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CITATION SEARCH MAP IMAGE ZOOMABLE
STANDARD VIEW MARC VIEW
MAP SERIES NO. 60
UNITED STATES DEPARTMENT OF THE INTERIOR
FLORIDA DEPARTMENT OF NATURAL RESOURCES
published by BUREAU OF GEOLOGY
HYDROLOGY OF LAKE TARPON
NEAR TARPON SPRINGS, FLORIDA
by J.D. HUNN
UNITED STATES GEOLOGICAL SURVEY
in cooperation with the
SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT
BUREAU OF GEOLOGY
FLORIDA DEPARTMENT OF NATURAL RESOURCES
Lake Tarpon, in northern Pinellas County, has an area of about
4 square miles and is near several urban areas. It is used principally for
boating, fishing, and swimming. The lake was used for water supply
only during a 4-year period between March 1926 and May 1930. It has
not been used for this purpose since because of the frequent inflow of
saline water through a sinkhole on the west side of the lake. To prevent
this inflow, an earthen ring dike with four gated culverts was designed
by the Southwest Florida Water Management District and constructed
around the sinkhole in May 1969. An outlet canal leading south to
Tampa Bay was completed in July 1967 as an element of the Four
River Basin Project of the Southwest Florida Water Management
District and the U.S. Army Corps of Engineers. An earthen dam in the
canal was partly removed in July 1969 to allow discharge during high
water. In August 1971 the earthen dam was replaced and gated culverts
were constructed in the lower part of the dam.
Since the enclosure of the sink, the lake water has become less
saline on the average although it is still more saline than at times before
the enclosure. If other sinkholes with direct connections to saline water
are present in the lake bottom, the lake water may never become fresh
unless a high lake level is maintained. The purpose of this report is to
describe the physical, geologic, and hydrologic features of the lake, in
order to aid in management of the lake.
The lake basin is a depression in the land surface that may be a
consequence of a depression in the underlying limestone. Such a
depression is shown beneath or near Lake Tarpon on bedrock-surface
maps by previous investigators.
The bedrock is the upper part of the Floridan Aquifer (Parker and
others, 1955), an really extensive limestone aquifer that supplies large
quantities of ground water for municipal use in Pinellas and
Cohesive clayey deposits as much as 70 feet thick overlie the
limestone. These deposits confine water in the Floridan Aquifer and
probably lie close to the lake bed in the deeper parts of Lake Tarpon.
Because of its cohesiveness, this unit, where breached by collapse of
solution cavities in the underlying limestone, can maintain steep-sided,
A fine to medium silty sand overlies the clay and is the bed material
on most of the lake bottom. The sand is as much as 50 feet thick
(Heath and Smith, 1954). Where saturated the sand contributes seepage
to the lake from all sides.
The lake bottom is a shallow trough modified in some places near the
shore by dredging. The dredged areas were not mapped in detail, and
the canals on the west side of the lake were not sounded. The
maximum natural depth of the lake is 15 feet. The bottom is smooth,
and the east side is slightly steeper than the west side. Mapping was
done when the lake level was about 3 feet above sea level.
Lake Tarpon Sink, through which saline water entered the lake is
118 feet deep. A search was made for other sinkholes, but no others
were found in the lake during this study, during an investigation by
R.N. Cherry in March 1968, or during depth mapping by Kenner (1964,
p. 72). Echo-sounding was done by W.O. Smith and R.L. Taylor in
1947 (Taylor, 1953, p.8). Small, steep-sided sinks are difficult to detect
by echo sounding, and the probability of any being discovered with
conventional depth-sounding methods is slight.
Since 1931, rainfall at Tarpon Springs ranged from 32.89 inches in
1956 to 83.20 inches in 1959 (National Oceanic and Atmospheric
Administration, 1951-72). Normal rainfall is 54.56 inches. In most
years, more than 50 percent of the rainfall occurred from June through
September. Mann (1971, p. 28) estimates the average annual
evaporation from open- water bodies in this area to be 50 inches. This
nearly equals the rain that falls on the lake surface.
The drainage basin of the lake is about 60 square miles (Taylor,
1953, p. 8). Surface-water inflow is primarily from Brooker Creek. This
stream has a drainage area of about 30 square miles and empties into
the south end of the lake. Inflow from Brooker Creek has ranged from
0 to 1,600 efs (cubic feet per second) and averaged 26.1 cfs during
1950-70 (U.S. Geological Survey, 1972). Other streams contribute a
small amount of inflow at points shown as light areas on figure 1.
Before the outlet canal was opened, the only known outlet was Lake
Tarpon Sink. The lake water would drain, at irregular intervals, into the
sink through the Floridan Aquifer to Tarpon Spring in Spring Bayou, a
small tidal estuary about 3 miles northwest of Lake Tarpon Sink in the
city of Tarpon Springs. At other times salt water from Spring Bayou
would move back through the aquifer, up through the sink, and into
Lake Tarpon. The mechanism of the flow was confirmed by dye studies
in 1946 and 1949 as reported by Taylor (1953) and was explained in
detail by Cooper (in Heath and Smith, 1954, p. 39). Taylor placed dye
in Lake Tarpon Sink during a high lake stage and detected it later in
Spring Bayou. In March 1968, dye, which was injected into the saline
water at the sink at Spring Bayou during a low stage of Lake Tarpon
and at the beginning of flow back into Lake Tarpon, was detected at
Lake Tarpon Sink. Results of attempts to detect the dye at other
observation points in the lake were inconclusive. The calculated
increase in volume of Lake Tarpon, resulting from the lake level being
raised, was reasonably consistent with the measured flow into the sink
at Spring Bayou.
Figure 2 shows the relation between lake level and volume. The
change in total stored water with change of the lake level is somewhat
greater than the graph shows, because of additional storage in the
extensive swampy areas in the Brooker Creek drainage basin, in other
small tributaries, and additional ground-water storage.
The salinity of the lake water has varied with inflow of saline water
through the Lake Tarpon Sink. In general, the chloride content, a
measure of salinity, increased when low lake levels permitted frequent
inflow of saline water. The chloride content of water from the lake has
ranged from near zero to nearly 5,000 mg/1 (milligrams per liter),
1953-71 (fig. 3). The Floridan Aquifer contains water with a chloride
content greater than 250 mg/l beneath the lake basin and in a mile-wide
zone extending from the Lake Tarpon Sink northwest to the Gulf of
Mexico (Cherry, 1966).
Water levels in the Floridan Aquifer near Lake Tarpon are affected
by tidal fluctuations. The pattern of the water-level fluctuations in
Lake Tarpon Sink is similar to the pattern of tidal fluctuations in
Spring Bayou in Tarpon Springs (fig. 4). The water level in Lake Tarpon
Sink represents the potentiometric surface of the Floridan Aquifer as
shown by nearly identical water-level fluctuations in the sink and at
well 1, and is usually above lake level at high tide and below lake level
at low tide. Tidal influence on water-level fluctuations in the aquifer
diminish as distance from the gulf increases as shown by smaller
amplitude and the time lag in the hydrograph of well 2 compared with
hydrograph of well 1 (fig. 4).
EFFECT OF THE SINK ENCLOSURE AND THE NEW OUTLET
Since enclosure of the Lake Tarpon Sink in May 1969, the chloride
content has decreased during summer rainy seasons and has increased
somewhat during fall and winter dry seasons (fig. 2). This increase
cannot be explained entirely by evaporation of lake water and
consequent concentration of dissolved salt. The chloride analyses (table
1) show that little residual salinity remains in deeper parts of the lake,
as values for samples collected at the lake surface and from near the
lake bottom are nearly the same. The uniformity of the chloride
content throughout the lake would suggest a general source of
additional chloride rather than a point source, such as another sinkhole.
On December 3, 1970 when the lake water contained about 600 mg/l
chloride (fig. 2), two core samples of sediment were taken near the west
shore of the lake, and the water contained in the sediment was analyzed
for chloride. One sample, taken near the lake gage, contained water
with 900 mg/I chloride. The other sample, taken 1,000 feet south of
Lake Tarpon Sink, contained water with 2,000 mg/1 chloride (W.R.
Murphy, written common., 1970). The bottom sediments, which were
saturated with water having a high chloride content before the sink was
enclosed, are probably the principal source of chloride contamination
of the lake water at the present time and probably will continue to
contaminate the lake for several years. Ground-water inflow, principally
from the shallow aquifer, is the probable mechanism by which the
residual saline water enters the lake. The difference between the
chloride content of water in the sediment and in the lake water is
attributable in part to siltation and consequent low hydraulic
conductivity of the lake-bottom sediments. Ground-water inflow
through these sediments is slow, and flushing of the saline water from
them will take a long time.
The lake level has been regulated since enclosure of the sink and
construction of the new outlet and dam. Minimum levels are not as low
as they had been, and maximum levels are not nearly as high (fig. 5).
Flooding, which had been a problem to owners of lake-shore property
in the past (Mohler and Bishop, 1961), probably will not be as frequent
in the future. Before the sink was enclosed the lake level fluctuated as
much as 0.05 foot in response to tides, usually twice daily, in Spring
Bayou (fig. 6). Since enclosure, response to tidal fluctuation either has
ceased or has diminished in magnitude below the sensitivity of the
recorder gage (fig. 4).
Seiches, short-term fluctuations of the lake level (fig. 6) can be
caused by wind or by the sudden inflow or outflow of water. The
frequency of occurrence of seiches in Lake Tarpon may have
diminished since the enclosure of the sink and construction of the
FUTURE DATA COLLECTION
Temporarily increasing the sensitivity of the recorder gage would
be necessary to determine whether the lake is subject to very small tidal
fluctuations. The time chosen for the change should be when a
maximum tidal range is expected along the Gulf Coast.
Core samples of sediment taken at the same places as those taken in
December 1970 would aid in assessing the amount of flushing of saline
water from the lake-bottom sediment. Collection of additional water
samples for chloride analysis would aid in monitoring the changes in
A plan has been finalized by the Southwest Florida Water
Management District for management of the lake level This plan
includes allowing a low lake level of 1 foot above mean sea level to
stand for 3 months every fifth year, beginning in 1974. During this
time, if the salinity of the water at several points in the lake is found
not to be uniform, an intensive search should be made for small sinks or
saline springs in the area of maximum salinity. During at least the first
recession to a level of 1 foot, the salinity of the lake likely will increase,
because of increased ground-water inflow from sediment containing
saline water. If the salinity of the lake water becomes greater than the
salinity of water in the sediment then residual saline water in the
sediment cannot be the only source of saline water.
1966 Florida in 1950 and 1960: Florida Board Conserv. Div.
Geology Map Series 20.
Cherry, R.N., Stewart, J.W., and Mann, J.A.
1970 General hydrology of the Middle Gulf area, Florida
Florida Dept. Nat. Resources, Bur. Geology Rept. Inv. 56.
Heath, R.C., and Smith, Peter C.,
1954 Ground-water resources of Pinellas County, Florida:
Florida Geol. Survey Rept. Inv. 12.
1961 Stage characteristics of Florida lakes: Florida Geol. Survey
Inf. Circ. 31.
1964 Maps showing depths of selected lakes in Florida: Florida
Geot. Survey Inf. Circ. 40.
1971 Hydrologic aspects of freshening upper Old Tampa Bay,
Florida: Florida Dept. Nat Resources, Bur. Geology Inf.
Mohler, F.C., and Bishop, E.W.,
1961 Gulf Coast basins reconnaissance study: Florida Dept.
Water Resources, Tallahassee, Florida.
National Oceanic and Atmospheric Administration,
Chimatological data. Florida: Annual summaries 1947-71.
Parker, Garald G., Ferguson, G.E., Love, S.K., and others,
1955 Water resources of southeastern Florida: U.S. GeoL Survey
Water-Supply Paper 1255.
1972 Sea-water intrusion in the upper part of the Floridan
aquifer in coastal Pasco County, Florida, 1969: Florida
Dept. Nat. Resources, Bur. Geology Map Series 47.
1968 Hydrologic effects of pumping from the Floridan aquifer
in northwest Hillsborough, northeast Pinellas, and
southwest Pasco Counties, Florida: U.S. Geol. Survey
open-f ile report.
Stewart, J.W., Mills, L.R., Knochenmus, D.D., and Faulkner, G.L.,
1971 Potentiometric surface and areas of artesian flow, May
1969, and change of potentiometric surface 1964 to 1969,
Floridan aquifer, Southwest Florida Water Management
District, Florida: U.S. Geol. Survey Hydrol. Inv. Atlas
1966 Artesian water in Tertiary limestone in the southeastern
states: U.S. Geol. Survey Prof. Paper 517.
Taylor, Robert L.,
1953 Hydrologic characteristics of the Lake Tarpon area,
Florida: U.S. Geol. Survey open-file report.
U.S. Geological Survey,
1970 Water resources data for Florida, part 2, water quality
records, 1968: U.S. GeoL Survey, Tallahassee, Fla.
1971 Water resources data for Florida, part 1, surface-water
records, vol. 3 lakes, 1969: U.S. Geol. Survey, Tallahassee,
1972 Water resources data for Florida, part 1, surface-water
records, voL 1, streams-northern and central Florida,
1970: U.S. Geol. Survey, Tallahassee, Fla.
Vernon, R.O., and Puri, H.S., compilers,
1965 Geologic map of Florida: Florida Board Conserv., Div.
Geology Map Series 18.
1965 Reconnaissance of springs and sinks in west-central
Florida: Florida Geol. Survey Rept. Inv. 39.
Figure 1. Infrared imagery of Lake Tarpon flown by NASA May 15, 1969. The light and dark areas indicate differences in thermal radiation, and therefore
surface temperature. Dark areas are colder than light areas. Surface-water inflow from Brooker Creek and smaller tributaries (indicated by arrows) is
warmer than the lake water. No indication of leakage is shown at the sink enclosure.
Figure 3. Graph of chloride content of month-end water samples taken near the gage in Lake Tarpon.
Table 1. Chloride content of water samples taken April 25, 1972
(values in milligrams per liter)
Site number 1 2 3 4 5 6
and Dredged hole Dredged hole Near sink Dredged hole Dredged hole Natural
description depth: 18 feet depth: 20 feet depth: 10 feet depth: 18 feet depth: 30 feet depression
depth: 13 feet
300 300 300
310 320 300
Figure 7. Location map.
14 15 16
Figure 6. Graph showing example of seiches and tidal fluctuations of
the level of Lake Tarpon before enclosing the sink.
LAKE VOLUME. CUBIC FEET
Figure 2. Graph of stage-volume relation for Lake Tarpon.
4 -- -- -- 1- ----------------------- -- -- -- ---
1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971
Figure 5. Graph of month-end water levels m Lake Tarpon, 1945-71.
300 300 300
350 330 310
Well I about 30C'0 ."2 /'
feel west of edge //'
SSalmons \ / LAKE
8 < \\ Bay ,\ "
8 7 ARP,. /ON
.' Lake Tarpon 14
i Recorder / .
0\' / / "
I O, ly 1
,1~,. .. .
- -~ \
c: ~ '~
I .. .. ... I
1000 2000 30-i.0 4000 FEET Outfoll Cana I,
general depth I
12 to I8 feel
May 10, 1972 M. 11,1972
Figure 4. Graphs showing comparison of tidal fluctuations in
Spring Bayou with water levels in Lake Tarpon Sink,
observation wells and Lake Tarpon.
DEPARTMENT OF NATURAL RESOURCES
BUREAU OF GEOLOGY
This public document was promulgated at a .total
cost of $480.00 or a per copy cost of $.30 for the
purpose of disseminating hydrologic data.
Depth contour, May, 1972.
Contour interval 4 feet. Dashed supplementary 10-foot
Datum is lake stage of 3 feet above mean sea level
6 Sampling site and number shown in table 1.
'z Uana suLiace and
-23 top, confining bed
-92 top, Floridan aquifer
Altitude in feet above (+) or below (-) mean sea level
Geologic datum point. Detailed log available.
.-i.m rpt, M aT. 1i'? byKucera
Associates, In~., Xkeland, Florida
171-13IDA GE3OLO)GIC SURVEY MA~PSEFRIES & & ./
salt 5eR, ld
Lake OA n
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