UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY


MAP SERIES NO. 73


FLORIDA DEPARTMENT OF NATURAL RESOURCES
published by BUREAU OF GEOLOGY


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POTENTIOMETRIC SURFACE AND
AREAS OF ARTESIAN FLOW OF THE
FLORIDAN AQUIFER IN FLORIDA,
MAY 1974
by
Henry G. Healy
Prepared by the
U. S. GEOLOGICAL SURVEY
in cooperation with the
BUREAU OF GEOLOGY
FLORIDA DEPARTMENT OF NATURAL RESOURCES
Tallahassee, Florida
1975
INTRODUCTION
The configuration of the potentiometric surface and the areas of
artesian flow of the Floridan aquifer in Florida as of May 1974 are shown
on the large map. The potentiometric surface is the level to which water
will rise in tightly cased wells that tap the aquifer. The level of this
surface is referenced to sea level. The position of the potentiometric
surface for May 1974 is based on water levels measured in 670 wells in
Florida. The water levels were measured and the map prepared as an
integral part of the continuing cooperative program of monitoring the
ground-water resources of the state.

THE FLORIDAN AQUIFER
The Floridan aquifer is part of an artesian aquifer system that
extends over 82,000 mi2 in Florida, southern Alabama, southeastern
Georgia, and part of South Carolina, and is a principal source of ground-
water supplies in these areas.
In Florida, the aquifer consists chiefly of limestone formations
ranging from middle Eocene to Miocene in age. It was given the name
"Floridan" by Parker (1955). The aquifer ranges in thickness from about
500 ft in north-central Florida in Citrus and Levy Counties to about 2,000
ft in northeast Duval County. The top of the aquifer is at land surface in
north-central Florida and at least 1,200 ft below land surface in the
extreme northwestern part of the State (fig. 1).
Water in the Floridan aquifer occurs under artesian conditions
except where the aquifer is close to land surface. In those areas where
the aquifer lies close to the surface, the aquifer is under water-table or
unconfined conditions. Where the aquifer is overlain by confining beds it
contains water under sufficient pressure so that water in wells will rise
above the top of the Floridan aquifer and wells will flow if the petentio-
metric surface in the well is above land surface.
The Floridan aquifer is the major source of fresh ground-water
supplies in the State of Florida. In 1970, the aquifer supplied about 280
mgd for municipal use, 710 mgd for agricultural use, and 360 mgd for
industrial use. This total demand of 1,350 mgd represents 47 percent of
the total daily fresh ground-water use in the State (Healy, 1972; Pride,
1973).
POTENTIOMETRIC SURFACE OF
THE FLORIDAN AQUIFER
The potentiometric surface of the Floridan aquifer as of May 1974 is
shown on the large map by a series of contours that connect points of
equal altitude of water levels in wells that tap the aquifer. The earliest
portrayal of the altitude of the potentiometric surface of a limestone
aquifer in Florida was made by Gunter (1929) who prepared a
potentiometric map for the northern part of the Florida peninsula.
Stringfield (1936) prepared a potentiometric map that covered a
segment of the State extending from the Suwannee River to the Florida
Keys. The first potentiometric map to show artesian pressure conditions
for the state as a whole was prepared by Healy (1962) from water-
level information collected during July, 1961.
The potentiometric surface continually fluctuates in response to
changes in rates of recharge to and discharge from the aquifer. If
recharge exceeds discharge the pressure surface rises and if discharge
exceeds recharge the surface declines. The surface also fluctuates in
response to many natural phenomena other than rainfall, some of which
include changes in atmospheric pressure, wind, ocean tides and earth-
quakes. Because the contours reflect the hydrologic regimen of the
aquifer they, accordingly, denote such hydrologic features, in general,
as areas of natural recharge and discharge, and areas of withdrawal by
wells. Also, the contoured surface of the aquifer can be used in conjunc-
tion with other hydrologic data for quantitative determinations of
hydrologic parameters such as the coefficient of leakance (Bermes,
1963), velocities of ground-water movement (Faulkner, 1973), and rates
of recharge to the aquifer (Clark, 1964).
The principal source of recharge to the Floridan aquifer is rainfall.
The aquifer is replenished by infiltration of rain over about 13,000 mi2
(Cooper, 1953). In addition to rainfall in Florida, recharge also occurs by
underground flow of water from Alabama and Georgia and in some
areas from stream flow. Annual rainfall ranges from about 52 in. in
central Peninsular Florida to more than 64 in. in northwestern Florida
(Hughes, 1971). However, most of the rainfall is lost by evaporation and
stream runoff; only a fraction is available to recharge the aquifer.
Recharge to the aquifer occurs (1) where the top of the aquifer
intercepts land surface, (2) where sediments covering the aquifer are
relatively thin and permeable, and (3) where the overlying impermeable
formations have been breached by erosional processes and sinkholes.
Mounds or highs in the potentiometric surface generally indicate
recharge areas of the aquifer; however, the existence of a mound or high
in the potentiometric surface does not necessarily indicate that recharge
is significant. At least two significant hydrogeologic conditions can
produce potentiometric highs in the Floridan aquifer: (1) the cover of
impermeable formations is thin or discontinuous and (or) numerous sink
holes are open to the aquifer. Such conditions are favorable for
significant recharge. The mound (on the large map) in parts of Alachua,
Bradford, Clay, and Putnam Counties is an example of this condition
(Clark and others, 1964, p. 124). (2) the Floridan is of relatively low
permeability and silt and clay in abundance form a continuous cover.
Recharge, generally small, can maintain a potentiometric high because
the rate of lateral migration of water in the Floridan is low. The mound
on the surface in the north part of Liberty County is an example of this
condition. Yields of wells that tap the Floridan there generally are less
that 250 gpm (Pascale, 1975).
Of the two conditions cited above, the second, concerned with low
permeability in the Floridan, may be relatively common. That recharge
is being rejected in the areas of some potentiometric highs is obvious:
land surface #n these areas is wet or swampy much of the year.
Discharge from the Floridan aquifer occurs as seepage or by spring
flow into streams, withdrawals by wells, ground-water outflow to the
ocean or to the Gulf, and evapotranspiration where the aquifer is near
land surface. Some of the discharge areas are defined by the
configuration of the potentiometric contour lines on the large map. For
example, the many depressions, nearly circular (of which some are
emphasized by hachures), indicate the existence of wells or well fields
where ground-water withdrawal is significant. The depression at the
coast in Okaloosa County is a result of such withdrawal.
The trough in the potentiometric surface paralleling the Suwannee
River is the result of draining the aquifer by springs discharging to and in
the bed of the river. Approximately 34 springs discharge to the river
(Rosenau, 1974). According to Ferguson (1947), spring discharge to the
Suwannee River in the 32-mi reach from White Springs to Ellaville is 385
mgd. Discharge from the aquifer includes flow of 5,000 mgd from at least
200 springs (Rosenau, 1974) which is nearly four times the total estimated
water use from the Floridan aquifer in 1970.


DEPARTMENT OF NATURAL RESOURCES
BUREAU OF GEOLOGY

This public document was promulgated at a total
cost of $390.00 or a per copy cost of $.156 for the
purpose of disseminating hydrologic data.


EXPL N. ATI
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Interval 200 feet. Datum is mean
sea level. AN I '-













FIGURE 1. oDepth f p of the Floridan aquifer in Florida (Adapted from
Vernon, 1973.)


130


A comparison of the potentiometric surfaces of the Floridan aq.- d .-r
for 1961 (Healy, 1962) and 1974 reveals most of the major features of the
potentiometric surface have not changed appreciably during the
intervening 14-yr period. Areas where the potentiometric surface is
declining, the average annual rate of decline for the 14-yr period, and the
net decline of ground-water levels are shown on figure 2. The average
annual rate of decline of water levels in individual wells in the Floridan
aquifer ranged from 2 ft/yr to less than 1 ft/yr during 1961-74. In five
areas where withdrawal of ground water has been heavy the
potentiometric surface has declined markedly: 10 ft or more in the
north-central part of Orange County, in Putnam and St. Johns Counties,
and in the northern part of Hillsborough County; and 20 ft or more in the
coastal parts of Nassau and Okaloosa Counties and in the west-central
part of Polk County (fig. 2).
The potentiometric surface also changed in Bay, Liberty, and
Walton Counties. However, this change was probably the result of
additional control data, obtained since 1961, that allowed better
definition of the potentiometric surface in 1974.
The potentiometric surface declined markedly in several areas
before 1961. A decline of at least 60 ft occurred during 1946-61 along the
coast in Okaloosa County; about 65 ft during 1939-61 in Nassau County;
and about 25 ft during 1947-61 in Polk County. In Okaloosa County the
decline has not yet resulted in sea-water encroachment possibly because
the transmissivity of the aquifer decreases seaward. In Nassau County,
sea-water encroachment began in the early 1940's, possibly because of
the observed decline in potentiometric surface there. In Polk County,
the decline has not resulted in any noticeable upward movement of saline
water from depth. There, the confining beds below or in the lowermost
part of the Floridan seem to be effective in preventing such upward
movement.
AREAS OF ARTESIAN FLOW
The areas in which water in the aquifer is under sufficient pressure
to rise above land surface and, therefore, to flow from open wells and
springs, is shown on the large map.
Water will flow from wells that tap the Floridan aquifer in 19,700
mi2 or 36 percent of Florida. Areas of flow include the Atlantic coast
extending from Nassau to Dade County, the southern third of the
peninsula and the Gulf Coast from Monroe County and the Keys
northward as far as coastal parts of Taylor, Jefferson and Wakulla
Counties, and many areas in northwest Florida. Inland, flow areas
parallel many of the major rivers and springs but are too small to be
included on the map.
Just as the potentiometric surface fluctuates because of changing
quantities of recharge (prolonged dry or wet periods) and discharge
(increased pumping, spring flow) so must the areas of artesian flow
increase and decrease-in response to these changes-where the
altitude of the potentiometric surface is equal to or nearly equal to the
altitude of land surface. If the potentiometric surface rises, the area of
flowing wells will increase and, if the surface declines, the area of flow
will decrease. Thus, the area and amount of flow will decrease during a
drought and increase during or after periods of heavy rainfall. Prolonged
dry periods and (or) periods of heavy pumping can lower the potentio-
metric surface to the extent that wells and springs in the center of,
rather than marginal to, the area of flowing wells will cease to flow. For
example, Kissengen Spring flowed 28.2 mgd in 1933 and ceased to flow in
February 1950 as a result of the decline of the potentiometric surface
below the spring outlet (Peek, 1951). Comparison of the area of flow in
1961 (Healy, 1962) with that of 1974 as delineated by the large map in this

report reveals that the area of artesian flow has decreased only slightly
in many areas in spite of major declines in the potentiometric surface
locally. In other words, the declines were not regional.

SELECTED REFERENCES
Bermes, B. J., Leve, G. W., and Tarver, G. R.,
1963 Geology and ground-water resources of Flagler, Putnam, and
St. Johns Counties, Florida: Florida Geol. Survey, Rept. Inv. 32.
Clark, W. E., Musgrove, R. H., Menke, C. G., and Cagle, J. W., Jr.,
1964 Water resources of Alachua, Bradford, Clay, and Union
Counties, Florida: Florida Geol. Survey, Rept. Inv. 35.
Cooper, H. H., Jr., Kenner, W. E., and Brown, Eugene,
1953 Ground-water in central and northern Florida: Florida Geol.
Survey, Rept. Inv. 10.
Faulkner, G. L.,
1973 Geohydrology of the cross-Florida Barge Canal area, with
special reference to the Ocala vicinity: U. S. Geol. Survey Water
Resources Inv. 1-73.
Ferguson, G. E., Lingham, C. W., Love, S. K., and Vernon, R. 0.,
1947 Springs of Florida: Florida Geol. Survey Geol. Bull. 31.
Gunter, Herman, and Ponton, G. M.,
1929 Need for conservation and protection of our water supply with
special reference to waters from the Ocala Limestone: Florida
Engineer and Contractor, v. 6, p. 74, May-June.
Healy, Henry G.,
1962 Piezometric surface and areas of artesian flow of the Floridan
aquifer, July 7-16, 1961: Florida Geol. Survey Map Series 4.
Healy, Henry G.,
1972 Public water supplies of selected municipalities in Florida, 1970:
Florida Dept. Nat. Resources, Bur. Geology Inf. Cire. 81.
Healy, Henry G.,
1974 The observation-well network of the U. S. Geological Survey in
Florida: Florida Dept. Nat. Res., Bur. Geology Map Series 65.
Hughes, G. H., Hampton, E. R., and Tucker, D. F.,
1971 Annual and seasonal rainfall in Florida: Florida Dept. Nat.
Resources, Bur. Geology Map Series 40.
Parker, G. G., and others
1955 Water resources of southeastern Florida with special reference
to the geology and ground water of the Miami area: U. S. Geol.
Survey Water Supply Paper 1255.
Pascale, C. A.,
1975 Estimated yield of fresh-water wells in Florida: Florida Dept.
Nat. Resources, Bur. Geology, Map Series 70.
Peek, Harry M.,
1951 Cessation offlow ofKissengen Spring in Polk County, Florida, in
water resource studies, Part III: Florida Geol. Survey Rept.
Inv. 7.
Pride, R. W.,
1973 Estimated use of water in Florida, 1970: Florida Dept. Nat.
Resources, Bur. Geology, Inf. Circ. 83.
Rosenau, J. C., and Faulkner, G. L.,
1974 An index to springs of Florida: Florida Dept. Nat. Resources,
Bur. Geology Map Series 63.
Stringfield, V. T.,
1936 Artesian water in the Florida peninsula: U. S. Geol. Survey
Water-Supply Paper 773-C.
Vernon, Robert 0.,
1973 Top of the Floridan artesian aquifer: Florida Dept. Nat.
Resources, Bur. Geology Map Series 56.
I


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EXPLANATION

Net decline in feet.

S1- 5

o 5-10

S10 -20

* 20 -40

2 Average annual rate of
decline in feet.
( < means "less than").


EXPLANATION
- 50- POTENTIOMETRICCO I'll R ho.- alnaludeai u hi, h o dai-r
level would have sto.d 'n telhil ca-.d at.l- itha potiniratt
the Floridan aquifer. L%. 1974 ,Cntour iDiersaJl- Ii and f?
feet, changing at sea -irl latlum i- mean --ad 1-sI
D AREA OF ARTESIAN F 1. 1. F ctn and di-hatriiion oI drad-
of artesian flow vary nb h nlutiuSton.- sni ih p.ir niam* ir c
surface. Areas of arnt-ian flus ddlstt-ni ito -prinur- mans
rivers, and coastal beai h ridge ari-a hair Or. bton n- lud-d


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FIGURE 2. Decline of the potentiometric surface of the Floridan aquifer in areas
of heavy withdrawal of ground water, 1961-74.


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Graphics by D. F. Tucker 1' 3931
------------------G 3931
FL-.IF.I'DA GEOLOGIC SURVEY MAP S CI

No 73
1974


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