Saline-water intrusion from deep artesian sources in the McGregor Isles area of Lee County, Florida ( FGS: Information circular 75 )

STATE OF FLORIDA
DEPARTMENT OF NATURAL RESOURCES
Randolph Hodges, Executive Director

DIVISION OF INTERIOR RESOURCES
Robert O. Vernon, Director

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

Information Circular No. 75

SALINE-WATER INTRUSION
FROM DEEP ARTESIAN SOURCES IN THE
McGREGOR ISLES AREA OF LEE COUNTY, FLORIDA

By
C. R. Sproul
Formerly with the Florida Department of Natural Resources
Bureau of Geology

D. H. Boggess
U.S. Geological Survey

H. J. Woodard
Florida Department of Natural Resources
Bureau of Water Resources

Prepared by the
UNITED STATES GEOLOGICAL SURVEY
and the
FLORIDA DEPARTMENT OF NATURAL RESOURCES
in cooperation with the
COUNTY COMMISSIONERS OF LEE COUNTY

TALLAHASSEE
1972

Completed manuscript received
February 1, 1972
Printed for the Florida Department of Natural Resources
Division of Interior Resources
Bureau of Geology
by Rose Printing Company
Tallahassee, Florida

Tallahassee
1972

ii

CONTENTS

Abstract .............
Introduction .........
Purpose and Scope . .
Acknowledgements . .
Description of the area .
Well numbering system .
Description of aquifers .
Water-table aquifer ....
Sandstone aquifer .
Upper Hawthorn aquifer
Lower Hawthorn aquifer
Suwannee aquifer ....
Deeper aquifers . .
Evidence of faulting ..
Water quality and the effects
Lower Hawthorn aquifer
Upper Hawthorn aquifer
Other aquifers . .
Mechanics of intrusion ...
Control procedures .....
Summary and conclusions .
References . ...

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. . . ., .I

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. . . . . .
. . . . .
. . . .

.o..f .an wae. nrso
. . . . .
. . . . .
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of salie water intrusion .
. . . .
. . .

ILLUSTRATIONS

Figure
1 Map of Lee County showing the location of the McGregor Isles area ........

2 Map showing the location of wells . . . . . . .

3 Geologic column showing lithology, aquifers, and typical gamma ray log of
formations underlying McGregor Isles . . . ...... . ..

4 Geologic section showing faults based on interpretations of gamma ray logs ..

5 Map of McGregor Isles showing the approximate location of faults . .

6a Map of McGregor Isles showing the extent of saline water intrusion into the lower
Hawthorn aquifer .....................................

6b Map showing the effects of intrusion on water temperatures in the lower
Hawthorn aquifer . . . . . . . . . .

7 Graph showing changes in chloride content of water from the lower Hawthorn
aquifer, 1946-1968 .....................................

Page

Page
4

5

7

13

14

17

18

19

8 Map showing the chloride content of water from wells in McGregor Isles,
1967-1969 ............. .......... ...... .......... 21

9 Graph showing changes in chloride content of water from well 17-1 on April 15,
1969 ............. .... .......... ...... ..... 23

TABLES

Table Page
1 Record of wells in the McGregor Isles area . . . .... At end of report

2 Chemical analyses of water from wells in McGregor Isles and at Hot
Springs ............... ...... ............ At end of report

3 Comparison of the arithmetic mean of chemical constituents for
wells 22-1 and Hot Springs, with the chemical analysis for well 16-7 . ... 15

SALINE-WATER INTRUSION FROM DEEP ARTESIAN SOURCES
IN THE McGREGOR ISLES AREA OF LEE COUNTY, FLORIDA

By

C. R. Sproull, D. H. Boggess2, and H. J. Woodard3

ABSTRACT

Upward leakage of saline water from an artesian aquifer below 1,500 feet
has caused an increase in chloride concentration in the lower Hawthorn aquifer
from less than 1,000 mg/1 (milligrams per liter) to values ranging from about
1,300 to 15,000 mg/1. Similarly the higher temperatures of the intruding water
has caused an increase in water temperatures in the aquifer from 82"F to values
ranging from 83 to 93"F. The intruding water moves upward either through the
open bore hole of deep wells or test holes, or along a fault or fracture system,
which has been identified in the area. From these points of entry into the lower
Hawthorn aquifer, the saline water spreads laterally toward the south and
southeast, but is generally confined to components of the fault system.
The saline water moves upward from the lower Hawthorn aquifer into the
upper Hawthorn aquifer through the open bore hole of wells, which connect the
aquifers. This movement has resulted in an increase in chloride from less than
200 mg/1 in the unaffected parts of the upper Hawthorn aquifer to values
commonly ranging from about 300 to more than 3,000 mg/1 in parts of the
aquifer affected by upward leakage. The upper Hawthorn aquifer is the principal
source of ground-water supply for public water-supply systems in western Lee
County.
Similar effects have been noted in the water-table aquifer, where chloride
increased from less than 100 to concentrations ranging from about 500 to more
than 5,000 mg/1. This was caused by the downward infiltration of water
discharged at land surface from wells tapping the lower Hawthorn aquifer.
The spread of saline water throughout most of the McGregor Isles area is
continuing as of 1971.

INTRODUCTION

Nearly all of southwest Florida is underlain at shallow depths by permeable

1Formerly with the Florida Dept. of Natural Resources, Bureau of Geology
2U.S. Geological Survey
3Florida Dept. of Natural Resources, Bureau of Water Resources and Conservation

BUREAU OF GEOLOGY

strata which are sources of water supply for domestic, municipal, agricultural,
and industrial purposes. Strata at greater depths, although equally permeable
contain highly mineralized water under artesian pressure high enough that a head
difference exists between the deeper and shallower aquifers. Because the deeper
aquifers are normally under higher artesian pressure, the existence of any path or
conduit of high permeability between the two will result in upward movement
of more highly mineralized water into the overlying aquifers. Under natural
conditions, the water in these different formations is in a state equilibrium and is
prevented from intermixing by relatively impermeable beds which separate
them. Lowering of the artesian pressure in the shallower aquifers by pumping
increases the difference in head between the aquifers.
Water from the deeper strata can then move upward into the shallow strata
in at least two different ways. First, penetration of the impermeable beds by
drilling, whereby both the shallow and deeper strata are interconnected through
the open well bore, will allow the movement of water from the deeper strata
(under higher artesian pressure) into the shallow strata (under lower pressure).
Second, the existence of faults, extending downward at least through the Ocala
group1, can provide a conduit through which the saline water can move upward.
Both these possibilities will be explored later in the report.

PURPOSE AND SCOPE

The problems of saline water movement into the shallow aquifers by
upward leakage from deep artesian sources is of considerable magnitude in Lee
County, where an estimated 2,500-3,000 deep artesian wells and test holes have
been drilled. The purpose of this report is to present the results of an analysis of
available geologic and hydrologic data for a small area in Lee County where
saline water from deep artesian sources has moved upward into several different
aquifers.
From an analysis of the available data, the authors attempt to define not
only the source of the highly mineralized water, but also to describe the
mechanism through which upward leakage occurs and the effects of intrusion on
water quality in each of the aquifers underlying the area. The effects of upward
leakage of saline water through the open bore hole of existing wells connecting
aquifers at depths of less than 300 feet and those which occur to depths of
about 1,000 feet is evident from the data presented herein and from similar
studies conducted in other parts of Lee County. However, the mechanism
responsible for upward leakage of saline water from an artesian aquifer below
1,500 feet into the aquifers between 400 and 1,000 feet is not well known and
can only be surmised from the available data. That such leakage does occur is
evident from the information presented herein.
1 The nomenclature used in this report conforms to that of the Bureau of Geology, Florida
Division of Interior Resources, Department of Natural Resources, and not necessarily to
that of the U.S. Geological Survey.

BUREAU OF GEOLOGY

strata which are sources of water supply for domestic, municipal, agricultural,
and industrial purposes. Strata at greater depths, although equally permeable
contain highly mineralized water under artesian pressure high enough that a head
difference exists between the deeper and shallower aquifers. Because the deeper
aquifers are normally under higher artesian pressure, the existence of any path or
conduit of high permeability between the two will result in upward movement
of more highly mineralized water into the overlying aquifers. Under natural
conditions, the water in these different formations is in a state equilibrium and is
prevented from intermixing by relatively impermeable beds which separate
them. Lowering of the artesian pressure in the shallower aquifers by pumping
increases the difference in head between the aquifers.
Water from the deeper strata can then move upward into the shallow strata
in at least two different ways. First, penetration of the impermeable beds by
drilling, whereby both the shallow and deeper strata are interconnected through
the open well bore, will allow the movement of water from the deeper strata
(under higher artesian pressure) into the shallow strata (under lower pressure).
Second, the existence of faults, extending downward at least through the Ocala
group1, can provide a conduit through which the saline water can move upward.
Both these possibilities will be explored later in the report.

PURPOSE AND SCOPE

The problems of saline water movement into the shallow aquifers by
upward leakage from deep artesian sources is of considerable magnitude in Lee
County, where an estimated 2,500-3,000 deep artesian wells and test holes have
been drilled. The purpose of this report is to present the results of an analysis of
available geologic and hydrologic data for a small area in Lee County where
saline water from deep artesian sources has moved upward into several different
aquifers.
From an analysis of the available data, the authors attempt to define not
only the source of the highly mineralized water, but also to describe the
mechanism through which upward leakage occurs and the effects of intrusion on
water quality in each of the aquifers underlying the area. The effects of upward
leakage of saline water through the open bore hole of existing wells connecting
aquifers at depths of less than 300 feet and those which occur to depths of
about 1,000 feet is evident from the data presented herein and from similar
studies conducted in other parts of Lee County. However, the mechanism
responsible for upward leakage of saline water from an artesian aquifer below
1,500 feet into the aquifers between 400 and 1,000 feet is not well known and
can only be surmised from the available data. That such leakage does occur is
evident from the information presented herein.
1 The nomenclature used in this report conforms to that of the Bureau of Geology, Florida
Division of Interior Resources, Department of Natural Resources, and not necessarily to
that of the U.S. Geological Survey.

INFORMATION CIRCULAR NO. 75

ACKNOWLEDGMENTS

The authors are indebted to the landowners and residents of the McGregor
Isles area for providing information on wells and for permitting the logging and
other measurements on privately owned wells. The authors acknowledge the
assistance of local well drillers, particularly Joseph M. Maharrey, for providing
valuable data on the location and construction of wells. Test hole logs provided
by the Humble Oil and Refining Company and the Mobil Oil Corporation were
helpful in the identification of geologic formations.
The interest and continued support of the County Commissioners of Lee
County in the study described herein is greatly appreciated.

DESCRIPTION OF THE AREA

The McGregor Isles area is about 5 miles southwest of Fort Myers in Lee
County, Florida. The 9-square-mile area is bounded on the east by U.S. Highway
41 (Tamiami Trail) and on the west by the Caloosahatchee River (figs. 1, 2).
Drainage ditches or canals form the northern and southern limits.
McGregor Isles, where the problems of salt-water intrusion were first
recognized and studied in detail, is a small waterfront development on the
Caloosahatchee River. The name has been applied to the entire report area,
although there are several other subdivisions within the area.
Between 1940 and 1958 a large number of deep, flowing artesian wells
were drilled to provide water for irrigation during the winter growing season.
Much of the land was used for truck crops, flower farms, citrus groves, and plant
nurseries, and the use of ground water increased rapidly. Since 1958, urban
development has largely displaced agriculture and most of the deep artesian wells
have been abandoned. Most of the homes were supplied with water from small
diameter wells until recently, when public water-supply systems were installed.
As of 1970 the small diameter wells are used primarily for lawn irrigation,
although a few wells continue to provide water for domestic use.
To obtain data for this study, a fairly complete inventory of the deep,
artesian wells was made, together with a scattered sampling of the newer,
shallow domestic wells. Not all the domestic wells were visited because of the
large number of such wells. All the wells inventoried are listed in table 1 and
their location is shown on figure 2.

WELL NUMBERING SYSTEM

Each well plotted on figure 2 is identified by a number designating the

INFORMATION CIRCULAR NO. 75

ACKNOWLEDGMENTS

The authors are indebted to the landowners and residents of the McGregor
Isles area for providing information on wells and for permitting the logging and
other measurements on privately owned wells. The authors acknowledge the
assistance of local well drillers, particularly Joseph M. Maharrey, for providing
valuable data on the location and construction of wells. Test hole logs provided
by the Humble Oil and Refining Company and the Mobil Oil Corporation were
helpful in the identification of geologic formations.
The interest and continued support of the County Commissioners of Lee
County in the study described herein is greatly appreciated.

DESCRIPTION OF THE AREA

The McGregor Isles area is about 5 miles southwest of Fort Myers in Lee
County, Florida. The 9-square-mile area is bounded on the east by U.S. Highway
41 (Tamiami Trail) and on the west by the Caloosahatchee River (figs. 1, 2).
Drainage ditches or canals form the northern and southern limits.
McGregor Isles, where the problems of salt-water intrusion were first
recognized and studied in detail, is a small waterfront development on the
Caloosahatchee River. The name has been applied to the entire report area,
although there are several other subdivisions within the area.
Between 1940 and 1958 a large number of deep, flowing artesian wells
were drilled to provide water for irrigation during the winter growing season.
Much of the land was used for truck crops, flower farms, citrus groves, and plant
nurseries, and the use of ground water increased rapidly. Since 1958, urban
development has largely displaced agriculture and most of the deep artesian wells
have been abandoned. Most of the homes were supplied with water from small
diameter wells until recently, when public water-supply systems were installed.
As of 1970 the small diameter wells are used primarily for lawn irrigation,
although a few wells continue to provide water for domestic use.
To obtain data for this study, a fairly complete inventory of the deep,
artesian wells was made, together with a scattered sampling of the newer,
shallow domestic wells. Not all the domestic wells were visited because of the
large number of such wells. All the wells inventoried are listed in table 1 and
their location is shown on figure 2.

WELL NUMBERING SYSTEM

Each well plotted on figure 2 is identified by a number designating the

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C,

0

0

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LEE COUNTY
LOCATION MAP

GULF

INFORMATION CIRCULAR NO. 75

R 24 E

28 *

I 0.5 0 I MILE
APPROX.SCALE

Figure 2. Map showing the location of wells.

BUREAU OF GEOLOGY

section in which it is located followed by a number assigned sequentially within
each section. For example, well 21-5 is the fifth well inventoried in section 21;
well 15-5 is the fifth well inventoried in section 15. In section 16, where 46 wells
were inventoried, the well numbers range from 16-1 to 16-46.

DESCRIPTION OF AQUIFERS

The formations underlying McGregor Isles were identified by the use of
geophysical logs and other information obtained on existing wells. Some of these
data-chiefly geophysical logs and test-hole data-have been used in preparing
the composite geologic column shown in figure 3. The usage of formation names
that appear in figure 3 conforms generally to that of Pur and Vernon (1964, p.
43) except for the usage of the term Tampa Limestone, which conforms to that
of Cooke (1945, p. 111-121).
Using these data, 6 and possibly 7 different aquifers also were identified.
The stratigraphic positions of these aquifers are shown on figure 3. The names
which have been assigned the aquifers refer to the geologic formations in which
they occur, except the two uppermost, the water table and sandstone aquifers.
All the aquifers shown on figure 3 probably occur in other parts of Lee County.
The gamma ray log included in figure 3 serves chiefly to illustrate
characteristic features which make possible the identification of formations from
this log. The radiation intensity at any point in a well depends principally upon
the kinds and concentrations of radioactive materials in the formation
surrounding the well (Patten and Bennett, 1963, p. 45). In McGregor Isles, as
well as much of Florida, the highest radiation levels, and therefore the highest
peaks on gamma ray logs, are caused by the existence of phosphorite-bearing
zones. The phosphorite in these zones exhibits relatively high radioactivity
because it contains a small but significant percentage of uranium (Altschuler,
Clarke and Young, 1958). Clay, which is slightly radioactive due to the presence
of a radioactive isotope of potassium (potassium-40), is represented by lower
peaks. Clean sand, shell, or limestone is indicated on gamma ray logs by a low
level of radioactivity. Certain peaks on the logs, when matched with the
lithology of the rock units determined from test hole data, provide useful
correlation markers as shown in figure 3.

WATER-TABLE AQUIFER

The water-table aquifer consists of sand, sandy limestone, and calcareous

UppHerathorn

Low Hawthorn
aquifer

I Suwonnee aquiter

Aquifer?
Not pnetlroled by wells
in study area

Salt water
(Chrides 15P,00-20,000
man)

Figure 3. Geologic column showing lithology, aquifers, and typical gamma ray log of

formations underlying McGregor Isles.

BUREAU OF GEOLOGY

sandstone ranging in thickness from less than 10 feet to about 30 feet. Its base
almost everywhere is not more than 30 feet below land surface although some
localized shell beds which occur at greater depths are included as part of this
aquifer.
The aquifer, under atmospheric pressure, is recharged directly from
rainfall. Water levels rise in response to recharge by rainfall and fall in response
to discharge as base flow to streams, or by evapotranspiration or pumping.
Although the annual range in fluctuation of the water table has not been
established, the maximum range is estimated at 5 or 6 feet in areas of higher
elevation and only 2 or 3 feet in the low lying areas. Seasonally, water levels
normally are low in May or June near the end of the dry season and high in
September or October.

SANDSTONE AQUIFER

The sandstone aquifer consists of calcareous sandstone and loose quartz
sand, which in places grades downward into a sandy limestone. The aquifer
probably is present throughout the report area although it is nonproductive in
some places. Its thickness ranges from a few feet to a maximum of about 35
feet. The aquifer is separated from the overlying water-table aquifer by 50 feet
or more of green sandy clay. The stratum of green sandy clay underlies most of
the county, including McGregor Isles, and provides an effective barrier against
the downward movement of salt water from the Caloosahatchee River or from
tidal inland canals.
The sandstone aquifer is under slight artesian pressure and probably
receives recharge from rainfall in the eastern part of Lee County. Water levels in
wells tapping the sandstone aquifer fluctuate seasonally in about the same
manner as those tapping the water-table aquifer.

UPPER HAWTHORN AQUIFER

The Hawthorn Formation contains two well defined water-bearing zones
designated herein as the upper and lower Hawthorn aquifers. The upper
Hawthorn aquifer consists of a gray-white limestone containing numerous small
grains of black and brown phosphorite. This aquifer may be hydraulically
connected with the overlying sandstone aquifer at McGregor Isles, and, of
course, with underlying permeable formations containing saline water, for
without such continuity it would not have become contaminated. The upper

BUREAU OF GEOLOGY

section in which it is located followed by a number assigned sequentially within
each section. For example, well 21-5 is the fifth well inventoried in section 21;
well 15-5 is the fifth well inventoried in section 15. In section 16, where 46 wells
were inventoried, the well numbers range from 16-1 to 16-46.

DESCRIPTION OF AQUIFERS

The formations underlying McGregor Isles were identified by the use of
geophysical logs and other information obtained on existing wells. Some of these
data-chiefly geophysical logs and test-hole data-have been used in preparing
the composite geologic column shown in figure 3. The usage of formation names
that appear in figure 3 conforms generally to that of Pur and Vernon (1964, p.
43) except for the usage of the term Tampa Limestone, which conforms to that
of Cooke (1945, p. 111-121).
Using these data, 6 and possibly 7 different aquifers also were identified.
The stratigraphic positions of these aquifers are shown on figure 3. The names
which have been assigned the aquifers refer to the geologic formations in which
they occur, except the two uppermost, the water table and sandstone aquifers.
All the aquifers shown on figure 3 probably occur in other parts of Lee County.
The gamma ray log included in figure 3 serves chiefly to illustrate
characteristic features which make possible the identification of formations from
this log. The radiation intensity at any point in a well depends principally upon
the kinds and concentrations of radioactive materials in the formation
surrounding the well (Patten and Bennett, 1963, p. 45). In McGregor Isles, as
well as much of Florida, the highest radiation levels, and therefore the highest
peaks on gamma ray logs, are caused by the existence of phosphorite-bearing
zones. The phosphorite in these zones exhibits relatively high radioactivity
because it contains a small but significant percentage of uranium (Altschuler,
Clarke and Young, 1958). Clay, which is slightly radioactive due to the presence
of a radioactive isotope of potassium (potassium-40), is represented by lower
peaks. Clean sand, shell, or limestone is indicated on gamma ray logs by a low
level of radioactivity. Certain peaks on the logs, when matched with the
lithology of the rock units determined from test hole data, provide useful
correlation markers as shown in figure 3.

WATER-TABLE AQUIFER

The water-table aquifer consists of sand, sandy limestone, and calcareous

INFORMATION CIRCULAR NO. 75

Hawthorn aquifer is separated from the lower Hawthorn aquifer by relatively
impermeable clay and marly limestone, except where these are penetrated by
wells or displaced by faults.
The upper Hawthorn aquifer nearly everywhere in the report area is within
the depth range 100-300 feet below land surface. The aquifer is under artesian
pressure. Records from observation wells in less highly developed parts of the
county indicate that under natural conditions the water level in this aquifer at
McGregor Isles may have reached a maximum altitude of about 20-25 feet above
mean sea level or about 15 feet above land surface. As of 1970, because of
pumping from the aquifer, water levels are considerably lower. For example,
records from observation well 14-1 (fig. 2 and table 1) show that the highest
water level recorded since October 1968 was about 6 feet below land surface,
which represents a decline of about 20 feet from pre-development water levels at
that location. In 1969, the highest water level recorded in well 14-1 was about
10 feet below land surface, indicating a further lowering of 4 feet due to
increased pumping. The water level in this well is affected by the pumping of
nearby large-capacity wells (144 through 14-12). This decline is similar to that
which occurred at Cape Coral and adjacent areas over the same period. This
trend of declining water levels will continue as pumping draft increases. Wells 6
inches or more in diameter yield 100-200 gpm (gallons per minute); those 2-3
inches in diameter yield 10-30 gpm.
The upper Hawthorn aquifer is the principal source of water for public
water systems, domestic, and lawn irrigation uses in western Lee County. It is
presently (1970) used as a source of supply for water systems which serve Cape
Coral, Pine Island, Fort Myers Beach, and other offshore islands, and for
thousands of small diameter domestic wells. Maximum pumpage occurs during
the winter and spring, coinciding with the period of minimum recharge. An
estimated 6 mgd (million gallons per day) were withdrawn from the aquifer for
public-water supply during the period of maximum demand in 1969.

LOWER HAWTHORN AQUIFER

The lower Hawthorn aquifer as defined herein, includes the lower part of
the Hawthorn Formation and the upper part of the Tampa Limestone. This
limestone aquifer consists of sediments similar in appearance to those in the
upper Hawthorn aquifer.
Confined above and below by clay and marly limestone this aquifer has
sufficient permeability and is under sufficient artesian pressure to provide
300-500 gpm to large diameter wells by natural flow. Both the artesian pressure
and flow rates vary from well to well. This variation is related to differences in
construction of individual wells and in hydraulic properties of the aquifer
penetrated by the well. Because wells that tap this aquifer nearly always are

BUREAU OF GEOLOGY

hydraulically connected to the upper Hawthorn aquifer through the uncased
section of the bore hole, the pressure and discharge measurements usually
represent a composite of conditions in both aquifers. On the basis of
measurements made in the eastern part of Lee County, where the artesian head
within the aquifer is about 50 feet above mean sea level, it is estimated that
under natural conditions at McGregor Isles the artesian head may have been
30-35 feet above mean sea level. Earlier records of wells at the McGregor Isles
tend to confirm this estimate: In well 16-4 in October 1957, the artesian head
was about 32 feet above mean sea level; in well 16-9 in February 1934, the head
was about 37 feet above. The highest water level measured in recent years was at
well 23-3 where, in April 1969, the artesian head was 27 feet above. A review of
all available records indicates that the artesian head within the aquifer at
McGregor Isles has fallen about 10-15 feet.
Only small quantities of water are withdrawn from the lower Hawthorn
aquifer at the present time (1970). However, water is discharged from this
aquifer by leakage upward from the uncased portion of wells. The amount of
leakage in individual wells, as measured by geophysical logging methods, ranged
from about 30 gpm to nearly 100 gpm. Flows less than 30 gpm could not be
measured reliably with the instruments used, but it may be assumed that such
flow does occur in most wells penetrating the aquifer. Assuming an average
leakage rate of only 30 gpm per well, and that at McGregor Isles 40 wells are
open to both the upper and lower Hawthorn aquifers, about 1.7 mgd (million
gallons per day) is discharged from the lower aquifer as vertical leakage. The
quantity of water discharged from the lower aquifer either through wells or
along faults probably will increase as the head in the shallower aquifers is
lowered by pumping.

SUWANNEE AQUIFER

The Suwannee aquifer as the term is used herein, consists of a permeable
zone in the upper part of the Suwannee Limestone. As indicated in figure 3, the
top of the Suwannee Limestone is readily determined from gamma ray logs by
the decrease in radioactivity, and from test-hole data by the absence of
phosphorite. Relatively impermeable beds above and below separate the
Suwannee aquifer from the lower Hawthorn aquifer and those occurring at
greater depths.
Flow rates up to 400 gpm may be obtained from large-diameter wells
drilled to the Suwannee aquifer, although well yields at McGregor Isles are
generally lower. The low discharge rate of 30 gpm measured from well 16-14,
where no leakage to upper formations was apparent, indicates that this well
penetrated a zone of low permeability within one or more of the aquifers
penetrated.

INFORMATION CIRCULAR NO. 75

Under natural conditions, the artesian head within the aquifer probably
ranged from 35 to 40 feet above mean sea level at McGregor Isles. The level in
well 16-14 in September 1944 was 36 feet above mean sea level, 29 feet above
land surface. In February 1967, the head in this well was 23 feet above mean sea
level, indicating a reduction in artesian head of 13 feet. This reduction probably
has not occurred throughout the aquifer; in April 1969 the level in well 10-2,
about a mile distant, was 30 feet above mean sea level.
Wells in the Suwannee aquifer usually are hydraulically connected to both
the lower and upper Hawthorn aquifers through the uncased sections of the well
bores. The distribution of artesian pressure within the well bore is such that
water can move upward from the Suwannee aquifer into the overlying aquifers.
Only about 18 wells have been drilled to the Suwannee aquifer in the
report area, less than half as many as have been drilled to the lower Hawthorn
aquifer and only a few are presently used (1970) for irrigation.

DEEPER AQUIFERS

Little is known about the water-bearing properties of formations under-
lying the Suwannee Limestone. The deepest well in the report area, number
16-14, drilled to a depth of 1,106 feet, reportedly did not penetrate
water-bearing zones beneath the Suwannee aquifer. Well 15-11, a 1,360-foot test
well, penetrated limestone of the Ocala Group at a depth of 1,150 feet. This well
was subsequently plugged back to 590 feet, and no information is available
concerning the possible existence of water-bearing zones between 590-1,360
feet. Records of water wells in nearby areas indicate that a water-bearing zone is
present within the upper 50-100 feet of the Ocala Group. These records also
suggest that water from this zone is more mineralized than water from the
Suwannee aquifer. Data concerning the water-bearing properties of still deeper
aquifers was obtained principally from geophysical logs and drillers reports of
nearby oil exploratory wells. Geophysical logs of two wells drilled just beyond
the eastern boundary of the study area show salt water present below a depth of
1,570 feet in the northernmost well and 1,500 feet in the southernmost well.
The electric log of a well about 5 miles southeast of McGregor Isles
(outside the report area) shows salt water present at a depth of 1,570 feet.
Strong flows of salt water have been reported from depths ranging from 1,518
feet to 1,707 feet in other parts of the county, and salt water is flowing (1970)
from a well 1,641 feet deep at Hot Springs (fig. 1) in Charlotte County, 18 miles
north of McGregor Isles. On October 17, 1957, its shut-in pressure was 39 feet
above mean sea level.
From these data it is generally concluded that water from these deeper
aquifers, particularly at depths greater than about 1,500 feet, is highly

BUREAU OF GEOLOGY

mineralized and unsuitable for most purposes. The artesian pressure within these
aquifers probably is higher than in any of the overlying aquifers under natural
conditions, and considerably higher than in those aquifers where the pressure has
been lowered by pumping.

EVIDENCE OF FAULTING

A study of gamma ray logs obtained during the study shows vertical
offsetting of beds. The offset is apparently caused by a series of faults. Figure 4
shows a geologic section based on correlation of distinctive features on the
gamma ray logs. One particularly distinctive peak which occurs on all the gamma
ray logs has been selected as a point of correlation between wells to show the
presence of faults. This peak, herein referred to as the gamma ray correlation
marker, represents the uppermost bed identifiable on the logs which shows
substantial displacement caused by faulting. This marker is indicated by a dotted
line in figure 4.
The altitude of gamma ray correlation marker, the approximate location
of faults and of the geologic section are shown in figure 5.
As shown in figures 4 and 5, the vertical displacement of comparable beds
ranges from about 50 to 110 feet. The depth to which the faults extend has not
been determined. It is assumed that the faults extend at least through the Ocala
Group, and probably deeper. The available data seem to indicate that most, but
not all, of the displacement occurred after the unit represented by the gamma
ray correlation marker was deposited, and prior to deposition of the upper part
of the Hawthorn Formation. Displacement of beds above the gamma ray
correlation marker is not so obvious from an examination of the logs. The
configuration of the Caloosahatchee River shoreline in the vicinity of the
northeast corer of section 17, and the alignment of a tributary to Whisky Creek
near the center of section 15 are suggestive of fault controlled features and may
indicate that some displacement of near-surface beds has occurred in compar-
atively recent times. Tanner (1964, p. 41) notes a fault in Lee County ...
active in the last 10,000 years, responsible for offset in the coast line." Tanner,
in the reference cited above, suggests the presence of two shear planes in south
Florida, oriented approximately N. 50 degrees E., and N. 70 degrees W. This
orientation, within a few degrees, is identical with that of the faults in McGregor
Isles.

WATER QUALITY AND THE EFFECTS OF SALINE-WATER INTRUSION

Complete or partial chemical analyses have been made on water from 15
wells in McGregor Isles as summarized in table 2.

A A'
28-1 22-1 22-2 16-8 16-7 16-11 16-14

PER MIOCENE BNEiR YOUNG BEDS
................ ... ." "

S 400- -400

w00

-jr

S ,00- EXPLANATION r 200I

'FAULT
UP THROWN SIDE UI O DOWN THROWN SIDE
S S T. GAMMA RAY CORRELATION MARKER
M VERTICAL EXASPERATION X6.6
APPROXSCALE

EXPLANATION
*2 Well and well number
(-250o Altitude of gamma ray correlation marker
Mean sea level datum

.U Upthrown side
D Douthrownside Fault, dashed where inferred
A, Downthrown side

0 0.5
APPRL SCALE

Figure 5. Map of McGregor Isles showing the approximate location of faults.

INFORMATION CIRCULAR NO. 75

Also included for purposes of comparison is a chemical analysis of water
from a well at Hot Springs in Charlotte County, about 18 miles northwest of
McGregor Isles (fig. 1). Additional temperature and chloride measurements for
wells are included in table 1. The analyses in table 2 are presented in descending
order of depth of the aquifers. Within each aquifer, the analyses are arranged to
show the increasing effects of saline-water intrusion.
Based on water quality data from the 1,641-foot well at Hot Springs, and
other data from wells near the study area, the authors believe that the primary
source of the saline water causing deterioration in water quality in the lower
Hawthorn aquifer is an artesian aquifer at a depth of 1,500-1,700 feet. Although
the chemical characteristics of the water from this aquifer have not been
determined in McGregor Isles, the analysis given for Hot Springs (table 2)
probably is generally representative of its water quality. The water is highly
mineralized, containing 34,000 mg/1 of dissolved solids and 18,700 mg/1 of
chloride. The water temperature in this aquifer, as measured at Hot Springs, was
960F.

LOWER HAWTHORN AQUIFER

Intrusion of highly saline water has caused deterioration in water quality
within the lower Hawthorn aquifer. The chemical character of water contained
in the unaffected part of the aquifer is generally represented by the analysis for
well 22-1 (table 2) where the chloride content was 560 mg/1. The analyses of
water from wells 21-3, 22-8, 16-4, and 16-7 show the progressively increasing
effects of the intruding water on the aquifer, with a range in chloride
concentration from 1,490 mg/1 to 10,200 mg/1. The greatest chloride
concentration determined from wells in the lower Hawthorn aquifer was 15,200
mg/1 for well 16-45 (table 1). It is interesting to note from table 3 that the

Table 3.-Comparison of the arithmetic mean of chemical constituents for wells 22-1 and Hot Springs, with
the chemical analysis for well 16-7. (Chemical constituents in milligrams per liter).
1/ 2/
Wells SiO2 Ca Mg Na K HCO3 SO4 Ci F DS- Sp.C
Average for 22-1 12 353 577 5366 202 168 1468 9630 1.7 17,730 27,370
and Hot Springs
16-7 14 428 640 5620 188 164 1370 10,200 1.7 18,600 29,500
/ DS = Sum of determined constituents
2/ Sp.C = specific conductance, micromhos at 25"C

BUREAU OF GEOLOGY

average of the analysis for well 22-1, in the unaffected part of the aquifer, and
the analysis for Hot Springs, is much like the analysis shown for well 16-7, in the
affected part of the aquifer. It is not to be expected that observed and
theoretical mixtures will be exactly the same because of chemical reactions
which can take place when waters of different origin become mixed within the
aquifer (Hem, 1959, p. 227). However, the comparison is a valid indicator of the
source and effects of the intruding saline water.
The chloride concentration in water is a reliable indicator of changes in
water quality and is readily measured with field or laboratory equipment. The
chloride content of water from most wells in McGregor Isles is indicated in table
I. A map showing the chloride content of water from wells in the lower
Hawthorn aquifer is shown in figure 6a. The lines of equal chloride content show
that the intruding water enters the aquifer in the central part of section 16 and
spreads laterally in the aquifer. The elongated paths of spreading toward the
southeast and southwest may be due to the permeable zones along the fault
planes. The effects of the intruding water seemingly are largely confined to an
area bounded by components of the fault system.
Another indicator of changes occurring within an aquifer is water
temperature. Ground-water temperatures generally increase with depth. From
the data included in table 1, water temperatures ranged from 74"F in the
water-table aquifer at a depth of about 20 feet, to 870F in the Suwannee aquifer
at a depth of about 900 feet. This represents an increase of about loF for each
additional 70 feet of depth. At this rate of increase, the water temperature at
1,600 feet would be about 100F higher than in the Suwannee aquifer, or about
97"F. The water temperature from the Hot Springs well in Charlotte County,
considered to be from about this depth, was 960F.
Significant upward leakage from this deep artesian aquifer would cause
some change in the normal temperature distribution within the intruded aquifer.
Figure 6b shows the distribution of water temperature in the lower Hawthorn
aquifer which clearly shows the effects of intrusion from this deep artesian
source. The normal water temperature in this aquifer as determined in this and
in other parts of the county was 820F. The highest temperatures occur in the
vicinity of wells 16-7 and 16-45 thus indicating, as does the chloride data in
figure 6a that the intruding water enters the aquifer in the central part of section
16. From there, the temperatures decrease laterally to normal or near normal
values. As in the case of chloride shown in figure 6a, the anamalous water
temperatures are largely confined to the area bounded by the NW-SE trending
components of the fault system, and the pattern of spread is elongated to the
southeast and southwest. The higher temperature strongly suggests that the
source of intruding water is a deep artesian aquifer below 1,500 feet.
Most of the chloride and temperature data shown on figures 6a and 6b
were obtained during the period 1967-69 and should not be considered

R24E / EXPLANATION
S(740)
86 o y\ *s2 Well and well number
8 900) (9 oo>Chloride content in milligrams per liter
5l .0 ----1500 '-'Line of equal chloride concentration in

f Up thrown side Fault,dashed where
II 1 P0oo Down thrown side inferred z
1o5 MILE
Ea S11420 } /,\APPROX.SCALE
1/ ,/ ago I
6 ( -19 9) D ) -.
00

SU Ooo 0) '. PARKWAY

I ri co yReso^4u '\ Asso; *^
oi'-0
I -*'(960) 0760) C
00. Z 0) oh?40001o ) (1 )

5! 86' 06/ / CP6 0) Io
2 .(1000 21 CYPRESS LAKE 22 Q -DRIVE 23
*074
0 0540)

r.A- Mr. INY IJ IIJ
*2 Well and well number 00
(teI Wter temperature in degrees Fahrenheit
5 to I I degrees above normal
^ 2 to 4 degrees above normal
/ I degree above normal
Upthrown side Fault, dashed where inferred
0 05 I MILE
APPRoX. SCALE ~

14

0
PARKWAY

/ -,

INFORMATION CIRCULAR NO. 75

representative of a single point in time. Resampling of several wells during this
period showed only small changes in water quality. However, this does not imply
that static conditions exist within the aquifer, only that changes probably occur
at a relatively slow rate. The long-term changes in the chloride content of water
from selected wells in the lower Hawthorn aquifer are shown in figure 7. Wells

Figure 7. Graph showing changes in chloride content of water from the lower Hawthorn aqui-
fer, 1946-68.
16-1 and 16-9 which are near the point where saline water enters the aquifer
have shown the greatest increase in chloride content. As indicated by the initial
chloride measurement on well 16-1 (1,520 mg/1), some change in water quality
in the aquifer had occurred prior to 1946. Well 22-5 showed a progressive
increase in chlorides since 1950, although this well is more than a mile from the
principal area of intrusion. In contrast, well 22-1, which is south of the fault
system, has shown little change in chloride content since 1950.
The advancing front of saline-water to the southeast is evident from
samples of water from wells 22-8 and 23-3 near the 1,500 mg/1 chloride line
shown on figure 6a. The chloride content of water from these wells increased
from 1,550 and 760 mg/1 in June 1967, to 1,940 and 920 mg/1 in May 1970.

UPPER HAWTHORN AQUIFER

The quality of water from the upper Hawthorn aquifer is generally good
except where affected by intrusion of saline water. The chemical analysis for

BUREAU OF GEOLOGY

well 16-35 (table 2), is generally representative of water quality in the
unaffected part of the aquifer. As shown by this analysis, the dissolved solids
content was 426 mg/1 with chloride content of 170 mg/1. Deterioration in
water quality is indicated by the analysis for well 16-23 in table 2, where the
dissolved solids were 3,470 mg/1 and the chloride wat 1,940 mg/1.
The chloride content of water from wells at McGregor Isles is shown on
figure 8. Changes in water quality in the upper Hawthorn aquifer are greatest
near wells drilled to the lower Hawthorn aquifer. For example, water from wells
16-20 through 16-25, all drilled into the upper Hawthorn aquifer, ranges in
chloride content from 500 to 2,160 mg/1. Chloride content generally decreases
with distance from well 16-2, which taps both the upper and lower aquifer.
Similar conditions exist near well 16-4 as indicated by the chloride content of
water from wells 16-18, 16-36, 16-42, and 16-43, which ranges from 440 to
1,560 mg/1.
Flowmeter surveys in numerous wells confirmed that water was flowing
from the lower Hawthorn and Suwannee aquifers into the upper Hawthorn
aquifer. Internal flows of nearly 100 gpm were measured in wells penetrating the
lower Hawthorn and deeper aquifers. The water from the lower aquifers was
entering the upper Hawthorn aquifer through the uncased part of the borehole.
The salinity of water from the upper Hawthorn aquifer is increasing in
some parts of the area. Water from well 16-20, formerly used for domestic
purposes, had a reported chloride content of 800 mg/1. In June 1969, chloride
content had increased to 2,160 mg/l, and in January 1970, to 3,050 mg/1. The
chloride content of water from well 16-36 increased from 715 mg/1 on October
31, 1967 to 1,100 mg/1 on June 5, 1969. In other parts of the area, attempts to
obtain usable water from the upper Hawthorn aquifer have been abandoned
because the water is too saline for use.
The continued spread of saline water within the upper Hawthorn aquifer
may cause a substantial change in the quality of water from wells 14-3 through
14-12 and 23-4 through 23-6 which supply water to Fort Myers Beach and
adjacent areas. The chloride content of water from these wells ranged from 81 to
183 mg/1 when drilled. An increase in chlorides has been noted in wells 14-8,
14-9, and 14-10. In well 14-8, the chloride content increased from 141 mg/1,
July 1967, to 376 mg/1, June 1970. Similarly in well 14-10, the chloride
increased from 105 mg/l, August 1967, to 224 mg/1, June 1970. The increase in
chloride in well 14-9 from 81 to 162 mg/1 from July 1967 to June 1970,
although of lesser magnitude, is equally significant in indicating the potential
changes which may occur.

OTHER AQUIFERS

The water-table aquifer normally contains water of relatively good quality

INFORMATION CIRCULAR NO. 75

cALOOSAc0'r

COLLEGE

2600

EXPLANATION
4 WELL NUMBER
4650 CHLORIDE CONTENT (mg/l)

) Well drilledtothelowerHawthorn
or Suwonnee aquifer.
Well drilled tothe upper Hawthorn aquifer.

e Well drilled to the sandstone aquifer.

o Well drilled to the water-table aquifer.

Figure 8. Map showing the chloride content of water from wells in McGregor Isles, 1967-69.

10,200

15 0
5750

7500

300 9 300 600 900 1200 FEET

454.2

BUREAU OF GEOLOGY

as indicated by the analysis for well 21-1 in table 2, which shows a total
dissolved solids content of 477 mg/1 and chloride content of only 96 mg/1. One
of the most objectionable characteristics of water from this aquifer is the high
concentration of iron. Although no analysis for iron has been made in the report
area, the typical metallic taste imparted to water by iron and staining of surfaces
sprayed with the water can be observed in many places. The water may also
contain organic compounds which cause taste or odor problems, or discoloration
as indicated by the color value of 30 in the analysis for well 21-1. Salt water has
entered this aquifer at places as shown by the analysis for well 16-15 (fig. 8 and
table 2) where the chloride content was 5,750 mg/1. These wells probably were
affected by water from well 16-7 (chloride content 10,200 mg/1) which has
been flowing uncontrolled for years into a ditch from which it percolates
downward to the water table. Saline water intrusion into the water-table aquifer
probably is general in areas immediately bordering the Caloosahatchee River,
and along the tidal reaches of surface streams and canals as a result of inland
movement of salt water from the river during the dry season.
Chloride content of water from the sandstone aquifer at McGregor Isles
and analyses of water from this aquifer in the eastern part of Lee County suggest
that the chemical characteristics are similar to water contained in the unaffected
part of the upper Hawthorn aquifer. Inasmuch as the two aquifers are
hydraulicaly connected to some extent at McGregor Isles, it is assumed that the
water quality is similar. However, saline-water intrusion into the sandstone
aquifer apparently has not progressed as rapidly as in the upper Hawthorn
aquifer, probably because all the deeper wells are cased through this aquifer. For
example, the chloride content of water from well 16-33 (sandstone aquifer) was
240 mg/1, whereas water from well 16-32 (upper Hawthorn aquifer) about 50
feet away, contained 1,100 mg/1 of chloride (see fig. 8). Similarly, wells 16-31
and 16-34, both tapping the sandstone aquifer, yield water containing 400 mg/1
chloride or less, even though wells nearby, tapping the lower Hawthorn aquifer
yield water whose chloride content is more than 3,500 mg/1. Locally, water in
the sandstone aquifer is less saline than that from the underlying upper
Hawthorn aquifer. This suggests that water of better quality may be developed
from the sandstone aquifer in places where water in the upper Hawthorn aquifer
is too saline for use. However, a significant increase in use of water from the
sandstone aquifer might cause an increase in leakage from the deeper aquifers,
and result in a progressive deterioration in its chemical quality.
The Suwannee aquifer contains water generally similar, although some-
what more highly mineralized, than that contained in the unaffected part of the
lower Hawthorn aquifer as shown by the analyses for wells 22-2 and 16-14 in
table 2, where the total dissolved solids range from 1,720 to 1,790 mg/1 and the
chloride concentration is about 700 mg/1. Apparently little intrusion of saline
water has occurred within this aquifer although, as shown in figure 3, it lies

INFORMATION CIRCULAR NO. 75

between the deep salt-water source and the highly saline lower Hawthorn
aquifer. Chloride data show that some salt invasion of this aquifer has occurred
in the vicinity of wells 16-11 and 16-45, but that the intruding water has not
spread beyond the immediate vicinity of these wells. The artesian pressure
within the Suwannee aquifer may remain sufficiently high to retard movement
of saline water, or the aquifer may contain zones of relatively low permeability
adjacent to avenues of upward leakage.
Wells which yield water from both the lower Hawthorn and Suwannee
aquifers show evidence of saline-water intrusion as indicated by the analyses for
wells 15-8 and 17-1, with chloride ranging from 1,325 to 2,100 mg/1. An
interesting feature of these multiple aquifer wells concerns the changes in water
quality which occur when the wells are allowed to discharge after they have been
inactive for some time. This phenomenon is illustrated in figure 9, from a test on
well 17-1, April 15, 1969. This well had been inactive for about a week prior to
the test. As shown on figure 9, the chloride content of the water remained
relatively constant at 840-860mg/1 for 10 minutes, then increased progressively
to about 1,600 mg/1 after 2 hours, and to 1,930 mg/1 after about 15 hours of
discharge. The flow rate was about 400 gpm. The chloride content continued to
increase over a period of about 3 days to a maximum of 2,060 mg/1.
Apparently this phenomenon is related to differences in water quality and
artesian pressure between the lower Hawthorn and Suwannee aquifers. During
the period when the well is closed, water under higher artesian pressure moves

2000

,.IBOO ,/---
i--

0-160C

J 1400

m 1200

800
1 10 100 1000
TIME, MINUTES SINCE FLOW BEGAN, APRIL 15,1969
Figure 9. Graph showing changes in chloride content of water from well 17-1 on April 15, 1969.

BUREAU OF GEOLOGY

upward from the Suwannee through the open well bore into the lower Hawthorn
aquifer. In this well it is believed that yield from the upper Hawthorn may be
minor. Under these conditions, the intruding water is of better quality, resulting
in a reduction in the chloride content of water in the lower Hawthorn aquifer
around the well. When the well is opened, the discharge consists largely of
Suwannee water from both aquifers, but with continued discharge, the
Suwannee water that had entered the lower Hawthorn aquifer becomes
exhausted and the contribution from the intruded lower Hawthorn aquifer
increases resulting in a progressive increase in chloride content. Flowmeter and
water-resistivity logs, run after the well had been flowing long enough for the
chloride content of the well discharge to stabilize, indicated that the Suwannee
aquifer was contributing about 10 percent of the flow to the well, of water
containing about 800 mg/1 of chloride. The lower Hawthorn aquifer trans-
missivity in the vicinity of well 17-1 doubtless is higher than either the
Suwannee or upper Hawthorn aquifer transmissivity so that it yields water to a
discharging well more freely than the other aquifers. Consequently, most of the
water discharged from the well comes from the lower Hawthorn even though the
Suwannee may have the higher head.

MECHANICS OF INTRUSION

The mechanism of intrusion responsible for the chloride concentration in
the lower Hawthorn aquifer has not been positively identified because of the
several possibilities that exist. Two hypotheses are here described to explain the
apparent hydraulic connection between this aquifer and the salt-water aquifer or
aquifers occurring at greater depths, for example, those of the Ocala. The first
hypothesis concerns the upward movement of saline water in a deep well or test
hole which provides a connection between the aquifers. Essentially, this
represents a point source of saline water, or where several wells are involved,
would represent several point sources. The saline water from the deeper aquifer,
under higher artesian pressure, would enter the lower Hawthorn aquifer at these
points and spread out laterally through the aquifer. The increase in chloride in
the lower Hawthorn aquifer would be greatest near these points and would
decrease with increased distance from these points. The lateral spread of saline
water would be controlled by pressure gradients, permeability distribution,
subsurface barriers, and other related factors.
This hypothesis would be consistent with most of the observed facts. The
date of drilling of such wells, or test holes would mark the beginning of the
intrusion, probably between 1940 and 1945. Although a detailed study has
failed to disclose any well or test hole that could be the source of the saline
water, this does not preclude the possibility that they exist although there is no

INFORMATION CIRCULAR NO. 75

longer any surface evidence of the well or wells. As mentioned earlier, less than
half as many wells tap the Suwannee aquifer than tap the lower Hawthorn, and
it has been shown that although Suwannee water can and has intruded the lower
Hawthorn, it has resulted in a freshening, rather than a deterioration of the
water in the lower Hawthorn. In summary, the point-source hypothesis appears
tenable in explaining a mechanism for the upward migration of water from the
lower to the upper Hawthorn aquifer. It does not, as implied above, provide a
realistic mechanism whereby the lower Hawthorn has become contaminated.
A second, and more tenable, hypothesis concerns the upward leakage of
saline water along the fault or fracture system which has been shown to exist in
the report area, and which can provide a hydraulic connection between the
lower Hawthorn aquifer and deeper aquifers containing saline water. It is
postulated that faulting has created paths of high vertical permeability through
what would otherwise be relatively impermeable sediments. Under these
conditions upward leakage could occur resulting in what may be considered as
point or line sources of saline water intrusion into the lower Hawthorn aquifer.
This process apparently occurs elsewhere in Florida. At Warm Mineral Springs in
Sarasota County, about 35 miles northwest of the report area, upward leakage of
saline water occurs along a fracture system to emerge at the surface as a spring
(S. R. Windham, oral commun., 1970). In St. Johns County, in northeastern
Florida, Bermes and others (1963, p. 88) found a chloride anomaly that he
ascribed to the upward leakage of water along a fault.
This hypothesis is consistent, as is the first one, with the fact that the
beginning of the intrusion of high-chloride water coincides with the period of
increased use of water from the lower Hawthorn aquifer, about 1940-45. The
lowering of artesian pressure within the aquifer increased the difference in head
between the lower Hawthorn and the saline-water aquifer, resulting in an
increase in upward leakage. Upon entering the lower Hawthorn aquifer, the
saline water was of high concentration near the points of entry and moved
laterally through the formation.
Additional information will be required to prove the validity of either of
the hypotheses described. In either case, saline water may enter the lower
Hawthorn aquifer in the vicinity of wells 16-7 and 1645 inasmuch as water from
these wells show the greatest effects of intrusion. The quality of the water from
well 16-45 (chloride 15,200 mg/1 and temperature 93F), indicates a more direct
hydraulic connection with the deep saline-water aquifer near this well site.
Apparent offset of beds, as determined from a study of the gamma ray logs,
suggests that well 16-45 is near a fault plane, which could be a zone of greater
vertical permeability. Zones of greater permeability developed along fault planes
may also account for the pattern of spreading of the intruding water as indicated
on figures 6a and 6b. The existence of unmapped faults could affect the water
quality, as well.

BUREAU OF GEOLOGY

The uncased wells constructed to the lower Hawthorn and Suwannee
aquifers provide a conduit through which water can flow to the upper Hawthorn
aquifer. Typical well construction in western Lee County includes the
installation of well casing to the top of the limestone that forms the uppermost
part of the upper Hawthorn aquifer. By seating the casing in this limestone, the
overlying sand is prevented from entering the well. After seating the casing, an
open hole is drilled until sufficient water is obtained for the required purpose.
Thus, wells drilled to the Suwannee aquifer are also connected to the upper and
lower Hawthorn aquifers through the open bore hole. Those drilled to the lower
Hawthorn are also connected to the upper Hawthorn aquifer.
Each well drilled to the deeper aquifers is a potential source of saline water
leakage to the upper Hawthorn aquifer. Where a large number of these wells
exist, the effects of a single well may be obscured. In the case of a somewhat
isolated well (16-2), the effects have been noted for a distance of about 1,000
feet.
The sandstone aquifer is not ordinarily directly connected to the deeper
aquifers through open well bores. As previously indicated, in constructing wells
to the Hawthorn aquifers, the casings usually are seated in limestone beneath the
sandstone aquifer to prevent sand problems. Except for faulty construction,
therefore, transfer of saline water to the sandstone apparently is the result of
upward leakage from the part of the upper Hawthorn aquifer through the thin
beds which separate them. (See also p. 13.) At places where the upper Hawthorn
aquifer contains salty water, water of better quality may be obtained from the
sandstone aquifer, but progressive changes in water quality may. occur with
increased use of water from the aquifer.
Water quality changes in the water-table aquifer may occur as a result of
intrusion of sea water from surface-water sources, or from the discharge of saline
water from artesian aquifers through wells. In McGregor Isles, deterioration in
water quality from the water-table aquifer results primarily from the discharge
or surface storage of saline water from the artesian aquifers. Where the water is
discharged into drainage or irrigation ditches, the effects may be noted for
considerable distances from the source. Discharge into a pond or other storage
reservoir would similarly affect the water-table aquifer in the surrounding area.
The lateral spread of saline water probably is accelerated during the winter and
spring when the water table reaches a seasonal low. Some dilution probably
occurs during the period of heavy rainfall, although it is unlikely that the saline
water is completely flushed from the water-table aquifer.

CONTROL PROCEDURES

Procedures for eliminating the intrusion of saline water from the artesian
aquifer below 1,500 feet into the lower Hawthorn and Suwannee aquifers

INFORMATION CIRCULAR NO. 75

cannot be developed without additional detailed information to identify the
mechanism of intrusion. However, the effects could be minimized-that is, the
transfer of saline water could be slowed somewhat-if the artesian pressure
within the lower Hawthorn and Suwannee aquifers was allowed to increase,
particularly if heads could be established comparable to those which existed
prior to the extensive development of water supplies from these aquifers. Placing
cement plugs in individual wells between the upper and lower Hawthorn aquifers
would prevent upward movement of saline water through the well into the upper
Hawthorn. It would also prevent draft from the lower Hawthorn and Suwannee
so that their potentiometric heads would have opportunity to recover. However,
this increase in head may force water in the Suwannee and lower Hawthorn to
the faults, from which it could continue its upward migration. In those parts of
the report area, where the saline water may be coming into the Suwannee,
plugging wells just below the lower Hawthorn aquifer doubtless would be at least
partially effective. To be effective, all deep wells in the McGregor Isles and
surrounding area would have to be plugged in this way. The proper positioning
of these cement plugs can be readily determined from geophysical logs, many of
which are available for wells at McGregor Isles.
By plugging the deep artesian wells where indicated, some of the salt water
now entering the upper Hawthorn, sandstone, and water table aquifers might be
eliminated. If these wells were plugged the salt water eventually might be diluted
or flushed from the aquifers above 300 feet. In some cases, improvement in
quality of water from wells in the water table, sandstone, or upper Hawthorn
aquifers may be obtained by plugging wells which have been identified as
localized sources of saline water. The proper positioning of plugs is important,
since plugging a well improperly could be a waste of time and, at most, could be
harmful: capping a well at the surface in no way diminishes the effects of the
intruding water into the upper Hawthorn or sandstone aquifers, and may
actually exacerbate the problem.
A monitoring program could determine the effectiveness of well plugging
and obtain information for the correction of similar problems in other areas.

SUMMARY AND CONCLUSIONS

There are six and possibly seven aquifers within the uppermost 1,700 feet
of sediments underlying McGregor Isles. Under natural conditions the artesian
pressure, temperature, and mineralization of the water generally increases with
depth. The aquifers which occur above depths of 300 feet normally contain
water suitable for public water supplies. The aquifers between 300 feet and
1,000 feet contain water that is too highly mineralized for public supplies, but at
some places, may be suitable for irrigation. The aquifer which occurs at depths

BUREAU OF GEOLOGY

upward from the Suwannee through the open well bore into the lower Hawthorn
aquifer. In this well it is believed that yield from the upper Hawthorn may be
minor. Under these conditions, the intruding water is of better quality, resulting
in a reduction in the chloride content of water in the lower Hawthorn aquifer
around the well. When the well is opened, the discharge consists largely of
Suwannee water from both aquifers, but with continued discharge, the
Suwannee water that had entered the lower Hawthorn aquifer becomes
exhausted and the contribution from the intruded lower Hawthorn aquifer
increases resulting in a progressive increase in chloride content. Flowmeter and
water-resistivity logs, run after the well had been flowing long enough for the
chloride content of the well discharge to stabilize, indicated that the Suwannee
aquifer was contributing about 10 percent of the flow to the well, of water
containing about 800 mg/1 of chloride. The lower Hawthorn aquifer trans-
missivity in the vicinity of well 17-1 doubtless is higher than either the
Suwannee or upper Hawthorn aquifer transmissivity so that it yields water to a
discharging well more freely than the other aquifers. Consequently, most of the
water discharged from the well comes from the lower Hawthorn even though the
Suwannee may have the higher head.

MECHANICS OF INTRUSION

The mechanism of intrusion responsible for the chloride concentration in
the lower Hawthorn aquifer has not been positively identified because of the
several possibilities that exist. Two hypotheses are here described to explain the
apparent hydraulic connection between this aquifer and the salt-water aquifer or
aquifers occurring at greater depths, for example, those of the Ocala. The first
hypothesis concerns the upward movement of saline water in a deep well or test
hole which provides a connection between the aquifers. Essentially, this
represents a point source of saline water, or where several wells are involved,
would represent several point sources. The saline water from the deeper aquifer,
under higher artesian pressure, would enter the lower Hawthorn aquifer at these
points and spread out laterally through the aquifer. The increase in chloride in
the lower Hawthorn aquifer would be greatest near these points and would
decrease with increased distance from these points. The lateral spread of saline
water would be controlled by pressure gradients, permeability distribution,
subsurface barriers, and other related factors.
This hypothesis would be consistent with most of the observed facts. The
date of drilling of such wells, or test holes would mark the beginning of the
intrusion, probably between 1940 and 1945. Although a detailed study has
failed to disclose any well or test hole that could be the source of the saline
water, this does not preclude the possibility that they exist although there is no

BUREAU OF GEOLOGY

below 1,500 feet probably contains water similar to that determined at Hot
Springs where the dissolved solids were 34,000 mg/1 with a chloride content of
18,700 mg/l and a water temperature of 960F.
The intrusion of saline water from the deep artesian aquifer has caused
deterioration in water quality in parts of the lower Hawthorn aquifer where a
maximum chloride concentration of 15,200 mg/1 and water temperature of
930F have been measured. The saline water from the deep artesian aquifer moves
upward, either through the open bore hole of as yet unidentified wells or test
holes which connect the aquifers, or along a fault or fracture zone which
provides a connection between them. In either case, the intruding saline water
apparently enters the lower Hawthorn aquifer along faults or otherwise in the
vicinity of wells 16-7 and 1645, and spreads laterally, with the effects
decreasing with increased distance from the source. The saline water has spread
over an area of about 2.5 square miles and continues unabated at the present
time (1970).
This saline-water that has migrated into the lower Hawthorn aquifer has, in
turn, begun to migrate into the upper Hawthorn aquifer. The maximum chloride
content of water from the upper Hawthorn aquifer was 3,050 mg/1 from well
16-20 in contrast to the 15,200 mg/1 for the lower. Each well drilled to the
lower Hawthorn aquifer is a potential source of saline water leakage into the
upper Hawthorn aquifer. There are a large number of such wells. Chemical
quality records of water from this and other wells indicates a progressive increase
in chlorides in the upper Hawthorn aquifer in some parts of the area, including
several public-water wells in section 14.
The high chloride content of water at places in the sandstone aquifer
probably is the result of upward leakage from the upper Hawthorn aquifer
through the thin beds which separate the aquifers. As of 1970, water within the
sandstone aquifer has not been seriously affected by migration of saline water;
this aquifer may be a suitable source of supply where the underlying aquifer
contains saline water. However, any significant increase in water use from the
sandstone aquifer may cause an increase in upward leakage rates as long as the
upper Hawthorn aquifer contains saline water under higher head.
The leakage of saline water into the upper Hawthorn, sandstone, and
water-table aquifers would be reduced or eliminated by preventing the upward
movement of water from the lower Hawthorn aquifer, and to a lesser extent
from the Suwannee aquifer. A control procedure that probably would be
effective in at least some parts of the report area involves setting cement plugs
within these wells to separate the aquifers. This procedure will prevent
intermixing of water from the different formations, where a major part of the
migrating waters are flowing upward through the well bores. The proper
positioning of these plugs can be readily determined from geophysical logs, and
the improper placement of these plugs may result in the well becoming a

INFORMATION CIRCULAR NO. 75

permanent source of salt-water leakage.
When drilling new wells to the lower Hawthorn or Suwannee aquifers,
extending the well casing to a depth at least 300 feet and sealing in place with
concrete grout would prevent any upward leakage through the open-well bore
into the upper Hawthorn and sandstone aquifers.
It is estimated that 30 deep wells yielding water with chloride concen-
trations of 1,000 mg/1 or more, are present in the area. The location of most of
these wells are included in this report. Although detailed records are not
available on all of these wells, most of them probably allow upward transport of
saline water into the water table, sandstone and upper Hawthorn aquifers.
On the basis of data currently (1970) available, the saline water doubtless
will continue to spread laterally into areas not presently affected as long as the
supply of saline water lasts, and as long as hydraulic and density gradients near
the sources of salt water remain sufficiently high. Within the lower Hawthorn
aquifer, the lateral movement probably will be toward the south, southeast, and
east. Within the upper Hawthorn aquifer, the saline water will continue to spread
laterally from wells open to the lower Hawthorn aquifer. Problems of the
greatest magnitude probably will occur in the vicinity of artesian wells which
contain high concentrations of saline water and where the pressure in the upper
Hawthorn aquifer has been significantly lowered by pumping. Similar effects
may be noted in the sandstone aquifer.
As previously indicated, separating the upper and lower Hawthorn aquifers
in existing wells, by plugging would be a good start toward corrective action.
Establishment of a monitoring program would provide data concerning the
effectiveness of a well plugging program. A well plugging and monitoring
program would require the coordinated efforts of public and private agencies, as
well as the cooperation of land owners and other residents of the area.
Corrective action will not prevent the saline water from spreading further
than it is now but would eventually limit its spread and, assuming continued
withdrawals from the upper Hawthorn, would decrease the salinity over large
areas if all man-made connections between the upper Hawthorn and the deeper
aquifers were sealed off.

INFORMATION CIRCULAR NO. 75

cannot be developed without additional detailed information to identify the
mechanism of intrusion. However, the effects could be minimized-that is, the
transfer of saline water could be slowed somewhat-if the artesian pressure
within the lower Hawthorn and Suwannee aquifers was allowed to increase,
particularly if heads could be established comparable to those which existed
prior to the extensive development of water supplies from these aquifers. Placing
cement plugs in individual wells between the upper and lower Hawthorn aquifers
would prevent upward movement of saline water through the well into the upper
Hawthorn. It would also prevent draft from the lower Hawthorn and Suwannee
so that their potentiometric heads would have opportunity to recover. However,
this increase in head may force water in the Suwannee and lower Hawthorn to
the faults, from which it could continue its upward migration. In those parts of
the report area, where the saline water may be coming into the Suwannee,
plugging wells just below the lower Hawthorn aquifer doubtless would be at least
partially effective. To be effective, all deep wells in the McGregor Isles and
surrounding area would have to be plugged in this way. The proper positioning
of these cement plugs can be readily determined from geophysical logs, many of
which are available for wells at McGregor Isles.
By plugging the deep artesian wells where indicated, some of the salt water
now entering the upper Hawthorn, sandstone, and water table aquifers might be
eliminated. If these wells were plugged the salt water eventually might be diluted
or flushed from the aquifers above 300 feet. In some cases, improvement in
quality of water from wells in the water table, sandstone, or upper Hawthorn
aquifers may be obtained by plugging wells which have been identified as
localized sources of saline water. The proper positioning of plugs is important,
since plugging a well improperly could be a waste of time and, at most, could be
harmful: capping a well at the surface in no way diminishes the effects of the
intruding water into the upper Hawthorn or sandstone aquifers, and may
actually exacerbate the problem.
A monitoring program could determine the effectiveness of well plugging
and obtain information for the correction of similar problems in other areas.

SUMMARY AND CONCLUSIONS

There are six and possibly seven aquifers within the uppermost 1,700 feet
of sediments underlying McGregor Isles. Under natural conditions the artesian
pressure, temperature, and mineralization of the water generally increases with
depth. The aquifers which occur above depths of 300 feet normally contain
water suitable for public water supplies. The aquifers between 300 feet and
1,000 feet contain water that is too highly mineralized for public supplies, but at
some places, may be suitable for irrigation. The aquifer which occurs at depths

) BUREAU OF GEOLOGY

REFERENCES

Aultschuler, Z. S.
1958 (and Clarke, R. S., Jr., and Young, E. J.) The Geochemistry of Uranium
in Apatite and Phosphorite: U.S. Geol. Survey Prof. Paper 314-D.

Bermes, B. J.
1963 (and Leve, G. W. and Tarver, G. R.) Geology and Ground-water
Resources of Flagler, Putnam and St. Johns counties, Florida: Fla.
Geol. Survey Rept. of Inv. 32.

Cooke, C. W.
1945 Geology of Florida: Fla. Geol. Survey Bull. 29.

Hem, J. D.
1959 Study and Interpretation of the Chemical Characteristics of Natural
Water: U.S. Geol. Survey Water-Supply Paper 1473.

Patten, E. P., Jr.
1963 (and Bennett, G. D.) Application of Electrical and Radioactive Well
Logging to Ground-Water Hydrology: U.S. Geol. Survey Water-Supply
Paper 1544-D.

Puri, H. S
1964 (and Vernon, R. O.) Summary of the Geology of Florida and a
Guidebook to the Classic Exposures: Fla. Geol. Survey Sp. Pub. 5.

Tanner, W. F.
1964 The Origin of the Gulf of Mexico in Trans. of Gulf Coast Assoc. of
Geol. Soc., v. XV.

Section
and well
number

9-1
9-2
10-1
10-2
10-3
10-4
11-1
14-1
14-2
14-3
14-4
14-5
144-
14-7
14-8
14-9

14-10

14-11
14-12

S

%as-r aa3(/lS~~

~o 51-

Latitude Longitude
number

263403N0815430.1
263403N0815430.2
263415N0815409.1
263417N0815323.1
263428N0815318.1
263404N0815413.1
263428N0815303.1
263323N0815224.1
263337N0815246.1
263325N0815213.1
263325N0815202.1
263317N0815244.1
263317N0815239.1
263317N0815233.1
263312N0815244.1
263312N0815238.1

263312N0815233.1

263312N0815228.1
263312N0815221.1

263329N0815412.1
263337N0815354.1
263317N 0815407.1
263336N 015343.1
263327N0815332.1

263311N0815342.1
263403N0815317.1
263317N0815400.1
263347N0815403.1
263352N0815356.1
263351N0815353.1
263351N0815439.1

263353N0815447.1
263317N0815447.1
263337N0815435.1
263335NO815431.1

105 100 2 6
168 120 2 6
6 8
880 150 6 10
4 5
6 8
27 24 2 14
225 138 8 9
270 126 4 8
235 121 8 7
225 136 8 8
186 134 8 7
187 134 8 7
235 138 8 8
183 130 8 7
197 124 8 7

184 127 8 8

206 130 8 8
225 126 8 9

6 6
6 6
626 130 6 7
640 240 4 6
6 6

6 6
600+ 6 11
861 119 6 7
200 140 4 7
160 2 8
590 100 6 8
583 142 4 6

6 5
6
520 132 6 7
600 6 9

+14.5

+15.5

4-1649 100F
175F
4-16-69 485F

228 769 SS
512 769 UH
83 740 6-58 (L), UH
87 730 4-68 (Su), LH, UH E, GR,C, F
85 716 6-58 (Su, LH), UH
82 940 448 (LH), UH
79 64 769 WT
109 4-68 UH
69 266 UH
UH
102 1067 UH
77 134 5.69 X UH
135 9-67 UH
UH
141 7-67 UH
81 7.67 UH
78 162 6-70
105 8-67 UH
79 224 6-70
87 867 UH
87 847 UH

84 5250 467
83 1900 647
83 4550 448'
83 1900 447
1300 6-58
1700 447
83 4150 6.67
87 720 6-58
84 1325 4-67
300 447
310 847

83 1520 4-46
5300 4-67
83 7360 269
1400 1957
85 4650 4-68
81 3600 10-68

(LH), UH
(LH), UH
(LH), UH E, OR, C, F, R
LH E, C, F
(LH), UH

(LH), UH
(Su), LH, UH
X (Su, LH), UH E, OR, C, F, R
UH
UH
(LH), UH
(LH), UH

(LHl), UH
(LH), UH
X (LH), UH E, OR, C, F
(LH), UH

a. Analysis doubtful; not shown on Figure 6A

Table 1. Record of wells in the McGregor Isles area.

Abbreviations used in table: Aquifers-WT (water table), SS (sandstone), UH (upper Hawthorn), LH (lower Hawthorn), and Su (Suwannee).
For wells which produce from more than one aquifer, the principal aquifer(s) is shown in brackets. Geophysical logs E (electric log), GR
(gamma ray), C (caliper), F (flowmeter), and R (resistivity).

200F
+13.5 4.16-69 50F
+13.0 4-1669 300F
15F
300F

+18.5 8-30-68 450P
+14.0 6-11-58 200P
225F

+22.3 4-8-46

+18.6 6.11-58
+19.5 4.16-69
+21.6 6-11-58
+23.6 10.22-57

7.7 10-3048

iI

lSection
end well Llltud Lonlltude
number number

16-6 263325N0815430.1 950 6 7 +18.0 6-12-58

16.7 263343N0815422.1
16-8 263338N0815416.1
16.9 263342N0815432.1

16-10 263347N0815428.1
16-11 263351N0815423.1
16-12 263359N0815418.1
16-13 263402N0815416.1
16-14 263403N0815417.1

16-15 263343N0815416.1
16-16 263325N0815437.1
16-17 263347N0815428.1
16-18 263337N0815436.1
16-19 263324N0815438.1
16-20 263354N0815452,1
16-21 263354N0815454.1
16-22 263353N0815453.1
16-23 263354N0815453,1
16.24 263353N0815452.1

16-21
16-22
16-23
16-24.
16-25
16.26
16.27
16-28
16-29
16-30
16-31
16-32
16-33
16-34
16-35
16-36
16-37
16-38

263354N 015454.1
263353N0815453.1
263354N'0815453.1"
263353N,0815452.1
263353N'0815449.1
263350N0815445.1
263354N0815453,1
263350N0815448.1
263350N 0815448.2
263355N0815441.1
263351N0815437.1
263353N 0815433.1
263353N0815433.2
263338N0815434.1
263343N0815457.1
263339N0815436.1
263348N0815416.1
263348N0815416,2

582 138 6 8
657 126 6 7
764 170 6 7 +30.3 2.10-34
+15,0 2-16-67
5 6
797 125 4 6 -4 4-2-68
4 9 +23.9 10-22-57
997 6 9 +22.5 10-22-57
116 120 6 7 +29.3 9-2544
+15.5 2-16.67
20 20 4 6 -6,5 4- 3-68

90 2 8
140 2 8
220 183 2 7
95 2 7
185 2 6
180 2 6
190 2 6
200 2 6

180 2 6
190 2 6
190 2 6
200 2 6
180 2 6
150 2* 6
180 2 6
185 2 6
60 2 6
90 2 6
92 2 6
167 2 6
93 2 6
80 2 7
189 160 2 6
190 2 7
100 2 9
40 4 9

- 2.3 7-28-69

- 1.5 7-2349

2044 6-58
83 2600 647
20F 87 10200 4-68 X
200P 85 7500 4468
475P 82 950 2-34
85 4750 267
1900 9.57
10 85 7700 448 X
125F 85 700 10-57
125F 85 760 10-57
30F 85 870 9-50 X
740 7-69
65 74 5750 4-68 X
510 847
670 8-67
30 79 1560 4-67

78 2160 6-69
78 780 5469
78 1980 569 X
1020 5-69

(Su, LI), UII

(LII), UlH E, GR, C, V
Su, (LII), UII H, GR, C, I'
Su, (LII), UII P, GR, C, I'

(LI), UIl
Su, (LII), Ull OR, C, P, R
(Su), LII, UII
(Su), LH, Ull
(Su), LH, Ull E, GR, C, F

WT E
88
Ull
UH
UH
UH
UIIH
UH
UH

78 780 569
78 500 5-69
78 1980 5469 X
1020 569
78 1900 549
500 569

78 420 549
520 549
260 569
200 549
78 1100 549
240 549
400 549
180 549 X
1100 669
106 749
76 620 7-69

Section
and well Latitude Longitude
number number

C~d1 8 jsB

o4~8- '7N 1tu

16.39
16-40
16-41
16-42
1643
1644
16.45
1646
17-1
20-1
21-1
21-2
21-3
21-4
21-S
21-6
22-1

22-2

22-3
22-4

22-5

22-6
22-7

22-8

22-9
22-10
22-11
22-12

263353N0815425.1
263358N0815432.1
263401NO815437.1
263333N0815437.1
263333N10815441.1
263327N0815441.1
263332N0815455.1
263324N70815446.1
263312N,0815513.1
263309N,0815513.1
263304N0815447.1
263244N0815501.1
263310N0815432.1
263302N0815447.1
263258N0815429.1
263222N0815504.1
263251N0815411.1

263304N0815409.1

263237N0815414.1
263304N0815338.1

263304N0815326.1

263252N0815337.1
263252N0815325.1

263300N0815317.1

263232N0815414.1
263242N0815349.1
263304N0815358.1
263248N0815347.1

94 2
200 2
168 147 2
200 2
150 2
150 140 2
710 252 6
165 141 2
682 137 6
582 136 6
60 42 4
383 129 8
538 121 6
803 130 6
938 146 5
697 130 6
626 130 6

897 172 6

206 137 6
629 128 6

6

6
599 172 6

6

670 6
677 151 6
596 148 6
155 126 3

250F
+ 2.5 11- 3-69 5F
+17.0 4-2567 400F
I50F

+18.3 10-25-57 100F

200F
+15 4-25-67 100F
200F
+ 9.5 4-26-67 60P

+14.5 4-14.67 400F
+23.5 9-13-50
+ 8.6 10-10-57 100F
300F

300F

+18.0 4-16-69 100F
+16.5 4-16-69 500F

200F

+ 9.5 10-23-67 200F
200F
+13.5 4-20-67 350F

260 7469
975 7.69
695 7-69
1080 7-69
440 7469
212 7-69
93 15200 10-69
705 10-69
83 2100 4-67
83 2100 5-67
90 5-67
81 1000 5-67
83 1550 6-67
84 900 5-67
84 700 4-67
84 1000 2-69
82 555 9-50
560 468

86 660 10-57
700 9-50
78 120 8-67
2000 10-57
83 4000 667
84 650 9-50
2300 4-68
83 1520 6-67
83 650 9-50
1280 468
83 1550 6-67
1940 5-70
84 540 6-67
82 570 2-69
83 1740 4-68
225 2-69

ss
SS
UYH
UH
UH
UH
UH
(Su, LH) E, GR,C, P
UH
X (Su, LH), UH E, GR, C, F
(LH), UH E, GR, C, F
X WT
(LH), UH E
X (LH), UH E, OR, C, F, R
(Su, LH), UH E, GR, C, F
(Su, LH), UH E, GR, C, F
(LH), UH E, OR, C, F, R
X (LH), UH E, GR, C, F

X (Su), LI, UI E, GR, C, F

UH E
(LH), UH E, GR, C, F

(LH), UH

(LH), UH
(LH), UH

X (LH), UH

(LH), UH
(LH), UH
(LH), UH
UH

Date
Section of
and well coll.
number ect-
ion

21-1
16-15
14-5
16-35
16-23
22-1
21-3
22-8
16-4
16-7
15-8
17-1
16-11
22-2.
16-14

7-14-69
4- 3-68
5.15-69
7-14-69
7-14-69
4- 3-68
4- 4-68
4- 4-68
4- 4-68
4- 2-68
4- 3-68
4- 4-68
4- 2-68
4- 4-68
4- 4-68

Hot
Springs 12-23-64

Hardness
-1- Dis- as CaCOa O
Mag- Pot- Str- car Chl- sol- Alk-
Aquifer(s) Sl. Cal. ne- Sod- as- ont- bon- Sul. or- Fluo- ved Ca. Non- all-
Ica clum slum ium slum lum ate fate ide ride, Sol- Mg. car- nity
Ids bon- as
(SO ) (Ca) (Mg) (Na) (K) (Sr) (HCOg) (SO4) (CI) (P) (sum) ate CaCO

WT
WT
UH
UH
UH
(LH), UH
(LH), UH
(LH), UH
(LH), UH
(LH), UH
(Su, LH), UH
(Su, LH), UH
Su (LH), UH
(Su), LH, UH
(Su), LH, UH

20 120 12 40 1.4 372 3.2 96 0.2 477 376 71 305 800
5750 11,500 17,700
24 42 33 59 7.5 1.8 242 0.0 134 1.2 420 243 44 198 750
16 52 31 60 3.9 180 170 1.4 426 263 116 148 800
15 171 153 876 17 87 176 202 1940 1.2 3470 1067 923 144 750
17 67 85 332 19 11 206 275 560 2,0 1470 529 360 169 6250
1490 3240 5400
1620 3420 5700
14 248 318 2460 77 38 176 624 4650 1.5 8530 1970 1830 144 14,300
14 428 640 5620 18.3 39 164 1370 10,200 1.7 18,600 3740 3610 135 29,500
17 111 120 668 29 19 180 344 1250 1.7 2650 792 644 148 4620
1960 4070 6800
18 340 494 4280 150 26 170 1180 7700 1.9 14,300 2910 2770 139 23,200
18 86 87 418 24 15 184 340 700 1.7 1790 590 438 151 3050
19 94 91 382 18 17 184 296 710 1.6 1720 628 478 151 3000
52,100
8 639 1070 10,400 385 131 2660 18,700 1.4 34,000 6330 6220 52,100

Col-
pH or

7.9 30

8.1 5
7.6 5
7.4 3
7.6 10

Table 2. Chemical analyses of water from wells in McGregor Isles and at Hot Springs.
(For description of aquifer codes see table 1).
Chemical constituents in milligrams per liter.

FLRD GEOLOSk ( IC SUfRiW

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MILLISECOND	CLASS.METHOD	MESSAGE
0	sobekcm_page_globals.constructor
0	sobekcm_page_globals.constructor	Application State validated or built
0	sobekcm_database.verify_item_lookup_object
0	sobekcm_page_globals.constructor	Navigation Object created from URI query string
0	sobekcm_database.verify_item_lookup_object
0	sobekcm_page_globals.display_item	Retrieving item or group information
0	sobekcm_page_globals.get_entire_collection_hierarchy	Retrieving hierarchy information
0	sobekcm_assistant.get_entire_collection_hierarchy
0	cached_data_manager.retrieve_item_aggregation
0	cached_data_manager.retrieve_item_aggregation	Found item aggregation on local cache
0	item_aggregation_builder.get_item_aggregation	Found 'all' item aggregation in cache
0	system.web.ui.page.page_load (ufdc.page_load)
0	sobekcm_page_globals.constructor.on_page_load
0	html_echo_mainwriter.add_style_references	Adding style references to HTML
0	html_echo_mainwriter.add_text_to_page	Reading the text from the file and echoing back to the output stream
67	html_echo_mainwriter.add_text_to_page	Finished reading and writing the file

	Title Page
	Table of Contents
	Abstract and introduction
	Purpose and scope
	Description of the area
	Description of aquifers
	Mechanics of intrusion
	Summary and conclusions
	References
	Tables