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    Title Page
        Title Page
    Table of Contents
        Table of Contents
    Introduction
        Page 1
        Page 2
        Page 3
    Geography
        Page 4
        Page 3
        Page 5
    Geology
        Page 6
        Page 5
        Page 7
    Ground water
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
    Summary and conclusions
        Page 20
        Page 21
    References
        Page 22
        Copyright
            Main

FLORIDA
STATE BOARD OF CONSERVATION
CHARLIE BEVIS, SUPERVISOR


FLORIDA


GEOLOGICAL


SU RVEY


HERMAN GUNTER, DIRECTOR


INFORMATION CIRCULAR No. 4


THE ARTESIAN WATER
OF HILLSBOROUGH
INTERIM


OF THE RUSKIN AREA
COUNTY, FLOR.IDA
REPORT


By

HARRY M. PEEK
U. S. GEOLOGICAL SURVEY


Prepared by the
GEOLOGICAL SURVEY
UNITED STATES DEPARTMENT OF THE INTERIOR
in cooperation with the
FLORIDA GEOLOGICAL SURVEY
and the
BOARD OF COUNTY COMMISSIONERS
of
HILLSBOROUGH COUNTY


1953









CONTENTS


Page


Introduction . . . . .
Previous investigations . . . .
Geography . . . . .
Geology . . . .. .
Ground water . . . ...
Wells . . . . .
Piezometric surface . . .....
Salt-water encroachment ..... . .
Chloride content of the artesian water .
Salt water in relation to depth and pressure
Summary and conclusions . . . .
References . . . . .


S 9 9 .


ILLUSTRATIONS


Page


,Figure 1.


2.

3.
4.

5.

6.

7.-


A-Map of Florida showing location of Hillsborough
County . . . . .
B-Map of Hillsborough County showing area investigated .
Generalized cross section'showing the formations
penetrated by wells . . . . .
Map of Ruskin area showing the distribution of wells .
Map of Ruskin area showing the piezometric surface
in September 1951 .. . .
Map of Ruskin area showing the chloride content of
water from wells . . ...
Graph showing relation between chloride content and
water level in well B-55 . . a .
Graph showing relation between chloride content and
water level in well H-133 .. . . .







THE ARTESIAN WATER OF THE RUSKIN AREA OF
HILLSBOROUGH COUNTY, FLORIDA
INTERIM REPORT

by

Harry M. Peek


INTRODUCTION

In several coastal areas of Florida where large quantities of ground

water are used, water levels have been lowered sufficiently to permit the

encroachment of sea water into the water-bearing formations. Encroachment

in some areas has been so extensive that an adequate supply of fresh water

has become difficult to obtain.

During recent years, rapid expansion and development of the farming

industry in the vicinity of Ruskin, in the southern part of Hillsborough

County, has greatly increased the use of artesian water for irrigation.

This increased draft has lowered artesian pressures considerably, thereby

creating a possibility of encroachment of salt water from Tampa Bay.

Recognizing this possibility, the Board of County Commissioners of

Hillsborough County requested the United States Geological Survey and the

Florida Geological Survey to make an investigation of the problem.

Accordingly, an investigation was begun in October 1950.

This report gives the results of the progress of that investigation

during the first year. The investigation was financed partly through a

cooperative agreement between the United States Geological Survey and the

Florida Geological Survey, and partly through an agreement between the

United States Geological Survey and the Board of County Commissioners of

Hillsborough County. The investigation was made by the author under the

supervision of H. H. Cooper, Jr., District Engineer of the United States

Geological Survey, Tallahassee, Florida.






The purpose of the investigation is to make a detailed study of the

geology and ground water in the Ruskin area, especially as related to the

problem of salt-water encroachment. The major objectives of the program

includes:

(1) An inventory of wells to determine their number and distribution,

their depths and diameters, and other pertinent information.

(2) A study of artesian pressures.

(3) Analyses of water from selected wells to determine the location
and extent of any areas in which the artesian water is salty.

(4) A study of the surface and subsurface geology as related to the

occurrence and movement of ground water.

(5) An estimate of the quantity of ground water withdrawn.

PREVIOUS INVESTIGATIONS

Several reports that include discussions of the geology and ground-

water resources of Hillsborough County have been published by the Florida

Geological Survey and the United States Geological Survey.

A report by Matson and Sanford (1913, pp. 320, 323, plate 5) (see

references at end of report) contains a generalized map of the Pleistocene

terraces, logs of wells, descriptions of the formations exposed at the

surface and a brief discussion of the ground water in Hillsborough County.

A report by Sellards and Gunter (1913, pp. 258-262) published in the same

year includes a discussion of the geology and ground water of Hillsborough

County and contains a map showing the area of artesian flow.

A report by Leverett (1931, pp. 18-20) contains a description and

map of the shore line of the Pensacola sea in Hillsborough County. The

geology and ground water of Hillsborough County were discussed in a report

on the ground water of Florida by Stringfield in 1936 (pp. 127, 128, and

152). This report includes maps showing the area of artesian flow, of






artesian water at moderate depth, chloride in excess of 100 ppm, and the

first published piezometric map of the water in the principal artesian

aquifer.

A report by Parker and Cooke (1944, plate 3) contains a map showing

the general pattern of Pleistocene terraces in southern Florida, including

Hillsborough County.

The formations penetrated by wells and exposed at the surface in the

County are described in some detail in a report on the geology of Florida

by Cooke (1945, pp. 34, 42, 47, 125, 208, 222, 290, and 305). A report

by MacNeil (1949, p. 105, plate 19) on the Pleistocene shore lines in

Florida and Georgia includes a map showing the general configuration of

these shore lines. A recent report by Vernon (1951, figs. 11, 33 and

plate 2) contains maps showing the subsurface features of some of the

formations underlying Hillsborough County.


GEOGRAPHY

For the purpose of this report, the Ruskin area comprises about 140

square miles in the southwest corner of Hillsborough County adjacent to

Tampa Bay (fig. 1).

The mean temperature in the area is about 72' F. according to the

Annual Summary of Climatological Data published by the United States

Weather Bureau in 1951. The average annual rainfall is about 55 inchesand,

as in most of Florida, the season of heaviest rainfall is in the summer and

early fall. The rainfall from June to September is usually more than half

of the total during the year.

The topography of the area may be generally divided into two units:

a relatively flat coastal area and an area of rolling sand ridges. The

coastal area is about three to five miles wide and extends inland from

Tampa Bay to an escarpment that marks the Pamlico shore line, a former

3

























ATLANTIC
OCEAN


GULF OF MEXICO


FIGURE I. A-MAP OF FLORIDA SHOWING LOCATION
OF HILLSBOROUGH COUNTY.


B-MAP OF HILLSBOROUGH COUNTY
SHOWING AREA OF INVESTIGATION.


I






artesian water at moderate depth, chloride in excess of 100 ppm, and the

first published piezometric map of the water in the principal artesian

aquifer.

A report by Parker and Cooke (1944, plate 3) contains a map showing

the general pattern of Pleistocene terraces in southern Florida, including

Hillsborough County.

The formations penetrated by wells and exposed at the surface in the

County are described in some detail in a report on the geology of Florida

by Cooke (1945, pp. 34, 42, 47, 125, 208, 222, 290, and 305). A report

by MacNeil (1949, p. 105, plate 19) on the Pleistocene shore lines in

Florida and Georgia includes a map showing the general configuration of

these shore lines. A recent report by Vernon (1951, figs. 11, 33 and

plate 2) contains maps showing the subsurface features of some of the

formations underlying Hillsborough County.


GEOGRAPHY

For the purpose of this report, the Ruskin area comprises about 140

square miles in the southwest corner of Hillsborough County adjacent to

Tampa Bay (fig. 1).

The mean temperature in the area is about 72' F. according to the

Annual Summary of Climatological Data published by the United States

Weather Bureau in 1951. The average annual rainfall is about 55 inchesand,

as in most of Florida, the season of heaviest rainfall is in the summer and

early fall. The rainfall from June to September is usually more than half

of the total during the year.

The topography of the area may be generally divided into two units:

a relatively flat coastal area and an area of rolling sand ridges. The

coastal area is about three to five miles wide and extends inland from

Tampa Bay to an escarpment that marks the Pamlico shore line, a former

3






level of the sea during late Pleistocene time. The coastal area slopes

gently toward the bay from the base of the escarpment, which is about 25

feet above sea level. Most of the coastal area is between five and fifteen

feet above sea level, but it contains a few low ridges with elevations up

to 20 feet or more. The area of gently rolling sand ridges extends eastward

from the Pamlico escarpment, gradually increasing in elevation to more than

100 feet in the vicinity of Wimauma. Most of the ridge area is between 40

and 70 feet above sea level.

The principal drainage in the area consists of the Little Manatee

River, the Alafia River, Bullfrog Creek, and their tributaries, all of

which empty into Tampa Bay, as do several other smaller streams. Numerous

ponds, depressions and swamps occur throughout the area.


GEOLOGY

Previous investigations of geology in southern Hillsborough County

have been devoted primarily to the surface formations. The lack of well

cuttings prior to the beginning of the current investigation has prevented

a detailed study of the subsurface characteristics of the formations. Well

cuttings and electric logs that have recently become available have not yet

been studied; therefore a detailed description of the subsurface geology

will not be attempted in this report.

A generalized cross section showing the formations penetrated by

wells in the Ruskin area is shown in figure 2. The formations are separated

by erosional unconformities and generally dip toward the south. The Hawthorn

and younger formations thicken in that direction.

The Ocala limestone of Eocene age yields large quantities of water

throughout most of the State. In the Ruskin area, however, only a few wells

have been drilled into the Ocala limestone because the overlying formations














SW


E I
a


1-18STOoceM Gnu r-rorvnwec

io em) FLORIDAN AQUICLUDE
iH4WTthON FORMATION (Mief)


I00l-




0-




100



0-j






-ao0
400


5 00-


600


WWA^ U.MESTONE (m0ocene)











AOGAI ES E (TONEScene)



wooc o I



SCALE IN MILOS



FIGURE 2. GENERALZED CROSS SECTION SHOWING FORMATIONS PENETRATED
BY WELLS IN THE RUSKIN AREA


__


__ _~_ _~Lt ___ __






level of the sea during late Pleistocene time. The coastal area slopes

gently toward the bay from the base of the escarpment, which is about 25

feet above sea level. Most of the coastal area is between five and fifteen

feet above sea level, but it contains a few low ridges with elevations up

to 20 feet or more. The area of gently rolling sand ridges extends eastward

from the Pamlico escarpment, gradually increasing in elevation to more than

100 feet in the vicinity of Wimauma. Most of the ridge area is between 40

and 70 feet above sea level.

The principal drainage in the area consists of the Little Manatee

River, the Alafia River, Bullfrog Creek, and their tributaries, all of

which empty into Tampa Bay, as do several other smaller streams. Numerous

ponds, depressions and swamps occur throughout the area.


GEOLOGY

Previous investigations of geology in southern Hillsborough County

have been devoted primarily to the surface formations. The lack of well

cuttings prior to the beginning of the current investigation has prevented

a detailed study of the subsurface characteristics of the formations. Well

cuttings and electric logs that have recently become available have not yet

been studied; therefore a detailed description of the subsurface geology

will not be attempted in this report.

A generalized cross section showing the formations penetrated by

wells in the Ruskin area is shown in figure 2. The formations are separated

by erosional unconformities and generally dip toward the south. The Hawthorn

and younger formations thicken in that direction.

The Ocala limestone of Eocene age yields large quantities of water

throughout most of the State. In the Ruskin area, however, only a few wells

have been drilled into the Ocala limestone because the overlying formations






generally yield sufficient water. The Ocala is a creamy-white to buff,

soft, granular limestone containing abundant Foraminifera, and is probably

more than 100 feet thick in the vicinity of Ruskin.

The Suwannee limestone of Oligocene age is probably the most

productive of the water-bearing formations in the Ruskin area. It overlies

the Ocala limestone and is generally a creamy-white to brown, soft,

granular limestone containing abundant Foraminifera and other fossils. The

Suwannee limestone is about 150 to 200 feet thick in southern Hillsborough

County.

The Tampa limestone of Miocene age, which overlies the Suwannee, is

also a very productive source of artesian water. The Tampa generally

consists of white to gray, hard, dense, sandy limestone containing many

mollusk molds and some Foraminifera. It is generally silicified and inter-

bedded with thin layers of clay. The average thickness of the Tampa lime-

stone is about 200 feet in the Ruskin area.

Overlying the Tampa limestone is the Hawthorn formation, also of

Miocene age, consisting predominantly of phosphatic clay, but containing

beds of sand and sandy limestone. The sand and limestone beds yield

small quantities of water to a few shallow wells. However, as the thick

beds of clay cause the formation to have a very low vertical permeability,

it constitutes an effective confining bed for the artesian water in the

Ruskin area, as elsewhere in the State. Parker (1951, PP. 819-820) has

named this confining layer, which in some places contains other than

Hawthorn materials, the Floridan aquiclude.

The Hawthorn formation is overlain by Pliocene and Pleistocene

deposits of marl and sand which yield water to a few small domestic wells.

These deposits are generally 20 to 60 feet thick, but in a few places

along the larger streams they have been removed by erosion and the Hawthorn

is exposed.







GROUND WATER

Almost all the water withdrawn from wells in the Ruskin area is

derived from the permeable limestones that compose: the artesian aquifer

throughout most of Florida. These limestone are of several formations

and different ages, ranging from Eocene through Miocene (see figure 2),

however, they comprise a hydrologic unit to which the name Floridan

aquifer has been given (Parker 1951, p. 819). The water in the Floridan

aquifer is replenished by rainfall in areas where the aquiclude is absent

or sufficiently permeable to permit the passage of water from the surface

into the water-bearing strata. The artesian pressure gradient indicates

that the artesian water in the Ruskin area probably originates in the

lake region of Polk County, which, as Stringfield (1936, plate 12 and,

p. 148)revealed, is a large area of recharge and the source of much of

the artesian water in the State.

In the Polk County recharge area, water enters the Floridan aquifer

through numerous sinkholes, filled with permeable material, that penetrate

the Hawthorn and younger formations. The water collects in the sinks by

runoff from the land surface and also by seepage from the permeable beds

in the Hawthorn and younger formations; it then percolates downward to

recharge the Floridan aquifer.

From the recharge area, the water moves laterally through the pores

and cavities in the limestone toward areas of lower pressure (fig. 4) at

places either of natural or artificial discharge. From the intake area

in Polk County, the water moves underground for a distance of about 30

miles to the Ruskin area.

A few small domestic wells yield water from the shallow Pleistocene

sands or other permeable beds that lie in or above the Hawthorn formation.

The water in these beds is replenished by local rainfall, as there is no





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HILLSBOROUGH COUNTY
MANATEE COUNTY


24 19





25 30

LEGEND


Flowing well
o Nonflowing well
3 S6 IN MI
SCALE IN MILES


28 27


*

33 34


FIGURE 3. MAP OF THE RUSKIN AREA OF HILLSBOROUGH COUNTY


Showing the distribution of artesian wells
that have been inventoried


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HILLSBOROUGH COUNTY
MANATEE COUNTY


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I0MUM
-WIMAUMA


Well in which wofer-level measurement
30 *165 wos made. Number represents Ihe
altitude of woaer level in feet above
meon sen level.


SContour Imes represent approximate height
/ in feet above mean sea level that waler
31 \ 1 will rise in lightly closed wells. Broken
lines represent inferred position of contours.


SSCALE IN MILES


FIGURE 4. MAP OF THE RUoKIN AREA OF HILLSBOROUGH COUNTY
SHOWING THE PIEZOMETRIG SURFACE IN SEPTEMBER 1951


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overlying impervious layer to hinder percolation of water from the surface.

However, some of the water in the shallow aquifers may be derived from

the Floridan aquifer through wells that are cased to shallow depths, or

through the confining bed itself wherever it is sufficiently permeable to

allow leakage.


Wells

Figure 3 shows the distribution of more than 500 wells that have been

inventoried in the Ruskin area, almost all of which are used for irrigation.

Most of them have been drilled during the past 10 to 15 years, a period of

intensive agricultural development. They are concentrated in a zone about

three or four miles wide along the coast, which includes nearly all the

cultivated land. Most of the wells will flow, hence only a few are

equipped with pumps.

About 350 of the wells are six inches in diameter and about 125

are eight inches in diameter. Of the rest, some have diameters of four

inches or less and some have diameters of ten inches. The depths of the

artesian wells range from 100 and 700 feet, but most are between 350

and 500 feet.

Piezometric Surface

The contours on the map in figure 4 represent the piezometric

surface in the Ruskin area in September 1951. The piezometric surface is

an imaginary surface that represents the height above sea level that water

will rise in tightly-cased wells. The configuration of the contours on

such a map reveals the hydraulic gradients and provides a means of

determining the directions of the movement of the artesian water. The

water moves from the higher toward the lower pressures in the direction

of the steepest gradient, which is at right angles to the contours. If







there is any recharge in an grea, it is generally represented by a high

in the piezometric surface. Areas of discharge are indicated by

depressions in the piezometric surface.

The map in figure 4 shows no evidence of recharge in the Ruskin area,

and indicates that, in general, the water moves from the region of higher

pressures in the south and southeast toward the northwest. In the area

east of the 22-foot contour, where interference by discharging wells is

negligible, the piezometric surface has a fairly uniform gradient of about

two to four feet per mile, as indicated by the gentle curvature and

relatively uniform spacing of the contours. In the farming area long the

coast, the contours are irregular and unevenly spaced because of discharge

from wells during the period in which pressure measurements were made.

Areas of heavier withdrawals are indicated by the depressions in the piezo-

metric surface, as shown by the more pronounced distortion or the closure

of the contours.

The area of low pressure west of Sun City, indicated by the closed

14-foot contour, is a result of the combined drawdown of a large number

of wells that were discharging at the time measurements were made. See

figure 4 for location of wells within this and other low pressure areas.

A second area of low pressure, centered about a mile north of Ruskin at

another closure of the 14-foot contour, may also be the result of discharge

from wells. However, as there are many wells in this area that are cased

to only shallow depths, some reduction of pressure may result from leakage

of the artesian water into the shallow formations.

Local changes in the piezometric surface occur almost constantly, as

there are several factors that cause fluctuations of artesian pressures.

These fluctuations, their causes, and related phenomena have been discussed

in a recent paper by Parker and Stringfield (1950). The large daily and

seasonal variations in withdrawal of water from wells produce the more

12







significant fluctuations in the Ruskin area, although observable changes

in pressure are caused by rainfall, changes in barometric pressure, and

ocean tides. Pressures are measured at intervals of about six weeks in

12 selected observation wells distributed over the area, and from

automatic water-stage recording instruments on two wells. Also, about

twice a year, measurements are made in a large number of wells so as to

construct maps of the piezometric surface.


SALT-WATER ENCROACHMENT

The occurrence of salt water in artesian wells in coastal areas of

Florida may be a result of (1) encroachment of salt water into the aquifer

directly from the sea, or (2) contamination by highly mineralized water

that exists naturally in the aquifer or underlying formations. Contami-

nation from either source may be directly related to the lowering of

artesian pressures.

In coastal areas where the water-bearing formations crop out beneath

the sea, the artesian head must remain well above sea level to prevent

sea water from entering the water-bearing strata. The specific gravity

of sea water is slightly higher than that of fresh water so that the

fresh water "floats" almost completely submerged in salt water, in much

the same way that ice floats in water with only a small part of its mass

above the surface (Brown, 1925, p. 16). The specific gravity of sea water

is generally about 1.025, whereas, that of fresh water is, for practical

purposes, 1.000. It may be shown that with these specific gravities, a

column of fresh water 41 feet high will exactly balance a column of sea

water 40 feet high. Application of this principle to ground water in

areas adjacent to the sea indicates that for each foot of fresh water

above sea level there will be 40 feet of fresh water below sea level.

This relationship is applicable, however, only in aquifers of fairly

13






uniform geohydrologic characteristics, therefore, proper allowance must

be made for relatively impervious strata that may occur within or beneath

the water-bearing formations. See Brown and Parker (1945) and Parker

(1945). Proper allowance must also be made for the time lag between

changes in pressure and adjustment of the hydrostatic balance between the

salt and fresh water. Wentworth (1942, pp. 683-693) concluded that the

time lag is a period of at least several months in an aquifer of relatively

high permeability, and probably many years in an aquifer of relatively low

permeability. His conclusions were strengthened by the work of Brown and

Parker (1945, pp. 256 and 257) at Miami.

As shown on the map in figure 4, artesian pressures along the coast

range from less than 10 feet in the vicinity of Adamsville to more than

16 feet south of the Little Manatee River. Application of the 40 to 1

ratio indicates that, theoretically, the depth to salt water is about 400

feet at Adamsville, and increases southward to more than 600 feet south

of Little Manatee River. During periods of heaviest withdrawal, artesian

pressures in areas where the wells are concentrated are several feet lower

than those shown in figure 4. Thus, wells in these areas that are

drilled to only moderate depths might be susceptible to contamination

unless vertical migration of the salt water is hindered by the presence

of relatively impervious beds in the aquifer.

Available geologic information indicates that the artesian water in

the upper part of the Floridan aquifer occurs in permeable zones which

are often separated, at least locally, by relatively impervious layers that

retard or prevent vertical movement of the water. If encroachment were

occurring in the deeper formations, the impervious strata would form an

effective barrier against contamination from below. Thus the process of

encroachment would be slowed considerably, as each permeable zone could

become contaminated only by lateral migration of sea water from the out-
134






crop, or through wells that penetrated the underlying contaminated beds.

Much of the coastal land of Florida is underlain at moderate depths by

highly mineralized water which entered the formations prior to Recent

time, when the sea stood at higher levels. Thus, wells in many coastal

areas may yield salt water where geologic and artesian conditions would

not permit direct encroachment of water from the sea. In such areas,

the lowering of artesian pressures would cause vertical migration of the

salt water, except where such migration might be hindered by overlying

impervious beds.


Chloride Content of the Artesian Water

The chloride content of ground water is generally a reliable index

of contamination with salt water from the sea or from other sources. As a

part of the investigation, analyses of chloride have been made in about 250

selected wells. The results of these analyses are shown by symbols on the map

in figure 5. However, the symbols do not represent the exact amount of

chloride in each well, but show the limits within which the chloride

content of each well is included. It is planned that the exact chloride

contents of the wells will be given in tabular form in a subsequent report.

As indicated by the open circles in figure 5, the chloride content

of the artesian water over most of the area is no more than 30 parts per

million. In fact, most of the wells represented by the open circles had

chloride contents of about 25 p. p. m. or less. A chloride content of 20

to 30 p. p. m. is about what one might expect to find in water that is

uncontaminated by salt water from the sea or other sources, and may be

considered as being normal for the area. Thus, the circles that are partly

or completely filled indicate wells in which the aquifer has in some way

become contaminated with salt water. The wells in which contamination has

occurred are restricted to the northern half of the coastal area.

15







RIBE


A


0
0
320 0 330 34
O 0

AO


29 I 28 I 27
EXPLANATION
O 30 ports per million or less
0 31 to oo00 parts per million
* 101 to 250 ports per million
* 251 to 500 parts per million
* More than 500 parts per mSon
_-i -~-- .....


FIGURE 5. MAP OF THE RUSKIN AREA OF HILLSBOROUGH COUNTY
SHOWING THE CHLORIDE CONTENT OF WATER FROM ARTESIAN WELLS

SCALE IN MILES







As indicated in figure 5, the wells that yield water with a relatively

high chloride content are restricted to three small areas. One area is

about two miles north of Ruskin, chiefly in the northern part of Sec. 32,

and the southern part of Sec. 29, T. 31 S., R. 19 E. The water from one

well in this area contains more than 500 p.p.m. of chloride. A second

area, somewhat smaller than the first, in Sec. 21 about four miles north of

Ruskin, includes wells yielding water with a chloride content ranging up

to 250 p.p.m. The third area extends an undetermined distance northward

from the vicinity of Adamsville, along the shore of Tampa Bay. Water

from two wells at Adamsville contained more than 1,000 p.p.m. of chloride,

but each of these wells are more than 700 feet deep. Most of the wells

near Adamsville, having depths of about 400 feet or less, yield water

having contents of no more than 300 p.p.m.


Salt Water in Relation to Depth and Pressure

The areas in which wells yield water containing chloride contents

above the normal are all adjacent to Tampa Bay and in the vicinity of

areas of relatively low artesian pressure, where saline contamination

from the sea or other sources would be most likely to occur. Some areas

of low pressure, however, such as the one west of Sun City (fig. 4) contain

no contaminated wells. The reason for the absence of contaminated wells in

this area has not been determined, but there are several possible explana-

tions, among which are:

1. The wells in this area may not penetrate the strata that yield

salty water to wells in other low pressure areas.

2. Impervious beds may prevent contamination from the underlying

strata or from the sea.

3. Artesian pressures between the area and the submarine outcrop

may be sufficiently high to prevent sea water from entering the aquifer.






4. Contamination may not have reached this area because of the

slow rate at which salt water moves through the aquifer.

The wells that yield water of highest chloride content generally

penetrate the deeper formations, which indicates that an increase in

chloride with depth is the normal situation. These wells are generally

cased to only shallow depths and below the bottom of the casing are open

to all the permeable strata they penetrate. However, most of the water

yielded is probably from the deeper formations where artesian pressures

are slightly higher than in the shallow strata. Because of this difference

in pressure, there would be a tendency for the water in the lower part

of the aquifer to move up through the wells and, in the uncased part,

flow out into the shallow formations. Thus, contamination of the shallow

formations in the vicinity of the deeper wells is possible. This

possibility would account for the higher than normal chloride contents in

nearby wells that penetrate only the shallow formations.

Periodic chemical analyses indicate that the chloride content of

water from wells in this area varies with major changes in artesian

pressure. Generally, a decrease in artesian pressure is accompanied by

an increase in the chloride content, and vice versa, as shown in figures

6 and 7. The analyses have not covered a period of sufficient length to

allow for determination of whether or not the chloride content is

progressively increasing, but they indicate that no impervious formation

separates the fresh and salt water in the deeper part of the aquifer, and

that migration of the salt water is probably hindered only by the fresh

water in the aquifer. Further lowering of the artesian pressure probably

would bring the salt-water contact nearer to the land surface and thereby

extend the contaminated area.










Pab or~l A~wI ~1951


7 -1-- S-ep iT ti. uOc Au. pt. Uct. Nov. Dec.
JI--- I I A It I I


aJ WATER LEVEL


u-g~~~~~~ 1 ---- --- ----- --- ---- ----_____ ____________





150




130 __ CHLORIDE CONTENT OF WATER
--o-______ __

90 o______
70
w __-- -__ ____ ____

0. _0------------------------------- __ _____________


10) ------------------- ---- ---- ---- ____ __________________


FIGURE 6-RELATION BETWEEN CHLORIDE CONTENT AND WATER LEVEL IN WELL H-55
(WELL LOCATION: SW/4 of SE/4 of SEC. 3, T31S, R19E)





1951
Jan. Feb. Morch April May June July Au Sept Oct. Nov. Dec.


------- ------- ------- ----_----_-------- ___ ___ ___ ____________



19-_______ ____ _____ _____-- _0
o FI -WATER LEVEL-







?I
S- --- --------- ---- --------- _______ __
7 ------ 0--- ---- --- ---- ------ ------------__________ ____ ____








24 _-..._^O CHLORIDE CONTENT OF WATER








140 0


FIGURE 7- RELATION BETWEEN CHLORIDE CONTENT AND WATER LEVEL IN WELL H--13
(WELL LOCATION: NE/4 of SW/4 of SEC.21, T31S, RISE)


AAml... .






SUMMARY ANd CONCLUSIONS

The field work during the period covered in this report consisted

of:

1. An inventory of wells.

2. Periodic water-level measurements in selected observation

wells for use in studying fluctuations of artesian pressures.

3. Establishing altitudes of measuring points on wells for use

in mapping the piezometric surface.

4. Mapping the piezometric surface.

5. Making chloride analyses of water from about 250 wells in

order to determine areas in which the water is highly mineralized.

6. Periodic chloride analyses of water from selected wells in

order to determine the relation between artesian pressures and salinity.

7. Collection of geologic data for use in preparing maps of the

subsurface formations and for aid in determining their water-bearing

properties.

8. An inventory of the estimated yield from wells for use in

calculating the total quantity of ground water used in the area.

As the investigation was still incomplete at the end of the period

covered in this report, available data are not sufficient to reach

definite conclusions concerning the.problem as a whole. However, the re-

suits obtained do allow tentative conclusions to be drawn concerning certain

aspects of the problem, and they provide a basis for future study.

Records of fluctuation of artesian pressure show that local pressures

are lowered as much as eight feet during periods of heaviest withdrawal.

This lowering of pressure creates relatively large depressions in the piezo-

metric surface, such as those shown in figure 4, where either encroachment

of sea water, or migration of salty water from the deeper formations, is

possible. Extension of the cultivated area and the drilling of new wells
20






indicates an increase in withdrawal of ground water from the aquifer.

This will result in further lowering of artesian pressures.

Wells in three small areas yield water with a chloride content above

the normal (for the area of this report) of 20 to 30 p.p.m. Most of these

wells penetrate the deeper water-bearing strata, and are in or near low pres-

sure areas where contamination by salt water would be most likely to occur.

Although the chloride content of water from wells varies with major

changes in artesian pressure, the information obtained during the relatively

short period of observation is not sufficient to allow for determination of

whether the chloride content is progressively increasing.

Plans for completing the investigation require the continuation of

periodic water-level measurements and chloride analyses in order to allow for

determination of long-range trends or any major changes in ground-water

conditions.

Future studies will also include:

1. Pumping and recovery (aquifer performance) tests to determine

the capacities of the formations to transmit and store water.

2. Analyses of water samples taken at various depths in wells to

determine the chloride content of water in the different formations.

3. Exploration of wells with current meter and resistivity instru-

ments to determine the depths of producing strata and the depths at which

changes in the salinity of the water occur.

4. A detailed study of well cuttings and electric logs in order to

construct maps of the subsurface formations and to aid in the determination

of the water-bearing properties of these formations.

5. A study of the use of ground water and the making of an estimate

of the annual withdrawal.

6. Complete chemical analyses of water samples from selected wells

to determine the general chemical properties of the water and their relation

to the chloride content.








REFERENCES


1. Brown, J. S., 1925, A study of coastal ground water with special
reference to Connecticut: U. S. Geol. Survey Water-Supply Paper
537.
2. Brown, R. H., and Parker, G. G., 194, Salt-water encroachment in
limestone at Silver Bluff, Miami, Florida: Econ. Geol. Jour.,
vol. 40, no. 4.

3. Cooke, C. W., 19.45 Geology of Florida: Florida Geol. Survey Bull.
29.

4. Leverett, Frank, 1931 The Pensacola terrace and associated beaches
and bars in Florida: Florida Geol. Survey Bull. 7.

5. MacNeil, F. S., .19 Pleistocene shore lines in Florida and Georgia:
U. S. Geol. Survey Prof. Paper 225-F.

6. Matson, G. C., and Sanford, Samuel, j191 Geology and ground waters
of Florida: U. S. Geol. Survey Water-Supply Paper 319.

7. Parker, G. G., and Cooke, C. W., 1944, Late cenozoic geology of
southern Florida with a discussion of the ground water: Florida
Geol. Survey Bull. 27.

8. Parker, G. G., 1945, Salt-water encroachment in southern Florida:
Am. Water Works Assn. Jour., vol. 37, no. 6.

9. Parker, G. G., and Stringfield, V. T., 1950, Effects of earthquakes,
trains, tides, winds, and atmospheric pressure changes on water in
the geologic formations of southern Florida: Econ. Geol. Jour.,
vol. 45, no. 5.

10. Parker, G. G., p195 Geologic and hydrologic factors in the
perennial yield of the Biscayne aquifer: Am. Water Works Assn.
Jour., vol. 43, no. 10.

11. Sellards, E. H., and Gunter, Berman, 1913, The artesian water supply
of eastern and southern Florida; Florida Geol. Survey 5th Annual
Report.

12. Stringfield, V. T., 1936, Artesian water in the Florida Peninsula:
U. S. Geol. Survey Water-Supply Paper 773-C.

13. Vernon, R. 0., 1951, Geology of Citrus and Levy counties, Florida:
Florida Geol. Survey Bull. 33.

14. Wentworth, C. K., 1942, Storage consequences of the Ghyben-Herzberg
theory: Am. Geophys. Union Trans. 1942, part 2.










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