UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY


MAP SERIES NO. 44


FLORIDA DEPARTMENT OF NATURAL RESOURCES
publhed by BUREAU OF GEOLOGY


GROUND WATER IN LAKE COUNTY, FLORIDA

by
Darwin D. Knochenmus
Prepared by
UNITED STATES GEOLOGICAL SURVEY
in cooperation with the
BUREAU OF GEOLOGY
FLORIDA DEPARTMENT OF NATURAL RESOURCES, \
BOARD OF COUNTY COMMISSIONERS OF LAKE COUNTY
and the
OKLAWAHA BASIN RECREATION AND WATER CONSERVATION I / 7-,s
AND CONTROL AUTHORITY 29' 10' 29 10' 29'" ', 29'0 1 '

TALLAHASSEE, FLORIDA | \ \


INTRODUCTION

An investigation of the water resources of Lake County was begun
by the U. S. Geological Survey in 1967 in cooperation with the Board
of County Commissioners of Lake County to determine the mode of
occurrence, quantity, and quality of water in the county. As aquifers
are the source of most water used in Lake County for municipal,
domestic, irrigation, and industrial supplies, this map report was
prepared to give a general description of the ground-water features. The
maps, sections, and graphs show some of the important hydrogeologic
features, the quality of water, and fluctuations of the potentiometric
surface of the Floridan aquifer.
Lake County, an area of 1,150 square miles, is in central peninsular
Florida (location map). This part of Florida is characterized by
numerous lakes and north-trending sand ridges (fig. 1), which locally
have relief as great as 225 feet in Lake County. Two such ridges, the
Lake Wales and Mount Dora ridges, are prominent features. Interridge
areas slope gently northward and are drained by the Palatlakaha River
and other headwater streams of the Oklawaha River system on the west
and by the St. Johns River on the east (fig. 1).
The climate is subtropical. Average annual rainfall is 51 inches at the
National Weather Service (U. S. Weather Bureau) station at Clermont.
Rainfall in any given year, however, can vary greatly from the average.
For example, 68 inches was recorded in 1959, whereas only 32 inches
was recorded in 1961. Seasonally the amount of rainfall also varies
greatly; about twice as much rain falls in the summer as in the spring.
The principal industry is agriculture. Of the various crops, citrus fruit
is the most important. Most citrus fruit is grown on the ridges, where
higher altitudes provide frost protection and the well-drained soils are
suited to citrus culture. Very little rain runs off the ridges, and water
that is not lost to evapotranspiration percolates through the sand to the
water table, where it recharges the ground-water reservoir.
The major use of water in Lake County is for irrigation. Of the
143,000 acres of citrus groves (1970), 36,000 acres, or about 25
percent, is irrigated. Seventy-five percent is irrigated from the Flondan
aquifer and 25 percent from lakes and streams (Jackson Haddox, Lake
County Agricultural Agent-oral communication, 1970). Most irrigation
water is withdrawn from March to June, when rainfall is low and
evapotranspiration is high and the fruit is just beginning to develop.

HYDROGEOLOGYl

Lake County is underlain by unconsolidated to semiconsolidated
elastic sediments of Pleistocene through Miocene age, which have a
combined thickness ranging from 25 to 300 feet. Underlying the elastic
sediments is a 2,000-foot section of permeable limestone and dolomitic
limestone of early Miocene to middle Eocene age. These rock materials
may be classified into two hydrogeologic units: 1) a elastic upper unit
of unconsolidated sand and semiconsolidated clayey sand, herein called
the elastic aquifer, and a sandy clay (Hawthorn Formation) composing
a confining bed; and 2) a lower carbonate unit of limestone and
dolomitic limestone named the Floridan aquifer (Parker, 1955). Figure
2 and the hydrogeologic sections show the depth to the Floridan
aquifer or the approximate combined thickness of the elastic unit.
The sediments of the elastic aquifer generally become finer with
depth. The top 5 to 10 feet, which mantles most of the county, is a fine
well-sorted sand having a high infiltration capacity. Below this sand is
from 30 to 80 feet of clayey sand, which slightly retards the downward
movement of water, but not sufficiently to cause the occurrence of
perched water. Water percolates downward from the land surface to a
water table in the clayey sand. Underlying the clayey sand is the sandy
and silty clay of the Hawthorn Formation, except where it is absent
under part of the Lake Wales ridge (fig. 1). This material of low
permeability, which has a maximum thickness of 75 feet, directly
overlies the Floridan aquifer and partly confines water in the Floridan
aquifer.
In the northeastern part of the county, a limestone bed at the base of
the Hawthorn Formation rests on and is hydraulically connected to the
underlying Eocene limestone and is therefore included as part of the
Floridan aquifer (hydrogeologic section B-B'). Throughout most of the
county, however, the uppermost limestone formations of the Floridan
aquifer make up the Ocala Group of late Eocene age. The Ocala Group,
which has a maximum thickness of just over 100 feet, is thin compared
with the underlying limestone of middle Eocene age. The Ocala Group
is thin or absent where erosion of the Ocala-capped fault blocks has
been most active (hydrogeologic section C-C) or where solution has
removed the upper limestone formations (hydrogeologic section B-B').
Underlying the Ocala Group is the Avon Park Limestone, having a
thickness of 400 to 1,000 feet. Near Clermont and Lakes Eustis and
Harris, the Ocala Group is missing, and the Avon Park forms the upper
part of the aquifer. The Avon Park overlies the Lake City Limestone,
which is the lowermost formation of the Floridan aquifer, according to
Stringfield (1966, p. 97). The Lake City Limestone is 500 feet thick in
southern Lake County, based on oil- test records.
The irregular surface of the Floridan aquifer was formed by solution
of the limestone, a process which is still active today. As the limestone
is dissolved by percolating ground water, cavities in the limestone grow
and finally collapse from the weight of the overlying elastic material.
The elastic materials flow into the cavity during collapse, thereby partly
filling the solution channels and pipes in the upper part of the aquifer
with sand and clay. Consequently, the upper part of the Floridan
aquifer has lower permeability than the unfilled cavernous zones below.
The contours on figure 2 indicate the approximate altitude of the
top of the Floridan aquifer. The common method of water-well
construction in central Florida is to seal off the elastic sediments by
seating the casing in the upper part of the Flondan aquifer. Thus the
length of casing needed will vary for different parts of the county; its
length can be estimated from figure 2. The estimate of required casing
length will have greater reliability if the proposed well site is on a hill
rather than in a depression. Depressions are usually surface expressions
of solutional collapses-sites that require longer casing lengths to reach
the sand-free part of the Floridan aquifer.
The depth to the Floridan aquifer is greatest in the vicinity of the
large lakes in the northern part of the county (fig. 1 and hydrogeologic
section B-B'). The area of greatest depth is delineated by the minus
50-foot contour on figure 2 and has the configuration of a buried
stream valley. This configuration suggests that limestone was removed
by stream erosion as well as by ground-water solution. The sandy
alluvium that subsequently filled the valley is coarser, thicker, and more
permeable than the fine sand and clayey sand of the elastic deposits
described previously. Hydrologically, the sandy alluvium is considered
here as part of the elastic aquifer. The alluvium is generally about 200
feet thick, although some wells have penetrated as much as 400 feet of
it. Because of its relatively thick saturated section and good
permeability, the sandy alluvium is capable of yielding more water to
wells than the rest of the elastic aquifer.

GROUND WATER

Most of the ground water presently used in Lake County is drawn
from the Floridan aquifer. Some water is available from the shallow
elastic aquifer, which at present is used principally for domestic
supplies. Except for the alluvial section, the saturated thickness of the
plastic aquifer is generally less than 100 feet, as compared with about
2,000 feet for the Floridan aquifer. The permeability of the elastic
aquifer is also lower than that of the more permeable zones of the
Floridan aquifer, so the Floridan aquifer has the greater potential as a
source of water.
Water in a well that penetrates the Floridan aquifer rises above the
top of the aquifer to a level that coincides with the potentiometric
surface at that point. The depth to water in a well drilled into the
Floridan aquifer is less than 50 feet, in Lake County except under some
of the highest ridges, where it is as much as 200 feet below the land
surface (fig. 3). The potentiometric surface is above the land surface in
many places, as is shown on figure 3, and wells in those areas will flow.
The depth-to-water map (fig. 3) can be used to predict the approximate
depth to water inha well that taps the Floridan aquifer in any part of
the county. If the altitude of a specific location is known, the depth to
water can be predicted with greater accuracy than that shown on the
map by subtracting the altitude of the potentiometric surface from the
altitude of the land surface.

The geologic nomenclature is that of the Bureau of Geology, Florida
Department of Natural Resources.


The gross direction of water movement in the Floridan aquifer is
generally down the slope of the potentiometric surface, which is about
100 feet above mean sea level in the southeastern part of the county
and 15 feet above the St. Johns River (fig. 3). Therefore, water in the
Floridan aquifer moves generally in a north-northeastward direction. In
an area of Clermont, reduced permeability along faults apparently
retards the northward movement of water, as indicated by a closer
spacing of contour lines on the potentiometric map in the area of
faulting (fig. 3). To a lesser degree, the closer spacing of potentiometric
contours along the St. Johns River valley also indicates a retardation of
water movement caused by faults. Some ground water flows into the
county from the Green Swamp in Polk County to the south, but most
of the water in the Floridan aquifer infiltrates from the overlying elastic
aquifer. Water leaves the Floridan aquifer within the county by
withdrawals from numerous irrigation, municipal, and domestic wells
and by spring discharge and seepage along Lake Harris, Lake Griffin,
Black Water Creek, and St. Johns River.
Water-level fluctuations in live wells open to the Floridan aquifer are
shown by the hydrographs. These wells have the longest record of
water-level changes in or near the county. The period shown, 1959-69,
includes the highest water levels for the past 30 years. Lowest water
levels of record occurred during 1956-57 and were about 1 foot lower
than the water levels in 1962, 1963, and 1968. Levels during the 1960's
were slightly below normal but appeared to be approaching normal
levels in 1969. These short records indicate no overall decline of
ground-water levels.
The Floridan aquifer in Lake County is semiconfined, and recharge
may occur wherever the water level in the elastic aquifer is higher than
the potentiometryc surface. Where the potentiometric surface is only a
few feet below the land surface (such as in wells 844-146, 832-154.1,
and 822-149.1), it fluctuates much less than where it is tens of feet
below the land surface (wells 910-138 and 857-138). Where the
potentiometric surface is near the land surface, the elastic aquifer is
generally full and therefore rejects recharge most of the time.
Conversely, where the potentiometric surface is deep and water- level
fluctuations are greater, such as in the ridge areas, more aquifer storage
is available and recharge there is greatest. Thus the potential for
recharge is greatest in the ridge ridgareas.
The yield of a well is primarily related to the ability of the aquifer to
transmit water and to the thickness of aquifer penetrated by the well.
To a lesser degree, the yield is related to the well diameter, because a
larger diameter well exposes more area of aquifer and therefore offers
less frictional resistance to the flow of water into the well bore.
Large-diameter wells (12-20 inch) that are open to 400-600 feet of the
Floridan aquifer in Lake County generally yield 1,500-2,000 gallons per
minute. No high-capacity wells have been constructed in the elastic
aquifer, but completing high-capacity screened wells in the thick
alluvial sediments (fig. 2) may be feasible.
In 1965, about 80 percent of the 45 mgd (million gallons per day) of
water used in Lake County was ground water. About 16 mgd of this
total was for irrigation, 90 percent of which was used on citrus groves.
Another 19 mgd was used by industry in processing various citrus
products. The number of irrigation wells has increased greatly since
1965, and it is estimated that ground water is the source of 85 percent,
or 46 mgd, of the 55 mgd used in the county in 1970.

WATER QUALITY

Water in the Flondan aquifer is of good quality, except in the St.
Johns River valley, where it contains dissolved solids that exce that exceed the
maximum of 500 mg/l (milligrams per liter) recommended for public
supplies by the U. S. Public Health Service. The areal distributions of
the dissolved solids and chloride content in water of the Floridan
aquifer are shown on figures 4 and 5.
On figure 4 areas of low concentration of dissolved solids indicate in
a general way where the aquifer receives the greatest recharge. Water
that has just entered the aquifer has had less time to dissolve minerals
than water that has traveled for some distance through the aquifer.
Areas of greater or lesser recharge, however, can only be generalized
from water-quality maps, because other factors, such as the mineralogy
of aquifer materials and the type of water recharging the aquifer, affect
water quality.
The hardness of water, a quality characterized by the amount of soap
required to produce a lather, is caused predominantly by the presence
of calcium and magnesium ions. Water having a hardness of 0-60 mg/l is
considered soft; 60-120 mg/l, moderately hard; 120-180 mg/l, hard; and
over 180 mg/l, very hard (Durfor, 1964). Water in the Floridan aquifer
is of a calcium bicarbonate type and is hard. The real distribution of
hardness would be similar to the pattern shown on the dissolved-solids
map (fig. 4) because calcium and magnesium ions (hardness) account
for most of the dissolved-solids content. Water in Lake County having
dissolved solids between 50-150 mg/I is moderately hard, that having
between 150-250 mg/I is hard, and water having more than 250 mg/I is
very hard. Thus, hardness of water in the Floridan aquifer may be
closely approximated by use of figure 4.
Chloride indicates the presence of saline water. The U. S. Public
Health Service recommends that chloride concentrations should not
exceed 250 mg/I in water used for domestic purposes. The chloride
content of the water in the Floridan aquifer, as shown on figure 5,
exceeds 250 mg/I only in the St. Johns River valley. The two
reentrants, shown on figure 5, of poorer quality of water along the St.
Johns River valley occur at Alexander Springs and Black Water Creek.
Other mineral constituents commonly dissolved in the Flondan aquifer
water are silica, iron, sodium, potassium, bicarbonate, sulfate, fluoride,
nitrate, and phosphate.
Data used in preparing the water-quality maps represent water from
wells ranging from 70 to 750 feet in depth. In general, the variation of
water quality was found not to be related to well depth, thus the
variation represents the real distribution of quality of water in the
upper one-third of the Floridan aquifer. An exception to this
relationship occurs along the St. Johns River, where water quality may
change with well depth. In the proximity of the St. Johns River, the
fresh- salt-water interface approaches the land surface, where the
potentiometric head is lowered by ground-water discharge along the
valley, and where faults allow upward movement of deep saline water.
Fresh water occurs in the aquifer to a maximum depth of about 2,000
feet, based on data from an oil-exploratory well drilled in the southwest
corner of the county.
Data used in preparing the water-quality maps represent water
from wells ranging from 70 to 750 feet in depth. In general, the
variation of water quality was found not to be related to well depth,
thus the variation represents the real distribution of quality of water in
the upper one-third of the Floridan aquifer. An exception to this
relationship occurs along the St. Johns River, where water quality may
change with well depth. In the proximity of the St. Johns River, the
fresh- salt-water interface approaches the land surface, where the
potentiometric head is lowered by ground-water discharge along the
valley, and where faults allow upward movement of deep saline water.
Fresh water occurs in the aquifer to a maximum depth of about 2,000
feet, based on data from an oil-exploratory well drilled in the southwest
corner of the county.

SELECTED REFERENCES


Durfor, C. N.,
1964 (and Becker, Edith) Public water supplies of the 100
largest cities in the Uiited States, 1962: U. S. Geol. Survey
Water-Supply Paper 1812.

Lichtler, W. F.,
1968 (and Anderson, Warren, Joyner, B. F.)Warer Resources of
Orange County, Florida:Fla. State Bd of Conservation,
Div. of GeoL, Rept. of Inv. no. 50.

Parker, G. G.,
1955 (and Ferguson, G. E., Love, S. K. and others) Water
resources of southeastern Florida:U.S. Geol. Survey
Water-Supply Paper 1255.

Pride, R. W.,
1966 (and Meyer, F. W., and Cherry, R. N.)Hydrology of the
Green Swamp area in central Florida: Florida Geol. Survey
Rept. of Inv., no. 42.


29 00'


81" 50'


SEX PLANATION
WATER LEVEL IN RELATION TO LAND SURFACE
ABOVE LAND SURFACE

BEELOW LAND SURFACE, FEET
j:,' -= 0-50
a so-oo



HYDROLOGIC DATA WELLS

seo
1 GEOLOGIC DATA WELLS


( Showl a~itud or potnisiometni surface)
28* 30' Co ur rval 5 f
I.I ..ean .. I n.. r

LINE OF HYOROGEOIC SECTION

FAULT
u Uphrown side
SPRCABLE FAULT
0 5 10 MILES


:6, :.


E XP L AANAT.ON





OMNE THAN 00

SAMPLED WELL
0 5 10 MILES
Ic usar j


28' o-




I,


81 50'


Figure 1. Culture, drainage and physiographic features.


Puri, H. S.,
1964 (and Vernon, R. O.)Summary of the geology of Florida
and a guidebook to classic exposures: Florida Geol. Survey
Special Pub. 5.

Stringfield, V. T.,
1966 Artesian water in Tertiary limestone in the southeastern
states:U.S. GeoL Survey, Prof. Paper 517.

U. S. Dept. Health, Education and Welfare,
1962 Public Health Service drinking water standards:Publ. no.
956.

Vernon, R. 0.,
1951 Geology of Citrus and Levy Counties, Florida: Florida
Geol. Survey Geol. Bull. no. 33.

Wyrick, G. G.,
1960 The ground water resources of Volusia County, Florida:
Florida Geol. Survey Rept. of Inv. no. 22.


I I.


LOCATION MAP


Figure 2. Depth to top and configuration of the top of the Floridan Aquifer.


S A WELL NO. 822-149.1
-2 -

-4-

-6
1959 1961 1963 1965 1967 1969


WELL HYDROGRAPHS


Figure 3. Depth to water and potentiometric surface of the Floridan Aquifer, May 1968.


HYDROGEOLOGIC SECTIONS


Figure 4. Dissolved solids of water from the Floridan Aquifer, 1968.


29 10'


i


29* o'1


'l U 0 ,a'


(4 ..,








ars
-.QU


o81 o::


EXPLAIN ACTION
LORIDE CONTENT, MILUGRAMS PER LITER
0- o- 25


f 1oo-2so
MORE THAN 250

SAMPLED WELL
0 5 to MILES
I I I


40'











28' 30


5'

Figure 5. Chloride content of wafer from the Floridan Aquifer, 1968. -'o'


I
-75'


-'_- ,., '-,- "II "


$LORIDA GEOLOGIC SURVEY MASRE ItEl


WELL NO. 910-1388


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