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The shallow fresh-water system of Sanibel Island, Lee County, Florida ( FGS: Report of investigations 69 )
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 Material Information
Title: The shallow fresh-water system of Sanibel Island, Lee County, Florida ( FGS: Report of investigations 69 ) with emphasis on the sources and effects of saline water
Series Title: ( FGS: Report of investigations 69 )
Uncontrolled: The shallow fresh-water system of Sanibel Island ..
Physical Description: vii, 52 p. : ill. ; 23 cm.
Language: English
Creator: Boggess, Durward H
Geological Survey (U.S.)
Lee County (Fla.) -- Board of County Commissioners
Publisher: State of Florida, Dept. of Natural Resources, Division of Interior Resources, Bureau of Geology
Place of Publication: Tallahassee
Publication Date: 1974
 Subjects
Subjects / Keywords: Hydrology -- Florida -- Sanibel Island   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by D. H. Boggess ; prepared by the U.S. Geological Survey, in cooperation with Bureau of Geology, Division of Interior Resources, Florida Department of Natural Resources, and the County Commissioners of Lee County.
Bibliography: Bibliography: p. 45.
 Record Information
Source Institution: University of Florida
Rights Management:
The author dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law and all related or neighboring legal rights he or she had in the work, to the extent allowable by law.
Resource Identifier: aleph - 000109628
oclc - 01273232
notis - AAM5254
lccn - 74623397
System ID: UF00001256:00001

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STATE OF FLORIDA
DEPARTMENT OF NATURAL RESOURCES
Harmon Shields, Executive Director



DIVISION OF INTERIOR RESOURCES
R. O. Vernon, Director



BUREAU OF GEOLOGY
Charles W. Hendry, Jr., Chief



Report of Investigation No. 69


THE SHALLOW FRESH-WATER SYSTEM OF SANIBEL ISLAND,
LEE COUNTY, FLORIDA, WITH EMPHASIS ON THE SOURCES
AND EFFECTS OF SALINE WATER



SBy
D. H. Boggess




Prepared by the
U. S. GEOLOGICAL SURVEY
in cooperation with
BUREAU OF GEOLOGY
DIVISION OF INTERIOR RESOURCES
FLORIDA DEPARTMENT OF NATURAL RESOURCES
and the
COUNTY COMMISSIONERS OF LEE COUNTY

TALLAHASSEE, FLORIDA


1974






DEPARTMENT
OF
NATURAL RESOURCES





REUBIN O'D. ASKEW
Governor


RICHARD (DICK) STONE
Secretary of State




THOMAS D. O'MALLEY
Treasurer




FLOYD T. CHRISTIAN
Commissioner of Education


ROBERT L. SHEVIN
Attorney General




FRED O. DICKERSON, Jr.
Comptroller




DOYLE CONNER
Commissioner of Agriculture


HARMON SHIELDS
Executive Director






LETTER OF TRANSMITTAL


Bureau of Geology
Tallahassee
February 27, 1974


Honorable Reubin O'D. Askew, Chairman
Department of Natural Resources
Tallahassee, Florida 32304


Dear Governor Askew:

The Department of Natural Resources, Bureau of Geology, is publishing as
its Report of Investigation No. 69 the report entitled, "The Shallow Fresh-water
System of Sanibel Island, Lee County, Florida, with Emphasis on the Sources
and Effects of Saline Water," by D. H. Boggess of the U. S. Geological Survey.

Sanibel Island for many years has been a major tourist attraction. The
resident population and the number of tourists visiting the island have increased
greatly over the last decade and projections indicate a much greater increase in
the future. Development of land on the island has generally paralleled the rapid
development in other parts of Lee County. As a result of this rapid growth,
numerous land and water-resource problems have occurred on the island.

The purpose of this report is to provide a generalized description of the
geology and hydrology of the surficial sediments and the surface-water network
which together form the shallow fresh-water system of the island.

Respectfully yours,



C. W. Hendry, Jr., Chief
Bureau of Geology




















































Completed manuscript received
January 21, 1974
Printed for the Florida Department of Natural Resources
Bureau of Geology
by Ambrose the.Printer
Jacksonville, Florida

Tallahassee
1974


iv






CONTENTS

Abstract ............................................ 1
Introduction .......................................... 2
Purpose and scope .......... .......... ................ 3
Acknowledgments . . . . . . . . 3
Previous investigations . . . ..... . . . .. 4
Location and description of the area . . . .... . ... 4
Climate ............................ .. .......... 5
Rainfall ............ ... ............ ............ 5
Well construction, inventory, and numbering system . . . . 8
Water-bearing formations .................. .... .......... 10
Deep artesian aquifers ............... ....... ........ 10
Shallow artesian aquifer ............. ........ ......... .. 12
Water level fluctuations . . . . . .. 14
Chloride concentrations ................... ......... 17
Water-table aquifer ............... .... ........... 17
Water-level fluctuations ................... ......... 18
Chloride concentrations ............................ 19
Surface water ......................... ............... 25
Sanibel River ........... .. ..... ......... ......... 25
Ponds, lakes, and canals ............. ..... .... ...... 33
Excavations .......... ................... ............ 38
Souces of saline water ................... ........ ......... 39
Summary and conclusions ................... .............. 41
References ........ ................. ....... ........... 45
Appendix ............................................ 47






ILLUSTRATIONS

Figure Page
1. Map of Lee County showing location of Sanibel Island . . . 5

2. Map of Sanibel Island showing road network, interior drainage system, and
other features or names used in the report . . . . 6

3. Map showing location of wells and test holes on Sanibel Island . 9

4. Log showing the geologic formations, lithology, aquifers, and chloride
concentrations in water from test hole L-1533 . . . ..... 11

5. Lithology sections showing thickness of surficial sediments ........ 13

6. Tracing of records from the tide gage at Point Ybel and well L-1408,
August 4-9, 1971 ............................... .. 14

7. Graph showing water-level fluctuations in wells L-1408 and L-1415 and
tide at Point Ybel, August 1971 July 1972 . . . ..... 15

8. Map of Sanibel Island showing chloride concentrations in water from the
upper part of the shallow artesian aquifer . . . ..... 16

9. Graph showing fluctuations of the water table in well L-1403 and rainfall
at station 2 (Sanibel Post Office) ........................ 18

10. Profile across Sanibel Island between wells L-1404 and L-1410 showing
seasonal water levels in the water-table aquifer . . . ..... 20

11. Hydrographs showing fluctuations of the water table in wells L-1412,
L-1414,and L-1416, in 1971 72 . . . ..... ....... 21

12. Map of Sanibel Island showing highest and lowest altitude of the water
table in 1972 .................................. 22

13. Graphs showing increase in chloride content of water with depth in the
water-table aquifer ................... .............. 23

14. Graph showing variation in chloride content in wells L-1147 and L-1158,
June 1970- May 1972 ................... ......... 24

15. Map of Sanibel Island showing location of selected surface-water sampling sites 26

16. Graph showing variation in river stage at Beach Road and chloride
concentrations in water at sites S-1 and S-2 (upstream) November
1970-December 1972 ........................................ 28

17. Graph showing chloride concentrations at sites S-3 and S4 (downstream)
June 1970-December 1972 ................ ........... 29

18. Graph showing chloride concentrations at sites S-4 (upstream), S-10 and
S-lI June 1970-December 1972 ........................ 30
vi







ILLUSTRATIONS continued

19. Graph showing chloride concentrations at sites S-13, S-18 and S-21, April
1970-December 1972 ................. ............. 32

20. Graph showing chloride concentrations at sites S-7, S-8, and S-9, June
1970 December 1972 ................................ 34

21. Graph showing chloride concentrations at sites S-5, S-12, and S-14. June
1970-December 1972 ................................ 36

22. Graph showing chloride concentrations at sites S-16 and S-17, October
1970-December 1972 ................... .......... 37

23. Graph showing chloride concentrations at sites S-15, S-19, S-20, S-22, and
S-23, October 1970 December 1972 ................ ...... 38


TABLES

Table Page
1. Monthly rainfall totals in inches for stations 1 and 2 on Sanibel Island and
Fort Myers, 1971 72 ..................... .. ....... 7

2. Records of wells on Sanibel Island ......................... 48









THE SHALLOW FRESH-WATER SYSTEM OF SANIBEL ISLAND,
LEE COUNTY, FLORIDA, WITH EMPHASIS ON THE SOURCES
AND EFFECTS OF SALINE WATER
By
D. H. Boggess




ABSTRACT

The Sanibel Island fresh-water system includes the water-table aquifer and
the hydraulically connected surface network of streams, ponds, lakes, and canals
in the interior of the island. The aquifer and surface-water network react
similarly to recharge and discharge factors and generally form a hydrologic unit.
Beneath the water-table aquifer, a-shallow artesian aquifer with widely different
characteristics, occurs at depths ranging from about 29 to 34 feet below land
surface and probably extends to depths of more than 100 feet. A clay or marl
stratum of variable thickness separates the aquifers and retards the movement of
water between them. Other artesian aquifers occur at greater depths beneath the
island.

The fluctuation of water levels in the water-table aquifer follows a seasonal
pattern; levels are highest near the end of the wet season in September or
October and lowest near the end of the dry season in May or June. The annual
fluctuation of the water table ranged from about 1 to 3 feet in most parts of the
island in 1972. Within the shallow artesian aquifer, water levels respond to the
loading and unloading of tides in the Gulf of Mexico. The tidal efficiency of
wells tapping this aquifer range from about 10 to 77 percent. The artesian
pressure in the aquifer is related to the tide; therefore, lowest water levels occur
during the winter months when mean tide levels are lower. It is estimated that
water levels in the aquifer annually fluctuate over a range of about 3 feet.

Chloride concentrations in the water-table aquifer range from 12 to
23,200 milligrams per liter; concentrations are highest in or adjacent to the
salt-marsh areas along the north half of the island. Generally the chloride
concentration increases with increasing depth of penetration in the aquifer,
although the rate of increase is variable. Periodic sampling of streams, lakes,
ponds, and canals in the interior of the island indicates a range in the chloride
concentration from about 500 to 16,100 miligrams per liter. Within the shallow
artesian aquifer, the chloride concentrations range from 2,250 to 30,900
miligrams per liter; concentrations are highest under the north half of the island.

The sources of saline water or the mechanisms through which this water
enters the interior fresh-water system include: direct inland flow through the






BUREAU OF GEOLOGY


Sanibel River from adjacent tidal water bodies because of leakage or overtopping
the control structures during high tides; inland movement through the
water-table aquifer when fresh-water levels are at or below mean sea level;
upward leakage from the shallow artesian aquifer during low stages of the water
table; discharge into the interior drainage system of water from the lower part of
the water-table aquifer and from the shallow artesian aquifer during dewatering
operations while excavating ponds, lakes, and canals; and discharge of water
from wells or test holes which tap saline water zones in the deep artesian
aquifers and which are improperly cased or sealed.

Most of the problems caused by intrusion of saline water into the interior
fresh-water system can be resolved. The inland movement of saline water
through the Sanibel River from adjacent tidal water bodies could largely be
averted if the control structures were modified to prevent their being topped by
high tides. Dewatering of excavations should be restricted to those areas where
no fresh water occurs within the water-table aquifer. Excavations should be
limited to depths which will not improve the hydraulic connection between the
water-table and shallow artesian aquifers. All wells or test holes which penetrate
any of the artesian aquifers should be plugged with cement after serving their
purpose.



INTRODUCTION


Sanibel Island for many years has been a major tourist attraction. The
resident population and the number of tourists visiting the island have increased
greatly over the last decade and projections indicate a much greater increase in
the future. Development of land on the island has generally paralleled the rapid
development in other parts of Lee County. As a result of this rapid growth,
numerous land and water-resource problems have occurred on the island.

In June 1970, saline water was encountered in several parts of the interior
drainage system. Although the sources of the intruding saline water were
identified, it was evident that additional detailed information was needed for
more effective management of the fresh water in the interior of the island.

In October 1970, the U. S. Geological Survey started a more detailed
investigation of the-geologic-hydrologic characteristics of the island. The
investigation was made in cooperation withliheLfe-County Board of County
Commissioners and the Florida Department of Natural Resources, Bureau of
Geology.







REPORT OF INVESTIGATION NO. 69


PURPOSE AND SCOPE

The purpose of this report is to provide a generalized description of the
geology and hydrology of the surficial sediments and the surface-water network
which together form the shallow fresh-water system of the island. The
ground-water part of the investigation is based on information obtained from
numerous test holes, observation wells, and private wells. Information on the
surface-water system was obtained from existing ponds, lakes, and canals and
those under construction and the interior drainage system of the Sanibel River.
Most of the ponds, lakes, and canals were excavated to obtain fill for increasing
the altitude of adjacent land areas. The methods of excavation and the depth of
penetration into the underlying sediments frequently cause the quality of the
water in the excavation to deteriorate. Thus, greater emphasis is placed on these
excavations in the -report. Particular attention has been given to the
identification of sources of saline water and the effects of the saline water on the
fresh-water system on the island.

Originally, the investigation was restricted to an area of about 2 square
miles near the east end of the island where basic elements of the hydrologic
cycle could be investigated in detail. However, it was determined early in the
investigation that surface runoff could not be measured accurately because of
existing and proposed changes in the drainage system. Thus, the scope of the
investigation was extended to provide greater, but less detailed, coverage beyond
the 2-square-mile area.

ACKNOWLEDGMENTS

The author gratefully acknowledges the cooperation of residents of
Sanibel Island, other land owners, and public and private firms for permitting
construction of observation wells, measurement of water levels, collection of
water samples, geophysical logs, and providing other essential geologic and
hydrologic information. Several drilling firms provided drill cuttings and water
samples from some of the deeper wells. Most of the geophysical logs from deeper
wells on the island were provided by the Florida Department of Natural
Resources.

Special thanks are given to Mr. Robert L. England for the volunteer
operation of the rain gages on the island and providing the rainfall records used
in this report.

The continued interest and support of the Lee County Board of County
Commissioners and the Sanibel-Captiva Conservation Foundation, Incorporated
is gratefully acknowledged.






BUREAU OF GEOLOGY


PREVIOUS INVESTIGATIONS

The results of two investigations have added measurably to an
understanding of the geology and hydrology of the island.

An investigation by Provost (1953) established the relation, between
fluctuations of ground-water and surface-water levels on the breeding cycle of
the salt marsh mosquito. The investigation also provided valuable data on
fluctuations of the water table resulting from cyclic recharge in discharge. The
report served as a basis for the present water-control system which has been a
major factor in reducing the mosquito population.

The study by Missimer (1972), conducted concurrently and utilizing some
of the subsurface data collected during the investigation described herein
provides a rational hypothesis concerning the origin and depositional history of
Sanibel Island. Missimer also provides a summary of earlier investigations which
mentioned some facet of the island's geology and hydrology as part of more
generalized studies.

LOCATION AND DESCRIPTION OF THE AREA

Sanibel Island is one of a series of offshore barrier islands bordering on the
Gulf of Mexico along the west coast of Florida. Unlike most other islands in this
chain which generally parallel the mainland, Sanibel Island forms an arc, one end
of which points eastward toward the mainland and the other northwestward.
The 18-square-mile island is along the coast of Lee County as shown on figure 1.

Most of the north half of the island bordering on Pine Island Sound
consists of mangrove swamps and other low lying areas subject to tidal flooding.
A large part of this area is in the J. N. "Ding" Darling National Wildlife Refuge
(fig. 2).

Beach ridge remnants in the upland interior of the island (along State
Road 867) are 4 to 7 feet above mean sea level and form a barrier to the
movement of surface water. The beach ridges parallel to the present shoreline of
the Gulf of Mexico similarly form a hydrologic boundary on the south side of
the island. Between these upland areas, the land generally slopes toward an
interior lowland, commonly referred to as the "Sanibel Slough."

Within the slough a central drainageway consisting of natural and
improved channels runs along most of the length of the island. This shallow,
narrow stream which flows only occasionally, is referred to as the "Sanibel
River" by local residents. This name is used throughout this report to identify
the main channel of the interior drainageway.







REPORT OF INVESTIGATION NO. 69


Numerous ponds, lakes, and canals have been excavated throughout the
island, primarily to obtain fill to increase the altitude of adjacent land areas for
development. An extensive network of shallow drainageways has been excavated
both in the interior and in the salt-marsh areas for mosquito control.

CLIMATE

The climate of Sanibel Island is subtropical; temperature extremes are
modified by the tempering influence of the Gulf. The estimate average annual
temperature is 74F -- similar to that at Fort Myers. Average monthly
temperatures range from 64F in January to 830F in August.

The prevailing wind direction is east and usually of relatively low velocity
except during the passage of thunderstorms, tropical storms, or hurricanes.

RAINFALL

Monthly rainfall totals at two locations on Sanibel Island and at Page Field
near Fort Myers are summarized in table 1. Station 1 on the island is near Beach
Road and station 2 is 2.5 miles west, at the post office (fig. 2). Both stations are
equipped with standard 8-inch metal gages, one of which is of the automatic
recording type.

82015'" 82000' g8145'
o" ___ __ ." CHARLOTTE

26045' CHARLOTTE 4RBOR g \








2 b 0 I b'i


mopfrom U.S.G.S. Topographic map
Figure 1. Map of Lee County showing location of Sanibel Island.











82 12' 30" 10' 7'10"I 5 2'30" 82O0'
26030' O I






44io 0 SAN CARLOS BY


27'30" EXPLANATION POINT

APPROXIMATE BOUNDARY 0
J.N."DINGDARUING NATIONAL C.p 0
WILDLIFE REFUGE. u l,
pef ** al <, I


MMILE G oo ci

.0"25'





Figure 2. Map of Sanibel Island showing road network, interior drainage system, and other features or nannies used
in the report.






TABLE 1. MONTHLY RAINFALL TOTALS IN INCHES FOR STATIONS 1 AND 2 ON SANIBEL ISLAND
AND PAGE FIELD NEAR FORT MYERS, 1971- 72.
(Records for Page Field from Environmental Data Service, U. S. Department of Commerce.)

1971

ion Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Total

Island
n 1 2.45 1.90 0.14 0.90 1.33 2.67 3.59 9.50 15.25 5.16 0.33 1.14 44.36

Island
n 2 6.36 17.88 5.40 0.42 1.06


0.85 1.55 0.55 0.70 3.97 6.18 9.50 8.06 9.21 6.49 0.16 0.30 47.32.


1972


Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Total


1.32 2.07 5.35 0 0.52 7.82 3.84 8.49 4.17 1.03 5.41 1.32 42.34


1.19 1.77 4.75 0 0.57 7.34 3.57 3.57


3.61 0.94 5.26 1.30 37.75


0.77 2.14 4.72 0.27 5.20 7.86 9.72 16.22 2.33 2.20 3.85 1.43 56.71


Locate

Sanibel
Station

Sanibel
Station


Page Field
near Ft. Myers


0'
H


Location

Sanibel Island
Station 1

Sanibel Island
Station 2

Page Field
near Ft. Myers


0

O
I






BUREAU OF GEOLOGY


The seasonal distribution of rainfall typical of south Florida where wet
and dry seasons alternate is evident. For example, the cumulative rainfall for
station I is about 36 inches for June to November 1971 and about 11 inches, or
about one-fourth of the annual total, from November 1971 to June 1972. The
seasonal pattern at Page Field is similar.

P monthly rainfall totals for stations 1 and 2 are somewhat different
particularly during the wet season. In August 1971, this difference was about 3.5
inches. Most of this difference resulted from afternoon thunderstorms on August
5-7 and I I when 4.8 inches was recorded at station 1 and 1.7 inches at station 2.
These differences are largely related to the erratic path of thunderstorms which
cause heavy rainfall in some areas and little or none elsewhere.

Comparison of records from Sanibel Island with those from Fort Myers
provides an even greater contrast. Table 1 shows wide difference in monthly
totals for June, July, and September 1971 and May, July, and August 1972.
These differences may nearly balance over longer periods as indicated by the
annual totals for 1971, or may result in a difference in annual totals of more
than 15 inches such as in 1972.

Continuous records from station 2 show that the rainfall intensity was
maximum on March 31, 1972 when 3.5 inches was recorded between noon and
1:00 p.m. By 2:00 p.m. the total had reached 4.5 inches. In September 1971,
the intensity ranged from about 1.3 to 2.1 inches per hour.

WELL CONSTRUCTION, INVENTORY, AND NUMBERING SYSTEM

During the investigation, 54 observation wells were drilled or driven on the
island (fig. 3). Most of these were constructed in pairs about 2 feet apart. One
well of each pair was used to obtain core samples to depths ranging from about
25 to 36 feet, to collect water samples, and to measure water level. The other
well, equipped with a screen, was driven to depths ranging from about 8 to 13
feet and was used to measure water levels and collect water samples. At two of
these sites, a third well was driven to a depth of about 17 feet. Four wells were
drilled to depths of 10-12 feet along. a north alignment across the island. All of
the shallower wells were used for the collection of water samples and
measurement of ground-water levels.

In addition to the observation well network, existing wells or those under
construction supplies geologic and hydrologic information. Selected information
from wells or test holes inventoried during the investigation is summarized in
table 2. (See appendix.)











82.00'






BUREAU OF GEOLOGY


The well numbers used in this report are part of a county-wide system
where numbers are assigned in sequence as each well is inventoried. The letter
prefix L refers to Lee County. Thus, well L-405 indicated that information had
been obtained on 404 wells in the county at the time this well was added to the
inventory. In table 2, a second series of numbers is given which more precisely
identifies the well location to the nearest second of latitude and longitude.

WATER-BEARING FORMATIONS

DEEP ARTESIAN AQUIFERS

Relatively little information is available on the lithology of the deeper
sediments underlying Sanibel Island. The deepest test hole (L-1533) on which
detailed information was obtained during the investigation was drilled to a depth
of 895 feet at the Island Water Association water plant. Based on drill cuttings
and geophysical logs obtained on this test hole, the geologic formations and
aquifer underlying the island have tentatively been identified on the log shown on
figure 4. Information from other deep wells on the island, however, indicate that
the log for test hole L-1533 may not by typical. The sediments comprising the
Tamiami Formation are thicker than elsewhere on the island and sediments that
occur at shallower depths are of different composition.

Although making a complete inventory of deep artesian wells was not
within the scope of this investigation, some information on 18 deep wells or test
holes is included in this report. Many of these wells were drilled to the
Hawthorn Formation or the underlying Tampa Limestone. The water-bearing
zone which includes the lower part of the Hawthorn Formation or the
underlying Tampa Limestone. The water-bearing zone which includes the lower
part of the Hawthorn Formation and the upper part of the Tampa Limestone is
termed the lower Hawthorn aquifer by Sproul and others, (1972, p. 9). Wells
more than 700 feet deep may also tap the Suwannee aquifer which includes the
upper part of the Suwannee Limestone and the lower part of the Tampa
Limestone (Sproul op. cit.). Both the lower Hawthorn and Suwannee aquifers
are generally present beneath the island. Based on well depths and available
geologic information, most of the deep artesian wells on Sanibel Island tap the
lower Hawthorn aquifer.

The highest head measured during the investigation was for well L-585
near the eastern tip of the island. In November 1970, the head was 27.4 feet
above the land surface, or about 32.4 feet above mean sea level. A recorder
installed on this well showed a direct response to tidal loading, with a change in
head of about 1 foot during a tide cycle. Levels in other wells drilled to the
lower Hawthorn aquifer have artesian heads ranging from about 16 to 32 feet







REPORT OF INVESTIGATION NO. 69


EXPLANATION
SAND CLAY OR MARL LIMESTONE OR PHOSPHORITE
DOLOMITE


Figure 4. Log showing the geologic formations, lithology, aquifers, and
chloride concentrations in water from test hole L-1533.







BUREAU OF GEOLOGY


above mean sea level. The lower artesian head in wells in some areas may be
related to previous discharge from the well or continued discharge from other
wells in the same general area. The lower head may also indicate that if the metal
well casing has deteriorated, water could be leaking from the well into other
aquifers.

Saline water as defined by the U.S. Geological Survey is water which
contains more than 1,000 mg/1 (milligrams per liter) of dissolved solids. Highly
saline water refers to concentrations of dissolved solids exceeding 10,000 mg/1.
The chloride concentration which is one of the chemical constituents comprising
the dissolved solids, is used in the report as an indicator of salinity.

Highly saline water was encountered at two depth intervals in test hole
L-I533 (fig. 4). The peak chloride concentration occurred at a depth of about
100 feet within the shallow artesian aquifer. The high chloride concentrations in
the depth interval 350-500 feet occurs near the contact between the Tamiami
and Hawthorn Formations, or entirely within the latter. Well drillers commonly
report this zone of highly saline water in other parts of the island. A water
sample from well L-1512 near the east end of the island has a chloride content
of 13,100 mg/1 at a depth of 372 feet.

The water from the lower Hawthorn aquifer is less highly mineralized near
the west or northwest end of the island than near the center or on the east end.
These differences in water quality may be related to vertical displacement of the
formations resulting from minor faults, one of which is indicated by correlation
of gamma ray logs of wells on the east end of the island. Because few wells tap
the Suwannee aquifer, little is known about the water quality. As indicated by
the data shown on figure 4, the chloride concentrations can be expected to
increase with depth in the Suwannee aquifer.

Because of these saline-water zones, 350 to 500 feet of casing are required
in constructing wells on Sanibel Island where as on the mainland, wells drilled to
comparable depths seldom contain more than about 150 feet of casing. These
fully cased wells provide a more accurate indication of artesian pressure in the
deeper aquifers than most wells on the mainland.

SHALLOW ARTESIAN AQUIFER

Core samples were obtained from a series of wells drilled throughout the
island. Most of these wells penetrated a limestone stratum at depths ranging
from about 29 to 34 feet below land surface. The core samples were used in the
preparation of figure 5, which shows the approximate altitude of the top of the
limestone beneath the island. None of these wells penetrated the full thickness







REPORT OF INVESTIGATION NO. 69 13

of the limestone. However, drill cuttings from wells L-1472 and L-1512 (not
shown on fig. 5) near the east end of the island, show that the limestone stratum
extends to a depth of about 130 feet in that area. Near the center of the island,
drill cuttings from test hole L-1533 and well L-1597 indicate that the limestone
extends to a depth of only 41 feet, where it is underlain by about 100 feet of
green and tan sand. Although the limestone is of variable thickness, it apparently
underlies all of Sanibel Island.

The water-bearing parts of this limestone, together with the permeable
shell bed immediately above and the hydraulically connected sandy sediments
below, are referred to herein as the shallow artesian aquifer. The aquifer is
overlain by clay or marl deposits which act as a confining bed and which
separate the aquifer from the overlying water-table aquifer.


Figure 5. Lithologic sections showing thickness of surficial sediments.


WATE- TABLE
GOJIFER







14 BUREAU OF GEOLOGY

WATER-LEVEL FLUCTUATIONS

Typically, fluctuations of water level in the shallow artesian aquifer reflect
only very small changes in ground-water storage. Water levels in the shallow
artesian aquifer fluctuate primarily in response to the tides in the Gulf as shown
on figure 6. On the incoming tide, the additional weight of water compresses the
aquifer causing an increase in artesian pressure and a rise in water levels in wells
penetrating the aquifer. This mechanism reverses during the out going tide. Thus,
the fluctuation in water levels is largely related to tidal loading rather than to an
exchange of water between the aquifer and water in the Gulf. This is virtually
the same mechanism responsible for the tidal variations in artesian pressure
within the lower hawthorn aquifer as previously described.
z.5
WELL L-1408







05




25
~ TIDE AT POINT YBEL
20











-0.5






Figure 6. Tracng of records from the tide gage at Point Ybel and well L-1408,
Aguust 4 9, 1971.






REPORT OF INVESTIGATION NO. 60 15

AUG SEPT OCT NOV DE. JAN FEB MAR APR MAY JUNE JULY

3
WATER-TABLE AQUIFER
WELL L-1415






2 -- SHALLOW ARTESIAN AOIFER
% /><_ V ^WELL L-1400
S. I I .. 1 1 I .. .. .. .r ...o A l I ,



. ..
No record


I ;I
AUG SEPT OCT NOV DEC JAN FEB MAR IAP2 MAY JUNE JULY

Figure 7. Graph showing water-level fluctuations in wells L-1408 and L-1415
and tide at Point Ybel, August 1971 July 1972.


The water level in well L-1408 fluctuates about 43 percent of the range in
tide levels (fig. 7). This is referred to herein as the tidal efficiency of the well,
although it is recognized that minor effects resulting from changes in barometric
pressure are also included. The tidal efficiency of other wells drilled to the
shallow artesian aquifer was from about 10 percent in well L-1479 to 77 percent
in well L-1456 and averaged 43 percent.

Inasmuch as water levels in a confined aquifer are related to the tide, it
follows that the water levels are highest at highest tide stages and lowest during
low tide stages. The highest water level recorded over the period of record was
3.45 feet above mean sea level in well L-1456 on June 19, 1972 with a tide level
3.60 feet above mean sea level. The lowest level recorded was 1.80 feet below
mean sea level in the same well on January 16, 1972 with a low tide level 3.03
feet below mean sea level. This indicates a range in fluctuation of water level in
the shallow artesian aquifer of 5.25 feet at the location. However, based on the
tidal efficiency of each well versus the maximum range in tide levels, it was
estimated that water levels in the aquifer fluctuate annually over a range of only
about 3 feet. Although water levels in the aquifer fluctuate in response to
changes in tide levels, they do not occur at the same time. Generally the change
in water level within the aquifer lags the change in tide level from 10 minutes to
about 3 hours.












8e2oo'


0 O, SAN CARLOS BAY



POINT
YBEL 0
EXPLANATION I'l
WELL 24,500 2p0P
NUMBER IS CHLORIDE )
CONCENTRATION, MILLIGRAMS t"
PER LITER. 0

APPARENT BOUNDARY WHERE C
CHLORIDE CONCENTRATIONS I
EQUAL OR EXCEED SEA WATER. O MILE GF







Figure 8. Map of Sanibel Island showing chloride concentrations in water form the upper part of the shallow
artesian aquifer.







REPORT OF INVESTIGATION NO. 69


Over a year, as shown on figure 7, the fluctuations in water levels within
the shallow artesian aquifer are responsive to changes in the tide level rather than
to changes in water levels in the overlying water-table aquifer. The tide level is
lower during the winter months; water levels in the shallow artesian aquifer are
lower during this period than at any other time.


CHLORIDE CONCENTRATIONS

The chloride concentrations in water from the shallow artesian aquifer
ranged from 2,250 mg/1 (milligrams per litter) in well L-1493 to 30,900 mg/1 in
well L-1477. The distribution of chloride in the upper part of the aquifer is
shown on figure 8. The water samples were collected while the wells were being
constructed, December 1970 to November 1971. The chloride concentrations in
the aquifer along the north half of the island equals or exceeds that of sea water
(see fig. 8). This suggests the presence of evaporite beds in the upper part of the
aquifer resulting from the concentration of sea water while the sediments were
being deposited. The chloride concentration of 24,600 mg/1 in water from well
L-1447 at a depth of 62 feet suggests that similar beds also occur at greater
depth.

Apparently some flushing of the upper part of the aquifer has occurred
within recent geologic time as indicated by the lower chloride concentrations in
some areas. Perhaps this occurred during lower stand of the sea when difference
in head between the water table and shallow artesian aquifers was greater.
However, it is unlikely that the effects of flushing have significantly altered the
predominantly saline character of water in the lower part of the aquifer.

WATER-TABLE AQUIFER

The shallow subsurface sediments underlying Sanibel Island are
predominately sandy. On the basis of lithology and apparent hydrologic
characteristics, these sediments may generally be divided into an upper, middle,
and lower zone. The upper zone, consisting of fine to medium grained sand and
shell, is of relatively high permeability. The middle zone, of fine sand, is of
relatively low permeability. The lower zone, with the fine sand, containing
varying percentages of silt and clay sized particles, is of relatively low
permeability. Collectively, the saturated part of these sediments comprises the
water-table aquifer. The water table is the upper surface of the zone of
saturation.

The strata underlying the water-table aquifer consist predominantly of silt
and clay size sediments, apparently of low permeability. These clay strata







18 BUREAU OF GEOLOGY

function primarily as a barrier to the movement of water either downward from
the water-table aquifer, or upward from the underlying shallow artesian aquifer.

WATER-LEVEL FLUCTUATIONS

Changes in the position of the water table indicate changes in the volume
of water in storage. The water table fluctuates primarily in response to recharge
from rainfall and discharge as base flow to streams, ponds, canals, and other
surface water bodies and to natural processes of evaporation and transpiration.
Fluctuations of the water table in well L-1403 are related to rainfall at station 2
as shown on figure 9. Note that the water table rises during periods of heavy
rainfall and declines during periods of little rainfall. Thus, during the annual
cycle the water table is highest near the end of the wet season in September or
October and lowest near the end of the dry season in May or June.
Superimposed on this annual recharge cycle are the effects of evaporation and
transpiration which probably reach a maximum during the summer and a
minimum in January.


Figure 9. Graph showing fluctuation of the water table in well L-1403 and
rainfall at station 2 (Sanibel Post Office).






REPORT OF INVESTIGATION NO. 69


The water table, generally following the topography of the land surface, is
higher beneath the upland beach ridge areas and lower in the interior lowlands or
along the coastal margins. Profiles of the water table across the island at
different times are shown on figure 10. Water-level data from the four wells
shown, indicate the general changes that occur at different water stages.

The water level in well L-1403, on the upland area along Casa Ybel Road,
was consistently higher than in other wells. The water table was near or below
mean sea level for about 10 weeks in April, May, and June 1971 (fig. 10). Levels
in observation wells L-1411 through L-1416 in adjacent areas also were low.

During these low-water stages, saline water from tidal water bodies
surrounding the island may move inland into the water-table aquifer. The extent
of this inland movement is controlled in part by the hydraulic gradient and the
length of time over which these conditions prevail.

The annual range in fluctuation of the water table is variable depending on
recharge and discharge factors. As shown by the hydrographs for wells L-1412,
L-1414, and L-1416 on figure 11, the water table reached both lower and higher
altitudes in 1971 than in 1972. The range in fluctuation of the water table of 3.6
to 4.8 feet as indicated in these wells in 1971, probably represents near extreme
conditions of high dnd low water levels in this part of the island.

An observation well network in the water-table aquifer was completed in
November 1971. The highest and lowest observed water-levels in 1972 are shown
on figure 12. Levels were generally lowest during June 1972, except for welis
L-1455, L-1457, and L-1503 at the east end of the island, and wells L-1484,
L-1486, and L-1488 at the northwest end of the island whose levels were lowest
in March 1972. The highest and lowest levels shown on figure 12 represent
extremes over a very short period of record and levels both higher and lower
should be expected as shown on figure 11.

Not all the high water levels shown on figure 12 occurred at the same time
because of the erratic distribution of rainfall. However, most occurred during or
immediately following the wet season.

Short-term records from most of the observation wells indicate an annual
fluctuation of the water table from about 1 to 3 feet.

CHLORIDE CONCENTRATIONS

Water from the water-table aquifer ranged in chloride from 12 mg/l in well
L-1401 to 23,200 mg/l in well L-1516 (table 2). This extreme range is related to






































1800 -'. 2400


3000


3600


4200


4800


0


DISTANCE, FEET FROM THE GULF OF MEXICO







Figue 10. Iofile acro Sanibe Island between wells L-1404 and -1410 owing seasonal water levels in the
wate-table aquifer.


S I I I I I' 1 1 I I If Ii I 1 1 p I f i 1i f ; I If I9 i I p "_

SEE FIGURE 3 FOR LOCATION OF PROFILE
WELL L-1403 WELL L-r405
WELL L-mIK








_u .. ._ .-Ul i n- _
-
SFAM- N


I ~ ~ l.. J ',,' l 11 0 1 i p l E ll 1 1 1 9 1 I I I I I I "


-2


-4
0


600


1200


I


5400






REPORT OF INVESTIGATION NO. 69


4



WELL L-1414










4 ---
3






WELL L-1416



0



I I I I I I I I I I I I I I I I I l
J F M A M J J A S 0 N D J F M A M J JA S 0 N
1971 1972




Figure 11. Hydrographs showing fluctuations of the water table in wells
L-1412, L-1414, and L-1416, in 1971 72.



















SO 4 SAN CARLOS BAY

EXPLANATION
27'30*- WELL 1.14 HIGHEST
-011 LDWEST pa POIN
NUMBERS INDICATE ALTITUDE
OF WATER TABLE IN 1972, IN FEET o
ABOVE OR BELOW(-)MEAN SEA LEVEL.




tenar*I I I I
iue 12 Map of besl showing highest adlowestaltude of the water table in 1972.
26025'- o





Figure 12. Map of Sanibel island showing highest and lowest altitude of the water table in 1972.





REPORT OF INVESTIGATION NO. 69


several different factors. Concentrations are generally higher in the low lying
areas along the north half of the island. Concentrations generally are lower
beneath the interior upland areas.


In most parts of the island, the chloride concentrations increase with
depth in the water-table aquifer as indicated on figure 13. For example, water
O:r- or-


2000 4000 6000 8000


WELLS L-1503 AND 1504 ARE
I-FOOT APART AND SCREENED
AT DIFFERENT DEPTHS.
L-1503 WELLS L-1496AND 1497 SIMILARLY
LOCATED AND SCREENED.


WELL L-1504


WELL L-1497


I I I I I I
0 2000 4000 6000 8000 10,000


L-1559


I I I I I I I I I I I I


S 2000 4000 6000 8000 10,000 12,000 14,000 16poo 18,000 20,000
CHLORIDE CONCENTRATATION, MILLIGRAMS PER LITER


2200 24,000


Figure 13. Graphs showing increase in chloride content of water with depth in
the water-table aquifer.


20 L
0

0 -

2

4-

6

8 -

10

12

14

16

18

20


--






24 BUREAU OF GEOLOGY


from well L-1559, near the mangrove swamp .on the north side of the island,
increased progressively in chloride from 13,250 mg/1 at a depth of 4 feet to
20,050 mg/l at 14 feet. Water from well L-1516 in a similar environment near
the northwest end of the island, increased in chloride from 2,250 mg/1 at a depth
of 8 feet to 23,200 mg/1 at 17 feet. Chloride concentrations greater than the
19,000 mg/l determined for water in the Gulf of Mexico, as stated earlier,
indicates the presence of evaporite beds where sea water has been concentrated
by evaporation. In the interior of the island, particularly beneath the beach
ridges, chloride concentrations increased less with depth. For example, the
chloride content in water from well L-1496 was 130 mg/1 at a depth of about 11
feet, whereas in well L-1497 the chloride was 375 mg/1 at a depth of about 17
feet. The two wells are 1 foot apart. In contrast wells L-1503 and L-1504, both
near the shore and of similar construction, showed a chloride content of 190'
mg/l at a depth of about 9 feet and 8,950 mg/l at about 17 feet. Other test wells
that tap the water-table aquifer on the island show similar increases in chloride
concentrations with depth but the rate of increase is highly variable.
1000 1 III-.----


500


Or 0

r-

C1 4000
2n
C" 3500
-J
2
-J
2 3000
z
0
1- 2500
I-
z
L 2000
0
1500


r

500


1 1 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 1













WELL L-1147


J J A S N D J F M A M J J A S ON D J F M A
1970 1971 1972

Figure 14. Graph showing variation in chloride content in wells L-1147 and
L-1158, June 1970 May 1972.






REPORT OF INVESTIGATION NO. 69


Chloride concentration of water at a given depth interval also varies with
time. The wide variation in chloride content shown in figure 14 for well L-1147
probably is not typical of the aquifer generally because of the proximity of this
well to the Sanibel River which contained highly saline water during part of the
period of record. In contrast, the relatively small variations in chloride in well
L-1158 may be typical of water only in the aquifer beneath the beach ridge
bordering the gulf. Nevertheless, periodic measurements in other water-table
wells indicated variations in chloride concentrations within the same depth
interval ranging from 25 to 1,550 mg/1.


SURFACE WATER

Surface water on the island includes the central interior drainageway of
the Sanibel River and connected secondary drainage ditches and other small
natural channels, canals, ponds, lakes, and other water-storage areas. Tide water
canals, channels, and ditches also form a part of the surface-water network
because they provide avenues for the inland movement of saline water, or
drainage routes for discharge of water from the interior of the island.

During the investigation, surface-water samples for chloride determination
were obtained at numerous sites throughout the inland (fig. 15). Generally the
chloride concentrations were determined from specific conductance-chloride
curves developed from water samples from the island. Because of the density
stratification in water which commonly occurs where saline and fresh water are
present, samples were obtained from near the surface and at the bottom at each
sampling site where the water depth exceeded 3 feet. Unless otherwise specified
chloride concentrations shown are those of bottom samples. In addition, during
low-water periods the culverts beneath road crossings along the Sanibel River did
not permit an exchange of water, so that it was necessary to collect both
upstream and downstream samples at some sites. By definition upstream refers
to the west, downstream to the east.


SANIBEL RIVER

The main channel of the Sanibel River is connected throughout most of its
length beginning at a pond at site S-23 toward the west end of the island to the
tidewater canal system at Beach Road (S-1) near the east end (fig. 15). The
length of the channel is about 8 miles, and the width varies from less than 10
feet to more than 50 feet depending on the extent of modification by dredging.
The altitude of the stream bed also is highly variable although nearly always at
or below mean sea level.











































Figure 15. Map of Sanibel Island showing location of selected surface-water sampling sites.


82o0d


26o25,






REPORT OF INVESTIGATION NO. 69


The stream is connected at all road crossings by culverts. The bottoms of
some culverts are several feet above the stream bed. These culverts, plus other
irregularities in the stream bed result in segmentation of the stream during low
water. During high water stages, water flows through the culverts to points of
discharge through control structures at Tarpon Bay or at Beach Road. Because
of relatively low gradients throughout the length of the stream, water may flow
toward either the Tarpon Bay or Beach Road control structures. However,
during most of the period of this investigation, the area west of site S4 on the
Casa Ybel Road drained toward the Tarpon Bay control structure because the
channel east of site S-4 was blocked by an earthen dam.


The river discharges to the bays only after heavy rainfall, and then usually
for only a short time. It is estimated that about 500 million gallons of water was
discharged through the control structure into Tarpon Bay after the heavy rainfall
in September 1971. About 100 million gallons was discharged into the tidal
canal at Beach Road over the same period.



Sanibel River was sampled at nine sites, S-1, S-2, S-3, S-4, S-10, S-11, S-13,
S-18, and S-21. Site S-1 is about 20 feet upstream from the control structure at
Beach Road. The chloride content at site S-2 (downstream) was virtually the
same as site S-1. The chloride concentrations at site S-1 were highest during the
low river stages in May, June, and July 1971 (fig. 16). The progressive increase in
chloride content from November 1970 until May 1971 was the result of
upstream leakage of saline water through the Beach Road control structure. The
intruding saline water moved upstream to the downstream side of site S-2 where
a roadway prevented further movement. The culverts beneath the road were
above the stream level over most of this period. The heavy rainfall in September
1971 flushed most of the saline water in the reach between sites S-1 and S-2
downstream. Modification of the control structure in September 1971,
combined with generally higher water stages,reducing the upstream leakage of
saline water, has lowered chloride concentrations in water in the reach between
sites S-1 and S-2.



The sharp increase in chloride content of water on the upstream side of
site S-2 in October 1971 was the result of continuous discharge of water into the
river from artesian well L-1472 which contained 5,850 mg/l of chloride. This
well was capped in October 1971 and the effects were largely dissipated by
November 1971 as indicated by the chloride samples at site S-2 (upstream).







28
Cn
2 12,000




-r




000



C.
0
-J
-J~j










z
LJ

+2

tr






W
U-
"r
2+

LU



LAI



0


BUREAU OF GEOLOGY


Figure 16. Graph showing variation in river stage at Beach Road and chloride
concentrations in water at sites S-1 and S-2-(Upstream) November
1970 December 1972.



The bottom chloride concentrations in water at sampling sites S-3 and S-4
downstream are shown on figure 17. Over most of the period of record, an
earthen dike near site S-4 prevented an interchange of water between these sites.
In addition, parts of the former river channel had been filled and an alternate
by-pass canal had been excavated. The initial high chloride of 11,000 mg/1 in
water at site S-3 on June 20, 1970, probably was caused by previous dewatering
operations during the excavation of the canal near site S-6. Water pumped from







REPORT OF INVESTIGATION NO. 69


29


this excavation drained into the Sanibel River through a series of connecting
ditches. By October 1970, the saline water at site S-3 had been dissipated as a
result of discharge at Beach Road. Since then, the records from site S-3 indicate
a seasonal increase in chloride concentrations during the day season which is
reduced during the wet season as a result of discharge from the river and dilution
by rainfall.

8000 I i i i I I II I i II 1

7000

6000

5000

4000
SITE S-4 (downstream)
3000 .-

w 2000

n1000
a.
0








S511,000
-J
S10,000

9000

z 8000

o 7000
w
1970 1971 1972
SSITE S-3
I 5000 1970 De r

4000

3000

2000

1000


J J A S 0 N D J F M A-M J J A S 0 N D J F M A M J J A S O N
1970 1971 1972



Figure 17. Graph showing chloride concentrations at sites S-3 and S-4
(downstream) June 1970- December 1972.







BUREAU OF GEOLOGY


4000


3000

2000

1000


0


(:
U')
a. 6000

< 5000
0
-I
- 4000

S3000


o 2000
z
LU


0
(-




' 3000


2000

1000


0


SITE S-li
















SS11TTEE SS10


Figure 18. Graph showing chloride concentrations at sites S-4 (upstream), S-10

and S-11, June 1970 December 1972.


I I I I I I I_ L I I I I I 1 I I I 1 I I I I I I I I I
J J A S N D J FMA M J J A S ON D J F M A M J J A S 0 N
1970 1971 1972






REPORT OF INVESTIGATION NO. 69


The high chloride concentration in water on the downstream side of site
S-4 in August 1971 resulted from the temporary failure of an earthen dike which
allowed saline water to move upstream from an area under excavation. The
break was quickly repaired and the heavy rainfall in the following month
reduced the chloride concentration to the lowest point of record.

The peak concentration on the upstream side of site S-4 in August 1971
(fig. 18) also stemmed from the temporary dike failure. Although the stream is
not normally connected across this roadway during low water stages because of
the altitude of the culverts, the failure of the earthen dike probably caused the
stream level to rise temporarily so that some of the saline water crossed the road.
However, the effects of this intrusion was much less on the upstream side as
indicated by comparing the graphs for site S-4 on figures 17 and 18.

The large progressive increase in chloride concentrations at site S-10
between October 1970 and July 1971 cannot be explained by any of the saline
water sources previously identified. As indicated by the records from sites S-4
(upstream) and S-11, the source of the saline water was between these sites.
The increase in chloride content of water at site S-10 coincided with a period of
declining water levels suggesting a possible upward migration of saline water
from the underlying aquifers. This, coupled with the normal increase in chloride
concentrations resulting from the reduction in water volume by evaporation and
transpiration during low water stages, may account for increase in chloride
content at site S-10. However, the evidence is inconclusive. Most of this saline
water was flushed from the area by October 1971.

The peak chloride concentration at sites S-4, S-10, and S-11 which
occurred in July and August 1971 (fig. 18) apparently all stemmed from a major
intrusion of saline water over the Tarpon Bay control structure on June 18, 19,
1972. On these dates maximum tide levels were 3.45 and 3.60 feet above mean
sea level at Point Ybel. At that time the top of the board spillway at the control
structure was about 2.25 feet above mean sea level. Thus, when the tide level
exceeded the height of the spillway, saline water from Tarpon Bay flowed
inland. Similar inflow did not occur at the Beach Road control structure
although the height of the spillway was set at 2.70 feet above mean sea level. An
alert local resident placed a temporary barrier in the spillway which increased its
height above the maximum tide levels.

The effects of intrusion of saline water from Tarpon Bay are also indicated
on the graphs for sites S-13, S-18, and S-21 on figure 19. The chloride
concentrations in water at site S-13 were more variable than at any other
sampling site. The progressive increase in chloride concentrations at site S-13
between December 1971 and June 1972 is largely attributed to leakage of saline







BUREAU OF GEOLOGY


8000


6000

4000

2000


0


12,000


10,000

8000


6000

4000


2000


0




16,000


14,000

12,000


10,000

8000


6000


4000


2000


0


1971 1972

Figure 19. Graph showing chloride concentrations at sites S-13, S-18 and S-21,
April 1970- December 1972.


SITE S-13




















A M J J A S 0 N D J F M A M J J A S 0 N


1 r


_






REPORT OF INVESTIGATION NO. 69


water through the loose fitting boards of the Tarpon Bay control structure. The
peak in June -- 15,700 mg/1 -- was determined several days after the tidal inflow
over the spillway. During the period of maximum tidal inflow, the chloride
concentration probably was similar to the composition of sea water, about
19,000 mg/l.

The intruding saline water of higher density flowed inland eventually
reaching the canal at sites S-11, and S-10 and S-4 (upstream) as shown on figure
18. The saline water also moved westward to sites S-18 and possibly to site S-21
as indicated by the peaks in June 1972. However, the increase in chloride
content at site S-21 may have resulted from some local inflow of saline water
from the Gulf when the high tide overtopped the beach ridge near site S-22.

The high chloride concentrations at sites S-13 and S-18 in April through
August 1971 (fig. 19), apparently were from two different sources. While the
pond at site S-17 was being excavated, water was pumped into the Sanibel River.
The chloride content of this water ranged from about 6,500 to 7,300 mgl/, or
about the same as that determined at site S-18. During this same period leakage
through the Tarpon Bay control structure had increased the chloride
concentrations in water at site S-13 to between 8,000 and 9,000 mg/l. Thus the
chloride concentrations in the reach between these sites represented a mixture of
saline water from both sources.

PONDS, LAKES, AND CANALS

Numerous ponds, lakes, and canals have been excavated on Sanibel Island
and many more are currently (1973) under consideration. The primary purpose
of most of these excavations is to obtain fill material for increasing the altitude
of adjacent land areas, usually residential housing development. The shapes,
sizes, and depths of the excavations are largely dependent on the quantity of fill
material needed, the area available for development, the location, and other
related factors.

Water samples for chloride determination were collected periodically from
ponds, lakes, and canals. Selected locations are shown on figure 15. Although
both surface and bottom samples were obtained at most sites, only the analyses
of bottom samples are shown.

The analyses of water samples from sites S-7, S-8, and S-9 in the canal
system are shown on figure 20. A more detailed investigation of this canal
system is currently underway (1973). The sustained high chloride concentrations
in the canal at site S-7 indicates a continued source of saline water although the
chloride content was generally lower in 1972 than in the previous year. This







34 BUREAU OF GEOLOGY


canal was dewatered during excavation and the water was discharged into the
Sanibel River before June 1970. This caused a large increase in chloride
concentrations in the river between sites S-1 and S-3. This means of disposal had
been discontinued several weeks before the first set of water samples was
collected on June 20, 1970. The water discharged to the river must have been
very saline. For example, samples collected at site S-6 indicated a density
stratification of the water with chloride concentrations of 5,300 mg/1 at the
surface and 16,100 mg/l at the bottom.


The canal at site S-8 is separated from the main canal at site S-7 by an
earthen dike which prevents a direct interchange of water between these sites.
During the early stages of excavation at site S-8, ground water entering the


5000

4000

3000

2000

I 1000

_1
CL
n 4000
'2
g 3000
3
- I
-4
2000

z
o



z


7000
LI
0


-5.
o 5000


4000

3000


Figure 20. Graph showing chloride concentrations at sites S-7, S-8, and S-9,
June 1970 December 1972.


I I SITI I I I I I I I IS-9 I I I I I I I I
SITE S-9

-


SITE S-7







J J A S N D J F MA M JJ A S ON D J F M A M J JASON
1970 1971 1972






REPORT OF INVESTIGATION NO. 69


bottom of the excavation had a chloride content of less than 200 mg/l. The
canal was dewatered by stages during excavation resulting in a progressive
increase in chloride concentration. A similar increase was noted within the same
canal during the excavation near site S-9. After the initial chloride
concentrations during the period of excavation, the chloride content has
generally stabilized.

The chloride concentrations in. water in ponds or lakes at sites S-5, S-12,
and S-14 are illustrated on figure 21. The chloride content of water in the lake at
site S-5 is inversely related to ground-water levels: the chloride increases when
the water level declines. A water sample obtained from observation well L-1405
near the lake at a depth of 11 feet showed a chloride content of 5,100 mg/1. This
suggests that the lake is underlain by saline ground water. After a heavy rainfall
such as that of September 1971, a density stratification of the water in the lake
is apparent. For example, on October 6, 1971 a water sample from the surface
contained 670 mg/l of chloride, whereas a sample from the bottom contained
more than 2,000 mg/1.

The pond at site S-14 illustrates the effects of saline-water intrusion from
other surface-water sources. As previously described, water from Tarpon Bay
over topped the control structure during the high tides of June 18-19, 1972.
This highly saline water moved inland through the main stream channel, from
which it spread into connecting secondary drainageways. One of these secondary
drainageways was connected to the pond at site S-14 which permitted saline
water to enter the pond. This resulted in a rapid increase in chloride
concentrations in the bottom of the pond from less than 2,000 to more than
12,000 mg/1, while near the surface the chloride content increases from 1,600 to
about 8,000 mg/1. Following this intrusion of saline water, the chloride content
at the bottom generally decreases although it was still above 7,000 mg/1 in
November 1972.

Water in the pond at site S-12 has varied only slightly in chloride content
over the period of record. The source of the saline water which has caused the
progressive increase in chloride during most of 1972 is probably the adjacent
Sanibel River which contained saline water over most of that period.

The chloride concentrations in water in two elongate ponds paralelled to
Rabbit Road are shown on figure 22. Both ponds have about the same shape.
Neither is directly connected to the adjacent Sanibel River. The chloride content
of water in the pond at site S-17 generally exceeded 5,000 mg/1; the chloride
content in the adjacent pond at site S-16 was less than 2,000 mg/1. As described
in the previous section on the Sanibel River, the pond at site S-17 was dewatered
during excavation. The water pumped from the excavation ranged in chloride








36 BUREAU OF GEOLOGY


content from about 6,400 to 7,300 mg/l. After the pond was excavated some
freshening occurred as a result of heavy rainfall in September 1971, although the
chloride concentrations have increased since then.






12.000 -

11,000 -

10,000 -

9000 -

000 -

7000

S6000 -
LU
SITE S-14
S5000 -
aC
M 4000-




2000-


C1
S 1000 I I I I I I I I I I I I I I I

i


00 -- -SITE 5-12 1



0







0 I -II III I


Figure 21. Graph showing chloride concentrations at sites S-5, S-12, and S-14,
June 1970 December 1972.


J J A S O N D J F M A M J J A S O N D J F M A M J J A S ON
1970 1971 1972







REPORT OF INVESTIGATION NO. 69 37

Water samples from well -1478 near sites S-17 indicated that the chloride
content of water in the water-table aquifer ranged between 1,350 and 1,500
mg/l at a depth of about 10 feet. Thus it appears unlikely that the saline water
(chloride 6,400 to 7,300 mg/1) pumped from the excavation at site S-17 was
from the water-table aquifer. However, samples from well L-1477 which was
drilled to the shallow artesian aquifer, contained 30,900 mg/1 of chloride at a
depth of 32 feet. Thus, upcoming of saline water from the artesian aquifer would
result in a mixture of water from the two aquifers which should readily account
for the saline water pumped from the excavation at site S-17. That similar
saline-water conditions did not exist during the excavation of the pond at site
S-16, is evident from the graph of figure 22.

a BO8000 I I I I I I i I I I I I I i I I I I I I I I I


W
-J 7000

sooo
6000

I 5000

4000
2F
0 3000

2000
z
a


o 0o

IC
8
Wt
0-


SITE S-17













M J J A S ON D J F MA M JJ A SON D J F M A M J A S ON D
1970 1971 1972


Figure 22. Graph showing chloride concentrations at sites S-16 and S-17,
October 1970 December 1972.


The chloride analyses for samples collected periodically from other lakes
and ponds are shown on figure 23. The lake at site S-15 has shown the least
variation and generally lower chloride concentrations than any other sampling
site on the island. Similarly, the ponds or lakes at sites S-20 and S-23 have varied
only slightly in chloride concentrations over the period of record. The rapid
increase in chloride content of water in the lake at site S-22 in July 1972, and
the progressive increase at site S-19 beginning at the same time, apparently were
related to the inflow of sea water during the high tides in June 1972. At site
S-19 the saline water entered from a secondary drainage channel connected to
Sanibel River. At site S-22, the saline water probably came directly from the
Gulf when one of the high tides overtopped the beach ridge. By October 1972
the effects of this intrusion at site S-22 were largely dissipated. In November
1972, however, the effects of intrusion at site S-19 were still evident.








38 BUREAU OF GEOLOGY


2000

1000

0
3000

2000

X 1000
LU
I-
-_ 0
0

3 2000
2000



3 1000


I-I

0
0
S3000


c--
C
S2000
7-^
0




000
0


I I I I II I I I I I I
SITE S-I







SITE S-


SI I II I f I I I I I I


I I I I I I I I I I I i i I i
SITE S-20










SITE S-22-







SITE S-23


SI I I l I I I I I I I I l I I I I I
S O N D J F M A M J J A S O N D J F M.A M J J A S 0 N
1970 1971 1972


Figure 23. Graph showing chloride concentrations at sites S-15, S-19, S-20,
S-22, and S-23, October 1970 December 1972.


EXCAVATIONS


The large number of excavations on the island both existing and proposed
warrants more detailed study and consideration. Based on the available
information, the sequence of events and the probable changes which occur in
water quality if ponds or lakes are dewatered during excavation are briefly
outlined as follows: after removal of the surface vegetation and soil cover,
pumps are installed when the water table is reached and the dewatering process
begins; as the depth of the excavation increases, the pumps are placed at
progressively lower levels and dewatering continues; near the final stages of
excavation, the pumps remove water from a sump several feet below the planned


)






REPORT OF INVESTIGATIONS NO. 69


depth of excavation; and finally upon reaching the planned depth, the pumps are
removed and the excavation allowed to refill with water. The dewatering process
may extend over a period of weeks or months depending on the size and depth
of the excavation.

The changes which may occur in water quality if the dewatering procedure
is used are related to the existing geo-hydrologic conditions beneath and
adjacent to the excavation site. Dewatering the excavation is comparable to
pumping a well of very large dimensions. A cone of depression develops around
the pumping site so that the affected area usually is much larger than the area of
excavation. As the depth of excavation increases and water levels are lowered,
the cone of depression tends.to expand outward from the pumping site. Under
these conditions, saline water may enter the excavation from several different
sources. Saline surface water within the cone of depression then moves toward
the pumping site. Beneath the excavation or in adjacent areas, more highly saline
water in the lower part of the water-table aquifer will move toward the
excavation. As the water table is lowered by pumping, the difference in head
between the water-table and shallow artesian aquifers increases, thereby creating
a much larger upward gradient than would occur under natural conditions, and
result in increased movement of saline water from the shallow artesian aquifer.
Thus, it is concluded that dewatering during excavation should be avoided in
areas where the water-table aquifer contains fresh water.

Any excavation which breaches the clay barrier or otherwise improves the
hydraulic connection between the water-table and shallow artesian aquifers, may
become a permanent source of saline-water contamination of the shallow
fresh-water system. This may require a depth limitation on excavations to avoid
this problem. Because of the variations in thickness, depth, and character of the
surficial sediments and the difference in water quality between the water-table
and shallow artesian aquifers, any fixed depth limitation for excavations on the
island would be arbitrary. However, considering that these excavations will
function as saline-water traps after invasions by sea water, as has occurred in the
past, suggests that they be constructed to minimum practical depths depending
on location.

SOURCES OF SALINE WATER

Some of the sources of saline water which affect the interior fresh-water
system of Sanibel Island have been identified. The tidal water bodies completely
surrounding the island represent a major source of saline water which may enter
the interior in several different ways. Extremely high tides generated by wind
action during the passage of hurricanes or other major storms, have in the past
resulted in widespread flooding of the interior with sea water. The effects of






BUREAU OF GEOLOGY


these massive invasions of saline water of the interior fresh-water system are
relatively unknown. However, based largely on theoretical factors, future
flooding of the interior with saline water will have a much greater impact on the
fresh-water system than past flooding.

During past invasions of the sea, few ponds were in existence, and these
were relatively shallow. Today, numerous ponds, lakes, and canals have been
excavated to greater depths and many more are under consideration. If an
invasion of the sea were to occur, the fresh water in these excavations would
largely be displaced by saline water. Because this water will not be removed by
gravity drainage, each excavation will function as a saline-water trap. This
entrapped sea water can, in turn, contaminate the underlying water-table
aquifer. Although this contamination would not be permanent the saline water
would be dissipated only over an extended period of time.

Leakage through or over the control structures at Tarpon Bay and Beach
Road represents another means by which sea water from the surrounding tidal
water bodies gains entry into the interior fresh-water system. The effects of this
intrusion within the shallow drainageway of the Sanibel River are more readily
apparent because of damage to the vegetation than they are in the deeper
excavations. However, the long-range effects on the fresh-water system from the
intrusion of saline water into the deeper excavations probably is of greater
significance.

Another major source of saline water is the shallow artesian aquifer. As
indicated by wells which tap the upper part of the aquifer, the salinity of the
water equals or exceeds sea water salinity at many places. In places where the
salinity in the upper part of the aquifer is relatively low (fig. 8), the water at
somewhat greater depths probably is more highly saline. A more detailed
investigation would be required to evaluate fully the effect of this saline-water
aquifer on the fresh-water system of the island. However, the available evidence
indicates that upward leakage of water from this aquifer is responsible in part for
the higher salinity of water in the lower part of the water-table aquifer. Upward
leakage from the shallow artesian aquifer also provides a reasonable explanation
for the highly saline water encountered in some areas during the excavation of
ponds and canals where dewatering procedures are used.

Other sources of saline water are the deep artesian aquifers. The highly
saline water zone in the upper part of the Hawthorn Formation, apparently
occurs beneath a large part of the island. The principal path by which water
moves upward from this deeper aquifer is through artesian wells or test holes.
Because most of the artresian wells are cased through the formations containing
highly saline water to tap formations containing water of relatively low salinity,






REPORT OF INVESTIGATION NO. 69


they have little effect on water quality in the shallow fresh-water system.
However, deterioration of the metal well casing with time can allow saline water
to enter and the well may become a source of contamination. Saline water may
also move to the surface through the uncased well bore of test holes or those
containing casing which have not been plugged after serving their intended
purpose.

Other known sources of saline water, or mechanisms through which this
water enters the fresh-water system of the island are beyond the scope of this
investigation. Among these, the direct inland movement of saline water from the
surrounding tidal water bodies during low-water stages warrants more detailed
investigation. During high stages of the water table, the hydraulic gradient is
toward the tidal water bodies and fresh water moves toward the sea. Conversely,
as the water table declines to near sea level or below, the hydraulic gradient is
reversed and sea water moves inland. The extent of this inland movement would
largely be dependent on the hydraulic gradient, the permeability of the
sediments, the density of the water, and the length of time over which these
conditions persisted. Some inland movement of sea water into the water-table
aquifer probably occurs along the shoreline as indicated by the water-level data
collected during this investigation.

SUMMARY AND CONCLUSIONS

The water-table aquifer contains the only -fresh water underlying Sanibel
Island and seldom is more than 25 feet thick. The aquifer thickness is largely
controlled by the clay deposits at the base and seasonal water level changes. thus
a 1-foot rise in the water table above mean sea level will result in 1-foot increase
in the thickness of the fresh-water zone.

Within the water-table aquifer, the dissolved solids content of the water
increases with depth; relatively fresh water occurs only in the upper part. The
lithologic and apparent hydrologic characteristics of the sediments, generally
grading downward from more permeable to less permeable materials, are not
conducive to adequate flushing by rainfall, particularly in the lower part of the
aquifer. Thus it is surmised that the brackish or saline water in the lower part of
the aquifer may be unflushed remnants of a former high stand of the sea.
However, an alternate and probably more tenable hypotheses concerns the
relationship between the water-table aquifer and the underlying shallow artesian
aquifer.

The clay and marl strata which separate the water-table and shallow
artesian aquifers are thin or absent in some areas and contain permeable sandy
sediments in other areas. Most likely these strata, where present, function as a






BUREAU OF GEOLOGY


"barrier" to the upward leakage or downward infiltration of water dependent
upon the head difference. During seasonal high stages of the water table, the tide
generated fluctuations of water level within the shallow arestian aquifer may be
continuously below those in the water-table aquifer. Under these conditions a
gradient would exsist such that some water would infiltrate downward. During
low stages of the water table a gradient would exist such that upward leakage
from the shallow artesian aquifer would occur. This hypothesis is consistent with
many of the observed conditions on Sanibel Island, including (1) the brackish or
saline water in the lower part of the water-table aquifer, (2) the source of
recharge to the shallow artesian aquifer, (3) the flushing of saline water from this
aquifer in some areas and the complete lack of flushing in other areas, and (4)
the observed effects of dewatering during the excavation of lakes, ponds, and
canals.

Although the head as indicated by water levels in the shallow artesian
aquifer follows the daily and seasonal variations in tide levels, it nevertheless
fluctuates at an altitude above mean sea level beneath most of the island. This
suggest that the altitude of the potentiometric surface in the shallow artesian
aquifer over an extended period may be similar to the long-term mean altitude
of the water table. It further infers that recharge to the shallow artesian aquifer
is from the overlying water-table aquifer during rainy periods when the water
table is generally higher.

The Sanibel River and smaller interconnected drainageways form an
integral part of the shallow fresh-water system on the island. The major
problems of saline-water contamination noted during the investigation can
largely be eliminated. The inland intrusion of sea water at Tarpon Bay and Beach
Road could be prevented by reevaluating the design and operation of the control
structures at those locations. The discharge of saline water into the interior
drainage system as a result of dewatering excavations has largely been curtailed,
although this remains as a possible future source of contamination.

The central drainageway of the stream is of shallow depth although deeper
pockets occur in some areas and deeper canals form part of the channel in other
areas. Saline water entering the central drainageway tends to collect and remain
more highly concentrated in the deeper parts of the channel. At times the
position or size of the culverts beneath road crossings prevents the spread of
saline water to other parts of the channel, whereas at other times they hamper
flushing of saline water from the stream, or restrict flow rates which cause
periodic flooding in some areas. Ideally, the central drainageway should be of
uniform shallow depth and connected throughout its length. Making the culverts
of adequate size would permit the rapid movement of water to points of
discharge. This would create a flow system which would minimize the effects of






REPORT OF INVESTIGATION NO. 69 43

flooding and provide an effective means of flushing saline water from the
interior of the island.

Generally it is concluded that the volume of water in storage in the
fresh-water system of the island is highly variable, reaching a maximum near the
end of the wet season in September or October and a minimum near the end of
the dry season in May or June. Little opportunity exists for increasing storage of
fresh water in either the water-table aquifer or the interior surface water bodies.
Thus, it is concluded that maintenance of the fresh-water system on the island is
largely dependent on the elimination or reduction in factors which adversely
affect water quality.





ri~f


::
I~
:




REPORT OF INVESTIGATION NO. 69


45


REFERENCES CITED

Missimer, T. M.
1972 The origin and lepositional history of Sanibel Island, Florida: B. A. Thesis,
Franklin and Marshall College, 108 p.

Provost, M. W.
1953 The water table on Sanibel Island, Florida: Florida State Board of Health,
Mimeograph report, 29 p.

Sproul, C. R., Boggess, D. H., and Woodard, H. J.
1972 Saline-water intrusion from deep artesian sources in the McGregor Isles area of
Lee County, Florida: Florida Bureau Geol. Inform. Circ. 75, 30 p.






46 BUREAU OF GEOLOGY






REPORT OF INVESTIGATION NO. 69


APPENDIX







TABLE 2, RECORDS OP WELLS ON SANIBEL ISLAND
Abbreviations mwed In tables!
Aqulfer WT (water table), SA (shallow water), LH (lower Hawthorn), Su (Suwannee).



Wel Latitude. Re s
number Lnitude Remarks
number is | |ih ]a
,,__ _____________
L- 405 262727N0820234.1 500 6 5 +26,6 12-29.7 F 960 12-70 LH Chloride 1035 mg/1 (1-46)
584 262606N0820448.1 421 393 4 6 LH Water salty, Well destraged
585 262711N0820053.1 475 335 6 5 +27.4 11.18-70 F 27 1500 11-70 LH Chloride 1530 mg/1 (1-64)
586 262823N0821008.1 735 415 8 4 +23.3 1-14-64 F 100 27 LH Well plugged back to 235 ft.
588 262538N0820457,1 557 403 4 3 +13.1 1.15-64 F 40 27 1000 6-70 LH
589 262546N0820458.1 609 6 3 +14.4 1-15-64 F 100 27 LH
590 262547NO820512.1 620 4 3 F 60 27 1150 1-64 LH
591 262822N0820912.1 654 405 6 5 +2566 1-16-64 F 60 27 950 12-70 LH Head +21.0 (12-8-70)
1022 262621N0820353.1 631 440 4 5 +20.5 11-18-70
1023 262611N0820735.1 490 461 6 7 LH
1147 262633N0820228.1 8.6 1% 4 -3,74 6-19-70 900 6-70 WT
1148 262632N0820229.1 7.4 1% 4 -2,62 10-2-70 WT
1149 262632N0820228.1 1% 4 400 10-70 WT
1150 262617N0820228.1 8 1% 5 380 10-70 WT Three drive points
1151 262631N0820225.1 7 1%4 5 26 110 10-70 WT
1162 262639N0820226.1 7.4 1 4 -2.80 10-8-70 WT
1153 262755N0820318.1 231 147 4 5 +1.91 10-8-70 420 10-70
1154 262528N0820401.1 400 389 2 Y 5 F 2 340 10-70 LH
1155 262547N0820341.1 8 1%A 4 180 10-70 WT
1158 262619N0820234.1 7 1% 7 420 10-70 WT Three drive points
1160 262617N0820237.1 7 1% 7 -2.25 10-23-70 WT
1185 262621N0820244.1 8 1% 6 80 10-70 WT








TABLE 2. RECORDS OF WELLS ON SANIBEL ISLAND (Cont'd)
Abbreviations used in table:
Aquifers WT (water table) SA (shallow water), LH (lower Hawthorn), Su (Suwannee).
- .. I I I I


Latitude-
Longitude
number


L-1186
1187
1188
1189
1190
1191
1193
1194
1196
1197
1198
1199
1401
1402
1403
1404
1405
1406
1407
1408


I


500


262633N0820327.1
262633N0820328.1
262552N0820342.1
262558N0820338.1
26.2553N0820337.1
262608N0820245.1
262602N0820334.1
262544N0820414.1
262833N0820940.1
262605N0820447.1
262613N0820425.1
262601 N0820451.1
262613N0820424.1
262613N0820424.2
262549N0820353.1
262540N0820351.1
262608N 0820354,1
262535N0820443.1
262540N0820451.1
262603N0820447.1


6
1%
1%
1%
1%/
6
2
2
6
4
5
4
1%
1%
4
4
4
2
2
2


- 4WI


4
4
4
5
4
4
5.6
5.0
5
4



5
5
6.1
2.6
5.4
4.7
4.1
2.2


+17.2





+24.5
-4.38
-3.63
+19.1






-4.39
-5.07
-2.29
-4.80
-3.62
-3.48
-1.93


11-16-70





11-16-70
12-23-70
12-29-70
12-1-70





1-27-71
2-10-71
2-12-71
2-12-71
2-15-71
2-17-71
2-17-71


90
a

It


700
9
7
8
8
600
28.5
29.9
600
600
8
7
7-17
7.3
11.9
9.8
11.6
29.2
29.9
25.4


4O
k140


1020
405
240
60
330
1500
3600
3900
980
600
75
1000
12


230
920
5100
5400
10750
4500


11-70
11-70
11-70
6-70
6-70
11-70
12-70
12-70
12-70
1-71
1-71
1-71
1-71

2-71
2-71
2.71
2-71
2-71
2-71


Remarks


LH Strong salt water flow 400 ft.
WT
WT
WT Two drive points
WT
WT Slotted pipe
WT Slotted pipe
WT Slotted pipe
SA
SA
SA


20
23

480
1%4
1%



12
10
12
29
20
19


h


B~3n
I


... L .._._.I.___ I I I I


I


100
35

F 5





3
10
20
5
1
4


I







TABLIE 2, RECORDS OF WELLS ON SANIBRL ISLAND (Cont'l)
Abbreviations used in table:
Aquifers WT (water table) SA (shallow water), LII (lower Iawthorn), Su (Suwannee),


Latitude- 5 A '
Well Longitude Is R
number I Remarks


L-1409 262612N0822420.1 26.4 19 2 3.6 -2.49 2-18-71 2 2800 2-71 SA
1410 262630N0820354.1 11,3 12 4 4.5 -4.25 2-19-71 23 2600 2-71 WT Slotted pipe
1411 262602N0820334.2 9.7 1% 5.6 -5.51 2.23-71 3 80 2-71 WT Chloride 1500 mg/1 (6-72)
1412 262544N0820414.2 10 1% 5.0 -3.87 2-23-71 3 535 2-71 WT Chloride 750 mg/1 (6-72)
1413 262636N0820443.2 8.5 1X 4.7 -3.60 2-23-71 3 23 580 2-71 WT Chloride 640 mg/1 (11-72)
1414 262540N0820451.2 7.5 1% 4,1 -2.62 2.23-71 3 1200 2-71 WT Chloride 1450 mg/1 (11-72)
1415 262603N0820447.2 7.5 1% 2.1 -1.19 2-23-71 3 21 1150 2-71 WT Chloride 900 mg/1 (6-72)
1416 262612N0820420.2 8.0 1A 3.5 -3.06 2-23-71 3 2325 2-71 WT Chloride 1900 mg/1 (6-72)
1429 262605N0820644.1 9 1% 5 1675 4-71 WT
1430 262614N0820738.1 10.5 1% 5 785 4-71 WT
1431 262627N0820758.1 10.5 1 5 15 220 4-71 WT
1447 262632N0820247.1 62 6 4 24600 6-71 SA Testhold
1450 262638N0820253.1 26.8 26 2 4.9 -4.37 7-26-71 2 3850 7-71 SA
1451 262638N0820253.2 12.6 1% 4.9 -4.65 7-26-71 3 1700 7-71 WT Chloride 450 mg/1 (11-71)
1452 262629N0820327.1 29.9 26 2 3.8 -3.14 7-26-71 60 25 5550 7-71 SA
1453 262629N0820327.2 12.6 1% 3.8 -3.67 7-26-71 3 350 7-71 WT Chloride 460 mg/1 (6-72)
1454 262646N0820226.1 34.6 34 2 5.7 -4.96 7-27-71 30 25 5100 7-71 SA
1455 262646N0820226.2 11.9 1% 5.7 -5.41 7-27-71 26 1350 7-71 WT Chloride 1528 mg/1 (6-72)
1456 262622N0820220.1 33.2 33 2 5.3 -3.81 7-27-71 50 25 2350 7-71 SA
1457 262622N0820220.2 10.8 1K 5.3 -4.22 7-27-71 3 26 205 7-71 WT Chloride 275 mg/1 (11-71)








TABLE 2. RECORDS OF WELLS ON SANIBEL ISLAND (Cont'd)
Abbreviations used in table:
Aquifers WT (water table) SA (shallow water), LH (lower Hawthorn), Su (Suwannee).




Wel Latitude- A
e Longitude Remarks
number *a1 ,l .| :
.1 t0 45 %a V01 QA d a 0 e Ego


L-1458
1459
1470
1472
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1491
1492


262611 N0820258.1
262611 N0820258.2
262603N0820253.1
262632N0820247.2
262618N0820538.1
262618N0820538.2
262633N0820630.1
262633N10820630.2
262651N0820712.1
262651 N0820712.2
262718N0820832.1
262718N0820832.2
262815N0820924.1
262815N0820924.2
262854N0820954.1
262854N0820954.2
262824N0821009.1
262824N0821009.2
262610N0820728.1
262738N0820910.1
262738N0820910.2


36.4
10.1
542
786
32.4
11.3
31.8
9.9
29.4
7.9
30.1
9.5
33.9
10.0
33.2
11.2
35.9
10.8
36.2
29.9
8.7


5.8
5.8
6
4
5.2
5.2
2.9
2.9
5.1
5.1
5.0
5.0
6.2
6.2
4.6
4.6
3.5
3.5
7.1
3.4
3.4


-5.47
-5.44
+16.7

-4.64
-3.15
-2.24
-1.44
-3.67
-3.29
-3.35
-2.74
-6.21
-5.67
-3.04
-2.78
-2.78
-2.54
-1.50
-2.19
-1.83


7-27-71
7-27-71
8-3T71

11-3-71
11-3-71
11-3-71
11-3-71
11-8-71
11-8,71
11.-871
11-8-71
11-9.71
11-9-71
11-9-71

11-9-71
11-10-71
11-10-71
11-17-71
11-10-71
11-10-71


F 125
40
5
15
3
3
5
4
3
12
3
12
3
4
3
50
50
3


2950
45
1050
5850
27100
4000
30900
1350
20700
1700
10100
1350
26300
1250
18900
7100
24500
4150
17400
15600
500


7-71
7-71
8071
10-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71
11-71


WT Chloride 120 mg/1 (6-72)
LH
LH-
Su
SA
WT
SA
WT Chloride 1500 mg/1 (6-72)
SA
WT Chloride 1400 mg/1 (6-72)
SA
WT Chloride 1000 mg/1 (6-72)
SA
WT
SA
WT Chloride 7600 mg/1 (6-72)
SA
WT Chloride 4750 mg/1 (6-72)
SA
SA
WT Chloride 600 mg/1 (6-72)


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TABLE 2, RECORDS OF WELLS ON SANIBEL ISLAND (Cont'd)
Abbreviations used in table;
Aquifers WT (water table) SA (shallow water), LH (lower Hawthorn), Su (Suwannee),


Latitude- I 1 0% V| S
number Longitude
el0 Ih11 M a IIJ|AIM -Z_______
number be Remarks

L-1493 282631N0820758.1 30.6 30 2 5.5 -3.33 11-11-71 3 26 2250 11-71 SA
1494 262631N0820768.2 10,7 114 5,5 -3.36 11-11-71 3 27 175 11.71 WT Chloride 100 mg/1 (6-72)
1496 262610N0820728.2 10.8 1% 7.1 -5.62 11-17.71 5 26 130 11-71 WT Chloride 180 mg/1 (6-72)
1497 262610N0820728.3 16.6 1% 7.1 -5.59 11-17-71 26 375 11-71 WT Chloride 350 mg/1 (6-72)
1498 262548N0820630.1 33.2 31 2 6.0 -4.48 11-15-71 22 26 9800 11-71 SA
1499 262548N0820630.2 10,6 1% 6.0 -4.01 11-17-71 3 29 670 11-71 WT Chloride 600 mg/1 (6-72)
1500 262536N0820548.1 31.8 31 2 4.6 -2.88 11-17-71 18 25 9050 11-71 SA
1501 262536N0820648.2 9.7 1% 4.6 -2.40 11-17-71 3 27 1950 11-71 WT Chloride 1600 mg/1 (6-72)
1502 262602N0820127.1 30.2 29 2 4.8 -2.55 11-17-71 50 26 18600 11-71 SA
1503 262602N0820127.2 9.3 1% 4.8 -2.60 11-17-71 3 27 190 11-71 WT Chloride 276 mg/1 (6-72)
1504 262602N0820127.3 17.1 1% 4,8 -2.60 11-17-71 26 8950 11-71 WT Chloride 8800 mg/1 (6-72)
1512 262552N0820327.1 566 465 4 6 F 25 26 1340 1-72 LH Chloride 12000 mg/1 at 220 ft.
1516 262813N0820916.1 16.2 1% 2.6 -2.36 1-6-72 23200 1-72 WT
1520 262808N0820924.1 13.2 1% 3.9 -2.82 1-10-72 1500 1-72 WT
1T29 262541N0820558.1 29 2 4.5 5200 1-71 SA Casing removed. Well cemented
1530 262541N0820558.2 14.6 1% 4.6 2500 1-71 WT
1633 262628N0820618.1 895 496' 4 5 240 28 S Plugged with cement.
1649 262632N0820752.1 15 1% 2.3 -1.20 2-4-72 6600 .2-72 WT
11558 262644N0820330.1 34 2 2.3 23700 3-72 SA Casing removed. Well cemented
159 262644N0820330.2 13.7 11 2.3 -2.73 3-8-72 20050 3-72 WT
i1597 262627N0820618.1 575 503 10 4 LH
S1654 262625N0820418.1 6.8 1% 2.2 -2.50 8-11-72 2U700 8-72 WT


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