carbonates, creating cavities. In many cases, the










water flow is focused along fractures, bedding planes, or joints. This type of sinkhole

usually forms gradually as a bowl-shaped depression (Beck and Sinclair 1986).






















Figure 1.1. Human-induced sinkhole development in the study area, Pasco and Hernando
counties (after Tihansky 1999).

Cover-subsidence sinkholes usually develop where the sediments are permeable

and contain sand (Tihansky 1999). Gradual traveling (downward migration of granular

sediments into openings in underlying carbonates) is known as piping, and the structures

formed are called solution pipes. As dissolution and infilling continue, a small surface

depression forms (Tihansky 1999). The overburden thickness may range from 15.2

meters (50 feet) or greater (Sinclair and Stewart 1985).

Cover-collapse sinkholes are usually in areas where he e ere is a thick covering of

sediments that contain a large amount of clay (Tihansky 1999). The clays usually

separate the sandy overburden from the underlying carbonates. Clays can provide some

cohesion to the overburden, and when a solution cavity develops below, the overburden












can act as a "bridge." The collapse occurs when the solution cavity can no longer support


the overburden and the "bridge" gives way. This kind of sinkhole can develop rapidly


and cause catastrophic damage (Beck and Sinclair 1986).


TYPES OF SINKHOLES -.-- .---.-.- I... ..


Dissolution of the limestone or dolomite
is most intensive where the water first con-
tacts the rock surface. Aggressive dissolution
also occurs where flow is focused in pre-
existing openings in the rock, such as along
joints, fractures, and bedding planes, and
in the zone ofwater-table fluctuation
where ground water is in contact with the
atmosphere.


Thin overburden Rain
--_z ]. Rainfall and surface water percolate through
L- joints in the limestone. Dissolved carbonate
I rock is carried way from the surface and a
i .L small depression gradually foa s.


' ,>o !_- _
Pt-I


L `-~--


On exposed carbonate surfaces, a depression
may focus surface drainage accelerating the
dissolution process. Debris carried into the
developing sinkhole may plug the outflow
ponding water and creating wetlands.


Gently rolling hills and shallow depressions caused by solution sinkholes
are common topographicfeatures throughout much of Florida.


Cover-subsidence sinkholes tend to
develop gradually where the covering sedi-
ments are permeable and contain sand.


Granular sediments spall A column of overlying sedi- Dissolution and infilling con- The slow downward erosion
into secondary openings ments settles into the tinue forming a noticeable eventually forms small sur-
in the underlying carbonate vacated spaces (a process depression in the land face depressions I inch to
rocks. termed "piping. surface- several feet in depth and
diameter.


m''n--i '-- -- '-- --'



In areas where cover material is thicker or sediments contain more clay, cover-subsidence
sinkholes are relatively uncommon, are smaller, and may go undetected for long periods.



Cover-collapse sinkholes may develop
abruptly (over a period of hours) and cause
catastrophic damages. They occur where the
covering sediments contain significant
amount of day.
Sediments spall into a cavity. As spelling continues the The cavity migrates up- The cavity eventually
cohesive covering sedi- ward by progressive roof breaches the ground sur-
ments form a structural collapse. face, creating sudden and
arch dramatic sinkholes.



-J -I



Overtime, surface drainage, erosion, and deposition ofsediment transform the steep-walled
sinkhole into a shallower bowl-shaped depression.

Figure 1.2. Types of sinkholes (from Tihansky 1999)









Purpose

The purpose of this study is to broaden existing sinkhole database inventories and

to assemble information into a Geographic Information System (GIS), making it

accessible to the many sectors of the community affected by the environmental dangers

and economic impact that sinkhole activity causes. The database covers 5 Florida

counties where sinkhole activity is high (Figure 1.3). Results in this study will be

compared to results of a similar study done by Dodek (2003) for any correlations.

A further objective is to determine whether or not increased sinkhole occurrence

is correlated to geologic features and anthropogenesis (i.e., aquifer withdrawal rates). To

achieve these objectives, the data are compared as spatial thematic coverage in separate

GIS layers, allowing data to be easily retrieved and compared for correlations. The

software used is ArcMap, GIS software that was developed by the ESRI company.

Specialists can use this information as a tool for identifying areas with high

potential for sinkhole activity, or to more accurately access the environmental impact that

can be endured safely. A Florida Department of Insurance (FDI) study in 1992 (Butler et

al. 1992) pointed out the importance of identifying and predicting sinkhole occurrence.

This database and study should be very beneficial for better understanding the rates and

distribution of karst evolution.















STUDY AREA


Figure 1.3. Map of the five-county study area (from Southwest Florida Water
Management District 1990)














CHAPTER 2
GEOMORPHOLOGY OF THE STUDY AREA

Structural and Depositional History

Early Mesozoic rifting and sea-floor spreading between the African and North

American plates (Hine 1997) formed the basement for the Florida peninsula, a pre-

Jurassic crystalline rock body overlain unconformably by Jurassic and Cretaceous

sediments (Smith and Lord 1997) with the thickest sediments occurring in the southern

part of the peninsula (Heatherington and Mueller 1997). Situated on the North American

Plate, Florida's coastline is currently a passive margin, but there are features within the

basement that indicate Florida has undergone some structural changes throughout its

geologic history.

The structural features (highs and lows) in Florida that influenced deposition of

sediments include the Ocala Platform (Uplift); the South Florida Basin; the Jacksonville

Basin, and the Chattahoochee Anticline in the northwest; the Sanford High in the east;

the Peninsular Arch; and the Georgia Channel System (which collectively, is the

Apalachicola Embayment, the Gulf Trough, and the Southeast Georgia Embayment)

(Schmidt and Scott 1984). Figure 2.1 shows some of the structural features of Florida

that suggest that the Florida platform responded to tectonic forces during the Mesozoic

Era (Scott 1997). During the Cenozoic, the Peninsular Arch appears to have been the

major high that affected sedimentation in the Paleogene; and the Ocala Platform had the

most influence on the deposition of late Paleogene and Neogene sediments.

































LEGEND \ \

AXIS OF POSITIVE
FEATURE -- SCE
AXIS OF NEGATIVE LOW
FEATURE
-APPROXIMATE UPDIP LIMIT
AND AREA UNDERLAIN BY THE
FLORIDAN AQUIFER SYSTEM
0 50 100 150 200 MILES

0 100 200 300 KILOMETERS
SCALE

Figure 2.1. Structural features of the Florida peninsula (from Scott 1997)

The deposition of the thick carbonate sequence covering the platform began in the

Jurassic, after the African plate rifted from the North American plate, with intermittent

periods of siliciclastic deposition until the Cenozoic. The first 35 million years of the

Cenozoic were dominated by deposition of carbonates, and evaporites (Scott 2001), until

the beginning of the Miocene, when transport of siliciclastic sediments over the peninsula

predominated (Walker et al. 1983). Up until then, the Gulf Trough's currents diverted









the Appalachian sediments away from the Platform. The reason for the renewed

siliciclastic influx is debatable, but its deposition, occurring on the platform in the Late

Oligocene, was due to the Gulf Trough's currents no longer occupying the Georgia

Channel System after the Late Oligocene sea level drop. The Appalachian siliciclastics

prevailed, covering the Florida Platform by the middle of the Pliocene. The supply

diminished by Late Pliocene (Scott 1997).

Hawthorn sediments are represented in almost the entire Miocene and part of the

lower Pliocene sections of the study area. These include the Arcadia Formation

(Hawthorn Group, with the Tampa and Nocatee members) and the Peace River

Formation (with the Bone Valley Member and Wabasso beds) (Scott 1997). Subaerial

exposure occurred during low-level stands in the Neogene and Quaternary (Scott 1997).

Deposition of Eocene or lower Oligocene, through Miocene sediments from the

Appalachian Mountains was blocked due to the Gulf Trough/Apalachicola Embayment.

The sediments exposed in the study area are mainly the Pleistocene-Holocene

undifferentiated sands, and the Hawthorn Group.

Geomorphology

Marine forces were the dominating factor in creating the landforms of Florida,

(Schmidt 1997). When the Florida Platform was covered by the sea, erosion and

deposition from the shallow marine currents left behind flat plains and scarps. Whenever

the sea covered the Florida Platform, the shallow marine currents and their associated

erosion and deposition shaped the shallow seabed, leaving subsequent erosional forces to

modify the geometry.

The exposed part of the Florida Platform is the State of Florida. It is approximately

half of the Platform and is about 650 km east to west and 800 km north to south. The









highest point in Florida is 104 meters above sealevel, in Walton County, northwest of the

study area. Land forms help to interpret the geologic history. Dissolution of bedrock has

formed basins for lakes in many parts of central Florida (Miller 1987). Both sea-level

and land-level changes influence the rate of deposition and erosion (Schmidt 1997),

continually changing the geomorphology. Varying sea-levels have alternately flooded

and exposed the Florida platform, and eroding and depositing sediments. When the water

table is low and the limestone bedrock is close to the surface, drainage is usually

"internal" (Schmidt 1997) with streams "disappearing" into the subsurface. This is karst

topography and is a common drainage characteristic in Florida which has developed from

dissolution of the carbonates that exist throughout the platform.

Water can drain through sinkholes rather than running off through stream systems.

Florida's extensive aquifer systems were "carved" out by the movement and chemical

action of the acidic waters. The reaction of the water with the limestone and dolostone, is

summed up by the following, which show the chemical reactions when rainwater comes

in contact with the land surface. The carbon dioxide needed for this reaction to occur is

obtained from the atmosphere and the soil. The disassociation of the carbonic acid

allows it to react with calcite to form karst features.

* CO2 gas + H20 -+ H2C03 carbonic acid
* H2CO3 + H+ +HCO3
* CaCO3 calcite + H2C03 < Ca2+ + 2HCO3

Figure 2.2 shows that the study area lies within several physiographic provinces

which are described by White (1970). The largest are the Gulf Coastal Lowlands,

occupying, 3350 km2, and the Polk Upland, 3119 km2. The smaller provinces are the









Coastal Swamps, Brooksville Ridge, and Zephryhills (Florida Department of

Environmental Protection (FDEP) 2000).

The Gulf Coastal Lowlands in the study area are mostly underlain by the Suwannee

Limestone and the more deeply buried Ocala Limestone (Green et al. 1995). The

lithology of the Suwannee Limestone, early Oligocene in age, is described as a packstone

or grainstone carbonate with trace amounts of sand and clay in the upper portions (Green

et al. 1995; Arthur et al. 2001).

The Polk Upland occupies the western portion of Hillsborough County, and most of

Polk County (Figure 2.2). Exposed Bone Valley Member, Peace River Formation

(Middle Miocene to Lower Pliocene ) sediments make up the western part of the Polk

Upland in eastern Hillsborough County, and the southeastern section of the Polk Upland

in Polk County. These sediments of quartz sand, silt, and clay contain phosphorite

grains, generally with poor consolidation. The Bone Valley Member has been

economically important in the phosphate industry of the United States (Scott 2001). On

the eastern edge of the Polk Upland lies the Lake Wales Ridge and the Osceola Plain,

both undifferentiated sands.

Hydrogeology

Hydrogeologic framework plays a large role in when and where sinkholes are

formed. In west-central Florida the framework is made up of three aquifer systems that

consist of carbonates and siliciclastics rock. The ground water levels (hydraulic head)

and chemistry determine where dissolution of the limestone will occur. Sinkholes also

influence the hydrogeologic framework (Tihansky 1999).

The Floridan (FAS), the Intermediate (IAS), and the Surficial (SAS) aquifers are

present in the study area. The FAS underlies most of the State (Hyde 1975; Wetterhall








12


1964) and is the main source of potable water in the study area. Table 2.1 shows the

regional hydrogeologic units' relationship to the major stratigraphic units in Florida.


N F Counties Lake Hcmny Ridge
SSinkholes Lake Upland
SPhysiopraphic Regions hFL- Wales dge
F-]Brooksvill- Ridge __LakelandRidge
Hernando Brook ee PolkUpland
S Coastal Swa ps opk. Pl
S "= GulfBarner Chain T. IerH i
C. ,C 9? Gulf Coastal LowlCds



;I hr r* r _Zephrl Gp
J 7''- --. --


V o
'-.'.. a





s :: II
imk j" .I *r... !.a *

I .I ... .Kilom e r
Figure 2.2. Location of sinkholes in physiographic provinces of the study area (after

Green et al. 1995), and is made up of undifferentiated sediments in the study area. These



















consist mainly of quartz sand and varying amounts of clay and shell material. And in
some of the area, it may also include the Bone Valley Member of the Peace River























Formation/Hawthorn Group. The Bone Valley Member contains significant amounts of
phosphorite (Scott 1992) When present, the SAS either overlies the IAS, or when this
Si a it vris t S. I t S ria r g
K, ..* .,.1 I' -


.It'" "Hillsborough Polk Upland e



0 20 40 80

Figure 2.2. Location of sinkholes in physiographic provinces of the study area (after
White 1970).

Most of the study area contains the SAS in varying thicknesses (Arthur et al. 2001;

Green et al. 1995), and is made up of undifferentiated sediments in the study area. These

consist mainly of quartz sand and varying amounts of clay and shell material. And in

some of the area, it may also include the Bone Valley Member of the Peace River

Formation/Hawthorn Group. The Bone Valley Member contains significant amounts of

phosphorite (Scott 1992). When present, the SAS either overlies the IAS, or when this

system is absent, it overlies the FAS. In the Southwest Florida Water Management









District (SWFWMD) the SAS is a limited water resource in the northern section, but of

increasing importance to the south of the study area (Scott 1992).

The Southeastern Geological Survey (SEGS 1986) defined the IAS as the rocks

that lie between the Surficial and Floridan aquifers, retarding water exchange between the

two. The IAS consists of siliciclastics of the Hawthorn Group interlayered with Miocene

and younger carbonates (Scott 1992). Cross-sections developed by Arthur (2001) and

Green et al. (1995) indicate that the IAS is present in part of Pinellas, Hillsborough, and

Polk counties and is mostly thin to absent in Hernando and Pasco counties.

Sinclair and Stewart (1985) note that the southern portion of the Southwest Water

Management District (SWFWMD has limited karst development and few karst conduits

penetrate the intermediate aquifer system. The Florida Sinkhole Research Institute

(FSRI) and this database confirm this pattern in karst development. The FAS lies deeper

in these regions.

The FAS underlies the entire study area and is dominated by Paleocene to Miocene

carbonate sediments (Miller 1986). The thickness of the Floridan within the state varies

from 100 feet to over 3,500 feet and within the study area, is between 1,400 and 3,400

feet thick ( USGS 1990). Its major units are the Avon Park and Oldsmar formations, and

also include the Ocala and Suwannee limestones (Scott 1992). The upper Floridan within

peninsula Florida, is made up mostly of the Ocala Limestone (Scott 1992). The FAS

supplied more than 60% of the total groundwater used in the state in 1985 (USGS 1990),

which was 2,503 million gallons of water per day (Mgal/d).

The Ground Water Atlas of the United States' (USGS 1990) publication shows that

within the study area, in 1980, daily withdrawal from the FAS in Polk County was 200 to









350 million gallons, Hillsborough withdrew 100 to 300 million Mgal/day, Pasco, 50 to

100 Mgal/day, and Pinellas and Hernando both withdrew 20 to 50 Mgal/day. Of the total

FAS withdrawal, about 30% is from 5 counties, and 4 of these are within the study area -

Polk, Hillsborough, Pasco, and Pinellas (USGS 1990).). Sinkholes are induced by

aggressive pumping (draw-down of wells). This is due to abrupt changes in ground water

levels which disturbs the equilibrium of karst features and overburden (Tihansky 1999).

Climate

Florida is located between latitudes 24.50 and 31.00 degrees north, (Henry et al.

1994) and the study area is within latitudes 27.65 and 28.70 degrees north. Florida is

classified as having both a subtropical and tropical climate, but all of the study area is

within the subtropical region ["humid subtropical" according to the K6ppen classification

system of 1918 (Henry et al 1994)]. It is also characterized as low relief and high rainfall

(Miller 1997). Precipitation recharges the aquifers in Florida. The average annual

rainfall for the twenty year period from 1981 thru 2001 for the study area ranges from 52

to 55 inches (132 to 140 cm) by county. Historically, between 1915 and 2001, the

average annual rainfall for the same areas ranges from 49 to 53 inches (124 to 135 cm),

(SWFWMD 2003; NOAA 2003). This illustrates that the trend for rainfall has varied

little over the past 85 years. The average temperature in the study area over the past 80

years has ranged from 72 to 740, with the average low during this period being 61 to 62

degrees and the average high between 82 to 840 (SWFWMD 2003).

Continentality and oceanicity are the terms used to classify an area as to whether

land mass or oceans (respectively) has the most influence over its climate. According to

Currey (1974), Florida is classified as Oceanic, and has the least continentality of the

eastern United States (Henry et al. 1994). This has an influence on the precipitation






15


occurring in the state, which is approximately 150 billion gallons per day (Henry et al

1994). Movement of these great quantities of acidic water through the

limestone/dolostone aquifer system has promoted the dissolution process.
















Table 2.1. Relationship of regional hydrogeologic units to measure stratigraphic units
Ir J iI -I -i rL i


PANHANDLE FLORIDA


SYSTEM SERIES LITHOSTRATIGRAPHIC UNIT HYDROSmATI-
GORAPHIC UNIT
i -... ...


QUARTERNARY


TERTIARY


CRETACEOUS
AND OLDER


HOLOCENE



PLEISTOCENE


PULIOCENE







MIOCENE


UNDIFFERENTIATED
PLEISTOCENE-HOLOCENE
SEDIMENTS


CITRONELLE FORMATION
MICCOSUKEE FORMATION
COARSE CLASTICS

ALUM BLUFF GROUP
PENSACOLA CLAY
INTRACOASTAL FORMATION
HAWTHORN GROUP


BRUCE CREEK LIMESTONE
ST.MARKS FORMATION
CHATTAHOOCHEE FORMATION

CHICKASAWHAY LIMESTONE
SUWANNEE LIMESTONE
OLIGOCENE MARIANNA LIMESTONE
BUCATUNNA CLAY


EOCENE


OCALA LIMESTONE
CLAIBORNE GROUP
UNDIFFERENTIATED SEDIMENTS


----


PALEOCENE


I II I I I I I


UNDIFFERENTIATED PALEOCENE ROCKS


UNDIFFERENTIATED


8URFICIAL
AQUIFER
SYSTEM






INTERMEDIATE
CONFINING
UNIT






FLORIOAN
AQUIFER
SYSTEM








BUB-FLORIOAN
CONFINING
UNIT

-, 5


NORTH FLORIDA


SOUTH FLORIDA


UTHOSTRATIGRAPHIC HYDROSTRATI- LITHOSTRATIGRAPHIC HYDROSTRAn-
UNIT GRAPnIC UNIITUNIT GRAPHIC UNIT
iI i. ..


UNDIFFERENTIATED
PLEISTOCENE-HOLOCENE
SEDIMENTS

MICCOSUKEE FORMATION
CYPRESHSEAD FORMATION
NASHUA FORMATION



HAWTHORN GROUP
STATENVILLE FORMATION
COOSAWHATCHIE FM.
MARK8HEAD FORMATION
PENNY FARMS FORMATION
ST MARKS FORMATION


SUWANNEE LIMESTONE


OCALA LIMESTONE
MON PARK FORMATION
OLDSMAR FORMATION



CEDAR KEYS FORMATION

UNDIFFERENTIATED


SURFICIAL
AQUIFER
SYSTEM





INTERMEDIATE
AQUIFER
SYSTEM OR
CONFINING
UNIT

'--I. -







FLORIDAN
AQUIFER
SYSTEM










CONFINING, ,
TUNIpT


UNDIFFERENTIATED
PLEISTOCENE-HOLOCENE
SEDIMENTS
MIAMI LIMESTONE
KEY LAROO LIMESTONE
ANASTASIA FORMATION
FORT THOMPSON FORMATION
CALOOSAHATCHEE FORMATION


TAMIAMI FORMATION



HAWTHORN GROUP
PEACE RIVER FORMATION
BONE VALLEY MEMBER
ARCADIA FORMATION

TAMPA- NOCATEE
MEMBERS


SUWANNEE LIMESTONE


OCALA LIMESTONE
AVON PARK FORMATION
LOSMAR FORMATION



CEDAR KEYS FORMATION

UNDIFFERENTIATED


.- .. p 3


SURFICIAL
AQUIFER
SYSTEM





INTERMEDIATE
AQUIFER
SYSTEM OR
CONFINING
UNIT










FLORIOWAN
AQUIFER
SYSTEM







SUB-FLORIDAN
CONFININGs ,,
UNIT, P


(from Fernald and Purdum 1998)


I I I =i i


L c


, ,















CHAPTER 3
METHODOLOGY

Methodology of Sinkhole Investigation

When there is an unexplained distress to a structure, subsurface evaluations are

usually requested by a property owner (or his representative) or an insurance company.

In areas that have high incidence of karst features, subsurface evaluations should also be

performed prior to land development. The evaluation is to determine the potential or

probability for sinkhole formation and damage to a future structure. Since a provision in

a homeowner's insurance policy covers the peril of sinkhole induced damage or loss,

determination of the cause of damage is pertinent. The rapid growth in Florida's

population may be responsible for the large increase in claims to insurance companies for

sinkhole related damage to property. The FDI Sinkhole Standards Summit could not

develop a uniform set of criteria for the investigation of subsurface disturbance and

concluded that no one uniform set of criteria could be universally applied to the

investigation of sinkhole claims, but is "site specific" (Butler et al. 1992). A list of

minimum standards that must be employed during a sinkhole investigation was compiled

for the geotechnical professionals involved in subsurface evaluations. Since

"shrink/swell clay activity may be the largest single cause of building damage in the

U.S." (Frank and Beck 1990 p.1), this Summit set forth criteria to help differentiate

between damage that occurred from sinkhole activity or shrink/swell clays. Other

deleterious soil conditions (e.g., organic) were also addressed.









Previous Studies

Many sinkhole studies in the past have been proprietary. Some have not included

studies of homes and other structures that have resulted in damage due to sinkhole

activity. One of the studies currently available to the public was conducted by the Florida

Sinkhole Research Institute (FSRI). The FSRI was founded in 1982 (and housed at the

University of Central Florida). The FSRI compiled an inventory of sinkholes that were

formed between 1960 and1996. These data are included in a 2001 report by Burwell and

Wilson. In 1992, when many issues concerning sinkhole damage were arising, in

compliance with a Florida Legislature directive, a study of sinkhole occurrence was

conducted by the Florida Department of Insurance (FDI) (Butler et al., 1992). One major

concern at that time was the role that the insurance companies would have to play in

restitution to the homeowner of a house damaged by sinkhole activity. There was no

centralized repository established for a database of sinkhole and traveling occurrence in

Florida. It was agreed upon by geologists, engineers, and academia that there was a great

and growing need for ongoing research for sinkhole occurrence in Florida, and a

centralized repository (Butler et al.1992). Although the Florida Geological Survey (FGS)

was appointed as the repository for the database, this depository has not been well

maintained, due mostly to lack of funding. The FGS provides a limited database. The

new data from this study significantly broadens the FGS database. Also, the database

from a 2003 study by Dodek is available to expand this new database.

Criteria for Sinkhole Recognition

* Criteria recommended by the FDI included the following:

* An on-site investigation that includes background history of the house, property,
surrounding properties, environment, and quality and workmanship of the structure.









* Familiarity with all the geological characteristics pertaining to the area. The
stratigraphy of an area is of major importance, since soil and rock type, and
thickness of the units is pertinent in a geological site analysis.

* Onsite soil sampling for shrink/swell clay and organic.

* Although geophysical studies are not a requirement, it has been the practice of the
geologic profession to consider geophysical surveys pertinent in an investigation to
meet the Florida Statutes in denying or verifying the presence of sinkhole activity.
The most common geophysical techniques employed in an investigation are ground
penetrating radar (GPR) and electrical resistivity (ER). The protocol for ER is
prescribed by the American Society for Testing and Materials (ASTM) and is
protocol G57-95a. The ASTM protocol for GPR methods is 6432-99.

Soil Testing

Soil samples are collected by two methods, either by hand augering or by a

standard penetration test. Standard penetration tests (SPT) involve a two-fold procedure.

Firstly soil samples are obtained and secondly, N values are recorded. An SPT is

conducted in accordance with ASTM standard D1586-99, entitled "Standard Test Method

for Penetration Test and Split-Barrel Sampling of Soils". The samples are usually

retrieved at 1.5 meter intervals. Samples are described by an on-site geologist and are

retained for further lab analysis when a need is indicated. In compliance with the ASTM

standards, an SPT boring is performed by advancing a drill bit into the ground using a

rotary drill rig. A sample is obtained by attaching a 2 inch long split-barrel to a

succession of drilling rods. The rods are advanced by the force of a 140 pound hammer

that is dropped freely from a 30 inch height. The number of blows required to advance

the bit six inches is recorded. To compute an "N" value, the sum of the blows for

advancing the second and third 6 inch increments provides the "N value". The

significance of the N value is for assigning a relative density to a soil, thereby

determining the relative stability of the subsurface material. The N value for each type of

soil has a different range of behavioral densities. For example, sand with an N value









between 0 and 4 is very loose but a limestone with an N value of a much wider range of

0 and 19 is very soft.

When appropriate, the samples might be subjected to laboratory analysis. Tests

include, but are not restricted to, clay swelling potential, grain size analysis, natural

moisture content, and percent organic. Randazzo and Smith (2003) explain the

importance of soil analyses during site investigations where property damage is suspected

to be sinkhole related.

* Expansive clay. The water content of clayey soil can affect the deformational
behavior of that soil. Atterberg limits (ASTM D4318) are a group of tests used in
determining the water content boundaries between the semi liquid and plastic states
(known as the liquid limit) and between the plastic and semi solid states (known as
the plastic limit) of a soil. This is also known as the plasticity of the soil mass.
Where the soil just begins to crumble when it has been rolled into a 1/8 inch thread,
it has reached its plastic limit, that is, it contains the percentage of moisture where
it can just retain this characteristic. When enough moisture has been added to a soil
to make it just begin to flow, it has reached its liquid limit. This range in moisture
content represents the plasticity index (PI). It is calculated by subtracting the
moisture content of a soil in the plastic state (PL) from the moisture content of that
soil in a liquid state (LL) (i.e., PI = LL PL). The United States Department of
Army (1983) recognizes a low PI to be less than 25, marginal PI to be between 25-
35, and greater than 35 to be highly plastic. Also, possible volume change of a
wetted soil should be determined. The volume change is the shrinkage factor limit
and the smaller the shrinkage limit, the greater the chance of volume change of the
soil.

* Grain Size. Clays must also be analyzed for grain size. This analysis is critical in
determining whether or not the clay's PI is relevant in causing damage to a
structure. The percentage of colloidal sized particles that make up a soil is a major
factor in the soils volume change and its plastic characteristics (Nelson and Miller
1992). Sridharan and Prakash (2000) have determined that a strong attenuation
factor in the shrinkage limit of a soil is the presence of coarse grain sizes (sand).
The acceptable sieve size for this analysis is the -200 mesh (ASTM D1140). A -
200 mesh filters out the clay and silt size particles and hence determines what
percentage of fines is contained in the material. There is more attenuation of the
shrink/swell behavior of the clay, the larger the grain size and the higher the
percentage of coarse material.

* Moisture. The natural moisture content of a soil is relevant when investigating a
site for sinkhole activity. The soil moisture percentage will indicate the relative
water saturation state. Soils with relatively low soil moisture percentages,









occurring during the rainy season or below the local water table, are indicative of
non-expansive soils (ASTM D2216).

* Hand auger borings. They are usually advanced to an approximate depth of 1.5
meters, in accordance with ASTM standards D1452-80, entitled "Standard Practice
for Soil Investigation and Sampling by Auger Borings. This results in a continuous
vertical sampling profile of the near surface materials. The technician looks for
clays and organic, and notes the water table, if it is penetrated. When clays are
present, it is very important to note at what depth they occur. Shrink/swell clay
within the upper 1.5 meters can be the cause of, or a contributing factor to,
structural damage (Chen 1988). Field identification is limited and can be
subjective, so soil samples are retained for lab analysis, which is more definitive.

* Organics. High organic content can be a contributing factor for structural damage
to both commercial buildings and homes. Organic material, especially when in a
dry environment, decomposes, decreasing the original volume of the soil. The
decrease in volume allows subsidence to occur. It is the subsidence that allows
movement of the structure and hence damage. The threshold for organic content
(ASTM D2974) in a soil before damage can take place is 5% (Frank and Beck
1990, p.20).

Geophysics

a) Ground penetrating radar (GPR) is a geophysical technique used by geologists

as an aid in interpreting the subsurface. The GPR device transmits microwave radiation

into the subsurface as it is dragged along a selected configuration on the surface of the

ground. When the radiation encounters a reflective surface, the waves are reflected back

and are collected by the GPR receiver. Materials are identified by the intensity of the

reflections they produce when the electromagnetic wave passes through them. Contacts

between rock types, foreign objects (debris and garbage for example), and most

importantly, voids and soil disturbance, can be detected. An anomaly in the subsurface is

indicated by the contrasting dielectric intensities of materials. Anomalous areas might be

indicators of raveled zones, or in filled paleosinks. These areas can be further

investigated by drilling. Where shallow clay layers are present, GPR signals can be









attenuated or absorbed, therefore limiting the effectiveness of the technique, and thus

necessitating other geophysical techniques to be used.

The GPR field data is downloaded to a computer system where it can be displayed

as a profile. A qualified geologist then interprets the profile to determine whether or not

anomalies exist and hence, whether or not sinkhole activity is present.

b) Electrical resistivity (ER) is another geophysical technique that can be

employed during a subsurface investigation. This technique is performed by placing

electrodes into the ground, using the configuration in accordance with the ASTM

standards. The Wenner electrode configuration, which is a 4 electrode procedure (ASTM

G57-95a), utilizes the four electrodes by connecting them to a direct current battery that

passes an electrical current into the ground. The resistance to flow is measured. Since

different earth materials, cavities, and manmade materials are characterized by their

resistance to flow, an evaluation of subsurface materials can be made. The depth to

which the current reaches is determined by the distance between the electrodes, and the

greater the distance, the greater the depth of flow.

One or both of these geophysical techniques was employed for all of the sinkhole

investigations included in this study.

Data

Acquisition of Data

Geotechnical related data from sites that have been investigated for sinkhole

activity from 1986-2001, provide the database for this study. This database includes 665

new sites within the study area, including 15 in Hernando, 75 in Hillsborough, 212 in

Pasco, 320 in Pinellas, and 43 in Polk (Table 3.1). The new data were obtained by

reviewing over 500 reports of geophysical investigations conducted by GeoHazards, Inc.,









a geotechnical consulting firm in Gainesville, Florida, and from the records of legal firms

within the study area. Most of the investigations were conducted at home sites, and the

remainders were located at private businesses. In order to develop a more comprehensive

study, these data have been combined with the Florida Geological Survey's database,

which documents cases from 1960 thru 1996, then displayed in a geographic information

system (GIS). This gives a total of 1,664 sinkholes and sites where sinkhole activity has

occurred.

A large percentage of these new (unpublished) cases are insurance-related sites

where sinkhole activity has occurred and sinkhole investigations have been performed.

As discussed in Randazzo and Smith (2003), and Zisman (2003), a geotechnical survey is

used to determine if sinkhole activity is occurring at a site. Geotechnical results are then

reviewed by the responsible insurance company, and if a settlement can not be negotiated

between the insurance company and the insured, litigation may ensue. When sinkhole

activity is recognized, an engineering company will be retained to evaluate mitigation of

the structure's damage.

The following is a description of the data used in this study, how it was obtained

and processed for review and analysis in GIS.

* More than 368 subsurface investigations performed by GeoHazards, Inc., from
1986 through 2001. The firm uses geophysics and other techniques to evaluate
subsurface conditions for the occurrence of sinkhole activity. The reports reviewed
contained detailed information of the investigation including on site descriptions
and diagrams of the site, hand auger results, lab analysis when applicable,
geophysics, and SPT's when applicable. Each site was identified by an
alphanumeric address and in order to display the sites on a map of Florida within
ArcView, the Excel file (Appendix 1) had to be put into a database format (dbf) and
then geocoded (converted into a latitudinal and longitudinal coordinate system).
The program used for geocoding was EZlocate and was downloaded from Teleatlas
at Teleatlac.com. The newest version of ArcView 8.3 (ArcMap) has included a
geocoding extension, which was utilized for geocoding additional addresses. An









additional 287sites reported by a legal firm are included, though they did not all
include all the parameters that were in the GeoHazards, Inc. reports.

* The Florida Sinkhole Research Institute's database of sinkhole occurrences,
between 1960 and 1996, is included in this study as an ArcView layer and occurred
in multiple Florida counties. In 1995 Spencer and Lane published the data in the
FGS Open File Report 58, "Florida Sinkhole Index". At that time it was available
in dBase format and is now current through 2001. It is expected to be updated with
additional cases that have been reported by private citizens, the Water Management
Districts, Department of Transportation, Sheriff's departments and the Departments
of Community Affairs. The FDEP verified all the reports prior to adding them to
the database. In recent years a private company, "Subsurface Evaluations, Inc."
converted the data into a Microsoft Excel spreadsheet and it is now available on the
FGS website. The file has been projected into Albers Spheroid Global Reference
System (GRS) 1983 HPGN (Central Meridian, -84 degrees; Central Parallel, 24
degrees; Standard Parallel 1, 24 degrees; and Standard Parallel 2) and was
downloaded from the Florida Geological Survey web site,
http://www.dep.state.fl/us/geology/gisdatamaps.

* The Geologic map of Florida published by the FDEP was downloaded from
http://www.dep.state.fl/us/geology. This map was in Albers HPGN projection. It
is included as an ArcView layer to enable the overlay of other data to identify
correlations that might exist between sinkhole occurrence and stratigraphy or
geology.

* The Florida base map and county maps were obtained from the Florida Geographic
Data Library (FGDL). These are available on the GeoPlan website,
http://www.fgdl.org/ or at the GeoPlan office at the University of Florida on CD.
The digital maps are in Albers projection and are based on HPGN.

* The County Potentiometric surface maps for 1975, 1985, 1994, and 2000 were
obtained from SWFWMD website at,
http://www.swfwmd.state.fl.us/data/dataonline/. This data layer was in UTM Zone
17, North American Datum 83/90 (NAD83) projection so had to be reprojected into
Albers HPGN, which was completed by a SWFWMD GIS technician. They were
then imported into this study as an ArcView layer.

* Thickness of the overburden to the top of the FAS maps were obtained from
ftp.dep.state.fl.us/pub/gis/data. These data had not yet been published and were
only available in Raster format. Therefore, to make them compatible with the other
themes, it was necessary to convert them into Polygon format and project them into
Albers.

* A Physiographic Provinces map was obtained from http://www.fgdl.org/. Like
other FGDL data, its projection was Albers, therefore compatible with the other
study themes.









* A map indicating areas of the recharge and discharge zones of the FAS was
obtained from SWFWMD. It was projected in UTM Zone 17, North American
Datum 83/90 (NAD 83), so in order to import into ArcView as a theme compatible
with the other data, it was converted to Albers by using the ArcView Projection
Utility. The values were then reclassified into workable categories.

* A map that shows the 30 top pumped wells in each county of the study area for
1980, 1985, 1990, 1996, and 2001was constructed from data supplied by
SWFWMD. The data were in Excel spreadsheet format and degree/minute/second
coordinates. The coordinate system was converted to decimal degrees and then
imported into ArcView as a theme.

* Other data that are included in this study but not available in GIS format, are
individual maps addressing ground water and the Floridan aquifer. The maps were
taken from the online version of "The Ground Water Atlas of The United States",
at http://www.capp.water.usgu.gov/, and include (i) a map of the thickness of the
Floridan aquifer system, (ii) a map showing the 3 areas in the state with the largest
withdrawal from the Floridan aquifer in the state. One of these lies within the
study area. (iii), a map showing the amount of freshwater withdrawal from the
Floridan aquifer, and (iv) a map of transmissivity of the Floridan aquifer.

The reports reviewed for this study contained physical addresses for each site. The

addresses were geocoded (assigned longitude and latitude), to enable projection of the

points onto a map within ArcView. By converting the data to a GIS format, multiple

layers of different variables were constructed and overlain. This allowed the layers to be

"turned-off and turned-on" so that any pattern of sinkhole occurrence could be more

easily recognized. Features that have correlations such as geomorphologic features and

sinkhole distribution, soil types, geology, and hydrogeologic parameters can be

recognized. The geology, stratigraphic data, rainfall data, potentiometric surfaces, and

water withdrawal from the Floridan aquifer can then be integrated with these features.

Processing of Data

After each set of data was collected it was then reviewed and converted to an

ArcView theme, when possible, or compared side-by-side with physical maps when

digital data was not available. By putting the data in a GIS format, layers could be









integrated, overlain, or removed. This made it possible to make correlations and identify

patterns in the distribution of occurrence of sinkholes/raveling, the geology, stratigraphy,

potentiometric surface, high magnitude springs (not in digital format), rainfall (not in

digital format), and the recharge and discharge of the Floridan Aquifer System. Each of

these layers is shown in the chapters that follow. For the data that were not in a digital

format (Springs, Rainfall) a side-by-side comparison was made to the digital data. Table

3.1 provides a summary of the main parameters of each county's data.

With the data in ArcView, analyses were made and exported to Excel, where tables

and graphs were generated for further analyses. Sinkhole distribution is the parameter to

which all other data was compared. Sinkhole density was calculated by dividing the

number of sinkholes by the total area for each parameter and sinkhole relative frequency

was determined by dividing the number of sinkholes per parameter by 1,664, the total

number of sinkholes in the study area (Figure 3.1). When analyzing the data, both

sinkhole density and sinkhole relative frequency have been considered. This was an

attempt to address some obvious biases. One is called a "collection bias". Collection (or

reporting) of data can be affected by certain factors, such as where population is greatest.

Where population is greatest reporting would be expected to be greatest. Use of relative

frequency can reduce reporting biases. Instead of simply using the number of sinkholes

per square kilometer (sinkhole density) utilization is made of the percentage of the

number of times the sinkholes occur within a given parameter (sinkhole relative

frequency). Despite the inherent biases of databases such as that developed in this study,

actual sinkhole locations have been identified on a regional scale and allow for causal

correlations.










Table 3.1. Parameters of the counties of the study area
Number of Relative Sinkholes
County Area km2 Sinkholes Frequency / km2

Pinellas 748.97 369 22.9% 0.49
Pasco 1980.95 431 25.9% 0.29
Hillsborough 2773.54 404 24.8% 0.15
Hernando 1276.41 203 12.2% 0.16
Polk 5208.30 257 15.4% 0.05
Totals 11,988.17 1664


I1lometers
0 20 40 80
Figure 3.1. Counties of the study area showing locations of sinkhole activity of both the
old database and new (after Florida Geographic Data Library 1990)














CHAPTER 4
RESULTS

The set of maps, charts, and tables presented in this chapter provide an effective

visualization of each parameter used for analyses with sinkhole distribution. Each

parameter was overlain on the sinkhole distribution map, calculations made and exported

to Excel where tables were constructed. The parameters in digital format include

hydrogeology, major well fields, physiographic provinces, recharge and discharge,

overburden to the FAS, subsurface geology, potentiometric surface (for 4 years),

thickness of the FAS, and population density for 2002. The non-digital data used for

determining correlations included rainfall and springs. The tables and charts show both

sinkhole density and its relative frequency.

Hydrogeology

Overburden Thickness to the top of the Floridan Aquifer System

Data for this theme were provided by the FDEP. The FDEP constructed the

model by subtracting the top of the FAS coverage from the Floridan digital elevation

model (DEM). They obtained the Top of the Floridan maps from multiple sources and

spliced the maps together in order to make a state-wide theme. Categories of

overburden thickness reflect generally acceptable divisions that provide a more useful

resolution of sinkhole distribution for this theme. The sinkhole theme was joined to the

overburden coverage and the results are in Table 4.1 and Figure 4.1.

Data are also presented in histogram format (Figures 4.2 and 4.3). Figure 4.2

indicates that there is a correlation between the number of sinkholes per km2 (sinkhole









density) and the thickness of the overburden (i.e. depth to the aquifer). There is a greater

distribution of sinkholes where the depth to the aquifer is less 75 feet or less. Though

sinkholes do occur in areas where the overburden is thicker than this, the occurrence is

much less. For example, where the thickness of the overburden is greater than 300 feet,

sinkhole occurrence is less than 1%. Sinkhole activity in these more thickly covered

areas may be due to extraordinarily large cavity systems residing below the overburden.

The sinkhole relative frequency results (Figure 4.3) indicate an even stronger

correlation between sinkhole distribution and overburden thickness. It indicates that

approximately 73% of the sinkholes occur in overburden less than 75 feet thick and 27%

in areas where the overburden is greater than 75 feet.

Table 4.1. Thickness of the overburden to the Floridan aquifer system, compared to
various sinkhole parameters
Overburden # of Sinkholes/ Relative
Depth in feet Sinkholes Area (km2) km2 Frequency
0-15 358 1,655 0.22 21.5%
16-30 314 960 0.33 18.9%
31-50 318 1,284 0.25 19.1%
51-75 226 589 0.38 13.6%
76-100 62 449 0.14 3.7%
101-150 95 1,532 0.06 5.7%
151-200 115 1,208 0.10 6.9%
201-300 162 2,439 0.07 9.7%
301-400+ 14 1,676 0.01 0.8%
















S*Sinkoles Ravelling 51-75

S Rivers 76-100
Floridan Overburden
101-150
0-15 ft
151-200
M16-30
31530 201-300
31-50
301400+


Kilometers
0 20 40 80


Figure 4.1. Distribution of sinkholes and overburden thickness to the Floridan aquifer system (after Florida Department Environmental Protection raster data 2003)


N


_ _~~__~~~ _~~_~


I














0.40 0.38



0.35- 0.33



0.30

025

E 0.25-
0.27


M 0.20-


C)
,. 0.14
C 0.15-


.10
0.10-,
.07
0.06



0 01










Figure 4.2. Sinkhole density in relation to thickness of overburden







32




25%










a) 1501 13 6%
t-
2.
U.
). 9.7"%


The 6.9Fh
5.70

591- 3.7


0 8'a


__- I-- II I -"
0-15 16-30 31-50 51-75 76-100 101-150 151-200 201-300 301-400+
Overburden Thickness (feet)


Figure 4.3. Relative frequency of sinkholes in relation to thickness of overburden to the
Floridan

The FAS is confined within some geographic areas and unconfined in others. It is

noted in the USGS Ground Water Atlas of the United States (1990) that large solution

cavities are present in areas where the confining unit is thin or absent. The map (Figure

4.4) representing the confining units, was not available in digital format, so an overlay

with the Sinkhole and Overburden themes was not possible. A side by side comparison

of Figure 4.1 and Figure 4.4, shows that the study area has three categories of

confinement (1) the upper confining unit thin or absent (2) upper confining unit generally

less than 100 feet thick, breached, or both (3) and areas where the confining unit is













generally greater than 100 feet and not breached. Where the sinkhole occurrence is high,


Figure 4.1 shows that the thickness of the overburden is 75 feet or less. Figure 4.4 also


indicates that in these areas, the upper confining unit of the Floridan is either absent or


thin, less than 100 feet or breached, or it is both.


In these situations where the overburden is thin, and the confining unit is thin or


absent, acidic waters can more easily leach units of the FAS and cause dissolution of the


limestone beds. As could be expected, the comparison also shows that where the


Floridan is confined and generally greater than 100 feet thick, that sinkhole occurrence is


less.


_- W
...-. ,' I' n ..' -- ,






EXPLANATION
SArea here Froridon aquifer systern is uronfined- icpr-
c ,rl.nmin L-n, ,% ahbs.rl r- hn
R Area here Floridan aquUer system is thInl) ronned- -pp*.r
,..'i'lirgn 1 Jr i s V y rc lallv Ic" i= 1 00 klct c'h br cch'-.d.
or both
Area where Fioridan aquifer system Is confined-- pper confin-
ing unit is qc rrally greater then 100 feet thick end urnrfe4ched

Lower Floridan aquifer confined by more than 200 feet of low-
permeabllty rocks
------ Approximate rnit of upper confining unit


The ciaycy rocks of the upper cnhinb~q unit of
the Hortdan aquifer system have been eroded aw~ay cornpletWy in
ptere~s and arr tes.t than 1() feet Ihkk in nilwt 4caceI ltarV
solution openings. some of which cause shnkhoks, we dtciloped
in the Floridan chiefly where this confining unit is thin or absent.


SCALE 1 5.000.00O
S LO 100 MILES
0 10 100 %jLWOlE1U4S


Figure 4.4. Confining units of the Floridan aquifer system. (after U.S. Geological
Survey "Ground Water Atlas of the United States, modified from Miller 1990)


,,
I~i;" ~''iMn(K1a(rPICU~t t98a
I)-









Physiographic Provinces

Fourteen Physiographic Provinces are joined with the sinkhole density theme in

ArcView (Figure 2.2). Table 4.2 indicates that the physiographic province with the

highest sinkhole density is the Coastal Swamps province (approximately 0.5/km2), an

area underlain by the Tertiary Suwannee Limestone. It is a bedded pure to slightly sandy

limestone (Randazzo 1997). The sinkhole density and relative frequency data from Table

4.2 are shown in Figures 4.5 and 4.6 respectively. The table indicates that the highest

sinkhole density is in the Coastal Swamps Province, 0.49 sinkholes per km2, and the next

is the Gulf Coastal Lowlands at 0.33 sinkholes per km2. Following the Coastal Swamps

Province and the Gulf Coastal Lowlands provinces, the next highest occurrence of

sinkholes (sinkholes per km2) is within the Lakeland Ridge and Winter Haven Ridge,

each with a density of approximately 0.20/km2. Although the greatest density of

sinkholes is in the Coastal Swamps province, the greatest relative frequency for sinkholes

(Table 4.2) is within the Gulf Coastal Lowlands region (66% vs only 7.6% for the

Coastal Swamps).

Within the database there are 1,106 documented occurrences of sinkholes in the

Gulf Coastal Lowlands and 234 in the Polk Uplands. Since there is a great difference in

total area for the provinces, the total relative frequency of sinkholes (% sinkholes per

kilometer) versus the total square kilometers in each physiographic province was

compared (Figure 4.7). Although the Gulf Coastal Lowlands and the Polk Uplands are

almost identical in size, the relative frequency of sinkholes for the Gulf Coastal Lowlands

is almost 5 times greater than it is for the Polk Uplands. The Polk Upland covers 26% of

the study area and has 14% of the total number of sinkholes. The Gulf Coastal Lowlands

covers 28% of the total study area but contains 66% of the total number of sinkholes.









Also, the Gulf Coastal Lowlands has a much greater density of sinkholes (Figure 4.5)

than the Polk Uplands (0.33 /km2 and 0.08/ km2 respectively).

For further analyses other parameters of the physiographic provinces were

compared. An overlay of maps depicting the physiographic provinces (Fig. 2.2) and the

subsurface geology (Figure 4.8) indicates that the Gulf Coastal Lowlands are mostly

covered by Quaternary age beach ridge dunes and undifferentiated sediments. And the

Bone Valley Member of the Peace River Formation (Hawthorn Group) underlies most of

the Polk Uplands (Campbell 1984; White 1970). The Bone Valley sediments are sand

and clayey fine sand, with montmorillonite and some concentration of phosphorite grains.

Within the Polk Uplands lie three north to south trending ridges, the Lakeland

Ridge, Lake Wales Ridge, and the Winter Haven Ridge. It was described by White

(1970) and later by Lane (1986) that the Lakeland Ridge and the Brooksville Ridge

(which is also within the study area) have star-shaped sinkholes on their edges. This

current GIS analysis also indicates that the Lakeland Ridge and the Winter Haven Ridge

have high sinkhole concentrations along their edges (Figure 2.2). But when sinkhole

relative frequency was computed, it indicated that they have only a relative frequency of

less than 3% each (Table 4.2). Figure 4.8 indicates that all of these ridges are

characterized by the Cypress Head Formation, surrounded by Bone Valley Member of

the Peace River Fm (Hawthorn Group) and reworked Cypress Head Undifferentiated

Sediments.











Table 4.2. Sinkhole parameters of the Physiographic Provinces
Area # of Sinkholes/ Relative Percent of
Province km2 Sinkholes km2 Frequency Total Area
Bombing Range Ridge 111 0 0.00 0.0% 0.9%
Brooksville Ridge 1,111 49 0.04 2.9% 9.3%
Coastal Swamps 256 126 0.49 7.6% 2.1%
Gulf Coastal Lowlands 3,351 1106 0.33 66.5% 28.0%
Lake Upland 541 2 0.00 0.1% 4.5%
Lake Wales Ridge 791 15 0.02 0.9% 6.6%
Lakeland Ridge 232 41 0.18 2.5% 1.9%
Osceola Plain 798 0 0.00 0.0% 6.7%
Polk Upland 3,119 234 0.08 14.1% 26.1%
Tsala Apopka Plain 156 1 0.01 0.1% 1.3%
Western Valley 362 5 0.01 0.3% 3.0%
Winter Haven Ridge 260 43 0.17 2.6% 2.8%
Zephyrhills Gap 680 42 0.06 2.5% 5.7%
Other 187 0 0.00 0.0% 9.2%

I


Figure 4.5. Sinkhole density in each of the Physiographic Provinces of the study area


S 0.30-

S 0.25-

0.20- 0.18

0.15-

0.10-

0.04
0.05 0.02
0.00 .0I00.. 0.000

Physc 0.00iog hic vin





Physiographic Province


0.00


~t~"~k~* g94


F













70% 66.5%



60%-



50%-






30% -


20% -
14.1%

10% 7.6%
00%/ 0 25% 2.6%
0.0 0.1 3% 0.0%
0%







Physiographic Province


Figure 4.6. Relative frequency of sinkholes in each physiographic provinces of the study
area







38



70% 66.5%


%- E0 Relative Frequency
60%-
E % of Total Area
50%-


O 40%-


28.0%
30%- 2 26.1%


20%
14.1%
9.3% 92%
10% 7.6% 6.6% 6.7% 2.6% 5.7%
2.9%
.24.5% 2.5/o 3% 3.0% 2.5%
0 0%9 0.0







Physiographic Province


Figure 4.7. Physiographic provinces versus sinkhole relative frequency and percentage
of total area















To


Ts




.**f .-
I.


.


* '


.1* *.".


Sibsltiiface Geoltlg\
Foi zin alltn


.. i I*.ene :e i

-. .mjEcr~rr~d,tf ~XirJ


T T -* !. '-: i ,.i -',e. seds
STQu-Undiff. seds
STQuc-Reworked Cypress Hd
C Tc-Cypress Head
S Th-Hawthorn Gp
C- That-Hawthor Gp/Arcadia Fm/Tampa Mem
C3 Thp-Hawthorn Gp/Peace River
7 Thpb-Hawthom GplPeace River/Bone Valley Mem
To-OcalaLS
li Ts-Suwanee LS


".0~ ~


4 .4 :.**E.~
4.


S.j
I-


* V


TQNu I~. *- i


LJ
&-


3 20


To -r

ZU.






'Ici n
Tm~






pbo.
:i~~ 4J ...;





I- .)



IT

S iN
9~~~j "'i 5N .f



= 2


S- Kilometers

40 80


Figure 4.8. Distribution of sinkholes and the Subsurface geology of the study area, with overlay of rivers (after Florida Department of Environmental Protection 2000)


AN









Recharge/Discharge in the Floridan Aquifer System

Annual recharge and discharge data are collected by the water management

districts, and represent the amount of water that enters and leaves the FAS annually. The

digital data for this theme were downloaded from the SWFWMD's website. The annual

data is reported in inches and has been categorized into generally accepted discharge and

recharge zones (Table 4.3). The statewide theme was clipped to represent the counties of

the study area, and then a spatial join with the sinkhole theme made it usable for

calculating how many sinkholes fall within each zone (Figure 4.10). Overlays with other

themes allowed for an overall examination of parameters that show correlations to the

amount of water entering or leaving the aquifer.

Table 4.3. Area of the discharge and recharge zones of the Floridan aquifer system in
relation to various sinkhole parameters
Recharge/ Relative
Discharge No. of AREA Sinkholes Relative Frequency
inches/yr Sinkholes km2 /km2 Frequency total
Discharge < 1 24 506 0.05 1.4%
Discharge 1 5 785 2,932 0.27 47.2% 49.5%
Discharge > 5 15 155 0.10 0.9%
Recharge < 1 84 1,817 0.05 5.1%
Recharge 1 10 501 4,804 0.10 30.1% 50.5%
Recharge > 10 255 1,773 0.14 15.3%

The study area has 1,664 sinkhole sites with 824 (0.23 sinkholes/km2) in the

discharge zones and 840 (0.10 sinkholes/km2) in the recharge zones. In Table 4.3 the

zone with the highest sinkhole density (0.27) and the highest relative frequency of

sinkholes is the discharge zone 1 to 5 inches/year, 47.2%. This is shown in histogram

format in Figures 4.10 (a) and (b). The second highest density of sinkholes falls where

recharge is >10, and the second greatest relative frequency of sinkholes is 30.1%, where

recharge is 1 to 10 inches/year (Table 4.3).









The FSRI noted in its report of sinkhole occurrence that there is a high correlation

between the density of sinkhole occurrence and the recharge rate in some Florida areas

(Frank and Beck 1990), but the new sinkhole data do not reflect this relationship. Since

dissolution of limestone from acidic water is a precursor to sinkhole formation, and

considering Frank and Beck (1990), it might be expected that the areas with the highest

amount of recharge, i.e., greater than 10 inches annually, would have the greatest number

of sinkholes. However the new data show that zones where the recharge and discharge is

less than 1, the sinkhole occurrence is least.

Findings by Dodek (2003) concur with Frank and Beck (1990) that there is a

relationship between high sinkhole activity and the recharge zones. Dodek (2003) found

that within his study area, the greatest relative frequency of sinkholes occurred where the

recharge is greater than 10 inches per year, and the highest sinkhole density is within the

zone of recharge 1 to 10 in/yr. Dodek (2003) concluded that the recharge zones are more

acidic than discharge areas, and hence, more dissolution should occur. When evaluating

these findings, location of the discharge areas must be considered. Within the study area,

the discharge zones with high sinkhole activity lie on the west coast, which may be a

contributing factor to their formation. According to Upchurch and Randazzo (1997), in

Florida, dissolution of limestone develops not only in recharge areas, but in the

saltwater/freshwater mixing zones, i.e. coastal areas.

When the Depth to the FAS theme (Figure 4.1) and the theme for

Recharge/Discharge (Figure 4.9) were overlain onto one another (Figure 4.11), it

indicated that the greatest sinkhole distribution occurred where (1) the FAS is 0 to 30 feet

and the discharge is 1 to 5 inches annually. (2) the depth to the FAS is 0 to 30 feet and the









recharge is > 10 inches per year, and (3) the thickness to the FAS ranged from 0 to 50

feet and the recharge to the FAS is 1 to 10 inches per year. Since there are no data

available regarding changes in discharge/recharge with time, the averages of

discharge/recharge were relied upon. The total relative frequency for the 3 discharge

zones is 49.5% and the total relative frequency for the 3 recharge zones is 50.5 %. Based

on this limitation, these time-averaged values may indicate that the natural recharge or

discharge to the aquifer systems is not a dominating factor in sinkhole development and

that the thickness of the overburden may have more of an influence.

When the major rivers and roads themes were overlain with the sinkhole theme

(Figure 4.9) and proximity to sinkhole locations was examined, no specific trend was

evident between the sinkhole locations and either the roads or the rivers. The rivers are

not included in the discharge theme created by FGS, but six of the seven major rivers of

the study area are located within discharge zones. Dodek (2003) also found that

discharge zones coincided with the rivers. His data showed a directional trend with one

of the major highways (US Highway 27), but a reporting bias was considered to have

affected the data. Much of his data were reported by the Florida Department of

Transportation (FDOT), hence, the sites were near roads. Within this study area, no

correlation was recognized between the sinkhole locations and the rivers and major roads.













A


n 20 40 80


- ajor Roads
Sinkholes
/ \/ Rivers
Counties
E Hernando
B Hillsborough
[ Pasco
Pinellas
D Polk


oiueters


Figure 4.9. Counties map with sinkhole locations, major roads and major rivers (after Florida Geographic Data Library 1990)
Figure 4.9. Counties map with sinkhole locations, major roads and major rivers (after Florida Geographic Data Library 1990)


_ ~ _~


olneters












/N' Major Rivers Discharge and Recharge


I !i -. ( *I.
inr^


0 20 IK
0 20 40 80


Ar


Figure 4.10. Recharge/Discharge zones measured in inches/year and sinkhole
distribution (after Southwest Florida Water Management District 2002)














.27
0.30

0.25-


E 0.20 .14

0.15- .0
) .04

0.10 .05 .05




0.00-




Recharge / Discharge inches / yr



(a)
Figure 4.11. Recharge/Discharge zones of the study area, (a) Sinkhole density (b)
Relative frequency of sinkholes







46







47.2%
50%
45%-i
40%-
%30.1%
35%
S 30% -
25%-
S2% 15.3%
20%-
S 15%
5.1%
10% 1.4% 0.9%
5 5%
0%'






Discharge/Recharge inches / year




(b)
Figure 4.11. Continued















I "-



; .
T .J' *. -.-
S*"' \ .
.S. .





J .. I ---. I -

*1, SI 5 o




.....-* ," .
*


5- 5A ** '




-a ~
-



+-. -.i ,
Ii

,. ,. 1 .. .






.- :, "1.I~ I'' I;


W Counties
Sinkholes
Overburden to Floridan
S0-15 feet
16-30 feet
Recharge/Discharge
S
QJ DISCHARGE/1 TO 5
SRECHARGE/1 TO 10


.aJFT Kilometers

f.l 0 10 20 40

.I I \

,I \
**


,I,; -. -.
1 p .

. *..
.* ..r : ." '* I *I *



*.,~,- .I. *.
%I. J I I.
: .. i m* ". .


S .. I. .
w ., .*' \ -

/ ''^ '* N" : '."y /


Figure 4.12. Overlay of the Thickness of the Overburden theme and the Recharge/Discharge theme with the sinkhole distribution theme









Potentiometric Surface

Potentiometric surface represents the total head of groundwater and is the level to

which water will rise in a tightly cased well. Potentiometric surface changes result from

recharge and discharge of the aquifers. This theme was constructed using data supplied

by the SWFWMD. Figures 4.13(a) and (b) display potentiometric surface data for 1975,

1985, 1994, and 2000. Categories of potentiometric surface span the years that the

sinkhole data was recorded and reflect years when rainfall was considered to be average

rainfall, low rainfall, and high annual rainfall (SWFWMD 2003). The rainfall values are

in Appendix (a).

Potentiometric surface maps were overlain with the sinkhole theme and

correlations identified. Figures 4.13(a) and (b) show that the highest density of sinkholes,

for all four years, is located where the elevation of the potentiometric surface is low (5 to

10 feet and 20 to 50 feet). Sinkhole formation is accelerated during periods of drought or

during artificial lowering of the potentiometric surface by over pumping of the aquifer

(Tihansky 1999). When potentiometric surface is low, there is a loss of buoyancy and a

reduced strength of the overburden; hence it can make the area conducive to sinkhole

formation. This suggests that sinkhole formation occurs during a drop in the

potentiometric surface. A lowered potentiometric surface may be a triggering factor in

sinkhole development (Upchurch and Randazzo 1997).












1974


(a)
Figure 4.13. The potentiometric surface values with sinkhole locations (a) for 1974 and
1985 (b) for 1994 and 2000 ( after Southwest Florida Watermanagement
District 2003)








































POTENTIOMETRIC SURFACE

Kiloineteas


Figure 4.13. Continued


A


2000


80













',
x
\.


Y









Geology

Themes for both the surficial and the subsurface geology were constructed. The

Polygon coverage was obtained from the FGDL (http://www.fgdl.org/). Geology

coverage for the counties in the study area were merged and then joined with the sinkhole

database, and analyses were made. The digital coverage for the subsurface and the

surficial geology showed very little variation from one another in terms of the geology.

Dodek (2003) also found this in his study area. Therefore, only the subsurface geology

was analyzed for this study.

Table 4.4 and Figure 4.14 indicate that the greatest sinkhole density occurs in the

Beach Ridge and Dune lithostratigraphic unit, which has 0.46 sinkholes per km2. The

really largest lithostratigraphic unit (Table 4.4) is the Hawthorn Group (3,654 km2),

which is also where the greatest relative frequency of sinkholes occurs (Figure 4.15).

The Hawthorn Group includes its Arcadia Formation (Miocene), Peace River Formation

(Miocene-Pliocene), and it's Bone Valley Member (Miocene-Early Pliocene). The

lithostratigraphic unit with the second highest relative frequency is the Quaternary

Undifferentiated Sediments lithostratigraphic unit (32.2%).

The Hawthorn Group's Arcadia Formation is predominately siliciclastic-bearing

carbonates, and the Peace River Formation, which overlies it, is a plastic unit (Compton

1997; Scott 1997). It contains variable amounts of carbonate with clay and quartz sand

(Compton 1997). This area contains 572 sinkholes and has a relative frequency of 34.4%

(Table 4.4), the greatest for all the geologic units.

By utilizing ArcView, an overlay of the sinkhole locations, the overburden

thickness theme (Figure 4.1), and the geology theme (Figure 4.8) were viewed. The GIS

representation reveals that the areas with the greatest density of sinkholes occur in the









Beach Ridge and Dune geologic unit and the Suwannee Limestone unit. The Beach

Ridge and Dune geologic units are mainly situated where the thickness of the overburden

to the FAS is 0 to 30 feet, and the Suwannee Limestone is occurs where the FAS is less

than 75 feet thick. Dissolution of limestone and dolomite is the precursor to sinkholes

and traveling. The Suwannee Limestone is principally a carbonate unit. The coverage

suggests a correlation between the high incidence of sinkholes, the geologic units and the

thickness of the overburden.

Table 4.4. Subsurface geology of the study area in relation to various sinkhole
parameters


Lithostratigraphic Unit
Holocene Sediments
T-Q Undiff. Sediments
Peace River Fm/Hawthorn Gp
Dunes
Plio-Pleist Shelly Sediments
Beach Ridge & Dunes
Arcadia Fm/Hawthorn Gp
Ocala Limestone
Suwannee Limestone
Cypress Head Fm
Hawthorn Group
Reworked Cypress Head
Bone Valley Mem/Peace River
Quaternary Undiff. Sediments
All Hawthorn (combined)


Area
km2
57
180
208
230
297
367
401
420
668
778
1,135
1,750
1,910
3,542
3,653


# of
Sinkholes
2
5
16
1
17
170
61
3
199
72
318
87
177
536
572


Sinkholes /
km2
0.04
0.03
0.08
0.00
0.06
0.46
0.15
0.01
0.30
0.09
0.28
0.05
0.09
0.15
0.16


Relative
Frequency
0.1%
0.3%
1.0%
0.16
1.0%
10.2%
3.8%
0.2%
12.0%
4.3%
19.1%
5.2%
10.6%
32.2%
34.4%


% Total
Area
0.5%
1.5%
1.7%
1.9%
2.5%
3.1%
3.4%
3.5%
5.6%
6.5%
9.5%
14.7%
16.0%
29.7%
30.6%













0.50- 0.46

0.45

0.40

0.35 -
0.30
2 0.30-

0.25-

i
0.20-
0.15 0.16
0.10.15
0.09

0.050. 004 003 06 0.05

S0.01
00.00 i-i R i ti 0.0








Lithostratigraphic Unit


Figure 4.14. Sinkhole density and lithostratigraphic units of study area











35%- 32.2%


30%-


25%-





10%-
S 5%12.0%




4%5.2%
5%-










Lithostratigraphic Unit


Figure 4.15. Relative frequency of sinkholes in lithostratigraphic units of study area

Major Wells in the Study Area

A theme was constructed within ArcView, showing the locations of the 30 most

highly pumped wells in each county of the study area in relation to the sinkhole

distribution (Figure 4.16). The data were supplied in Excel spreadsheet format by

SWFWMD and each well contained a longitude and latitude. These were then projected

onto the sinkhole distribution map in the same projection, Albers GCS North American

1983. In chapters 2 and 3 it was noted that increased sinkhole activity may result from

over pumping of the aquifer. An overlay of the sinkhole theme and geology theme were

used to identify correlations between the known sinkhole activity and the location of the









well. The data that are included were taken from collections in 1980, 1985, 1990, 1996,

and 2002. Some of the wells may have been active (and still active) in all of these years,

while other wells may have been capped (abandoned) and new wells put into existence.

Since 1980 there has been a significant increase in annual pumpage. The highest annual

pumpage in 1980 was in Polk County and was 1,784,834,700 gallons, and in 2002 the

highest annual pumpage occurred in Polk County, 5,928,020,000 gallons (SWFWMD

2002) (Table 4.5).

Table 4.5. Highest annual water pumpage (gallons) for top wells in each county of study
area for selected years
County 1980 1985 1990 1996 2002
Hernando 900,847,108 724,062,000 820,175,046 1,305,510,000 1,015.348.000
Hillsborough 818,505,000 2,001,658,016 1,813,487,502 861,005,600 1.241,900.000
Pasco 804,145,137 653,945,909 518,648,000 482,182,000 995,260,000
Pinellas 193,497,000 141,684,000 306,273,000 325,033,000 491,370.000
Polk 1.784,834,700 2,263,694,854 2,892,071,000 2,105,493,200 5,928,020.000


Within ArcView, it was possible to identify where sinkhole activity was high and

multiple wells existed. The four circles on Figure 4.16 show these areas. A correlation

between the large withdrawal from these wells and sinkhole distribution is generally

indicated. It appears to demonstrate that the wells may be a contributing factor, but since

some areas show a concentration of wells but not a concentration of sinkholes, other

parameters must also be considered as possible contributing factors.

When overlays of different parameters were viewed within ArcView, the

Subsurface Geology theme indicated (Figure 4.16) that the stratigraphic units in these

four areas of high sinkhole distribution are the Quaternary undifferentiated sediments,

Quaternary Beach Ridge and Dune, and the Suwannee limestone. Another important

parameter that may be a contributing factor to sinkhole formation is the thickness of the

overburden. By viewing the ArcView overlays, four areas were identified where there is









a high occurrence of sinkhole formation, a large number of highly pumped wells (Figure

4.16) and the overburden to the top of the Floridan aquifer is mostly 75 feet thick or less

(Figure 4.1).

Some biases to consider that may be having a large influence here are the

collection bias, and the limited information about the actual drawdown of each well. The

collection bias would indicate that the larger the population, the greater demand for water

and hence, more wells would be expected in the vicinity of larger populations. More

sinkholes are expected to be reported where population is greatest.













SW0 |__ HoWkeem se dime ts Cyp.IeaId Fm
S U d" ff- SdiM ri.s Hnarth Group
.T -a D.a.s Ha tH-GApl tr dbaFm
S rR Phao-RPeitslllyseds HarthomGpe River
.' .. -- \_ C.] Bm ay
SGgyd ffsedirw t f Hwatn eGpPee River




.41
Beach *dgld um *- RedxkidCygpashead : OcaIaLimestei




0 .
.* re .

oTo
i' .




I*
e *. \



f_ .s
***




J 4..

"TQu
J '' '*
-. .:Q
AI












( 0 20 40 80

Figure 4.16. Sinkhole distribution on the geology theme, with the 30 most highly pumped wells in each county for the study area. The circles indicate areas of high sinkhole
density and a large number of highly pumped wells (after Southwest Florida Water Management District 2003)
:,, ; --'# fll "" "
., -41,




2' 4"- .i' ",.
Figure~ 4.1.Snhl d :. o nth elg hm, ihte3 os ihypme wlsi ahcunyfrt 'ud ra h iclsidct raso ihs] l
dn it n ag ub o !ihl pupe well" '"te So:: .:s Flo:id, Wae Mangen ,.tc .


~









Population Density

Population density data, when applied to sinkhole distribution pattern, suffers from

biases that limit meaningful correlations. The 2000 US Census data is used in the

Population Density theme. The data were obtained from the FGDL. The US Census

Bureau delineates geographic areas into Census tracks which are made up of Census

block groups. The Census block groups are defined by the US Census Bureau as having

approximately 600 to 3,000 people, with an optimum size of 1,500 people in each Census

block group. The area of each track is in square meters. This area was then converted to

square kilometers and then broken into its associated Blocks. The population per square

kilometer was then calculated (Table 4.6). It is hypothesized that the areas with the

highest population density (people per square kilometer, Figure 4.17) would have the

greatest sinkhole occurrence. This is to be expected for many reasons. For example,

anthropogenesis is likely to be a strong contributing factor, and as mentioned before, can

be causing a reporting bias. For example many of the cases included in this thesis are

reported by homeowners that have structural damage to their home, and if the damage is

verified as being due to sinkhole activity, it might lead to other neighbors having similar

damage investigated. Hence, confirmation of sinkhole activity for several homes in one

neighborhood can increase the reported sinkhole density.

The US Census Bureau's block data (Figure 4.17) of population density was used

to compute sinkhole density and sinkhole relative frequency for specified ranges of

population (Table 4.6). Figure 4.18 shows a general trend of increased sinkhole activity

with an increase in population. Sinkhole relative frequency for the area does not indicate

as strong a correlation, but this may be explained by the U.S. Census Bureau's block

sizes. The blocks decrease in size as the population increases in size. This would









decrease the relative frequency for those blocks. This analysis agrees with Dodek (2003)

who also recognized a general trend between higher sinkhole densities and an increase in

population density. Where population is high, many anthropogenic factors may be

influencing sinkhole activity. For example, an increase of population means an increase

in water usage, which increases withdrawal from the aquifer. Figure 1.1 demonstrates the

affect of large water withdrawals. Population may also affect water drainage patterns

which may play a role in sinkhole distribution when certain other factors are present.

Table 4.6 indicates that the population range with the highest sinkhole density, 0.95

sinkholes per sq km, is 1,501 to 2,000 people/km2 range. The next three population

groups in descending order of sinkhole density are the 5,001 to 6,000 range, 3,001 to

4,000 range, and 2,001 to 3,000 range. Both the sinkhole density and the relative

frequency of sinkholes are within the 1,501 to 2,000 range (15.0%).

Analyses also show that the most populous areas, 3,000 to 6,000 people/sq km have

a much lower relative frequency of sinkholes than expected. This is the result of the

biases previously mentioned. The area where population is 3,000 to 6,000 people/sq km

represents only 33 km2 of the 11,987 km2. This is only 0.3% of the study area and might

be considered anomalous. The 0 to 50 and 101 to 200 ranges represent 67.5% of the total

study area with less than 0.02 to 0.14 sinkholes density, but have an average relative

frequency of 10.5% (Figures 4.18 and 4.19).

Overall, population density data are inadequate in addressing sinkhole distribution

issues, but they do provide interesting patterns for future analyses.













N _Population Density
II 0-50 pop km2
j50-150

S j.3. 50-500
S; .' 500-1000
'----'' oo1000-2000
S '- I 2000-3000
3000-6000
S.. .. 6000-27000



__- -,.^ -*I ','** :_ --. '\ --
-f \ -




-4,
S, -.-- 4 .4"
I j -. i 4. /
I"-L' -- ...... I --- + ..- t


II-- ., Kilometers
















0 20 40 80

Figure 4.17. Distribution of sinkholes and the US Census Bureau's Block Groups showing population density for 2000 ( from US Census Bureau 2000)
.. ',p-:1" I i ,-:-, .- _: -





.. ... -4 'j \








Table 4.6. Population density in relation to various sinkhole parameters

Range Number of Area Sinkholes / Relative
(pop/km2) Sinkholes km2 km2 Frequency
0-50 164 6,884 0.02 9.9%
51-100 87 1,090 0.08 5.2%
101-200 172 1,207 0.14 10.3%
201-300 96 588 0.16 5.8%
301-450 134 564 0.24 8.5%
451-600 98 362 0.27 5.9%
600-750 77 189 0.41 4.6%
751-1000 187 262 0.71 11.2%
1001-1500 233 380 0.61 14.0%
1501-2000 249 261 0.95 15.0%
2001-3000 140 167 0.84 8.4%
3001-4000 25 30 0.84 1.5%
4001-5000 1 3 0.33 0.1%
5001-6000 1 1 0.88 0.1%































Population Density /km2

Figure 4.18. Population density (km2) versus sinkhole density using US Census Bureau
block groups 2000







63




16%
15.0%

14.0%
14%- -


12%- 11.2%

>, 10.3%
,) 9.9%
C 10%-
S8.4%
8.1%
LL 8%-

.5.8% ,5.9%
m 6% 55.2% /


4%



... .. 1.5%.
2% 1.5%

i 0.1% 0.1%






Population Density I km2


Figure 4.19. Sinkhole relative frequency for population density 2000 (km2)















CHAPTER 5
ANALYSES

Utilization of a Geographic Information Systems analysis has made it possible to

test several hypotheses at once, recognizing contributing factors to sinkhole distribution.

In some cases, parameters could be evaluated without GIS themes, but in order to make a

stronger evaluation, a combination of factors hypothesized to be triggering factors to

sinkhole formation were overlaid by using themes a method for validating the results.

It is also a means to detect existing spatial biases. The complexities of sinkhole

distribution are demonstrated in Figures 5.1(a) and (b) and 5.2(a) and (b), which indicates

that sinkhole formation, is influenced by a combination of factors. The figures are a

complex representation of the parameters that exist in areas where the sinkhole relative

frequency and sinkhole density are greatest. Throughout this analysis, all parameters

were overlain on each other and each layer was "turned off and on" to examine for

correlations. All of the parameters addressed in Figures 5.1(a) and (b) and 5.2(a) and (b)

have been separately clipped from figures throughout this thesis to show the areas with

the greatest sinkhole activity and sinkhole relative frequency occur. The data are

summarized in Table 5.1.

Figure 5.1(a) represents the greatest sinkhole relative frequency for all the

parameters. It indicates that the relative frequency of sinkholes is greatest in the areas

where the overburden to the Floridan aquifer system is either missing or less than 30 feet

thick. The overburden thickness appears to play a large role in sinkhole formation, as

indicated by these results. This figure also shows that the physiographic province









containing the most sinkholes (greatest relative frequency) is the Gulf Coastal Lowlands.

The Gulf Coastal Lowlands has an overburden thickness that varies between 0 to 50 feet.

However, Table 5.1 displays that the highest density of sinkholes is in the Coastal

Swamps, which also has a very thin overburden to the FAS of only 0 to 15 feet (Figure

4.1 and Figure 2.2). Also, the differing results in the relative frequency and the density of

sinkholes is a reflection of the sizes of the provinces. The Gulf Coastal Lowland

represents 28% of the total study area, and the Coastal Swamps only 2%. This size bias

must be considered when making analyses. Also, the Polk Uplands represents 26% of the

total area and has the next highest relative frequency of sinkholes of only 14%, and a

sinkhole density of only 0 .08 per km2.

Figures 5.3(a) and (b) illustrate that both the relative frequency and the density of

sinkholes are greatest in the Discharge zone of 1 to 5 inches annually within the study

area. This does not correlate with the general idea that Recharge zones are the areas that

are expected to have the greatest number of sinkholes (Dodek 2003). ArcView allowed

the Recharge/Discharge theme to be overlaid by the Overburden theme (Figure 5.4) and it

shows that most of the discharge areas occur where the overburden is only 0 to 30 feet

thick, and that the discharge zones with the greatest sinkhole density are located along the

coast line (Figure 5.3) where saltwater/freshwater coastal mixing occurs (Upchurch and

Randazzo 1997). It is in these coastal areas that dissolution of limestone, and hence karst

formation, is preferentially developed (Upchurch and Randazzo 1997). The overlap of

figures also demonstrates that in many areas where the recharge zones occur, the

overburden is 100 to 300 feet thick. This suggests that the lower number of sinkholes

occurring in the recharge areas is due to the increased thickness of the overburden, and









that the high occurrence of sinkholes in the discharge areas could be related to the thin

overburden to the FAS.

Rock type has always been considered to be an important factor in sinkhole

formation. An example is the chemical weathering of carbonates (Lane 1986, p12; Beck

and Sinclair 1986; Galloway et al. 1999). As noted in Table 5.1, high sinkhole activity is

occurring where the Suwannee Limestone is at or near the surface and also a high relative

frequency of sinkholes occurs in the Hawthorn Group sediments, which is also carbonate

bearing (Compton 1997; Scott 1997). Although the Beach Ridge and Dunes and the

Suwannee Limestone units actually have the greatest sinkhole densities (Figure 4.14), the

Hawthorn Group and the Undifferentiated Quaternary Sediments units have the greater

sinkhole relative frequencies (Figure 4.15). This might be reflective of a size bias. The

Beach Ridge and Dunes and Suwannee Limestone are significantly smaller really than

both the Hawthorn Group and the Quaternary Undifferentiated Sediments (Table 4.4).

Relative frequency is a statistical operation that addresses the size bias of a sampling

population and it represents a valuable statistic accompanying sinkhole density. The

Hawthorn Group has a sinkhole relative frequency of 34.4%, but the Quaternary

Undifferentiated Sediments has a sinkhole relative frequency almost as great at 32.2%.

The Hawthorn Group contains a relatively higher percentage of carbonates (Scott 1997)

than the Quaternary Undifferentiated Sediments unit.

Other factors may be contributing to sinkhole distribution. The age (Oligocene,

Brooks 1981) and carbonate purity of the Suwannee Limestone, promote its dissolution.

Most of the Suwannee Limestone in the study area lies under an overburden of 0 to 15

feet along the coastline (Figure 4.1). The Beach ridge and Dune unit is composed of










sand, which is highly permeable, and also has an overburden of less than 30 feet thick

(Figures 4.1, 4.13).

As previously noted, population density data are extremely biased by many factors

and is temporal, but the results indicate (Table 4.4) that the greatest distribution of

sinkholes is in the zone where population density is 1,000 to 2,000. It was expected to

occur in the most highly populated areas, 3,000 to 6,000. It is noted that the US Census

Blocks are designed by population rather than area. This means that some blocks have a

much higher population density than others. This skews sinkholes per population

numbers, and when relative frequency is calculated the bias is exacerbated.

Table 5.1. Results from GIS analyses of sinkhole distribution and significant themes
Theme Highest Sinkhole Sinkholes/m2 Greatest Relative Relative
Density Frequency Frequency
Ov n 51-75 feet 0.38 0-15 feet 21.6%
Overburden
16-30 feet 0.33 31-50 19.1%
Physiographic Gulf Coastal
Physiographic Coastal Swamps 0.49 Lowlands 66.5%
Province
Gulf Coastal Lowlands 0.33 Polk Uplands 14.1%
Recharge/ Discharge 1-5 inches/yr 0.27 Discharge 1-5 47.9%
Discharge Recharge >10 0.14 Recharge 1-10 30.1%
Beach Ridge/Dune 0.46 All Hawthorn Seds 34.4%
Geology Suwannee LS 0.30 Quat. Undiff Seds 32.2%
Population 1501-2000 people/km2 0.95 1501-2000 15.0%
Density 5001-6000 0.88 1001-1500 14.0%











Sinkholes
E- Counties
Pop sqkm
1001 -100
50I1-00


Ph'. ooglz phIc Provinces Geology Formation
F7 Cww.LOvrr a d


0-IS feI
16-3feet


DischargeRechaige

Reckmgel. 10


&, ~ [I Other
- fl a'~t0'p M'. Vsky


* S


SI Kilometers


N
A













\


(a)
Figure 5.1. GIS overlay of all the areas of highest sinkhole relative frequency for all digital parameters used in the analyses, (a) with population density (b) without population density


I















S Smikholes

I Counties
Discharge/Rechaige
| Discharge/1 5 inches/yr
Recharge/ l- 10
SOther


Physiographic Provinces
W GulfCoastal Lowlands
Polk Uplands

OCverburden
0-15 feet
16-30 feet


Geology Fonnation
3 Undiff Seds
X Hawthorn Gp
SHawthorn/Arcadia Fm
SHawthom/Peace River
SHawthom/P River/Bone Valley


IU


SKilometers
0 20 40 80


Figure 5.1. Continued


'**









Geology Formation
] Beach Ridge/Dune
Hawthorn Gp
77 Hawthorn/Arcadia Fm
Hawthorn/Peace River
Hawthom/P River/Bone Valley
SSuwanee LS


( '


I]Kilometers
0 20 40 80
(a)
Figure 5.2. All digital parameters with the greatest sinkhole density for the study area (a) with the Census' blocks of population density, (b) without the Census' blocks of population density


z





71






S Sinkholes Physiographic Provinces Geology Formation
S; .. -- Counties Coastal Swamps Beach Ridge/Dune
Overburden Gulf Coastal Lowlands 3 Hawthorn Gp
S15-30 ft Dischare Recharge Hawthom/ArcadiaFm
i' --50-75 ft Discharge/1 to 5 Hawthom/Peace River
.L Recharge/Greater than 10 Hawthom/P River/Bone Valley
Other Suwanee LS



-" ..






F u 5" Co.t .-n.ed
13


"" *, I "
F r



J %*






Figure 5.2. Continued








0 20 40 80


Figure 5.2. Continued















































Sinkhole Relative Frequency
Sinkholes
SCounties
Di shcarge/Recharge
mches/yr
SDischarge/l to 5
Recharge/l tolO


Kilometers

S-- 10 20 40


". ,

* i .! .rL.
i. Ti**


II =


..-r:.'


Figure 5.3. Recharge and Dishcarge zones with sinkhole distribution. Relative
frequency and density of sinkholes


Sinkholes
7 Counties
RechargeDischarge
mches/yr
Discharge/l to 5
Recharge/Greater Than 10

Kilometers

0 10 20 40


A














aI .





aaka
S** *t* .


aI a
a *f t a, a


SI" .r *



a. --




,a, ,.- ., .- -
-i- ,,. .i









N4 0
S"''
H <'..7 -/ -- __- ; ,-:



i I L M O
"- a "





.. .: ", ..... aa
\ 1-. i ....


r, \ l C. I "..




e .' i ?


Figure 5.4. Recharge/Discharge theme overlaid with the Depth to the Floridan Aquifer System theme


I IIII en I. Fi I


,1. lll ll i 1'.' h l i 0u
,, 1 I nr .


Rediall-u e Di-,. i-t "e cul \

m t'. :.:har..- 1 I .
F i .l hrr" I I I"'


i.


a

I __________
I a I
a I


i I a a I

a a
Sm 1 -. .


*I


I .
I. I,
a I lii


aKilomelers

0 10 20 4C


-


".'. ,. .
i If .

"I i 1 ..


";--,--
II






\1 *


\
,.
1.
Ma


-'


'i:









Figure 4.16 suggests that the location of the 30 highest pumped wells in each

county correlate with the occurrence of sinkholes in at least four areas. This may

illustrate that well fields are one more influencing factor in sinkhole formation and

distribution, while a conclusion can not be made on this analysis it is useful when

examining other local factors.

Figure 5.5 shows the result of overlaying all the GIS themes and clipping out the

areas that intersect all of them. These (small) areas are outlined in red. The result

demonstrates that the formation of sinkholes does not require all of the theme conditions

to be present, but instead a combination of factors controls sinkhole distribution. From

the overlays in this analysis, tabulated in Table 5.1, it is indicated that the strongest

influencing factors for sinkhole development are discharge areas of 1 to 5 inches annually

(occurring 47% of the time), areas where the aquifer is shallow (50 feet or less), and areas

where the geologic units consist of permeable and easily dissolved materials. These

parameters are found most often in the Gulf Coastal Lowland physiographic province,

where the aquifer is close to the surface or exposed, the discharge is 1 to 5 inches

annually, and the near-surface geologic units are the Suwannee Limestone and the

Quaternary Undifferentiated Sediments.

This database can be combined with other studies to increase the accuracy of

predicting future sinkhole development. For example, an enlargement of the database

would help to substantiate findings. This can be done by adding newly collected data and

non-reported data, which for example, could be obtained by maps developed through

remote sensing. Further studies could include additional factors for parameters in this

study, which showed weak correlations to sinkhole distribution, i.e. the theme showing









Well locations in each county. Additional information could include the extent of the

well field's draw-down.

These results suggest that the thickness of the overburden to the FAS plays the

strongest role in sinkhole formation, and when combined with other sinkhole triggering

factors, sinkhole density and sinkhole relative frequency may increase. For example,

since sinkhole relative frequency is greatest where the overburden is less than 75 feet

thick and the Hawthorn Group is present (Figure 4.13 and Table 5.1) sinkhole activity

should increase. The physiographic provinces that had the greatest number of sinkholes

(Table 5.1) also coincided with areas of thin overburden and geologic units that are prone

to dissolution, and hence sinkhole formation.

A similar study by Dodek (2003) utilized the current FSRI database to increase his

database of sinkholes for 11 north-central Florida counties. His projections of future

sinkhole occurrence for several counties were based on results he determined were

influencing sinkhole development. These parameters were areas of high recharge to the

aquifer and a minimal depth to the aquifer system. He found that sinkhole locations

coincided with the presence of near-surface Ocala Limestone, a carbonate unit.

The results in this study found that there was a high density of sinkholes and a high

relative frequency of sinkholes coinciding with areas where the geologic units consisting

of near surface carbonates occurred (Table 4.4). This study did not agree with Dodek's

findings that sinkhole distribution is greatest in areas of high recharge. Factors, such as

the depth the FAS in the discharge zones, have an influence on sinkhole distribution.



















..* **




















3 Hilisborougyh
.. .












6 7.
., Polk


,*, '
.. *

*. *. __ ** ,. .


**i Hillsborou*h *

*.. ..

.. ** ***





HPinellao ,- .




Figure 5.5. GIS intersection of all the parameters that displayed the highest sinkhole relative frequency. The red areas represent the polygons that contain all the parameters













CHAPTER 6
CONCLUSIONS

A new sinkhole inventory has been created and includes the previously existing

FGS database. From this database, factors that were hypothesized to influence sinkhole

formation have been analyzed using a GIS, Esri's ArcView 8.3 (ArcView). After

calculating the sinkhole density and sinkhole relative frequency for each factor, they were

then coalesced into maps that allowed trends and patterns to be recognized. Results

indicate a correlation between sinkhole distribution and other factors, and are controlled

by a combination of parameters. New data has been obtained that enlarges the existing

database, and as this database grows, it strengthens results that are produced by any

future analyses.

Table 5.1 represents GIS results and indicate that when all parameters are

considered, sinkhole relative frequency is greatest in (1) the Gulf Coastal Lowlands (2)

overburden is 0 to 15 feet thick (3) Hawthorn sediments are present in the near-surface,

and (4) the aquifer discharge is 1 to 5 inches per year. Population density of 1,501 to

2,000 people per km2 may also be a factor. All of these parameters were extracted from

their ArcView themes and presented in Figures 5.1 (a) and (b).

When sinkhole density was computed, the results, summarized in Table 5.1, show

that the parameters where sinkhole density is greatest are where (1) the Depth to the

Floridan is 50 to 75 feet, (2) the Coastal Swamps province, (3) the discharge is 1 to 5

inches per year, and (4) the Beach Ridge and Dune subsurface geology is present.

Population density of 1500 to 2000 people per square kilometer may also be significant.









Other factors that may have an influence on sinkhole density include areas of low

potentiometric surface and water withdrawal from the aquifer system, i.e., well fields.

Figure 4.16 shows four areas where the wells with the highest withdrawal rates are

located in the vicinity where sinkhole occurrence is greatest. Withdrawal of large

quantities of water from the aquifer, contributes to the lowering of the potentiometric

surface, and may also result in sinkhole activity (Figure 1.1). Correlations of sinkhole

location and low potentiometric surface, suggests that lowering of the potentiometric

surface is another factor contributing to the formation of sinkholes. In the four years

depicted in Figures 4.11 and 4.12, the areas of highest potentiometric surface are those

where fewer sinkholes have formed (or have been reported).

The Gulf Coastal Lowlands has a sinkhole relative frequency value almost 5 times

larger than any other physiographic province, and although the Coastal Swamps has the

highest density of sinkholes, 0.49 per square kilometer (Table 5.1), the Gulf Coastal

Lowlands is close behind at 0.33. The Gulf Coastal Lowlands is a region where

Quaternary undifferentiated sediments are the most prominent geologic formation, and it

has an overburden to the FAS of 30 to 50 feet, well within the major mode for the

greatest sinkhole density and the highest relative frequency of sinkhole occurrence.

Population density is generally 1,000 to 2,000 people per square kilometer.

Biases are an important aspect in recognizing the significance of the various

parameters and sinkhole frequency. When an overlay of the major rivers and roads in the

area was made (Figure 4.10), no significant pattern or correlation was noted, but this

might be one of the biases that were discussed in prior chapters (i.e., the "collection

bias"). Of the seven major rivers, five of these are in areas that have a very low









population density, 0 to 50 people (Figure 4.17), or where population density is of 50 to

150 people per square kilometer. Within these ranges (people per km2), sinkhole density

is very low. It is only towards the mouth of the Hillsborough River that the sinkhole

density increases, and the population density in these areas also increases to 1,000 to

6,000 people per square kilometer (Figure 4.17). One could assume that the low number

of sinkholes is due in part to the reporting bias. In other words, where population is low,

reporting will be minimal, but as noted previously, population density data contains many

biases.

Future studies could increase the database by including newly reported sinkholes,

and non-reported sinkholes. Non-reported sinkholes could be obtained through digital

topographic maps, obtained by the use of remote sensing. With the increase in the

database, trends and patterns of occurrence will be more easily identified and hence, as

the database "grows", making predictions of future sinkhole distribution should be more

accurate.

In order to utilize the sinkhole database for predicting the high risk areas for

sinkhole formation, a theme that representing the distance-to-the-nearest-sinkhole was

constructed, using buffer zones of one, two, and three kilometers. In light of the large

study area, these buffers represent a reasonable category-distance resolution. The theme

is presented in Figure 6.1. It is hypothesized that the higher potential for sinkholes is

where sinkhole density is greatest and where the distance between sinkholes is least. In

Figure 6.1, that would be the 0 to 1 km zone, indicated by the blue buffer zone. Figure

6.2 also represents "nearest neighbor" but with buffers of 0.5, 1.0, and 1.5 kms. The

figures demonstrate that the highest risk areas would likely be in the north-western









section of Hillsborough County, the western section of both Pinellas and Pasco counties,

and central Polk County. The areas within Hillsborough, Pinellas, and Pasco are areas

where the FAS is close to the surface, 0 to 75 feet (Figure 4.1), and in Polk County, the

area where sinkhole density is greatest, is where the overburden thickness is 75 feet to as

great as 300 feet (Figure 4.1).

As population density grows, there will be an increasing demand for ground

water, and hence greater withdrawal from the FAS and lowering of the potentiometric

surface. This trend will impact areas where the significant factors for sinkhole formation

are concentrated, and future sinkhole density and sinkhole relative frequency will be

greatest.










Nearest Sinkhole
1:;. 0-1 km
SI 1-2km
1 C2-3 km
-- Counties


POLK


0 20 40


8'


Kilometers
0


Figure 6.1. Proximity to the nearest sinkhole, indicated by one, two, and three kilometer buffer zones


A


_~ ___ ~ __ ~ ~ ~ ~


0 7777










Nearest sinkhole
I "I 111111e
Neamest Sinkhole
0-5km
5-1km
1 1.5km


POLK


0v


77 -Kilometers


0 20 40


AN


I


Av%~


80














APPENDIX A
GLOSSARY OF ARCMAP TECHNICAL TERMS

Esri's ArcMap 8.3 was used for processing data within this thesis. Some of the

tasks performed are known as Geoprocessing. The following are descriptions of these

procedures:

* Clip: a process where a specified layer, known as the cookie cutter, is used to cut
another layer, know as the input layer. The result is a new layer that has only the
polygons and attributes inside the clip layer, while those outside the clip layer do
not remain.

* Dissolve: the process of combining polygons with identical attributes. The
attribute is specified by the user.

* Geocoding: the process of matching street addresses with geographic coordinates

* Geographic coordinates: a measurement by latitude and longitude on the earth's
surface.

* Join: the process of attaching tabular data to a layer. The fields in the table are
appended to the layer using a common field. Join establishes a one-to-one, one-to
many or many-to-many relationship between map features and table attributes.

* Layer: Geographic information is displayed on a map as layers; each layer
represents a particular type of feature such as streams, lakes, or highways. Layers
are listed in the ArcView table of contents and can be further organized into data
frames. A layer references geographic data stored in a data source, such as
coverage, and defines how to display it.

* Layout: the design or arrangement of elements such as geographic data, elements
like north arrows, legends, scale bars, and text, in a digital map display or printed
map.

* Merge: combining polygons from two or more themes. Attributes with the same
name are retained.

* Projection: a mathematical formula that transforms feature locations from the
earth's curved surface to a map's flat surface. A projected coordinate system
employs a projection to transform locations expressed as latitude and longitude






84


values to x, y coordinates. Projections cause distortions in one or more of these
spatial properties: distance: area, shape, and direction.

S Theme: a category within a layer.















APPENDIX B
HISTORICAL RAINFALL FOR STUDY AREA


Table B.1. Rainfall
Year Hernando Pasco Pinellas Hillsborough Polk

1974 66.83 65.98 67.21 53.11 52.31
1985 48.83 47.96 42.43 44.65 43.29
1994 54.81 51.75 43.03 55.20 56.70
2000 44.55 43.81 39.78 44.31 38.96

Min 44.55 43.81 39.78 44.31 38.96
Max 66.83 65.98 67.21 55.20 56.70
Avg 53.76 52.38 48.11 49.32 47.82
















REFERENCES CITED


Arthur, J. D., R. A. Lee, and L. Li. 2001. Lithostratigraphic and Hydrostratigraphic cross
sections through Levy-Marion to Pasco counties, Southwest Florida. Florida
Geological Survey Open File Report no.81. 21 p.

Beck, B., and W. C. Sinclair 1986. Sinkholes in Florida: An Introduction. The Florida
Research Institute Report no.85-86-4. 17 p.

Brooks, H.K. 1981. Geologic Map of Florida, Institute for Food and Agricultural
Sciences, University of Florida, Gainesville, Florida.

Burwell, A.L. and W.L. Wilson. 2001. New sinkhole proximity maps for selected
Counties in Central Florida. Published by Subsurface Evaluations, Inc., Tampa,
Florida. 16 p.

Butler, A.M., B.A. Diskin, K.L. Eastman, D.H. Gatzlaff, R.B. Corbett, C.C. Lilly, and
P.F. Maroney. 1992. Executive Summary in Insurance study of sinkholes. Florida
State University Center for Insurance Research, Tallahassee, Florida. 13 p.

Campbell, Kenneth M. 1984. Geology of Hillsborough County. Florida Geological
Survey Open File Report no.6, Tallahassee, Florida. Florida Geological Survey.
19 p.

Casper, J., B. Ruth, and J. Degner. 1981. A remote sensing evaluation of the potential for
sinkhole occurrence. Remote Sensing Applications Laboratory, Department of
Civil Engineering, University of Florida, Gainesville, Florida. 103 p.

Chen, F.H. 1988. Foundations on Expansive Soils. Developments in Geotechnical
Engineering 54:65-69. New York. Elsevier Scientific Publishing Company.

Compton, J. S., 1997. Origin and paleoceanographic significance of Florida's deposits in
The Geology of Florida edited by A.F. Randazzo and D.S. Jones, 195-216.
Gainesville, Florida. University Press of Florida.

Currey, D.R. 1974. "Continentality of extratropical climates," Annals of the Association
of American Geographers 64:268-280.

Department of Army USA 1983. Foundation in Expansive Soils. Washington, DC, TM
5:818-7.









Dodek, B. 2003. A GIS Analysis of Sinkhole Activity in North-Central Florida: Causes
and Correlations. Master's Thesis University of Florida. Gainesville, Florida.
79 p.

Fernald, E.A and E.D. Purdum, editors 1998. Water resources atlas of Florida: Institute
of Science and Public Affairs, Tallahassee, Florida. Florida State University,
310 p.

Florida Department of Environmental Protection 2000. Florida Physiographic Provinces.
Florida Department of Environmental Protection ftp site, May 2003.
http://www.dep.state.fl/us/geology/pub.

Florida Department of Environmental Protection 2003. Raster data obtained from FDEP
staff (unpublished data), then it was converted to digital format for this map.

Florida Department of Environmental Protection 2000. Florida Subsurface geology.
Florida Department of Environmental Protection ftp site, May 2003.
http://www.dep.state.fl/us/geology/pub.

Florida Geographic Data Library 1990. Florida county boundaries: http://www.fgdl.org

Florida Geological Survey web site, November 2002. http://www.dep.state.fl/us/geology.

Frank, E.F., B.F. Beck 1990. An analysis of the cause of subsidence damage in the
Dunedin, Florida area. 59 p.

Galloway, D., D.R. Jones, and S.E. Ingebritse 1999. Sinkholes, West-Central Florida.
Excerpt from Circular 1182:121-140. United States Geological Survey.

Green, R., J.D Arthur, and D. DeWitt 1995. Lithostratigraphic and Hydrostratigraphic
cross sections through Pinellas and Hillsborough counties, Southwest Florida.
Open File Report no.61, 1-26. Florida Geological Survey, Tallahassee, Florida.

Healy, H.G. 1975. Terraces and shorelines of Florida. Florida Bureau of Geology Map
Series no.71.

Heatherington, A.L. and P.A. Mueller 1997. Geochemistry and origin of Florida crustal
basement terranes, in The Geology of Florida edited by A.F. Randazzo and D.S.
Jones, 27-37. University Press of Florida, Gainesville, Florida.

Henry, J.A., K.M Portier, and J. Coyne 1994. The Climate and Weather of Florida.
Sarasota, Florida. Pineapple Press, Inc. 249 p.

Hine, A.C. 1997. Structural and paleoceanographic evolution of the margins of the
Florida platform, in The Geology of Florida edited by A.F. Randazzo and D.S.
Jones, 169-194. University Press of Florida, Gainesville, Florida.









Hyde, L.W. 1975. Principal Aquifers in Florida, Map Series No. 16, United States
Geologic Survey and Bureau of Geology, Tallahassee, Florida.

Lane, E. 1986. Karst in Florida, Special publication # 29, Florida Geological Survey,
Tallahassee, Florida. 100 p.

Miller, J.A. 1986. Hydrogeologic Framework of the Floridan aquifer system in Florida
an in parts of Georgia, South Carolina, and Alabama, United States, professional
paper 1403-B. Geological Survey, Department of the Interior, Washington D.C.-
91 p

- 1997. Hydrogeology of Florida, in The Geology of Florida edited by A.F. Randazzo
and D.S. Jones, 69-88. University Press of Florida, Gainesville, Florida.

National Oceanic and Atmospheric Administration, National Weather Service 2003
http://www.srh.noaa.gov/tbw/climate, May 5, 2003.

Nelson, J.D. and D.J. Millerl992. Expansive Soils: problems and practice in foundation
and pavement engineering, New York. John Wiley and Sons, Inc. 43 p.

Pirkle, E.C., W.H. Yoho, and C.W. Hendry, Jr. 1970. Ancient Sealevel Stands in Florida
Bulletin no.52, 61 p. Published by the Bureau of Geology Tallahassee, Florida.
61 p.

Randazzo, A.F. 1997. The sedimentary platform of Florida: Mesozic to Cenozoic, in The
Geology of Florida edited by A.F. Randazzo and D.S. Jones, 39-56. Gainesville,
Florida. University Press of Florida.

- and D.L.Smith 2003. Subsidence-induced foundation failures in Florida's karst terrain.
Sinkholes and the engineering and environmental impacts of karst. Proceedings of
the ninth multidisciplinary conference, Huntsville, Alabama, 82-94. Edited by
Barry F. Beck, American Society of Civil Engineers.

Schmidt, W., 1997. Geomorphology and physiography of Florida, in The Geology of
Florida edited by A.F. Randazzo and D.S. Jones, 1-12. Gainesville, Florida,
University Press of Florida.

- and T.M. Scott, 1984. Florida karst-Its relationship to geologic structure and
stratigraphy. Proceedings of the First Multidisciplinary Conference on Sinkholes,
11-16.

Scott,T. M. 1992. A Geolocgical Overview of Florida. Florida Geological Survey Open
File Report no.50, Tallahassee, Florida. 77 p.

Scott, T.M. 1997. Miocene to Holocene history of Florida, in The Geology of Florida
edited by A.F. Randazzo and D.S. Jones. University Press of Florida, Gainesville,
Florida. Structural features of the Florida peninsula p 58.











- 1997. Miocene to Holocene history of Florida, in The Geology of Florida edited by
A.F. Randazzo and D.S. Jones, 57-67. University Press of Florida, Gainesville,
Florida.

- 2001. Text to accompany the Geologic map of Florida, Florida Geological Survey
Open File Report 80. Tallahassee, Florida.

Sinclair, W.C. 1982. Sinkhole development resulting from ground-water withdrawal in
the Tampa Area, Florida: U.S. Geological Survey Water Resources Division. 19 p.

-, J.W. Stewart, R.L. Knutilla, A.E. Kilboy, and R.L. Miller 1985. Types, features, and
occurrence of sinkholes in the karst of west-central Florida: U.S. Geological
Survey Water Resources Investigations.81-50. 19 p.

and J.W. Stewart. 1985. Sinkhole type development and distribution in Florida:
Florida Geological Survey Map Series 110. Tallahassee, Florida. Florida
Geological Survey.

Smith, D.L. and K.M. Lord. 1997. A Techtonic Evolution and Geophysics of the Florida
Basement. In The Geology of Florida edited by A.F. Randazzo and D.S. Jones,
13-26. Gainesville, Florida. University Press of Florida.

Southeastern Geological Society Ad Hoc Committee on Florida Hydrostratigraphic Unit
Definition 1986. Hydrogeological units of Florida: Florida Geological Survey
Special Publication 28. Tallahassee, Florida. 8 p.

Southwest Florida Water Management District 2002. Recharge and Discharge areas of
the Floridan aquifer system. http://www.swfwmd.state.fl.us/data/gis, November
20, 2003.

-2002. Map of five-county study area.
http://www.swfwmd.state.fl.us/data/gis/library/political.htm. May 2003.

-2003. Potentiometric base maps and potentiometric surface obtained separately from
http://www.swfwmd.state.fl.us/data/data/dataonline.htm. May 2003.

-2003. Well pumpage and locations were supplied in Excel format by SWFWMD
employee via email June 2003.

Spechler, R.M. and D.M. Schiffer 1995. United States Geological Survey fact sheet
FS-151-95, 1 sheet.

Spencer, S.M. and Ed Lane 1995. Florida Sinkhole Index. Open File Report no.58,
Talahassee, Florida. Florida Geological Survey. 18 p.









Sridharan, A. and K Prakash 2000. Shrinkage limit of soil mixtures. ASTM Geotechnical
Testing Journal 23:3-8.

Teleatlas: http://www.na.Teleatlas.com, November 3, 2002

Tihansky, A.B. United States Geological Survey. 1999. Human-induced sinkhole
development in the study area, Pasco and Hernando counties. Sinkholes, West-
Central Florida, A Link Between Surface Water and Ground Water, an excerpt
from Galloway, Devin, D.R. Jones, and S.E. Ingebritsen, Circular 1182, p 138.

-,United States Geological Survey. 1999. Sinkholes, West-Central Florida, A Link
Between Surface Water and Ground Water, an excerpt from Galloway, Devin, D.R.
Jones, and S.E. Ingebritsen, Circular 1182, p 127.

-,United States Geological Survey. 1999. Sinkholes, West-Central Florida, A Link
Between Surface Water and Ground Water, an excerpt from Galloway, Devin, D.R.
Jones, and S.E. Ingebritsen, Circular 1182, 121-139.

United States Census Bureau 2000, Population density by U.S. Census block groups:
http://www.census.gov/geo/www/cob/bg2000.html

United States.Geological Survey 1990. Confining units of the Floridan aquifer system.
(modified from Miler). Ground Water Atlas of the United States: Alabama,
Florida, Georgia, and South Carolina, publication HA 730-G

- 1990. Ground Water Atlas of the United States: Alabama, Florida, Georgia, and
South Carolina, publication HA 730-G. Online version;
http://capp.water.usgs.gov/gwa, November 18, 2003.

United States Geological Survey 1990. National Water Summary 1987: hydrologic
events and water supply and use. U.S. Geological Survey Water-Supply Paper
no. 23, 550-553.

Upchurch, S.B. and A.F. Randazzo. 1997. Environmental geology of Florida. In
Geology of Florida, edited by A.F. Randazzo and D.S. Jones, 217-48. Gainesville,
Florida. University Press of Florida.

Walker, K.R., G. Shanmugam, and S.C. Ruppel 1983. A model for carbonate to
terrigenous plastic sequences. Bulletin of the Geological Society of America
no. 94, 700-712.

Wetterhall, W.S. 1964. Geohydrologic Reconnaissance of Pasco and Southern Hernando
Counties, Florida. Report of Investigations No. 34, Tallahassee, Florida. Florida
Geological Survey. 12 p.

White, W.A., 1970. The Geomorphology of the Florida Peninsula:Bureau Geology.
Florida Department of Natural Resources, Bulletin no.51, 69 p.






91


Zisman, E.D. 2003. Guilty until proven innocent Sinkhole definition & identifying
features. Sinkholes and the engineering and environmental impacts of karst.
Proceedings of the ninth multidisciplinary conference, Huntsville, Alabama.
Edited by Barry F. Beck, American Society of Civil Engineers, 124-129.




Full Text

PAGE 1

SINKHOLEACTIVITY IN WEST -CENTRAL FLORIDA: A GEOGRAPHIC INFORMATION SYSTEMS ANALYSIS By KATHLEEN COMMINS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITYOFFLORIDA 2004 '"

PAGE 2

ACKNOWLEDGMENTS Although there are many to whomlowethanks, it is Dr. Randazzo to whom I give special thanks.Itwas his outstanding patience and guidance that kept me on the right track.Ithank Dr. Smith for being a committee member. Also, duringmytenure in Geological Sciences at the UniversityofFlorida; both Dr. Randazzo and Dr. Smith have always shown enthusiasm for geology and far exceeded their callofduty.Iwould also like to thank Dr. Meert for being such a conscientious committee member, and due to his input, the final product is a better one! Special thanks go to GeoHazards, Inc, for allowing me to access proprietary materials for this database, and for gainful employment while working on it.Iwould like to acknowledge all those at the Southwest Florida Water Management District for their patience and for supplying someofthe data within this thesis; and to Sam Palmer for his assistance, and for sharing his ArcView expertise with me. Without this selfless cooperationIwould still be trudging on! ii

PAGE 3

TABLE OF CONTENTS ACKNOWLEDGMENTS ii LIST OF TABLES v LIST OF FIGURES vi ABSTRACT viii CHAPTER 1 INTRODUCTION 1 Background 1 Sinkhole Architecture 2 Purpose 5 2 GEOMORPHOLOGY OF THE STUDY AREA 7 Structural and Depositional History 7 Geomorphology 9 Hydrogeology11Climate143 METHODOLOGY17MethodologyofSinkhole Investigation17Previous Studies18Criteria for Sinkhole Recognition18Soil Testing19Geophysics21Data 22 AcquisitionofData 22 ProcessingofData 25 4 RESULTS 28 Hydrogeology 28 Overburden Thickness to the topofthe Aquifer System 28111

PAGE 4

Physiographic Provinces 34 RechargelDischarge in the Floridan Aquifer System.40Potentiometric Surface48Geology51Major Wells in the Study Area 54 Population Density585 ANALYSES 64 6 CONCLUSIONS 77 APPENDIX A GLOSSARY OF ARCMAP TECHNICAL TERMS83B HISTORICAL RAINFALL FOR STUDY AREA85REFERENCES CITED 86 BIOGRAPHICAL SKETCH92iv

PAGE 5

LIST OF TABLES2.1Relationshipofregional hydrogeologic units to measure stratigraphic units163.1Parametersofthe countiesofthe study area274.1 Thicknessofthe overburden to the .Floridan aquifer system, compared to various sinkhole parameters 29 4.2 Sinkhole parametersofthe Physiographic Provinces 36 4.3 Areaofthe discharge and recharge zonesofthe Floridan aquifer system in relation to various sinkhole parameters 40 4.4 Subsurface geologyofthe study area in relation to various sinkhole parameters...52 4.5 Highest annual water pumpage (gallons) for top wells in each countyofstudy area for selected years554.6 Population density in relation to various sinkhole parameters61 5.1Results from GIS analysesofsinkhole distribution and significant themes67v

PAGE 6

LIST OF FIGURES Figure1.1Human-induced sinkhole development in the study area, Pasco and Hernando counties (after Tihansky 1999) 3 1.2 Typesofsinkholes (from Tihansky 1999).41.3Mapofthe five-county study area (from Southwest Florida Water Management District 1990) 62.1Structural features of the Florida peninsula (from Scott 1997) 8 2.2 Locationofsinkholes in physiographic provincesofthe study area123.1 Countiesofthe study area showing locationsofsinkhole activityofboth the old database and new ; 27 4.1 Distribution of sinkholes and overburden thickness to the Floridan aquifer system 30 4.2 Sinkhole density in relation to thicknessofoverburden.314.3 Relative frequencyofsinkholes in relationtothicknessofoverburdentothe Floridan 32 4.4 Confining unitsofthe Floridan aquifer system (after U.S. Geological Survey "Ground Water Atlas ofthe United States, modified from Miller 1990)334.5 Sinkhole density in eachofthe Physiographic Provincesofthe study area 36 4.6 Relative frequencyofsinkholes in each physiographic provincesofthe study area 37 4.7 Physiographic provinces versus sinkhole relative frequency and percentageoftotal area 38 4.8 Distribution of sinkholes and the Subsurface geologyofthe study area, with overlayofrivers 39 4.9 Counties map with sinkhole locations, major roads and major rivers .43VI

PAGE 7

4.10RechargelDischarge zones measured in inches/year and sinkhole distribution ...... .44 4.11 RechargelDischarge zonesofthe study area, (a) Sinkhole density (b) Relative frequencyofsinkholes 45 4.12 Overlayofthe Thicknessofthe Overburden theme and the RechargelDischarge theme with the sinkhole distribution theme .47 4.13 The potentiometric surface values with sinkhole locations .49 4.14 Sinkhole density and lithostratigraphic unitsofstudy area 53 4.15 Relative frequencyofsinkholes in lithostratigraphic unitsofstudy area .54 4.16 Sinkhole distribution on the geology theme, with the30most highly pumped wells in each county for the study area 57 4.17 Distributionofsinkholes and the US Census Bureau's Block Groups showing population density for 2000 60 4.18 Population density (km2 )versus sinkhole density using US Census Bureau block groups 2000 62 4.19 Sinkhole relative frequency for population density 2000(km2 )63 5.1 GIS overlayofall the areasofhighest sinkhole relative frequency for all digital parameters used in the analyses 68 5.2 All digital parameters with the greatest sinkhole density for the study area705.3 Recharge and Dishcarge zones with sinkhole distribution 72 5.4 RechargelDischarge theme overlaid with the Depth to the Floridan Aquifer System theme 73 5.5 GIS intersectionofall the parameters that displayed the highest sinkhole relative frequency 76 6.1 Proximity to the nearest sinkhole, indicatedbyone, two, and three kilometer buffer zones816.2 Proximity to the nearest sinkhole, indicatedby0.5, 1.0, and 1.5 kilometer buffer zones 82 vii

PAGE 8

AbstractofThesis Presented to the Graduate Schoolofthe UniversityofFlorida in Partial Fulfillmentofthe Requirements for the DegreeofMasterofScience SINKHOLE ACTIVITY IN WEST-CENTRAL FLORIDA: A GEOGRAPHIC INFORMATION SYSTEMS ANALYSIS By KathleenH.Commins May 2004 Chair: Anthony F. Randazzo Major Department: Geological Sciences Geographic Information Systems (GIS), involving spatial analysis, was used to examine correlations between locationsofsinkhole activity and the factors influencing sinkhole distribution in West Central Florida. For the analyses, new data were collected from private sources and combined with the existing databaseofthe Florida Geological Survey. The new data include 648 sites that were investigated from 1990-2001. Factors that were suspected to be influencing sinkhole distribution were overlain and examined for correlations. These factors were compared using sinkhole density and sinkhole relative frequency and included the following themes: thicknessofthe overburden overlying the Floridan aquifer system, physiographic provinces, discharge/rechargeofthe Floridan aquifer system, potentiometric surface data, Florida's subsurface geology, the major wells in each county, and the U.S. Census Bureau's population density block groups. Analyses showed some strong and some weak correlations. viii

PAGE 9

Certain factors appear to beofsignificance. Within the study area, the greatest sinkhole activity is where the thicknessofthe overburden to the Floridan aquifer system is 75 feet or less, where the near surface geology consistsofthe Hawthorn Group, the Quaternary undifferentiated sediments, or the Suwannee Limestone, and within the Gulf Coastal Lowlands and Coastal Swamps province. The new database was used to construct maps showing locationsofsinkhole activity, and the various themes that could affect sinkhole frequency and distribution.Inan effort to address predictionofareasoffuture sinkhole activity, a GIS applicationofnearest neighbor occurrencesofsinkholes was conducted.Itproduced a map showing areas where future sinkhole activity can be expected. These include mostofNorthern Pinellas, southwest Hernando, the western coastlineofPasco, and northwestern Hillsborough counties.IX

PAGE 10

CHAPTER 1 INTRODUCTIONBackgroundThe most commonly recognized featureofkarst topography is the sinkhole. Sinkholes are a significant geologic hazard and lead to a lossofmany millionsofdollars each year. Sinkhole-related land subsidence problems are global and occur within 45 states in the United States, affecting over 17,000 square miles (Gallowayetal. 1999). A better understandingofsinkhole development in Florida isofimportance to planners at all levels. These include homeowners; architects; and engineers who design both private and commercial structures; land developers and realtors; insurance agencies; bankers; and local governments that issue permits for construction, waste disposal, and water use. Awarenessofsinkhole development and areas most sensitive to its occurrence can be turned into a tool to be used to help avoid damage and financial loss caused by this geologic feature. Florida's rain water has a pHof4.77 on average (Upchurch and Randazzo 1997), and is an integral participant in the dissolutionoflimestone. As rain percolates through the soil, it obtains CO2from the metabolizationofhumus (from organic soils), lowering the pHofthe water entering the aquifer and dissolving Florida's Eocene to Miocene carbonate bedrock, particularly during sea-levellowstands. Dissolution has given the carbonate bedrock its high permeabilityor"swiss-cheese characteristic." One direct benefitofthis is the Floridan aquifer system (FAS), but a negative effect is the sinkhole1

PAGE 11

2 activity and the ravelingofoverburden into the voids left in the bedrockbythe ongoing dissolution process. Many sinkholes are anthropogenically induced and are mainly caused by either water withdrawal from wellsorconstruction activities (Lane 1986). It is these activities that can trigger instability, leading to sinkhole formation. For example, Sinclair (1982) documented that in Tampa, Florida over 30 small sinkholes occurred northofa municipal well-field within 1 yearofthe massive pumping from this well. One well field increased from 5 mgaIJd to14mgal/d, and 64 new sinkholes were reported within 1 month. Figure1.1shows some anthropogenically induced sinkholes.Sinkhole ArchitectureSinkholes formbytwo processes (Sinclairetal. 1985): raveling, which is downward erosionofunconsolidated sediments into preexisting cavities; and collapse, a process where weak acidic solutions dissolve carbonate bedrock, creating cavities that can collapse,orin which unconsolidated overburden sediments can migrate. Lithology and thicknessofthe overburden determine sinkhole morphology (Sinclair 1985; Schmidt and Scott 1984). According to Schmidt and Scott (1984) the frequencyofsinkhole occurrences in Florida, and their sizes are associated with the thicknessofthe overburden (i.e., depth below land surface) to the FAS. In other words, both frequency and karst feature size are directly related to stratigraphy. The three classic sinkhole types in Florida are Dissolution, Cover-collapse,and Cover-subsidence (Figure 1.2). Dissolution sinkholes are formed by weak acidic solutions infiltrating areas where carbonate is exposedoronly a thin layerofoverburden is present (Tihansky 1999). The acidic solution dissolves the underlying carbonates, creating cavities. In many cases, the

PAGE 12

3water flow is focused along fractures, beddingplanes, or joints. This typeofsinkhole usually forms gradually as a bowl-shaped depression (Beck and Sinclair 1986). In 1998, multiple sinkholes appeared suddenly during constructionofan irrigation well on a 20 acre site in Florida. Figure 1.1. Human-induced sinkhole developmentinthe study area Pasco and Hernando counties (after Tihansky 1999). Cover-subsidence sinkholes usually develop where the sediments are permeable and contain sand (Tihansky 1999). Gradual raveling (downward migrationofgranular sediments into openings in underlying carbonates)isknown as piping, and the structures formed are called solution pipes. As dissolution and infilling continue, a small surface depression forms (Tihansky 1999). The overburden thickness may range from 15.2 meters (50 feet) or greater (Sinclair and Stewart 1985). Cover-collapse sinkholes are usually in areas where thereisa thick coveringofsediments that contain a large amountofclay (Tihansky 1999). The clays usually separate the sandy overburden from the underlying carbonates. Clays can provide some cohesion to the overburden, and when a solution cavity develops below, the overburden

PAGE 13

4 can act as a "bridge." The collapse occurs when the solution cavity can no longer support the overburden and the "bridge" gives way. This kindofsinkhole can develop rapidly and cause catastrophic damage (Beck and Sinclair 1986).Dissolutionofthe limestoneordolomite is most intensivewherethe water first con tacts the rocksurface. Aggressive dissolution also occurswhereflowis fucussed inpre existing openingsinthe rock, suchasalong joints' fractures, andbeddingplanes, and in the :zone ofwater-table fluctuationwhere groWld waterisincontact with the atmosphere.On exposed carboniltesurfaces. a depressionmayfOOlS surface drainage,acceleratingthedissolutionprocess.Debriscarried intothe developingsinkhole mayplug theout!Iow,pendingwater and creating wetlands. Gentlyrollinghillsandshallowdepressions caused by solution sinkholes are common topographicfeature.Hhroughout muchofFlorida.Cover-subsidencesinkholes tend to develop graduallywherethe covering sedi ments are permeable and contain sand.Granular sedimentsspallintosecondaryopeningsinthe underlyingcarbonate. rocks. A column of overlyingsediDissolutionandinfilling conments settles intothe tinue, forminganoticable vacatedspaces (a processdepression inthelandtermed"piping). SlJrTace.The slow downwarderosioneventually fOnTIssmallsur facedepressions I inch to several feet indepthand diameter. Inareas where covermaterialisthickerorsediments contain more day, cover-subsidence sinkholes are relativel}runcommon. are smaller. and maygoundetected for long periods. Cover-collapse sinkholesmay develop abruptly (over a periodofhours) andcausecatastrophic damages. Theyoccurwhere the covering sediments contain a significant amountofday. Sedimentsspall into acavity. As spalIing continues.the. mhesive coveringsedi. mentsforma structuralarch.Thecavitymigriltes up ward by progressiveroofcollapse. The cavityeventuallybreaches the ground sur face,creating suddenand dramaticsinkholes. Overtime. surface drainage. erosion. and depositionofsedimenttransformthe steep-,\'3lled sinkhole into a shallower bowl-shaped depression.Figure 1.2. Typesofsinkholes (from Tihansky 1999)

PAGE 14

5PurposeThe purposeofthis study is to broaden existing sinkhole database inventories and to assemble information into a Geographic Information System (GIS), making it accessible to the many sectorsofthe community affected by the environmental dangers and economic impact that sinkhole activity causes. The database covers 5 Florida counties where sinkhole activity is high (Figure 1.3). Results in this study will be compared to resultsofa similar study donebyDodek (2003) for any correlations. A further objective is to determine whether or not increased sinkhole occurrence is correlated to geologic features and anthropogenesis (Le., aquifer withdrawal rates). To achieve these objectives, the data are comparedasspatial thematic coverage in separate GIS layers, allowing data to be easily retrieved and compared for correlations. The software used is ArcMap, GIS software that was developedbythe ESRI company. Specialists can use this information as a tool for identifying areas with high potential for sinkhole activity,orto more accurately access the environmental impact that can be endured safely. A Florida DepartmentofInsurance (FDI) study in 1992 (Butleretal. 1992) pointed out the importanceofidentifying and predictingsinkhole occurrence. This database and study should be very beneficial for better understanding the rates and distributionofkarst evolution.

PAGE 15

6NISTUDY AREAI __ -------.r IPinellassoFigure 1.3. Mapofthe five-county study area (from Southwest Florida Water Management District 1990)-------------------

PAGE 16

CHAPTER 2 GEOMORPHOLOGY OF THE STUDY AREAStructural and Depositional HistoryEarly Mesozoic rifting and sea-floor spreading between the African and North American plates (Hine 1997) formed the basement for the Florida peninsula, a preJurassic crystalline rock body overlain unconformably by Jurassic and Cretaceous sediments (Smith and Lord 1997) with the thickest sediments occurring in the southern partofthe peninsula (Heatherington and Mueller 1997). Situated on the North American Plate, Florida's coastline is currently a passive margin, but there are features within the basement that indicate Florida has undergone some structural changes throughout its geologic history. The structural features (highs and lows) in Florida that influenced depositionof sediments include the Ocala Platform (Uplift); the South Florida Basin; the Jacksonville Basin, and the Chattahoochee Anticline in the northwest; the Sanford High in the east; the Peninsular Arch; and the Georgia Channel System (which collectively, is the Apalachicola Embayment, the Gulf Trough, and the Southeast Georgia Embayment) (Schmidt and Scott 1984). Figure 2.1 shows someofthe structural featuresofFlorida that suggest that the Florida platform responded to tectonic forces during the Mesozoic Era (Scott 1997). During the Cenozoic, the Peninsular Arch appears to have been the major high that affected sedimentation in the Paleogene; and the Ocala Platform had the most influence on the depositionoflate Paleogene and Neogene sediments. 7

PAGE 17

8AXISOFPOSITNEFEATUREJACKSONVILLE BASIN ST.JOHNS PLATFORM 0so 100 ISO 200MILES I i0 100200JOO KILOMETERSSCALE0. \LEGENDAXISOFNEGATIVEFEATURE __ APPROXIMATEUPDJPLIMIT / ANDAREAUNDERLAINBYTHE .-/ FLORIDANAQUIFERSYSTEMFigure 2.1. Structural featuresofthe Florida peninsula(from Scott 1997)The depositionofthe thick carbonate sequence covering the platform began in the Jurassic, after the African plate rifted from the North American plate, with intermittent periodsofsiliciclastic deposition until the Cenozoic. The first 35 million yearsofthe Cenozoic were dominated by depositionofcarbonates, and evaporites (Scott 2001), until the beginningofthe Miocene, when transportofsiliciclastic sediments over the peninsula predominated (Walker et al. 1983). Up until then, the Gulf Trough's currents diverted

PAGE 18

9the Appalachian sediments away from the Platform. The reason for the renewed siliciclastic influx is debatable, but its deposition, occurring on the platform in the Late Oligocene, was due to the Gulf Trough's currents no longer occupying the Georgia Channel System after the Late Oligocene sea level drop. The Appalachian siliciclastics prevailed, covering the Florida Platform by the middleofthe Pliocene. The supply diminished by Late Pliocene (Scott 1997). Hawthorn sediments are represented in almost the entire Miocene and partofthe lower Pliocene sectionsofthe study area. These include the Arcadia Formation (Hawthorn Group, with the Tampa and Nocatee members) and the Peace River Formation (with the Bone Valley Member and Wabasso beds) (Scott 1997). Subaerial exposure occurred during low-level stands in the Neogene and Quaternary (Scott 1997). DepositionofEoceneorlower Oligocene, through Miocene sediments from the Appalachian Mountains was blocked due to the Gulf Trough! Apalachicola Embayment. The sediments exposed in the study area are mainly the Pleistocene-Holocene undifferentiated sands, and the Hawthorn Group. Geomorphology Marine forces were the dominating factor in creating the landformsofFlorida, (Schmidt 1997). When the Florida Platform was coveredbythe sea, erosion and deposition from the shallow marine currents left behind flat plains and scarps. Whenever the sea covered the Florida Platform, the shallow marine currents and their associated erosion and deposition shaped the shallow seabed, leaving subsequent erosional forces to modify the geometry. The exposed partofthe Florida Platform is the StateofFlorida.Itis approximately halfofthe Platform and is about 650kmeast to west and 800 km north to south. The

PAGE 19

10highest point in Florida is 104 meters above sealevel, in Walton County, northwestofthe study area. Land forms help to interpret the geologic history. Dissolutionofbedrock has formed basins for lakes in many partsofcentral Florida (Miller 1987). Both sea-level and land-level changes influence the rateofdeposition and erosion (Schmidt 1997), continually changing the geomorphology. Varying sea-levels have alternately flooded and exposed the Florida platform, and eroding and depositing sediments. When the water table is low and the limestone bedrock is close to the surface, drainage is usually "internal" (Schmidt 1997) with streams "disappearing" into the subsurface. This is karst topography and is a common drainage characteristic in Florida which has developed from dissolutionofthe carbonates that exist throughout the platform. Water can drain through sinkholes rather than runningoffthrough stream systems. Florida's extensive aquifer systems were "carved" out by the movement and chemical actionofthe acidic waters. The reactionofthe water with the limestone and dolostone, is summed up by the following, which show the chemical reactions when rainwater comes in contact with the land surface. The carbon dioxide needed for this reaction to occur is obtained from the atmosphere and the soil. The disassociationofthe carbonic acid allows it to react with calcite to form karst features. CO 2gas+H20 +-+ H2C03carbonic acid H2C03 +-+ H++HC03 CaC03calcite+H2C03 +-+ Ca2 ++2HC03Figure 2.2 shows that the study area lies within several physiographic provinces which are described by White (1970). The largest are the Gulf Coastal Lowlands, occupying, 3350km2,andthe Polk Upland, 3119km2 The smaller provinces are the

PAGE 20

11Coastal Swamps, Brooksville Ridge, and Zephryhills (Florida DepartmentofEnvironmental Protection (FDEP) 2000). The Gulf Coastal Lowlands in the study area are mostly underlain by the Suwannee Limestone and the more deeply buried Ocala Limestone (Green et al. 1995). The lithologyofthe Suwannee Limestone, early Oligocene in age, is described as a packstone or grainstone carbonate with trace amountsofsand and clay in the upper portions (Green et al. 1995; Arthur el al.2001). The Polk Upland occupies the western portionofHillsborough County, and mostofPolk County (Figure 2.2). Exposed Bone Valley Member, Peace River Formation (Middle Miocene to Lower Pliocene) sediments make up the western partofthe Polk Upland in eastern Hillsborough County, and the southeastern sectionofthe Polk Upland in Polk County. These sedimentsofquartz sand, silt, and clay contain phosphorite grains, generally with poor consolidation. The Bone Valley Member has been economically important in the phosphate industryofthe United States (Scott 2001). On the eastern edgeofthe Polk Upland lies the Lake Wales Ridge and the Osceola Plain, both undifferentiated sands. Hydrogeology Hydrogeologic framework plays a large role in when and where sinkholes are formed. In west -central Florida the framework is made upofthree aquifer systems that consistofcarbonates and siliciclastics rock. The ground water levels (hydraulic head) and chemistry determine where dissolutionofthe limestone will occur. Sinkholes also influence the hydrogeologic framework (Tihansky 1999). The Floridan (FAS), the Intermediate (lAS), and the Surficial (SAS) aquifers are present in the study area. The FAS underlies mostofthe State (Hyde 1975; Wetterhall

PAGE 21

12 1964) and is the main sourceofpotable waterinthe study area, Table 2.1 shows the regional hydrogeologic units' relationship to the major stratigraphic units in Florida. oCounties_Lake Henry R.dge Sinkholes LauUplandPh\"slogrnphlrRedous,...,LakeWale'R.dge -L-JLakelandIUdgeoBrook""UeIUdgePolltUplandoCoastal Swamps TubApopltaPla:mo Gulf Barner Ch"n 0Wln,erHavenR.dge DGulrcoastaiLowlands Z.phyrlull,Gap-,lakeUpland --40PolkUpland20 ,. :. -========:::JI Kilometers80oI" Pasce" .''. ,",Hernando.. -:;.8"V" ..,..::. IA.....',.,...,' .... Zephyrhillsap., z "\!C,:".:,-.;. .. ...... .. .. ...5..J ..... .... ....... :" .... "'t .,'""' '.':":I.... 'j"j:.'\,.' ",\'....': ..:I','. )::," ",.. :(, .: :. (tl ":""1:. 'r';'t:r"' "0 ',0)...\P Ol.
PAGE 22

\,.' 13District (SWFWMD) the SAS is a limited water resource in the northern section, butofincreasing importance to the southofthe study area (Scott 1992). The Southeastern Geological Survey (SEGS 1986) defined the lAS as the rocks that lie between the Surficial and Floridan aquifers, retarding water exchange between the two. The lAS consistsofsiliciclasticsofthe Hawthorn Group interlayered with Miocene and younger carbonates (Scott 1992). Cross-sections developedbyArthur (2001) and Greenetal. (1995) indicate that the lAS is present in partofPinellas, Hillsborough, and Polk counties and is mostly thin to absent in Hernando and Pasco counties. Sinclair and Stewart (1985) note that the southern portionofthe Southwest Water Management District (SWFWMD has limited karst development and few karst conduits penetrate the intermediate aquifer system. The Florida Sinkhole Research Institute (FSRI) and this database confirm this pattern in karst development. The F AS lies deeper in these regions. The FAS underlies the entire study area and is dominatedbyPaleocene to Miocene carbonate sediments (Miller 1986). The thicknessofthe Floridan within the state varies from 100 feet to over 3,500 feet and within the study area, is between 1,400 and 3,400 feet thick(USGS1990). Its major units are the Avon Park and Oldsmar formations, and also include the Ocala and Suwannee limestones (Scott 1992). The upper Floridan within peninsula Florida, is made up mostlyofthe Ocala Limestone (Scott 1992). The F AS supplied more than 60%ofthe total groundwater used in the state in 1985 (USGS 1990), which was 2,503 million gallonsofwater per day (Mgal/d). TheGroundWater Atlasofthe United States' (USGS 1990) publication shows that within the study area, in 1980, daily withdrawal from the FAS in Polk County was 200 to

PAGE 23

14 350 million gallons, Hillsborough withdrew 100 to 300 million Mgal/day, Pasco, 50 to 100 Mgal/day, and Pinellas and Hernando both withdrew 20 to 50 Mgal/day.Ofthe total FAS withdrawal, about 30% is from 5 counties, and 4ofthese are within the studyareaPolk, Hillsborough, Pasco, and Pinellas (USGS 1990).). Sinkholes are induced by aggressive pumping (draw-downofwells). This is due to abrupt changes in ground water levels which disturbs the equilibriumofkarst features and overburden (Tihansky 1999). Climate Florida is located between latitudes 24.50 and 31.00 degrees north, (Henryetal. 1994) and the study area is within latitudes 27.65 and 28.70 degrees north. Florida is classified as having both a subtropical and tropical climate, but allofthe study area is within the subtropical region ["humid subtropical" according to the Koppen classification systemof1918 (Henry et al 1994)].Itis also characterized as low relief and high rainfall (Miller 1997). Precipitation recharges the aquifers in Florida. The average annual rainfall for the twenty year period from 1981 thru 2001 for the study area ranges from 52 to 55 inches (132 to 140 cm)bycounty. Historically, between 1915 and 2001, the averageannual rainfall for the same areas ranges from 49 to 53 inches (124 to 135 cm), (SWFWMD 2003; NOAA 2003). This illustrates that the trend for rainfall has varied little over the past 85 years. The average temperature in the study area over the past 80 years has ranged from 72 to 74, with the average low during this period being61to 62 degrees and the average high between 82 to 84 (SWFWMD 2003). Continentality and oceanicity are the terms used to classify an area as to whether land massoroceans (respectively) has the most influence over its climate. According to Currey (1974), Florida is classified as Oceanic, and has the least continentalityofthe eastern United States (Henryetal. 1994). This has an influence on the precipitation

PAGE 24

15occurring in the state, which is approximately 150 billion gallons per day (Henryeta11994). Movementofthese great quantitiesofacidic water through the limestone/dolostone aquifer system has promoted thedissolution process.

PAGE 25

PANHANDLEFLORIDANORTHFLORIDASOUTHFLORIDASYSTEMSERIESLITHOSTRATIGRAPHICUNITHYDIIOSTRATI-lITHOSTRATIGRAPHIC HVOAOSTRATILITHOSTRATIGRAPHIC HYDROSTRAn-GRAPHIC UNIT UNITORAPtnoUNITUNIT GRAPtnClJIIT QUARTERNARYHOlOCENE UNOIFFEFIENTIATED1_____ UNDIFFERENTIATEDUNDIFFEFIENTIATEDSIJRFICIALPlEISTOCENEHOLDCENl!SURFICIAL SEI>IMENTS PLEISTOCENe-HOlOCENE800FICIAL PLEISTOCENE-HOLOCENEAQUIFER MIAMIUMES'l'Ct>lEACUFEA PLEISTOCENE SEDIUENTS "'QUlFER SEDIMENTSSYSTEM KEY LARGOLIMESTONESYSTEMANASTASIAFORM ... TlON 8YSTEMMICCOSUKEE FORT THOMPSONFORMATION".'CALOOSAHATCHEEFOAMATIOff"."TERTIARY CITRONEll.E FORMATION CYPRESSHEAO fOAMATION,,;" PUOCENEMICCOSUKEEFORMATION COARSECLASTICS NASHUAFORM ...TlON ;;TAMlAUI FORMATION 100.......i______ 1..;INTERMEOlATIiINTERMEDIATEALULIBLlJ'FGROUPACUIFERAQUIFERPENSACOLA CLAY INTERMEOIATESYSTeMORSYSTEMOR INTRACOASTAL FORMATIONCONFINlN(JHAWlliORN GROUP CONFININ(JHAWTMORNQflOUPCOliFINlNGlNT STATENVILUl FORMATION UNIT P6CI!IIIVERFORMAT1ONUNIT MIOCENE HAWTHORNGROUPCOOSAWHATCHIEnt..........BONE VALLEY MEMBERMAftKSHEADFORMATION--""ARCADIAFORNATIOH1-----_.100 ......SRUCE CREEl< LIMESTONE I'EHHYFARMSFOfIMATION .ST ....ARKS FOR ......TIONSTMAAKSFORMATIONTA!4PA-NOCATEE ..-CH... nAHOOCHEE FORMATION MEMBERSFLORIDAN "C ..CKASAWHAYLllAESTONESUWANNEELIMESTONEAQUIFER OLIGOCENE MAAIANNALIMESTONESYSTEMSUWANNEE LIMESTONE FlORIDANSUWANNEELIMESTONEBUcATUNNACLAY ... QUlFERFlORIDANSYSTEMAQ\.MFERSYSTEM--...OCALA LIMESTONE .... OCALA LIMESTONEOCALALlM!STOtIE EOCENE .......NONPARKFOIWAtIONAVON"AI\lCFOIWAnOH CLAIBOAN!GROUPUNDII'FI!Pl!NnATlmSEDIMENTSOLDSMARFOAMATIONOLDSMARFOf'lMAtlONCONFINlN(J,."...EOCeHE UNDIFFERENTIATEDPALEOCENEROCKSUNlr CEDARKIvaFORMATION1-------CEDARKEYSFORMATIONSUB-FLOAtMH CRETACEOUS .......CONFINING.... 'AND OlDERUNDIFFERENTIATED ....ttI'UNDIFFERENnATED UNIT .......UNDIFFERENTIATED........ _....'L......... Table 21 Relationshipofregional hydrogeologic units to measure stratigraphic units (from Fernald and Purdum 1998)

PAGE 26

CHAPTER 3 METHODOLOGYMethodologyofSinkhole InvestigationWhen there is an unexplained distress to a structure, subsurface evaluations are usually requested by a property owner (or his representative) or an insurance company. In areas that have highincidenceofkarst features, subsurface evaluations should also be performed prior to land development. The evaluation is to determine the potential or probability for sinkhole formation and damage to a future structure. Since a provision in a homeowner's insurance policy covers the perilofsinkhole induced damageorloss, determinationofthe causeofdamage is pertinent. The rapid growth in Florida's population may be responsible for the large increase in claims to insurance companies for sinkhole related damagetoproperty. The FDI Sinkhole Standards Summit could not develop a uniform setofcriteria for the investigationofsubsurface disturbance and concluded that no one uniform setofcriteria could be universally applied to the investigationofsinkhole claims, but is "site specific" (Butler et al. 1992). A listofminimum standards that must be employed during a sinkholeinvestigation was compiled for the geotechnical professionals involved in subsurface evaluations. Since "shrink/swell clay activity may be the largest single causeofbuilding damage in the U.S." (Frank and Beck 1990p.l),this Summit set forth criteria to help differentiate between damage that occurred from sinkhole activity or shrink/swell clays. Other deleterious soil conditions (e.g., organics) were also addressed.17

PAGE 27

18Previous Studies Many sinkhole studies in the past have been proprietary. Some have not included studiesofhomes and other structures that have resulted in damage due to sinkhole activity. Oneofthe studies currently availabletothe public was conducted by the Florida Sinkhole Research Institute (FSRI). The FSRI was founded in 1982 (and housed at the UniversityofCentral Florida). The FSRI compiled an inventoryofsinkholes that were formed between 1960 and1996. These data are included in a 2001 report by Burwell and Wilson.In1992, when many issues concerning sinkhole damage were arising, in compliance with a Florida Legislature directive, a studyofsinkhole occurrence was conducted by the Florida DepartmentofInsurance (FDI) (Butler et aI., 1992). One major concern at that time was the role that theinsurance companies would have to play in restitution to the homeownerofa house damaged by sinkhole activity. There was no centralized repository established for a databaseofsinkhole and raveling occurrence in Florida.Itwas agreeduponby geologists, engineers, and academia that there was a great and growing need for ongoing research for sinkhole occurrence in Florida, and a centralized repository (Butler et al.1992). Although the Florida Geological Survey (FGS) was appointed as the repository for the database, this depository has not been well maintained, due mostly to lackoffunding. The FGS provides a limited database. The new data from this study significantly broadens the FGS database. Also, the database from a 2003 study by Dodek is available to expand this new database. Criteria for Sinkhole Recognition Criteria recommended by the FDI included the following: An on-site investigation that includes background historyofthe house, property, surrounding properties, environment, and quality and workmanshipofthe structure.

PAGE 28

19 Familiarity with all the geological characteristics pertaining to the area. The stratigraphyofan area isofmajor importance, since soil and rock type, and thicknessofthe units is pertinent in a geological site analysis. Onsite soil sampling for shrink/swell clay and organics. Although geophysical studies are not a requirement, it has been the practiceofthe geologic profession to consider geophysical surveys pertinent in an investigation to meet the Florida Statutes in denyingorverifying the presenceofsinkhole activity. The most common geophysical techniques employed in an investigation are ground penetrating radar (GPR) and electrical resistivity (ER). The protocol for ER is prescribed by the American Society for Testing and Materials (ASTM) and is protocol G57-95a. The ASTM protocol for GPR methods is 6432-99. Soil Testing Soil samples are collected by two methods, either by hand augeringorby a standard penetration test. Standard penetration tests (SPT) involve a two-fold procedure. Firstly soil samples are obtained and secondly, N values are recorded. An SPT is conducted in accordance with ASTM standard D1586-99, entitled "Standard Test Method for Penetration Test and Split-Barrel SamplingofSoils". The samples are usually retrieved at 1.5 meter intervals. Samples are described by an on-site geologist and are retained for further lab analysis when a need is indicated. In compliance with the ASTM standards,anSPT boring is performed by advancing a drill bit into the ground using a rotary drill rig. A sample is obtained by attaching a2 inch long split-barrel to a successionofdrilling rods. The rods are advanced by the forceofa 140 pound hammer that is dropped freely from a 30 inch height. The numberofblows required to advance the bit six inches is recorded. To compute an "N" value, the sumofthe blows for advancing the second and third 6 inch increments provides the"Nvalue". The significanceofthe N value is for assigning a relative densitytoa soil, thereby determining the relative stabilityofthe subsurface material. The N value for each typeofsoil has a different rangeofbehavioral densities. for example, sand with an N value

PAGE 29

20 between 0 and 4 is very loose but a limestone with an N valueofa much wider rangeofoand19is very soft. When appropriate, the samples might be subjected to laboratory analysis. Tests include, but are not restricted to, clay swelling potential, grain size analysis, natural moisture content,and percent organics. Randazzo and Smith (2003) explain the importanceofsoil analyses during site investigations where property damage is suspected to be sinkhole related. Expansive clay. The water contentofclayey soil can affect the deformational behaviorofthat soil. Atterberg limits (ASTM D4318) are a groupoftests used in determining the water content boundaries between the semi liquid and plastic states (known as the liquid limit) and between the plastic and semi solid states (known as the plastic limit)ofa soil. This is also known as the plasticityofthe soil mass. Where the soiljustbegins to crumble when it has been rolled into a 1/8 inch thread, it has reached its plastic limit, that is, it contains the percentageofmoisture where it can just retain this characteristic. When enough moisture has been added to a soil to make it just begin to flow, it has reached its liquid limit. This range in moisture content represents the plasticity index(pn.It is calculated by subtracting the moisture contentofa soil in the plastic state (PL) from the moisture contentofthat soil in a liquid state (LL) (Le.,PI=LLPL). The United States DepartmentofArmy (1983) recognizes a low PI to be less than 25, marginal PI to be between 25 35, and greater than 35 to be highly plastic. Also, possible volume changeofa wetted soil should be determined. The volume change is the shrinkage factor limit and the smaller the shrinkage limit, the greater the chanceofvolume changeofthe soil. GrainSize. Clays must also be analyzed for grain size. This analysis is critical in determining whetherornot the clay's PI is relevant in causing damage to a structure. The percentageofcolloidal sized particles that make up a soil is a major factor in the soils volume change and its plastic characteristics(Nelson and Miller 1992). Sridharan and Prakash (2000) have determined that a strong attenuation factor in the shrinkage limitofa soil is the presenceofcoarse grain sizes (sand). The acceptable sieve size for this analysis is the -200 mesh (ASTM DI140).A200 mesh filters out the clay and silt size particles and hence determines what percentageoffines is contained in the material. There is more attenuationofthe shrink/swell behaviorofthe clay, the larger the grain size and the higher the percentageofcoarse material. Moisture. The natural moisture contentofa soil is relevant when investigating a site for sinkhole activity. The soil moisture percentage will indicate the relative water saturation state. Soils with relatively low soil moisture percentages,

PAGE 30

21occurring during the rainy season or below the local water table, are indicativeofnon-expansive soils (ASTM D2216). Handaugerborings.They are usually advanced to an approximate depthof1.5 meters. in accordance with ASTM standards D1452-80, entitled "Standard Practice for Soil Investigation and Sampling by Auger This results in a continuous vertical sampling profileofthe near surface materials. The technician looks for clays and organics, and notes the water table,ifit is penetrated. When clays are present, it is very important to note at what depth they occur. Shrink/swell clay within the upper 1.5 meterscan be the cause of, or a contributing factor to, structural damage (Chen 1988). Field identification is limited and can be subjective, so soil samples are retained for lab analysis, which is more definitive. Organics.High organic content can be a contributing factor for structural damage to both commercial buildings and homes. Organic material, especially when in a dry environment, decomposes, decreasing the original volumeofthe soil. The decrease in volume allows subsidence to occur.Itis the subsidence that allows movementofthe structure and hence damage. The threshold for organic content (ASTM D2974) in a soil before damage can take place is 5% (Frank and Beck 1990, p.20). Geophysicsa)Ground penetrating radar (GPR) is a geophysical technique used by geologists as an aid in interpreting the subsurface. The GPR device transmits microwave radiation into the subsurface as it is dragged along a selected configuration on the surfaceofthe ground. When the radiation encounters a reflective surface, the waves are reflected back and are collected by the GPR receiver. Materials are identified by the intensityofthe reflections they produce when the electromagnetic wave passes through them. Contacts between rock types, foreign objects (debris and garbage for example), and most importantly, voids and soil disturbance, can be detected. An anomaly in the subsurface is indicated by the contrasting dielectric intensitiesofmaterials. Anomalous areas might be indicatorsofraveled zones,orin filled paleosinks. These areas can be further investigated by drilling. Where shallow clay layers are present, GPR signals can be

PAGE 31

22 attenuated or absorbed, therefore limiting the effectivenessofthe technique, and thus necessitating other geophysical techniques to be used. The GPR field data is downloaded to a computer system where it canbe displayed as a profile. A qualified geologist then interprets the profile to determine whether or not anomalies exist and hence, whetherornot sinkhole activity is present. b) Electrical resistivity (ER) is another geophysical technique that canbe employed during a subsurface investigation. This technique is performed by placing electrodes into the ground, using the configuration in accordance with the ASTM standards. The Wenner electrode configuration, which is a4 electrode procedure (ASTM G57 -95a ), utilizes the four electrodesbyconnecting them to a direct current battery that passes an electrical current into the ground. The resistance to flow is measured. Since different earth materials, cavities, and manmade materials are characterized by their resistance to flow, an evaluationofsubsurface materials canbe made. The depth to which the current reaches is determined by the distance between the electrodes, and the greater the distance, the greater the depthofflow. One or bothofthese geophysical techniques was employed for allofthe sinkhole investigations included in this study.Data AcquisitionofDataGeotechnical related data from sites that have been investigated for sinkhole activity from 1986-2001, provide the database for this study. This database includes 665 new sites within the study area, including15in Hernando, 75 in Hillsborough, 212 in Pasco, 320 in Pinellas, and 43 in Polk (Table 3.1). The new data were obtained by reviewing over 500 reportsofgeophysical investigations conducted by GeoHazards, Inc.,

PAGE 32

23 a geotechnical consulting firm in Gainesville, Florida, and from the recordsoflegal firms within the study area. Mostofthe investigations were conducted at horne sites, and the remainders were located at private businesses. In order to develop a more comprehensive study, these data have been combined with the Florida Geological Survey's database, which documents cases from 1960 thru 1996, then displayed in a geographic information system (GIS). This gives a totalof1,664 sinkholes and sites where sinkhole activity has occurred. A large percentageofthese new (unpublished) cases are insurance-related sites where sinkhole activity has occurred and sinkhole investigations have been performed. As discussed in Randazzo and Smith (2003), and Zisman (2003), a geotechnical survey is used to determineifsinkhole activity is occurring at a site. Geotechnical results are then reviewed by the responsible insurance company, andifa settlement can not be negotiated between the insurance company and the insured, litigation may ensue. When sinkhole activity is recognized, an engineering company will be retained to evaluate mitigationofthe structure's damage. The following is a descriptionofthe data used in this study, how it was obtained and processed for review and analysis in GIS. More than 368 subsurface investigations performed by GeoHazards, Inc., from 1986 through 2001. The firm uses geophysics and other techniques to evaluate subsurface conditions for the occurrenceofsinkhole activity. The reports reviewed contained detailed informationofthe investigation including on site descriptions and diagramsofthe site, hand auger results, lab analysis when applicable, geophysics, andSPT'swhen applicable. Each site was identified by an alphanumeric address and in order to display the sites on a mapofFlorida within ArcView, the Excel file (Appendix1)had to be put into a database format (dbf) and thengeocoded (converted into a latitudinal and longitudinal coordinate system). The program used for geocoding was EZlocate and was downloaded from Teleatlas at Teleatlac.com. The newest versionofArcView 8.3 (ArcMap) has included a geocoding extension, which was utilized for geocoding additional addresses. An

PAGE 33

24 additional 287sites reported by a legal firm are included, though they did not all include all the parameters that were in the GeoHazards, Inc. reports. The Florida Sinkhole Research Institute's databaseofsinkhole occurrences, between 1960 and 1996, is included in this study as an ArcView layer and occurred in multiple Florida counties. In 1995 Spencer and Lane published the data in the FGS Open File Report 58, "Florida Sinkhole Index". At that time it was available in dBase format and is now current through 2001. It is expected to be updated with additional cases that have been reported by private citizens, the Water Management Districts, DepartmentofTransportation, Sheriff's departments and the DepartmentsofCommunity Affairs. The FDEP verified all the reports prior to adding them to the database. In recent years a private company, "Subsurface Evaluations, Inc." converted the data into a Microsoft Excel spreadsheet and it is now available on the FGS website. The file has been projected into Albers Spheroid Global Reference System (GRS) 1983 HPGN (Central Meridian, -84 degrees; Central Parallel, 24 degrees; Standard Parallel1,24 degrees; and Standard Parallel 2) and was downloaded from the Florida Geological Survey web site, http://www.dep.state.fl/us/geology/gisdatamaps. The Geologic mapofFlorida published by the FDEP was downloaded fromhttp://www.dep.state.fl/us/geology.ThismapwasinAlbersHPGNprojection.Itis included as an ArcView layer to enable the overlayofother data to identify correlations that might exist between sinkhole occurrence and stratigraphy or geology. The Florida base map and county maps were obtained from the Florida Geographic Data Library (FGDL). These are available on the GeoPlan website, http://www.fgdl.org/ or at the GeoPlan office at the UniversityofFlorida on CD. The digital maps are in Albers projection and are based on HPGN. The County Potentiometric surface maps for 1975,1985,1994, and 2000 were obtained from SWFWMD website at, http://www.swfwmd.state.fl.us/data/dataonline/.This data layer was in UTM Zone 17, North American Datum 83/90 (NAD83) projection so had to be reprojected into Albers HPGN, which was completed by a SWFWMD GIS technician. They were then imported into this study as an Arc View layer. Thicknessofthe overburden to the topofthe F AS maps were obtained from ftp.dep.state.fl.us/pub/gis/data. These data had not yet been published and were only available in Raster format. Therefore, to make them compatible with the other themes, it was necessary to convert them into Polygon format and project them into Albers. A Physiographic Provinces map was obtained from http://www.fgdl.org/. Like other FGDL data, its projection was Albers, therefore compatible with the other study themes.

PAGE 34

25 A map indicating areasofthe recharge and discharge zonesofthe F AS was obtained from SWFWMD.Itwas projected in UTM Zone 17, North American Datum 83/90 (NAD 83), so in order to import into ArcView as a theme compatible with the other data, it was converted to Albers by using the Arc View Projection Utility. The values were then reclassified into workable categories. A map that shows the 30 top pumped wells in each countyofthe study area for 1980,1985, 1990,1996, and 2001was constructed from data supplied by SWFWMD. The data were in Excel spreadsheet format and degree/minute/second coordinates. The coordinate system was converted to decimal degrees and then imported into ArcView as a theme. Other data that are included in this study but not available in GIS format, are individual maps addressing ground water and the Floridan aquifer. The maps were taken from the online versionof"The Ground Water AtlasofThe United States", at http://www.capp.water.usgu.gov/, and include (i) a mapofthe thicknessofthe Floridan aquifer system, (ii) a map showing the 3 areas in the state with the largest withdrawal from the Floridan aquifer in the state. Oneofthese lies within the study area. (iii), a map showing the amountoffreshwater withdrawal from the Floridan aquifer, and (iv) a mapoftransmissivityofthe Floridan aquifer. The reports reviewed for this study contained physical addresses for each site. The addresses were geocoded (assigned longitude and latitude), to enable projectionofthe points onto a map within ArcView.Byconverting the data to a GIS format, multiple layersofdifferent variables were constructed and overlain. This allowed the layers to be "turned-off and turned-on" so that any patternofsinkhole occurrence could be more easily recognized. Features that have correlations such as geomorphologic features and sinkhole distribution, soil types, geology, and hydrogeologic parameters can be recognized. The geology, stratigraphic data, rainfall data, potentiometric surfaces, and water withdrawal from the Floridan aquifer can then be integrated with these features. ProcessingofData After each setofdata was collected it was then reviewed and converted to an ArcView theme, when possible, or compared side-by-side with physical maps when digital data was not available. By putting the data in a GIS format, layers could be

PAGE 35

26 integrated, overlain,orremoved. This made it possible to make correlations and identify patterns in the distributionofoccurrenceofsinkholes/raveling, the geology, stratigraphy, potentiometric surface, high magnitude springs (not in digital format), rainfall (not in digital format), and the recharge and dischargeofthe Floridan Aquifer System. Eachofthese layers is shown in the chapters that follow. For the data that were not in a digital format (Springs, Rainfall) a side-by-side comparison was made to the digital data. Table 3.1 provides a summaryofthe main parametersofeach county's data. With the data in ArcView, analyses were made and exported to Excel, where tables and graphs were generated for further analyses. Sinkhole distribution is the parameter to which all other data was compared. Sinkhole density was calculated by dividing the numberofsinkholes by the total area for each parameter and sinkhole relative frequency was determined by dividing the numberofsinkholes per parameter by 1,664, the total numberofsinkholes in the study area (Figure 3.1). When analyzing the data, both sinkhole density and sinkhole relative frequency have been considered. This was an attempt to address some obvious biases. One is called a "collection bias". Collection (or reporting)ofdata can be affected by certain factors, such as where population is greatest. Where population is greatest reporting would be expected to be greatest. Useofrelative frequency can reduce reporting biases. Insteadofsimply using the numberofsinkholes per square kilometer (sinkhole density) utilization is madeofthe percentageofthe numberoftimes the sinkholes occur within a given parameter (sinkhole relative frequency). Despite the inherent biasesofdatabases such as that developed in this study, actual sinkhole locations have been identified on a regional scale and allow for causal correlations.

PAGE 36

27 Table 3.1. Parametersofthe countiesofthe study area NumberofRelative Sinkholes County Areakm2Sinkholes Frequency/km2Pinellas 748.97 369 22.9% 0.49 Pasco 1980.9543125.9% 0.29 Hillsborough 2773.54 404 24.8% 0.15 Hernando 1276.41 203 12.2% 0.16 Polk 5208.30 257 15.4% 0.05 Totals 11,988.17 1664" ..-...,...... a: '..t ...... 'II"'.'I.,'t 'I,,:. I. .I: .....:" ..... .. ,"'" .'.'. ,-, .,, ..'. :', '.I,.:",,,, ""I,",, .', : ",'" 0:,.",._" t. .'. ,I:,, :.,., ..: ,, I"','V''II' I:' "":,: i1lsbol'ough ...,.'. .,. ,"y: :J .'..::" ... :-..:I" .. \,,:',' :: ... :.":''s".,'". ,'",III c::=====::::JIKilometers00 Figure 3.1. Countiesofthe study area showing locationsofsinkhole activityofboth the old database and new (after Florida Geographic Data Library 1990)

PAGE 37

-CHAPTER 4 RESULTS The setofmaps, charts, and tables presentedinthis chapter provide an effective visualizationofeach parameter used for analyses with sinkhole distribution. Each parameter was overlain on the sinkhole distribution map, calculations made and exportedtoExcel where tables were constructed. The parameters in digital format include hydrogeology, major well fields, physiographic provinces,recharge and discharge, overburdentothe FAS, subsurface geology, potentiometric surface (for 4 years), thicknessofthe FAS, and population density for 2002. The non-digital data used for determining correlations included rainfall and spIings. The tables and charts show both sinkhole density and its relative frequency.Hydrogeology Overburden Thickness to the topofthe Floridan Aquifer SystemData for this theme were provided by the FDEP. The FDEP constructed the model by subtracting the topofthe FAS coverage from the Floridan digital elevation model (DEM). They obtained the Topofthe Floridan maps from multiple sources and spliced the maps togetherinordertomake a state-wide theme. Categoriesofoverburden thickness reflect generally acceptable divisions that provide a more useful resolutionofsinkhole distribution for this theme. The sinkhole theme was joinedtothe overburden coverage and the results are in Table 4.1 and Figure 4.1. Data are also presented in histogram format (Figures 4.2 andA.3). Figure 4.2 indicates that there is a correlation between the numberofsinkholes perkm2(sinkhole28

PAGE 38

29 density) and the thicknessofthe overburden (i.e. depth to the aquifer). There is a greater distributionofsinkholes where the depth to the aquifer is less 75 feetorless. Though sinkholes do occur in areas where the overburden is thicker than this,the occurrence is much less. For example, where the thicknessofthe overburden is greater than 300 feet, sinkhole occurrence is less than 1%. Sinkhole activity in these more thickly covered areas may be due to extraordinarily large cavity systems residing below the overburden. The sinkhole relative frequency results (Figure 4.3) indicate an even stronger correlation between sinkhole distribution and overburden thickness.Itindicates that approximately 73%ofthe sinkholes occur in overburden less than 75 feet thick and 27% in areas where the overburden is greater than 75 feet. Table 4.1. Thicknessofthe overburden to the Floridan aquifer system, compared to various sinkhole parameters Overburden#ofDepth in feet Sinkholes 0-15 358 16-30 314 31-50 318 51-75 226 76-100 62 101-150 95 151-200 115 201-300 162 301-400+141,655 960 1,284 589 449 1,532 1,208 2,439 1,676 Sinkholes! km20.22 0.33 0.25 0.38 0.14 0.06 0.10 0.07 0.01 Relative Frequency 21.5% 18.9% 19.1% 13.6% 3.7% 5.7% 6.9% 9.7% 0.8%

PAGE 39

30201-300 101-150 151-200 301-400+ 76-10080 Sinkhole a -elliug51-75 /'./R!yers Floridan 0 -erburdellII0-15ftII16-30 31-50 4020 c=============:JK1lometer oFigure 4.1. Distributionofsinkholes and overburden thickness to the Floridan aquifer system (after Florida Department Environmental Protection raster data 2003)

PAGE 40

310.40 0.38.......................... 0.35 0.20 0.10 0.30 O.OO-j-od!-0.25 0.05 0.15Overburden Thickness (feet) Figure 4.2. Sinkhole density in relation to thicknessofoverburden

PAGE 41

3225% 21.5% 20% >(Jc::CI) 15% ::sC'" U. CI) > i 10% CI) a:5%.........I0.8% 0-15 16-30 31-50 51-75 76-100 101-150 151-200 201-300 301-400+Overburden Thickness (feet) Figure 4.3. Relative frequencyofsinkholes in relation to thicknessofoverburden to the Floridan The FAS is confined within some geographic areas and unconfined in others.Itis noted in the USGS Ground Water Atlasofthe United States (1990) that large solution cavities are present in areas where the confining unit is thinorabsent. The map (Figure 4.4) representing the confining units, was not available in digital format, so an overlay with the Sinkhole and Overburden themes was not possible. A side by side comparisonofFigure 4.1 and Figure 4.4, shows that the study area has three categoriesofconfinement (1) the upper confining unit thin or absent (2) upper confining unit generally less than 100 feet thick, breached, or both (3) and areas where the confining unit is

PAGE 42

k., .33generally greater than 100 feet and not breached. Where the sinkhole occurrence is high, Figure4.1shows that the thicknessofthe overburden is75feet or less. Figure 4.4 also indicates thatinthese areas, the upper confining unitofthe Floridaniseither absent orII f-.. Ii '.thin less than 100 feet or breached, or itisboth.Inthese situations where the overburdenisthin, and the confining unitisthin or less.1 SllSCALE1:5.000.000 50lOaWILEStI Arellwhen: Floridan lIqulrersyslcmis"n(onned-Upper connlngnilI.bsIorhinAra whereFlorlnilisgererllUy I 100fc<, hie br""chcd. orboth Ar"wh e Floridan aqu f",s}'Slem Is conflned-p(I'.'han 100 {pe/'II innt/wI piaLdfye solulion openings. someof whir:hcauseslnkhol Medevloped In !he Floridan chiC/I!! where Ih conrrningur1J! I., Ihin r ab:;enl. Aproximal"Umltof ppcrc nflnlnUunlEXPLANATIONFloridanisconfmed and generally greater than 100 feet thick, that sinkhole occurrenceislimestone beds. As could be expected, the comparison also shows that where the absent, acidic waters can more easily leach unitsofthe FASand cause dissolutionofthe.. '7 } Figure 4.4. Confining unitsofthe Floridan aquifer system. (after U.S. Geological Survey "Ground Water Atlasofthe United States, modified from Miller 1990) r )

PAGE 43

----------34------Physiographic ProvincesFourteen Physiographic Provinces are joined with the sinkhole density theme in ArcView (Figure 2.2). Table 4.2 indicates that the physiographic province with the highest sinkhole density is the Coastal Swamps province (approximately 0.5/km2),an area underlainbythe Tertiary Suwannee Limestone.Itis a bedded pure to slightly sandy limestone (Randazzo 1997). The sinkhole density and relative frequency data from Table 4.2 are shown in Figures 4.5 and 4.6 respectively. The table indicates that the highest sinkhole density is in the Coastal Swamps Province, 0.49 sinkholes perkm2 ,and the next is theGulfCoastal Lowlands at 0.33 sinkholes perkm2 .Following the Coastal Swamps Province and theGulfCoastal Lowlands provinces, the next highest occurrenceofsinkholes (sinkholes perkm2 )is within the Lakeland Ridge and Winter Haven Ridge, each with a densityofapproximately 0.20/km2 .Although the greatest densityofsinkholes is in the Coastal Swamps province, the greatest relative frequency for sinkholes (Table 4.2) is within theGulfCoastal Lowlands region (66% vs only 7.6% for the Coastal Swamps). Within the database there are 1,106 documented occurrencesofsinkholesintheGulfCoastal Lowlands and 234 in the Polk Uplands. Since there is a great difference in total area for the provinces, the total relative frequencyofsinkholes(%sinkholes per kilometer) versus the total square kilometers in each physiographic province was compared (Figure 4.7). Although theGulfCoastal Lowlands and the Polk Uplands are almost identical in size, the relative frequencyofsinkholes for the Gulf Coastal Lowlands is almost 5 times greater than it is for the Polk Uplands. The Polk Upland covers 26%ofthe study area and has 14%ofthe total numberofsinkholes. The Gulf Coastal Lowlands cover 28%ofthe total study area but contains66.%ofthe total numberofsinkholes .

PAGE 44

35 Also, theGulfCoastal Lowlands has a much greater densityofsinkholes (Figure 4.5) than the Polk Uplands (0.33/km2and 0.08/km2respectively). For further analyses other parameterofthe physiographic provinces were compared. An overlayofmaps depicting the physiographic provinces (Fig. 2.2) and the subsurface geology (Figure 4.8) indicates that theGulfCoatal Lowlands are mostly coveredbyQuaternary age beach ridge dunes and undifferentiated sediments. And theBoneValleyMemberofthe Peace River Formation (Hawthorn Group) underlies mostofthe Polk Uplands (Campbell 1984; White 1970). The Bone Valley sediments are sand and clayey fine sand, with montmorillonite and some concentrationofphosphorite grains. Within the Polk Uplands lie three north to south trending ridges, the Lakeland Ridge, Lake Wales Ridge, and the Winter Haven Ridge. It was described by White (1970) and later by Lane (1986) that the Lakeland Ridge and the Brook ville Ridge (which is also within the study area) have starhaped sinkholes on their edges. This current GIS analysis also indicates that the Lakeland Ridge and the Winter Haven Ridge have high sinkhole concentrations along their edges (Figure 2.2).Butwhen sinkhole relative frequency was computed, it indicated that they have only a relative frequencyofless than 3% each (Table 4.2). Figure 4.8 indicates that allofthese ridges are characterized by the Cypress Head Formation, surrounded by Bone ValleyMemberofthe Peace RiverFm(Hawthorn Group) and reworked Cypre Head Undifferentiated Sediments.-----------------.-,------------

PAGE 45

36 Table 4.2. Sinkhole parametersofthe Physiographic ProvincesProvince Bombing Range Ridge Brooksville Ridge Coastal Swamps Gulf Coastal Lowlands Lake Upland Lake Wales Ridge Lakeland Ridge Osceola Plain Polk Upland Tsala Apopka Plain Western Valley Winter Haven Ridge Zephyrhills Gap Other Area#ofSinkholes/ Relative Percentofk:m2Sinkholesk:m2Frequency Total Area 111 0 0.00 0.0% 0.9% 1,111 49 0.04 2.9% 9.3% 256 126 0.49 7.6% 2.1% 3,351 1106 0.33 66.5% 28.0% 541 2 0.00 0.1% 4.5% 791150.02 0.9% 6.6% 232 41 0.18 2.5% 1.9% 798 0 0.00 0.0% 6.7% 3,119 234 0.08 14.1% 26.1% 156 1 0.01 0.1% 1.3% 362 5 0.01 0.3% 3.0% 260 43 0.17 2.6%2.8% 680 42 0.06 2.5% 5.7%1870 0.00 0.0% 9.2%0.50-0.450.400.35NE 0.30r.I:JQ,l 0.25'0 .c ..::.::c: 0.20-.... rr, 0.10-0.00 .I......0.33 0.18 n::::: :::: n:;\:.::.:.::.::.1;.;:08:-:-:.::-:-::;:H 0 02 }:\1;:C11::.tWII IPhysiographic Province0,00 IFigure 4.5. Sinkhole density in eachofthe Physiographic Provincesofthe study area __...__ ._._ ...

PAGE 46

----------------------70% 60% 50% >.c<:1.1::l 40% 0'"<:1.1s..<:1.1 30% ....eoaQj 20%10%0% --'----1.---,__3766_5%-----Physiographic ProvinceFigure 4.6. Relative frequencyofsinkholesineach physiographic provincesofthe study area __________________________________. .-.J

PAGE 47

3870%66.5% 60%
PAGE 48

39That-Hawthorn Gp/ArcadlaFmtrampaMem CJ Thp-Hawthorn GplPeace River TQsu-PlIo-Pletst shelly seds TQu-UndUf seds TQuc-Reworked CypressHd CJ Tc-Cypress Head Q1-Holocene Seds Sands Ts-SuwaneeLS o Thpb-Hawthorn GplPeace RiverlBone Valley Mem TQd-Bch RldgelDuneTo-ocalaLS Th-Hawthorn Gp DQ>d-Bch Ridge DuneFormation A./RJ-ef' Sinkholes Subsm1ace Geology40 20_________ =================:::J Kilometers80oFigure 4.8. Distributionofsinkholes and the Subsurface geologyofthe study area, with overlayofrivers (after Florida DepartmentofEnvironmental Protection 2000)

PAGE 49

40RechargelDischargeinthe Floridan Aquifer System)Annual recharge and discharge data are collected by the water management..districts, and represent the amountofwater that enters and leaves the FAS annually. The. .., digital data for this theme were downloaded from the SWFWMD's website. The annual data is reported in inches and has been categorized into generally accepted discharge and ) ,recharge zones (Table 4.3). The statewide theme was clippedtorepresent the countiesofthe study area, and then a spatial join with the sinkholetheme made it usable for } .calculating how many sinkholes fall within each zone (Figure 4.10). Overlays with other themes allowed for an overall examinationofparameters that show correlationstothe amountofwater entering or leaving the aquifer. recharge is > 10, and the second greatest relative frequencyofsinkholesis30.1%,where zone with the highest sinkhole density (0.27) and the highest relative frequency of sinkholesisthe discharge zone 1to5 inches/year, 47.2%. Thisisshown inhitogram 49.5% 50.5% Relative Frequency total 1.4% 47.2% 0.9% 5.1% 30.1% 15.3% Relative Frequency 0.05 0.27 0.10 0.05 0.10 0.14 Sinkholes/km2506 2,9321551,817 4,804 1,773 AREAkm224 7851584501255o.ofSinkholes Discharge<1 Discharge 1-5 Discharge> 5 Recharge10 Recharge/ Discharge inches/yr recharge is 1to10 inches/year (Table 4.3). The study area has 1,664 sinkhole sites with 824 (0.23 sinkholeslkm2 )inthe format in Figures 4.10 (a) and (b). The second highest densityofsinkholes falls where discharge zones and 840 (0.10 sinkholes/km2 )in the recharge zones.InTable 4.3 the Table 4.3. Areaofthe discharge and recharge zonesofthe Floridan aquifer systeminrelationtovarious sinkhole parameters.... r r'L _----_.--._ ..--._-----------.---._----

PAGE 50

) ) ? ;> ) ,-) .))..-., 41The FSRI noted in its reportofsinkhole occurrence that there is a high correlation between the densityofsinkhole occurrence and the recharge rateinsome Florida areas (Frank and Beck 1990), but the new sinkhole data do not reflect this relationship. Since dissolutionoflimestone from acidic water is a precursortosinkhole formation, and considering Frank and Beck (1990), it might be expected that the areas with the highest amountofrecharge, i.e., greater than10inches annually, would have the greatest numberofsinkholes. However the new data show that zones where the recharge and dischargeisless than1,the sinkhole occurrenceisleast. Findings by Dodek (2003) concur with Frank and Beck (1990) that there is a relationship between high sinkhole activity and the recharge zones. Dodek (2003) found that within his study area, the greatest relative frequencyofsinkholes occurred where the rechargeisgreater than 10 inches per year, and the highest sinkhole densityiswithin the zoneofrecharge 1to10 in/yr. Dodek (2003) concluded that the recharge zones are more acidic than discharge areas, and hence, more dissolution should occur. When evaluating these findings, locationofthe discharge areas must be considered. Within the study area, the discharge zones with high sinkhole activity lie on the west coast, which may be a contributing factor to their formation. According to Upchurch and Randazzo (1997),inFlorida, dissolutionoflimestone develops not only in recharge areas, but in the saltwater/freshwater mixing zones, i.e. coastal areas. When the Depth to the FAS theme (Figure 4.1) and the theme for RechargelDischarge (Figure 4.9) were overlain onto one another (Figure 4.11), it indicated that the greatest sinkhole distribution occurred where (1) the FASis0to30 feet and the dischargeis1 to 5 inches annually. (2) the depthtothe FASis0to30 feet and the

PAGE 51

--------142 recharge is>10inches per year, and (3) the thicknesstothe FAS ranged from 0 to 50 feet and the rechargetothe FASis1to10inches per year. Since there arenodata available regarding changesindischarge/recharge with time, the averagesofdischarge/recharge were relied upon. The total relative frequency for the 3 discharge zonesis49.5% and the total relative frequency for the 3 recharge zones is 50.5%.Based on this limitation, these time-averaged values may indicate that the natural recharge or dischargetothe aquifer systems is not a dominating factor in sinkhole development and that the thickness of the overburden may have moreofan influence. When the major rivers and roads themes were overlain with the sinkhole theme (Figure 4.9) and proximity to sinkhole locations was examined,nospecific trend was evident between the sinkhole locations and either the roads or the rivers. The rivers are not includedinthe discharge theme created by FGS, but six of the seven major riversofthe study area are located within discharge zones. Dodek (2003) also found that discharge zones coincided with the rivers. His data showed a directional trend with oneofthe major highways (US Highway 27), but a repOlting bias was considered to have affected the data. Much of his data were reported by the Florida DepartmentofTransportation (FDOT), hence, the sites were near roads. Within this study area,nocorrelation was recognized between the sinkhole locations and the rivers and major roads.

PAGE 52

43Major Roads Swkholes /'VRivers Counties0Hernando0 Bill sborougb0Pasco0Pmellas0Polk4020================::JIKtlometers80o Q Figure 4.9. Counties map with sinkhole locations, major roads and major rivers (after Florida Geographic Data Library 1990)

PAGE 53

44 Figure 4.10. Recharge/Discharge zones measuredininches/year and sinkhole distribution (after Southwest Florida Water Management District 2002) Dischaqze and Rechargeo Discharge< IlPJyr Remorge 1-10 Remorge > 10 Remorge 5 /'../?!ajor Rivers Sinkholes COIulties 4020 =======::11 Kilometers80o ..:...., :,. ..... .... .... _11 ,, ... ....... ,...1'...)1-. ..

PAGE 54

450.30 0.25 C\I 0.20E U) 0.15 CI) 0 0.10 c:fI) 0.05 .05.27 .. :-:.: ... I .04 I. .14 0.00+-------,-------,-------,-------,----,--------,GE.L "\E."\ 5E.75GE.L "\ "\ "\0E.7"\0OISCI-\P-r>-OISCI-\P-r>-GOISCI-\P-r>-Gr>-E.CI-\p-r>-r>-E.CI-\p-r>-GE.r>-E.CI-\p-r>-G RechargeIDischarge inchesIyr(a)Figure 4.11. RechargelDischarge zonesofthe study area, (a) Sinkhole density (b) Relative frequencyofsinkholes

PAGE 55

4647.2%50%45%15.3% 5.1% 0.9%1.4% 5% 10%20%30.1% 25%40%30% 35% 15% 0%-+1----,.-----,--------,----,.-----,--------,GE.LiE.i.5GE.'75GE.Li i .iQ E.'7 iQ Q\scrlp.p. Q\scrlp.p.G Q\scrlp.p.p.E.Crlp.p.p.E.Crlp.p.GE.p.E.Crlp.p.G DischargelRecharge inches / year(b)Figure 4.11. Continued---_._-----------.. _---_._-_.

PAGE 56

4740D1SCHARGEl1TO5RECHARGEl1TO1020\Kilometers ... ..... o Counties SinkholesOverburden to Floridan 0-15 feet16-30feet .-....10o,. 1,\,\". \\\ '\.. '..,..',. ......,. .0....I.\.. .-...._-_ ... ..... /----..--_.----r \..".1 \\ '"\. i. I .. .... ,\ .:... ..., .. .. II .. ,; '.. .,.......'.',..,..Figure 4.12. Overlayofthe Thicknessofthe Overburden theme and the Recharge/Discharge theme with the sinkhole distribution theme

PAGE 57

48 Potentiometric Surface Potentiometric surface representsthe total headofgroundwater andisthe leveltowhich water will rise in a tightly cased well. Potentiometric surface changes result from recharge and dischargeofthe aquifers. This theme was constructed using data supplied by the SWFWMD. Figures 4.13(a) and (b) display potentiometric surface data for 1975, 1985, 1994, and 2000. Categoriesofpotentiometric surface span the years that the sinkhole data was recorded and reflect year when rainfall was considered to be average rainfall, low rainfall, and high annual rainfall (SWFWMD 2003). The rainfall values areinAppendix (a). Potentiometric surface maps were overlain with the sinkhole theme and correlations identified. Figures 4.13(a) and (b) show that the highest density of sinkholes, for all four years, is located where the elevationofthe potentiometric surfaceislow(5to10feet and 20to50 feet). Sinkhole formationisaccelerated during periods of drought or during artificial loweringofthe potentiometric surface by over pumpingofthe aquifer (Tihansky 1999). When potentiometric surfaceislow, there is a lossofbuoyancy and a reduced strength of the overburden; hence it can make the area conducivetosinkhole formation. This suggests that sinkhole formation occurs during a drop in the potentiometric surface. A lowered potentiometric surface may be a triggering factor in sinkhole development (Upchurch and Randazzo 1997).

PAGE 58

49 (a) Figure 4.13. The potentiometric surface values with sinkhole locations (a) for 1974 and 1985 (b) for 1994 and 2000 ( after Southwest Florida Watermanagement District 2003)

PAGE 59

501994 -to 20 KIlometers_____ 80oPOTEno1ETRIC SURFACE 2000 "... .). ,, Figure 4.13. Continued (b) ,.

PAGE 60

------"--._ _...__J.--L-.-_..__. 51Geology r \. I t \ .Themes forboth the surficial and the subsurfacegeology were constructed. The Polygon coverage was obtained from the FGDL (http://www.fgdl.org/). Geology coverage for the countiesinthe study area were merged and then joined with the sinkhole database, and analyses were made. The digital coverage for the subsurface and the surficial geology showed very little variation from one anotherintermsofthe geology. Dodek (2003) also found thisinhis study area. Therefore, only the subsurface geology was analyzed for this study. Table 4.4 and Figure 4.14 indicate that the greatest sinkhole density occursinthe Beach Ridge and Dune lithostratigraphic unit, which has 0.46 sinkholes per km2 .The areally largest lithostratigraphic unit (Table 4.4)isthe Hawthorn Group (3,654km2),which is also where the greatest relative frequencyofsinkholes occurs (Figure 4.15). The Hawthorn Group includes its Arcadia Formation (Miocene), Peace River Formation (Miocene-Pliocene), and it's Bone Valley Member (Miocene-Early Pliocene). The lithostratigraphic unit with the second highest relative frequency is the Quaternary Undifferentiated Sediments lithostratigraphic unit (32.2%). The Hawthorn Group's Arcadia Formationispredominately siliciclastic-bearing carbonates, and the Peace River Formation, which overlies it,isa clastic unit (Compton 1997; Scott 1997). It contains variable amountsofcarbonate with clay and quartz sand (Compton 1997). This area contains 572 sinkholes and has a relative frequencyof34.4% (Table 4.4), the greatest for all the geologic units. By utilizing ArcView, an overlayofthe sinkholelocations, the overburden thickness theme (Figure 4.1), and the geology theme (Figure 4.8) were viewed. The GIS representation reveals that the areas with the greate t density of sinkholes occurinthe -------.. _---------

PAGE 61

II ).... ..52 Beach Ridge and Dune geologic unit and the Suwannee Limestone unit.TheBeach Ridge and Dune geologic units are mainly situated where the thicknessofthe overburden to the FAS is 0 to 30 feet, and the Suwannee Limestone is occurs where the FAS is less than 75 feet thick. Dissolutionoflimestone and dolomite is the precursor to sinkholes and raveling. The Suwannee Limestoneisprincipally a carbonate unit.Thecoverage suggests a correlation between the high incidenceofsinkholes, the geologic units and the>thicknessofthe overburden.II Table 4.4. Subsurface geologyofthe study area in relation to various sinkhole parameters Area#ofSinkholes / Relative % Total Lithostratigraphic Unitkm2Sinkholeskrn2Frequency Area Holocene Sediments 57 2 0.04 0.1%0.5%! T-Q Undiff. Sediments 180 5 0.03 0.3% 1.5% l. Peace River FmlHawthomGp208 16 0.08 1.0% 1.7% Dunes 230 1 0.00 0.16 1.9%(PLio-PleistShelly Sediments 297170.06 1.0% 2.5% ,. Beach Ridge&Dunes 367 170 0.46 10.2% 3.1% Arcadia FmlHawthomGp401610.15 3.8% 3.4% .Ocala Limestone4203 0.01 0.2% 3.5% (" Suwannee Limestone 668 199 0.30 12.0% 5.6% Cypress HeadFm778 72 0.09 4.3% 6.5% Hawthorn Group 1,135 318 0.28 19.1% 9.5%\ Reworked CypressHead1,750 87 0.05 5.2% 14.7% Bone Valley MemlPeace River 1,910 177 0.09 10.6% 16.0%,Quaternary Undiff. Sediments 3,542 536 0.15 32.2% 29.7% All Hawthorn (combined) 3,653 572 0.16 34.4%30.6%. ,

PAGE 62

-----, --------_..---530.30 0.50 0.45 0.40 0.35 N 0.30E .......
PAGE 63

-------------------540_2% I5%-10%15%-20%25%-30%35%-Lithostratigraphic UnitFigure 4.15. Relative frequencyofsinkholes in lithostratigraphic unitsofstudy areaMajor Wells in the Study AreaA theme was constructed within ArcView, showing the locationsofthe 30 most higWy pumped wells in each countyofthe study areainrelationtothe sinkhole distribution (Figure 4.16). The data were supplied in Excel spreadsheet format by SWFWMD and each well contained a longitude and latitude. These were then projected onto the sinkhole distribution map in the same projection, Albers GCS North American 1983.Inchapters 2 and 3 it was noted that increased sinkhole activity may result from over pumpingofthe aquifer. An overlayofthe sinkhole theme and geology theme were used to identify correlations between the known sinkhole activity and the locationofthe----------------------------

PAGE 64

. 55 welL The data that are included were taken from collections in 1980, 1985, 1990, 1996, and 2002. Some of the wells may have been active (and still active) in allofthese years, while other well may have been capped (abandoned) and new wells put into existence. Since 1980 there has been a significant increase in annual pumpage. The highest annual pumpagein1980 was in Polk County and wa 1,784,834,700 gallons, and in 2002 the highest annual pumpage occurred in Polk County, 5,928,020,000 gallons (SWFWMD 2002) (Table 4.5). Table 4.5. Highest annual water pumpage (gallons) for top wellsineach countyofstudy area for elected year. t County 1980 Hernando900,847,108Hillsborough8J8 505,000Pa co804 145,137Pinellas193,497.000Polk1.784,834,7001985724,062,000 2,001,658016 653,945,909 141,684,000 2,263,694,8541990820,175,046 1,813,487 502 518,648,000 306,273,000 2,892,071,00019961.305,510,000 861,005,600 482,182,000 325,033,000 2,105,493,20020021,015,348.000 1.241 900,000 995,260,000 491.370.000 5,928,020.000 ,.;....> Within ArcView it was po sibletoidentify where sinkhole activity was high and multiple wells existed. The four circles on Figure 4.16 show these areas. A correlation between the large withdrawal from these wells and sinkhole distribution is generally indicated.Itappears to demonstrate that the wells may be a contributing factor, but since some areas show a concentration of wells but not a concentrationofsinkholes, other parameters must also be considered as possible contributing factors. When overlaysofdifferent parameters were viewed within ArcView, the Subsurface Geology theme indicated (Figure 4.16) that the stratigraphic unitsinthese four areasofhigh sinkholeditribution are the Quaternary undifferentiated sediments, Quaternary Beach Ridge and Dune, and the Suwannee limestone. Another important parameter that may be a contributing factor to sinkhole formation is the thicknessofthe overburden. By viewing the ArcView overlays, four areas were identified where thereis-----_.. _.-

PAGE 65

----------------------'-56 --------.. J ,-"A J, a high occurrenceofsinkhole formation, a large numberofhighly pumped wells (Figure 4.16) and the overburden to the topofthe Floridan aquifer is mostly75feet thick or less (Figure 4.1) Some biasestoconsider that may be having a large influence here are the collection bias, and the limited information about the actual drawdownofeach well. The collection bias would indicate that the larger the population, the greater demand for water and hence, more wells would be expected in the vicinityoflarger populations. More sinkholes are expectedtobereported where population is greatest.

PAGE 66

57 aM-thom GpJPuceRiwrH...,thomGlOQ.poH..,thomGplAEw.FmH..,thom GpIP ....RivuR.worktdCyp..,I...d0Oc.J.Limt,to,.oSUWUIHLilnMtone o Holocu.$Mune.li'OUIldiff.Hd-.>bDtu.u w"lls 4020___________ -============:::JIKilOll1eters 80o Q Figure 4.16. Sinkhole distribution on the geology theme, with the 30 most highly pumped wells in each county for the study area. The circles indicate areasofhigh sinkhole density and a large numberofhighly pumped wells (after Southwest Florida Water Management District 2003)

PAGE 67

58Population DensityPopulation density data, when appliedtosinkhole distribution pattern, suffers from biases that limit meaningful correlations. The 2000 US Census data is used in the Population Density theme. The data were obtained from the FGDL. The US Census Bureau delineates geographic areas into Census tracks which are made up of Census block groups. The Census block groups are defined by theUSCensus Bureau as having approximately 600to3,000 people, with an optimum sizeof1,500 peopleineach Census block group. The areaofeach track is in square meters. This area was then converted to square kilometer:s and then broken into its associated Blocks. The population persquare kilometer was then calculated (Table 4.6).Itis hypothesized that the areas with the highest population density (people per square kilometer, Figure 4.17) would have the greatest sinkhole occurrence. Thisistobe expected for many reasons. For example, anthropogenesis is likelytobe a strong contributing factor, and as mentioned before, can be causing a reporting bias. For example manyofthe cases included in this thesis are reported by homeowners that have structural damagetotheir home, and if the damage is verifiedasbeing due to sinkhole activity, it might leadtoother neighbors having similar damage investigated. Hence, confirmationofsinkhole activity for several homes in one neighborhood can increase the reported sinkhole density. TheUSCensus Bureau's block data (Figure 4.17)ofpopulation density was used to compute sinkhole density and sinkhole relative frequency for specified rangesofpopulation (Table 4.6). Figure 4.18 shows a general trendofincreased sinkhole activity with an increase in population. Sinkhole relative frequency for the area does not indicateasstrong a correlation, but this may be explained by the U.S. Census Bureau's block sizes. The blocks decrease in sizeasthe population increasesinsize. This would------------------------

PAGE 68

59 .-----"---._. decrease the relative frequency for those blocks. This analysis agrees with Dodek (2003) who also recognized a general trend between higher sinkhole densities andanincreaseinpopulation density. Where populationishigh, many anthropogenic factors may be influencing sinkhole activity. For example, an increaseofpopulation means an increase in water usage, which increa e withdrawal from the aquifer. Figure1.1demonstrates the affectoflarge water withdrawals. Population may also affectwater drainage patterns which mayplayarole in sinkhole distribution when certain other factors are present. Table 4.6 indicates that the population range with the highest sinkhole density, 0.95 sinkholes per sq km, is 1,501to2,000 people/km2range. The next three population groupsindescending orderofsinkhole density are the 5,001to6,000 range, 3,001to4,000 range, and 2,001 to 3,000 range. Both the sinkhole density and the relative frequencyofsinkholes are within the 1,501to2,000 range (15.0%). Analyses also show that the most populous areas, 3,000to6,000 people/sq km have a much lower relative frequencyofinkholes than expected. This i the resultofthe biases previously mentioned. The area where population is 3,000to6,000 people/sq km represents only33km2ofthe 11,987 km2 .This is only 0.3% of the study area and might be considered anomalous. The 0 to 50 and101to200 ranges represent 67.5%ofthe total study area withIes than 0.02to0.14 sinkholes density, but haveanaverage relative frequencyof10.5% (Figures 4.18 and 4.19). Overall, population density data are inadequateinaddressing sinkholeditribution issues, but they do provide interesting patterns for future analyses.

PAGE 69

60. SiukholespopulationDensityo0-50popkm250-150150-3500350-5000500-1000o1000-200002000-30003000-6000 6000-2700040 20 C=========:=JIKilometers 80oFigure 4.17. DistributionofsinkholesandtheUSCensus Bureau's Block Groups showing population density for 2000 ( from US Census Bureau 2000)

PAGE 70

61Table 4.6. Population densityinrelationtovarious sinkhole parameters Range NumberofArea Sinkholes / Relative (pop/km2 )Sinkholes km2km2Frequency 0-50 164 6,884 0.02 9.9% 51-100 87 1,090 0.08 5.2% 101-2001721,207 0.14 10.3% 201-300 96 588 0.16 5.8% 301-450 134 564 0.24 8.5% 451-60098362 0.27 5.9% 600-75077189 0.41 4.6% 751-1000187262 0.71 11.2% 1001-1500 233 380 0.61 14.0% 1501-2000 249 261 0.95 15.0% 2001-3000 140 167 0.84 8.4% 3001-40002530 0.84 1.5% 4001-5000 1 3 0.33 0.1% 5001-6000 1 1 0.88 0.1%

PAGE 71

620.600 0.7000.5001.000-0.200-0.3000.400-0.800-0.900-Figure 4.18. Population density (km2 )versus sinkhole density usingUSCensus Bureau block groups2000 -------------

PAGE 72

6316%-II1.5% j(i\ 0.1%0.1% -::::"..----' 4.6% I5.9% I5.2% 1::':: 9.9% 14%12%>0 c: 10%Q):::JC'"Q)8%LL. Q) > "';::m 6%Q)IX 4%-2%-0%Population DensityIkm2Figure 4.19. Sinkhole relative frequency for population density 2000(km2 ) --------,---,

PAGE 73

CHAPTER 5 ANALYSES Utilizationofa Geographic Information Systems analysis has made it possible to test several hypotheses at once, recognizing contributing factors to sinkhole distribution. In some cases, parameters could be evaluated without GIS themes, butinorder to make a stronger evaluation, a combinationoffactors hypothesized to be triggering factorstosinkhole formation were overlaid by using themes -a method for validating the results. It is also a meanstodetect existing spatial biases. The complexitiesofsinkhole distribution are demonstratedinFigures 5.I(a) and (b) and 5.2(a) and (b), which indicates that sinkhole formation, is influenced by a combinationoffactors. The figures are a complex representationofthe parameters that exist in areas where the sinkhole relative frequency and sinkhole density are greatest. Throughout this analysis, all parameters were overlain on each other and each layer was "turned off and on" to examine for correlations. Allofthe parameters addressed in Figures 5.I(a) and (b) and 5.2(a) and (b) have been separately clipped from figures throughout this thesis to show the areas with the greatest sinkhole activityand sinkhole relative frequency occur. The data are summarized in Table 5.1. Figure 5.I(a) represents the greatest sinkhole relative frequency for all the parameters. It indicates that the relative frequencyofsinkholes is greatest in the areas where the overburden to the Floridan aquifer system is either missing or less than 30 feet thick. The overburden thickness appears toplayalarge role in sinkhole formation, as indicated by these results. This figure also shows ,that the physiographic province64

PAGE 74

-----------------------65 containing themostsinkholes (greatest relative frequency) is theGulfCoastal Lowlands.TheGulfCoastal Lowlands has an overburden thickness that varies between 0 to50feet. However, Table 5.1 displays that the highest densityofsinkholes is in the CoastalSwamps,which also has a very thin overburden to the FASofonly 0 to15feet (Figure 4.1 and Figure 2.2). Also, the differing results in the relative frequency and the densityofinkhole is a reflectionofthe sizesofthe provinces.TheGulfCoastal Lowland represents 28%ofthe total study area, and the CoastalSwampsonly 2%. This size biasmustbeconsideredwhenmaking analyses. Also, the Polk Upland represents26%ofthe total area and has thenexthighest relative frequencyofsinkholesofonly 14%, and a sinkhole densityofonly0.08per km2 .Figures 5.3(a) and (b) illustrate that both the relative frequency and the densityofsinkholes are greatest in the Dischargezoneof1 to 5 inches annually within the study area.Thisdoenotcorrelate with the generalideathat Recharge zones are the areas that are expected to have the greatest numberofsinkholes (Dodek 2003).ArcViewallowedthe RechargelDischarge theme tobeoverlaid by the Overburden theme (Figure 5.4) and itshowsthatmostofthe discharge areasoccurwherethe overburden is only 0 to30feet thick, and that the discharge zones with the greatest sinkhole density are located along thecoastline (Figure 5.3) where saltwater/freshwater coastal mixing occurs (Upchurch andRandazzo1997).Itis in these coastal areas that di solutionoflimestone, and hence karst formation, is preferentially developed (Upchurch and Randazzo 1997).Theoverlapoffigures also demonstrates that in many areas where the recharge zones occur, the overburden is 100 to300feet thick. This suggests that thelowernumberofsinkholes occurring in the recharge areas isdueto the increased thickne softhe overburden, and --------------------

PAGE 75

-----------------------66 that the high occurrenceofsinkholes in the discharge areas could be relatedtothe thin overburdentothe FAS. Rock type has always been con ideredtobeanimportant factor in sinkhole formation.Anexample is the chemical weatheringofcarbonate (Lane 1986, p12; Beck and Sinclair 1986; Galloway et al. 1999).Asnoted in Table 5.1, high sinkhole activity is occurring where the Suwannee Limestoneisat or near the surface and also a high relative frequencyofsinkholes occurs in the Hawthorn Group sediments, whichisalso carbonate bearing (Compton 1997; Scott 1997). Although the Beach Ridge and Dunes and the Suwannee Limestone units actually have the greatest sinkhole densities (Figure 4.14), the Hawthorn Group and the Undifferentiated Quaternary Sediment units have the greater sinkhole relative frequencies (Figure 4.15). This might be reflectiveofa size bias. The Beach Ridge and Dunes and Suwannee Limestone are significantly smaller areally than both the Hawthorn Group and the Quaternary Undifferentiated Sediments (Table 4.4). Relative frequency is a statistical operation that addres es the size biasofa sampling population and it represents a valuable statistic accompanying sinkhole density. The Hawthorn Group has a sinkhole relative frequencyof34.4%, but the Quaternary Undifferentiated Sediments has a sinkhole relative frequency almost as great at 32.2%. The HawthornGroup contains a relatively higher percentageofcarbonates (Scott 1997) than the Quaternary Undifferentiated Sediments unit. Other factors may be contributing to sinkhole distribution. The age (Oligocene, Brooks 1981) and carbonate purityofthe Suwannee Limestone, promote its dissolution. Mo tofthe Suwannee Lime tone in the study area lies underanoverburdenof0to15feet along the coastline (Figure 4.1). The Beach ridge and Dune unit is composedof

PAGE 76

67sand, whichishighlypermeable, and also has an overburdenofless than 30 feet thick (Figures 4.1, 4.13). As previously noted, population density data are extremely biased by many factors and is temporal, but the results indicate(Table4.4) that the greatest distributionofsinkholesisinthe zone where population density is 1,000to2,000. It was expectedtooccur in the most highly populated areas, 3,000to6,000. It is noted that the US Census Blocks are designed by population rather than area. This means that some blocks have a much higher population density than others. This skews sinkholes per population numbers, and when relative frequencyiscalculated the biasisexacerbated. Table 5.1. Re ults from GIS analysesofinkholeditribution and significant themesThemeHighest Sinkholes/km2 Greatest RelativeRelative DenSity Frequency Frequency66.5% 14.1% 47.9% 30.1% 34.4% 32.2% 15.0% 14.0% 0.49 0.33 0.27 0.14 0.46 0.30 0.95 0.88 1501-2000 peoplelkm25001-6000 Coastal Swamps Gulf Coastal LowlandsDicharge1-5inches/yr Recharge>10Beach Ridge/Dune SuwanneeLS51-75 feet 0.38 0-15 feet 21.6% 16-30 feet 0.33 31-50 19.1%GulfCoastal Lowlands Polk Uplands Discharge1-5Recharge 1-10 All Hawthorn Seds Quat. Undiff Seds 1501-2000 1001-1500 Population Density Recharge/ Discharge Physiographic Province Overburden Geology ------------------.-

PAGE 77

68. ...'.'.....,. Sinkholes Ph)-siogI apluc ProuncesGeology Formation DischarlteRecharge 0CountiesD llIYCooota1Lowloods[QQl o Di",hargell 5 mcbeslyrPoplsqkInRochal&ell10 Oren",,"'.IawhlmJJcu.dalID. IOOI-lJOOo ... Eawh:aJlltIC.ta.D Other.16-30f... a_.-v.n.y ..... ".___ r-I_4020 r:===============:JIKllomelers 80o, C (a)Figure 5.1. GIS overlayofall the areasofhighest sinkhole relative frequency for all digital parameters used in the analyses, (a) with population density (b) without population density

PAGE 78

69 40 80Hawthorn Gp Hawthorn/Arcadia Fro HawthornlPcace River HawthornlP.RlVcrfBone Valley Polk UplandsOverburden0-15 feet 16-30feetPhysiographic Pro,iuces Geology FonuationoGulf Coastal Lowlands !QQI Undiff Seds....'....SUlkholesDCouuties DischargelRechargeDDischarge/I -5 incheslyr C Recharge/I 10 oOtber '......"..... I",.. ,..'.__ r-I_-Ij' Figure 5,1. Continued(b)

PAGE 79

70""-..... ..... '.Geology Fonnatioll o Beach RidgeIDune Hawthorn Gp Hawthorn/Arcadia Fm HawthornlPeace River Hawthorn/P.River/Bone Valley o SuwaneeLS:.. ,....' .. <00\.1,1.0'0'..-\'--......... ..., o Oth"l"DischargelRecharge o Dischargell to 5 o Recharge/Greater than10Sinkholes Physiographic Provinces Coastal Swamps o GulfCoastal Lowlands.",,, ,.,Overburden_15-30ft o 50-75ft o Counties Pop/sqkm o 1501-2000 o 5001-6000.. I.." ..II.... 1 \'-\" '.'\\ ,\(,..)\--'..\ ___ I4020 -==============::JI Kilometers80o,'.(a)Figure 5.2. All digital parameters with the greatest sinkhole density for the study area (a) with the Census' blocksofpopulation density, (b) without the Census' blocksofpopulation density

PAGE 80

71 C============::JIKilometerso204080Sinkholes Physiognphic Proyinces HawthornlPeace RiverGeology FOllnation o Beach RidgelDune Hawthorn Gp Hawthorn/Arcadia Fm HawthornlP.RiverlBone Valley o SuwaneeLS.. l,iiJ',1 Coastal SWIlmPSo Gulf Coastal LowlandsDischargelRecharge o Discharge/1to5 o Recharge/Greater than10.. .......Counties. ..,. .. ..,. o Overburden_15-30ft o 50-75ft c:=J Other.'.---I c-: I (. Figure 5.2. Continued(b)

PAGE 81

72 Slnkhlle5 DCClJntles incheslyr D Disc:hargellto5 Recharge IGreaterThan 10(a)N.. ". :.,:I : .... '. 'I# ',.:....'.Sinkhole Densitv ..'. :., .',".:" .\II, ',"..:.10 Kilometers20.\0Sinkholes oCounbesDishcargeIRecbarge mcheslyr D Duch"'llell to 5 Rechargelltol0N A '.Sinkhole Relative Frequencvo10 Kilometers 20.\0 Figure 5.3. Recharge and Dishcarge zones with sinkhole distribution. Relative frequency and densityofsinkholes

PAGE 82

7340'..,..... ,......'" 20 o Counties Sinkholes Onrburden to Floridan_ 0-15feet(0-4.5m 16-30 feet(4.9-9.1 m)Reeharoe isehargeelll 'ro oDIscharge 1 to 5 Recharge 1to10Kilometers10o'.\.\ '.'....... ... --------'-----,\,\\ .. ... ..,... ....I ... I, ........ "'. .. ... .\.,.... ..... ... ... ...... ......Figure 5.4. Recharge/Discharge theme overlaid with the Depth to the Floridan Aquifer System theme

PAGE 83

, tI,....OJ.. >. > 'y 74 Figure 4.16 suggests that the locationofthe 30 highest pumped wells in each county correlate with the occurrenceofsinkholes in at least four areas. This may illustrate that well fields are one more influencing factor in sinkhole formation and distribution, while a conclusion can not be made on this analysis it is useful when examining other local factors. Figure 5.5 shows the result of overlaying all the GIS themes and clipping out the areas that intersect allofthem. These (small) areas are outlined in red. The result demonstrates that the formation of sinkholes does not require allofthe theme conditionstobe present, but instead a combination of factors controls sinkhole distribution. From the overlays in this analysis, tabulated in Table 5.1, itisindicated that the strongest influencing factors for sinkhole development are discharge areasof1to5 inches annually (occurring 47%ofthe time), areas where the aquifer is shallow (50 feet or less), and areas where the geologic units consist of permeable and easily dissolved materials. These parameters are found most often in the Gulf Coastal Lowland physiographic province, where the aquifer is closetothe surface or exposed, the dischargeis1 to 5 inches annually, and the near-surface geologic units are the Suwannee Limestone and the Quaternary Undifferentiated Sediments. This database can be combined with other studiestoincrease the accuracyofpredicting future sinkhole development. For example, an enlargementofthe database would helptosubstantiate findings. This can be done by adding newly collected data and non-reported data, which for example, couldbeobtained by maps developed through remote sensing. Further studies could include additional factors for parametersinthis study, which showed weak correlationstosinkhole distribution, i.e. the theme showing

PAGE 84

75Well locations in each county. Additional information could include the extentofthe well field's draw-down. Thesereults suggest that the thickne softhe overburdentothe FAS plays the strongest roleininkhole formation, and when combined with other sinkhole triggering factors, sinkhole density and sinkhole relative frequency may increase. For example, since sinkhole relative frequencyisgreatest where the overburden is less than75feet thick and the HawthornGroup is present (Figure 4.13 and Table 5.1) sinkhole activity should increase. The physiographic provinces that had the greatest numberofsinkholes (Table 5.1) also coincided with areasofthin overburden and geologic units that are pronetodissolution, and hence sinkhole formation. A similar study by Dodek (2003) utilized the current FSRI database to increase his databaseofsinkholes for11north-central Florida counties. His projectionsoffuture sinkhole occurrence for several counties were based on results he determined were influencing sinkhole development. These parameters were areasofhigh rechargetothe aquifer and a minimal depthtothe aquifer system. He found that sinkhole locations coincided with the presenceofnear-surface Ocala Limestone, a carbonate unit. The results in this study found that there was a high densityofsinkholes and a high relative frequencyofsinkholes coinciding with area where the geologic units consistingofnear surface carbonates occurred (Table 4.4). This study did not agree with Dodek's findings that sinkhole distribution is greatest in areasofhigh recharge. Factors, such as the depth the FAS in the discharge zones, haveaninfluence on inkhole distribution.

PAGE 85

76SinkholesIntersectionofsinkhole relative frequency values..All Parameters Intersection. ... ............ .'a...." : ...... ... .... .. ...'...Polk. ... .......:....: .......'....Hillsborough.... ..Pasco..'.: ': ..............-... .. .. .,..... .......-....... i,_. .... .....:.t." S.Y" :,.: ..,.-:-.......-......,-... .... .. ...1_:.... ...;. ': ... .... ...e.-, 4 ,'. .. ,.. ... :.. .:.. .. _,--1 Kilometerso 00 !. J!' ... ';.";: : 't .,.1 ... ,.,..,..-\:.. ,.'J :... ':. .-.PinellasFigure 5.5. GIS intersectionofall the parameters that displayed the highest sinkhole relative frequency. The red areas represent the polygons that contain all the parameters

PAGE 86

CHAPTER 6 CONCLUSIONS A new sinkhole inventory has been created and includes the previously existing FGS database. From this database, factors that were hypothesizedtoinfluence sinkhole formation have been analyzed using a GIS, Esri's ArcView8.3(ArcView). After calculating the sinkhole density and sinkhole relative frequency for each factor, they were then coalesced into maps that allowed trends and patternstobe recognized. Results indicate a correlation between sinkhole distribution and other factors, and are controlled by a combinationofparameters. New data has been obtained that enlarges the existing database, and as this database grows, it strengthens results that are produced by any future analyses. Table 5.1 represents GIS results and indicate that when all parameters are considered, sinkhole relative frequencyisgreatest in(1)the Gulf Coastal Lowlands (2) overburdenis0 to15feet thick(3)Hawthorn sediments are presentinthe near-surface, and(4)the aquifer discharge is 1to5 inches per year. Population densityof1,501to2,000 people perkm2may also be a factor. Allofthese parameters were extracted from their ArcView themes and presented in Figures5.l(a)and (b). Whensinkhole density was computed, the results, summarizedinTable 5.1, show that the parameters where sinkhole density is greatest are where (1) the Depthtothe Floridanis50 to 75 feet, (2) the Coastal Swamps province,(3)the dischargeis1to5 inches per year, and(4)the Beach Ridge and Dune subsurface geology is present. Population densityof1500to2000 people per square kilometer may also be significant. 77

PAGE 87

78Other factors that may have an influence on sinkhole density include areasoflow potentiometric surface and water withdrawal from the aquifer system, i.e., well fields. Figure 4.16 shows four areas where the wellswith the highest withdrawal rates are located in the vicinity where sinkhole occurrence is greatest. Withdrawaloflarge quantitiesofwater from the aquifer, contributestothe loweringofthe potentiometric surface, and may also resultinsinkhole activity (Figure 1.1). Correlationsofinkhole location and low potentiometric surface, suggests that loweringofthe potentiometric surface is another factor contributing to the formationofsinkholes.Inthe four years depicted in Figures 4.11 and 4.12, the areas of highest potentiometric surface are those where fewer sinkholes have formed (or have been reported). The Gulf Coastal Lowlands has a sinkhole relative frequency value almost 5 times larger than any other physiographic province, and although the Coastal Swamps has the highest densityofsinkholes, 0.49 per square kilometer (Table 5.1), theGulfCoastal Lowlands is close behind at 0.33. The Gulf Coastal Lowlandsisa region where Quaternary undifferentiated sediments are the most prominent geologic formation, and it has an overburdentothe FASof30 to 50 feet, well within the major mode for the greatest sinkhole density and the highest relative frequencyofsinkhole occurrence. Population density is generally 1,000 to 2,000 people per square kilometer. Biases are an important aspect in recognizing the significanceofthe various parameters and sinkhole frequency. Whenanoverlayofthe major rivers and roadsinthe area was made (Figure 4.10),nosignificant pattern or correlation was noted, but this might be oneofthe biases that were discussedinprior chapters (i.e., the "collection bias").Ofthe seven major rivers, fiveofthese are in areas that have a very low ----

PAGE 88

79population density, 0 to 50 people (Figure 4.17), or where population density isof50to150 people persquare kilometer. Within these ranges (people per km2),sinkhole density is very low. It is only towards the mouthofthe Hillsborough River that the sinkhole density increases, and the population density in these areas also increasesto1,000to6,000 people per square kilometer (Figure 4.17). One could assume that the low numberofsinkholes is dueinpart to the reporting bia In other words, where populationislow, reporting will be minimal, butasnoted previously, population density data contains many biases. Future studies could increase the databa e by including newly reported sinkholes, and non-reported sinkholes. Non-reported sinkholes could be obtained through digital topographic maps, obtained by the use of remote sensing. With the increaseinthe database, trends and patternsofoccurrence will be more easily identified and hence, as the database "grows", making predictionsoffuture sinkhole distribution should be more accurate.Inordertoutilize the sinkhole database for predicting the high risk areas for sinkhole formation, a theme that representing the distance-to-the-nearest-sinkhole was constructed, using buffer zones of one, two, and three kilometers.Inlightofthe large study area, these buffers represent a reasonable category-distance resolution. The theme is presented in Figure 6.1. It is hypothesized that the higher potential for sinkholesiswhere sinkhole density i greatest and where the distance between sinkholesisleast.InFigure 6.1, that would be the 0to1 km zone, indicated by the blue buffer zone. Figure 6.2 also represents "nearest neighbor" but with buffersof0.5, 1.0, and1.5kms. The figures demonstrate that the highest riskareas would likely be in the north-western

PAGE 89

80 sectionofHillsborough County, the we tern ectionofboth Pinellas and Pasco counties, and central Polk County. The areas within Hillsborough, Pinellas, and Pasco are areas where the FASisclosetothe surface, 0to75 feet (Figure 4.1), and in Polk County, the area where sinkhole density is greatest,iswhere the overburden thickness is 75 feettoasgreat as 300 feet (Figure 4.1). As population density grows, there will be an increasing demand for ground water, and hence greater withdrawal from the FAS and loweringofthe potentiometric surface. This trend will impact areas where the significant factors for sinkhole formation are concentrated, and future sinkhole density and sinkhole relative frequency will be greatest.

PAGE 90

81POLKNearest Sinkhole0-1km D 1-2km2-3IanDCounties4020 -=========::::JI Kilometers80oFigure 6.1. Proximity to the nearest sinkhole, indicatedbyone, two, and three kilometer buffer zones

PAGE 91

82DcouutiesearetsinkholeNearest Slllkhole.O-5km o 5-1km.1-15kmPOLK _L..-IKilometerso204080Figure 6.2. Proximity to the nearest sinkhole, indicated by 0.5, 1.0, and1.5kilometer buffer zones

PAGE 92

APPENDIX A GLOSSAR Y OF ARCMAP TECHNICAL TERMS Esri's ArcMap 8.3 was used for processing data within this thesis. Some of the tasks performed are known as Geoprocessing. The following are descriptionsofthese procedures: Clip:a process where a specified layer, knownasthe cookie cutter, is usedtocut another layer, knowasthe input layer. The resultisa new layer that has only the polygons and attributes inside the clip layer, while those outside the clip layer do not remain. Dissolve: the processofcombining polygons with identical attributes. The attribute is specified by the user. Geocoding: the processofmatching street addresses with geographic coordinates Geographic coordinates: a measurement by latitude and longitude on the earth's surface. Join: the processofattaching tabular data to a layer. The fields in the table are appendedtothe layer using a common field. Join establishes a one-to-one, one-to many or many-to-many relationship between map features and tableattributes. Layer: Geographic information is displayed on a map as layers; each layer represents a particular typeoffeature such as streams, lakes, or highways. Layers are listed in the ArcView tableofcontents and can be further organized into data frames. A layer references geographic data stored in a data source, suchascoverage, and defines howtodisplay it. Layout: the design or arrangementofelements such as geographic data, elements like north arrows, legends, scale bars, and text, in a digital map display or printed map. Merge: combining polygons from two or more themes. Attributes with the same name are retained. Projection: a mathematical formula that transforms feature locations from the earth's curved surfacetoamap'sflat surface. A projected coordinate system employs a projection to transform locations expressed as latitude and longitude83

PAGE 93

-.,,....... ..---------------84 valuestox,y coordinates. Projections cause distortionsinone or moreofthese spatial properties: distance: area, shape, and direction. Theme:a category within a layer. .. -----

PAGE 94

-------------------------------------APPENDIXB HISTORICAL RAINFALL FOR STUDY AREA Table B.1. Rainfall Year Hernando Pasco Pinellas Hillsborough Polk 1974 66.83 65.98 67.21 53.11 52.31 1985 48.83 47.96 42.43 44.65 43.29 1994 54.81 51.75 43.03 55.2056.70 2000 44.55 43.81 39.78 44.31 38.96 Min 44.55 43.81 39.78 44.31 38.96 Max 66.8365.98 67.21 55.20 56.70 Avg 53.76 52.38 48.11 49.32 47.8285

PAGE 95

------------------REFERENCES CITED Arthur, J. D., R A. Lee, andL.Li. 2001. Lithostratigraphic and Hydrostratigraphic cross sections through Levy-Marion to Pasco counties, Southwest Florida. Florida Geological Survey Open File Report no.81.21p. Beck, B., and W. C. Sinclair 1986. Sinkholes in Florida: An Introduction. The Florida Research Institute Report no.85-86-4.17p.Brooks, H.K.1981. Geologic MapofFlorida, Institute for Food and Agricultural Sciences, UniversityofFlorida,Gainesville, Florida. Burwell, A.L. and W.L. Wilson. 2001. New sinkhole proximity maps for selected Counties in Central Florida. Published by Subsurface Evaluations, Inc., Tampa, Florida.16p.Butler, A.M., B.A. Diskin, K.L. Eastman, D.H. Gatzlaff,RB.Corbett, C.C. Lilly, and P.F. Maroney. 1992. Executive SummaryinInsurance studyofsinkholes. Florida State University Center for Insurance Research, Tallahassee, Florida.13p.Campbell, Kenneth M. 1984. GeologyofHillsborough County. Florida Geological Survey Open File Report no.6, Tallahassee, Florida. Florida Geological Survey. 19 p. Casper, J., B. Ruth, and1.Degner. 1981. A remote sensing evaluationofthe potential for sinkhole occurrence. Remote Sen ing Applications Laboratory, DepartmentofCivil Engineering, UniversityofFlorida, Gainesville, Florida. 103 p. Chen, F.H. 1988. Foundations on Expansive Soils. Developments in Geotechnical Engineeling 54:65-69.NewYork. Elsevier Scientific Publishing Company. Compton, J. S., 1997. Origin and paleoceanographic significanceofFlorida'sdeposits in The GeologyofFlorida edited by A.F. Randazzo and D.S. Jones, 195-216. Gainesville, Florida. University PressofFlorida. Currey,D.R1974. "Continentalityofextratropical climates," Annalsofthe AssociationofAmerican Geographers 64:268-280. DepartmentofArmyUSA1983. Foundation in Expansive Soils. Washington, DC,TM5:818-7. 86

PAGE 96

---""---"--"---------"--87Dodek, B. 2003. A GIS AnalysisofSinkhole Activity in North-Central Florida: Causes and Correlations. Master's Thesis University of Florida. Gainesville, Florida. 79p.Fernald, E.A and E.D. Purdum, editors 1998. Water resources atlasofFlorida: InstituteofScience and Public Affairs, Tallahassee, Florida. Florida State University,310p.Florida DepartmentofEnvironmental Protection 2000. Florida Physiographic Provinces. Florida DepartmentofEnvironmental Protection ftp site, May 2003. http://www.dep.state.fl/us/geology/pub. Florida Depattment ofEnvironmental Protection 2003. Raster data obtained from FDEP staff (unpublished data), then it was convertedtodigital format for this map. Florida DepartmentofEnvironmental Protection 2000. Florida Subsurface geology. Florida DepartmentofEnvironmental Protection ftp site, May 2003. http://www.dep.state.flIus/geology/pub. Florida Geographic Data Library 1990. Florida county boundaries: http://www.fgd1.org Florida Geological Survey web site, November 2002. http://www.dep.state.flIus/geology. Frarrk, E.F., B.F. Beck 1990. An analysisofthe causeofsubsidence damageinthe Dunedin, Florida area. 59p.Galloway, D., D.R. Jone and S.E. Ingebritse 1999. Sinkholes, West-Central Florida. Excerpt from Circular 1182: 121-140. United States Geological Survey. Green, R., J.D Arthur, andD.DeWitt 1995. Lithostratigraphic and Hydrostratigraphic cross sections through Pinellas and Hillsborough counties, Southwest Florida. Open File Report no.61, 1-26. Florida Geological Survey, Tallahassee, Florida. Healy, H.G. 1975. Terraces and shorelinesofFlorida. Florida BureauofGeology Map Series no.71. Heatherington, A.L. and P.A. Mueller 1997. Geochemistry and originofFlorida crustal basement terranes, in The GeologyofFlorida edited by A.F. Randazzo and D.S. Jones, 27-37. Univer ity PressofFlorida, Gainesville, Florida. Henry, J.A., K.M Portier, andJ.Coyne 1994. The Climate and WeatherofFlorida. Sarasota, Florida. Pineapple Press, Inc. 249p.Hine, A.C. 1997. Structural and paleoceanographic evolutionofthe margins of the Florida platform, in The GeologyofFlorida edited by A.F. Randazzo and D.S. Jones, 169-194. University Press of Florida, Gainesville, Florida.

PAGE 97

88Hyde, L.W. 1975. Principal AquifersinFlorida, Map SeriesNo.16, United States Geologic Survey and BureauofGeology, Tallahassee, Florida. Lane, E. 1986. Karst in Florida, Special publication#29, Florida Geological Survey, Tallahassee, Florida. 100p.Miller, J.A. 1986. Hydrogeologic Frameworkofthe Floridan aquifer systeminFloridaaninpartsofGeorgia, South Carolina, and Alabama, United States, professional paper 1403-B. Geological Survey, Departmentofthe Interior, Washington D.C.91p 1997. HydrogeologyofFlorida,inThe GeologyofFlorida edited by A.F. Randazzo and D.S. Jones, 69-88. University PressofFlorida, Gainesville, Florida. National Oceanic and Atmospheric Administration, National Weather Service 2003 http://www.srh.noaa.gov/tbw/climate, May 5,2003. Nelson, J.D. andDJ.Miller1992. Expansive Soils: problems and practice in foundation and pavement engineering, New York. John Wiley and Sons, Inc.43p.Pirkle, E.C., W.H. Yoho, and C.W. Hendry, Jr. 1970. Ancient Sealevel StandsinFlorida Bulletin no.52,61p.Published by the BureauofGeology Tallahassee, Florida.61p.Randazzo, A.F. 1997. The sedimentary platformofFlorida: MesozictoCenozoic,inThe GeologyofFlorida edited by A.F. Randazzo and D.S. Jones, 39-56. Gainesville, Florida. University PressofFlorida. and D.L.Smith 2003. Subsidence-induced foundation failures in Florida's karst terrain. Sinkholes and the engineering and environmental impactsofkarst. Proceedingsofthe ninth multidisciplinary conference, Huntsville, Alabama, 82-94. Edited by BarryF.Beck, American Society of Civil Engineers. Schmidt, W., 1997. Geomorphology and physiographyofFlorida, in The GeologyofFlorida edited by A.F. Randazzo and D.S. Jones, 1-12. Gainesville, Florida, University PressofFlorida. and T.M. Scott, 1984. Florida karst-Its relationshiptogeologic structure and stratigraphy. Proceedingsofthe First Multidisciplinary Conference on Sinkholes, 11-16. Scott,T.M.1992. A Geolocgical OverviewofFlorida. Florida Geological Survey Open File Report no.50, Tallahassee, Florida. 77p.Scott, T .M. 1997. MiocenetoHolocene historyofFlorida,inThe Geology of Florida editedbyA.F. Randazzo and D.S. Jones. University PressofFlorida, Gainesville, Florida. Structural featuresofthe Florida peninsula p58.

PAGE 98

89 1997. Miocene to Holocene historyofFlorida,inThe GeologyofFlorida edited by A.F. Randazzo and D.S. Jones, 57-67. University Pre sofFlorida, Gainesville, Florida. 2001. Texttoaccompany the Geologic mapofFlorida, Florida Geological Survey Open File Report 80. Tallahassee, Florida. Sinclair,W.e.1982. Sinkhole development resulting from ground-water withdrawalinthe Tampa Area, Florida: U.S. Geological Survey Water Resources Division.19p.-,J.W. Stewart,RL.Knutilla, A.E. Kilboy, andRL.Miller 1985. Types, features, and occurrence of sinkholesinthe karstofwest-central Florida: U.S. Geological Survey Water Resources Investigations.81-50.19p.-,and J.W. Stewart. 1985. Sinkhole type development and distributioninFlorida: Florida Geological Survey Map Series 110. Tallahassee, Florida. Florida Geological Survey. Smith, D.L. and K.M. Lord. 1997. A Techtonic Evolution and Geophysics of the Florida Basement.InThe GeologyofFlorida edited by A.F. Randazzo and D.S. Jones, 13-26. Gainesville, Florida. University PressofFlorida. Southeastern Geological Society Ad Hoc Committee on Florida Hydrostratigraphic Unit Definition 1986. Hydrogeological unitsofFlorida: Florida Geological Survey Special Publication 28. Tallahassee, Florida. 8p.Southwest Florida Water Management District 2002. Recharge and Discharge areasofthe Floridan aquifer system. http://www.swfwmd.state.fl.us/data/gis. November 20,2003.-2002.Mapoffive-county study area. http://www.wfwmd.state.fl.us/data/gis/library/political.htm. May 2003.-2003.Potentiometric base maps and potentiometric sUlface obtained separately from http://www.swfwmd.state.fl.us/data/data/dataonline.htm. May 2003.-2003.Well pumpage and locations were supplied in Excel format by SWFWMD employee via email June 2003. Spechler,RM.and D.M. Schiffer 1995. United States Geological Survey fact sheet FS-151-95, 1 sheet. Spencer, S.M. and Ed Lane 1995. Florida Sinkhole Index. Open File Report no.58, Talahassee, Florida. Florida Geological Survey.18p.------------

PAGE 99

.. ,',90 Sridharan,A.and K Prakash 2000. Shrinkage limitofsoil mixtures. ASTM Geotechnical Testing Journal 23:3-8. Teleatlas: http://www.na.Teleatlas.com. November3,2002Tihansky, A.B. United States Geological Survey. 1999. Human-induced sinkhole development in the study area, Pasco and Hernando counties. Sinkholes, West Central Florida, A Link Between Surface Water and Ground Water, an excerpt from Galloway, Devin, D.R. Jones, andS.B.Ingebritsen, Circular 1182, p 138.-,UnitedStates Geological Survey. 1999. Sinkholes, West-Central Florida, A Link Between Surface Water and Ground Water, an excerpt from Galloway, Devin, D.R. Jones, and S.E. Ingebritsen, Circular 1182, p 127.-,UnitedStates Geological Survey. 1999. Sinkholes, West-Central Florida, A Link Between Surface Water and Ground Water,anexcerpt from Galloway, Devin, D.R. Jones, and S.E. Ingebritsen, Circular 1182, 121-139. United States Census Bureau 2000, Population density by U.S. Census block groups: http://www.census.gov/geo/www/cob/bg2000.htrnl United States.Geological Survey 1990. Confining unitsofthe Floridan aquifer system. (modified from Miler). Ground Water Atlasofthe United States: Alabama, Florida, Georgia, and South Carolina, publication HA 730-G 1990. Ground Water Atlasofthe United States: Alabama,Florida, Georgia, and South Carolina, publication HA 730-G. Online version; http://capp.wateLusgs.gov/gwa, November 18, 2003. United States Geological Survey 1990. National Water Summary 1987: hydrologic events and water supply and use. U.S. Geological Survey Water-Supply Paper no. 23, 550-553 Upchurch, S.B. andA.F. Randazzo. 1997. Environmental geologyofFlorida.InGeologyofFlorida, edited by A.F. Randazzo and D.S. Jones, 217-48. Gainesville, Florida. University PressofFlorida. Walker, K.R.,G.Shanmugam, andS.c.Ruppel 1983. A model for carbonate to terrigenous clastic sequences. Bulletinofthe Geological SocietyofAmericano.94, 700-712. Wetterhall, W.S. 1964. Geohydrologic ReconnaissanceofPasco and Southern Hernando Counties, Florida. ReportofInvestigations No. 34, Tallahassee, Florida. Florida Geological Survey.12p.White, W.A., 1970. The Geomorphologyofthe Florida Peninsula:Bureau Geology. Florida DepartmentofNatural Resources, Bulletin no.51, 69p. ------------------.-.-----

PAGE 100

-----------------------------91Zisman, E.D. 2003. Guilty until proven innocent Sinkhole definition&identifying features. Sinkholes and the engineering and environmental impactsofkarst. Proceedingsofthe ninth multidisciplinary conference, Huntsville, Alabama. Edited by Barry F. Beck, American SocietyofCivil Engineers, 124-129.--------

PAGE 101

'It .. LBIOGRAPHICAL SKETCH Kathleen Commins wa born in the United States in the 20thcentury! After her grade school education at a public school in Florida, she movedtoAustralia where she graduated from a statehighschool. Before moving backtoFlorida and eventually attending the U ni versityofFlorida, she first found a bitoftime for some travel and lotsoftime for work experience. She attended the UniversityofFlorida both on a part time and a full time basis, until obtaining Bachelor and MasterofScience degrees in Geology .92

PAGE 102

----..... _----_.---'-.. ..-,'r I certify that I have read this study and that in my opinion it conforms to acceptable standardsofscholarly presentation and is fully adequate, in scope and quality,asa thesis for the degreeofMasterofScience. AnthonyF.andazzo, ChaianProfessorofGeological Sciences I certify that I have read this study and that in my opiruon it conformstoacceptable standardsofscholarly presentation and is fully adequate,inscope and quali ,as a thesis for the degreeofMasterofScience. DouglasL.S 'th ProfessorofGeological Sciences I certify that I have read this study and thatinmy opinion it conformstoacceptable standardsofscholarly presentation and is fully adequate,inscope and quality,asa thesis for the degreeofMasterofScience. /) /Joseph Meert Assistant ProfessorofGeological Sciences This thesis was subrllitted to the Graduate Facultyofthe DepartmentofGeological Sciences in the CollegeofLiberal Arts and Sciences and to the Graduate School and was acceptedaspartial fulfillmentofthe requirements for the degreeofMasterofScience. May, 2004 Dean, Graduate School ----------

PAGE 103

.' 'August 2007 Internet Distribution Consent A&reement In reference to the following dissertation:Author: ;t:fi7#LVv'dornfh1/5 Title: /l-e.n(/ITY/A/tUt:Sr c..e-tVTteALFLoe(D/l A GevGRA-;J,1/fC-/t<./FOICfi14notVf57YnrIfI/ALySt'PGblicationOats: I. K'fiTf/L.tJ!-I6&ey, ascopyright holderfortheaforementioned dissertation, hereby grantspecificandlimitedarchiveanddistributionrights to theBoardofTrusteesofthe University of Florida and it& agents. Iauthorize the Universityof Floridatodigitizeanddistributethedissertation descrIbedabovefornonprofit, educational purposesviatheInternetorsuccessive technologies.This is a non-exclusive grantofpermissions for specific off-lineand on-lineusesfor an Indefinite term. Off-line uses shall be limited tothosespecifically allowed by"FairUse"asprescrfbedbythetermsofUnitedStates copyright legislation(cf,TItle17,U.S.Code)aswell as to the maintenance and preservationofa digitalarchivecopy. Digitizationallows theUniversityof Floridato generate image-andtext-basedversionsasappropriate and toprovIdeandenhanceaccessusingsearchsoftware.Thisgrantofpermissions .:Jrohibits useofthedigitized versionsforcommercial use ororofit.Personalinformationblurred,.. ... __ ..... I r-.....,....'-'.-.. --.,,,,,,_1-'1'"""'.,'''''II........ l '-.I'V\J"'"I.tfll' I IVIUc:l/L.......g-P;0J"Date ofSi,gnatureReturnthis formto:CathyMartyniakPreservationDept., University ofFloridaLibrariesP.O.Box117007 GainesVille, FL 32611-7007.""" .'( 3" .. ::O'.j8e," ',',t"'\.


xml record header identifier oai:www.uflib.ufl.edu.ufdc:UF0008601900001datestamp 2009-02-16setSpec [UFDC_OAI_SET]metadata oai_dc:dc xmlns:oai_dc http:www.openarchives.orgOAI2.0oai_dc xmlns:dc http:purl.orgdcelements1.1 xmlns:xsi http:www.w3.org2001XMLSchema-instance xsi:schemaLocation http:www.openarchives.orgOAI2.0oai_dc.xsd dc:title Sinkhole activity in West-Central Floridadc:creator Commins, Kathleendc:publisher Kathleen Comminsdc:date 2004dc:type Bookdc:identifier http://www.uflib.ufl.edu/ufdc/?b=UF00086019&v=00001003087097 (alephbibnum)dc:source University of Floridadc:language English