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UNITED STATES DEPARTMENT OF TIE INTERIOR
FLoRDA DEPARIVENT OF WNAnWA
tsrerxred by BUREAU OF GEOWC~
A HYDROLOGIC DESCRIPTION OF LAKE THONOTOSASSA
NEAR TAMPA, FLORIDA
R.C. Reichenbaugh and J.D. Hunn
UNITED STATES GEOLOGICAL SURVEY
in cooperation with
FLORIDA BUREAU OF GEOLOGY
SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT
Lake Thonotosassa, a 1.29-square-mile (824 acres) lake in eastern
Hillsborough County typifies hundreds of Florida lakes, in that it helps
provide a desirable environment for people to live. Demands for
lake-front property in Florida have led to an increasing concern by
interested public officials toward maintaining the water quality of the
lakes at a level suitable for recreational use.
Degradation of lake water may result from the input of pollutants,
oxygen-demanding wastes, and excessive nutrients and from an increase
in turbidity. All these elements have degraded the water quality of Lake
Thonotosassa in recent years. Inflow to the lake through Baker Creek
consists of runoff from 60 square miles of agricultural land and
undeveloped marshland and includes waste effluents from Plant City,
the only city in the basin. Untreated industrial wastes (principally from
food-processing industries in the Plant City vicinity) and municipal
wastes had been entering Lake Thonotosassa for many years.
Consequently, in 1969 the lake became the site of the Nation's largest
single pollution-caused fish kill reported, when 26.5 million fish in the
lake died. (U.S. Department of the Interior, FWQA 1970).
Investigations at the time confirmed the cause of the kill to be low
concentrations of DO (dissolved oxygen) caused by untreated
oxygen-demanding wastes in the water (Hillsborough County Health
The U.S. Geological Survey, in cooperation with the Southwest
Florida Water Management District undertook the study of Lake
Thonotosassa in 1971. The objectives were to provide an understanding
of the hydrologic setting of the lake basin and to document
water-quality problems. This information will be useful in assessing the
present character of the lake and predicting the effects of water
management, such as the Four River Basins project proposed by the
U.S. Army Corps of Engineers (1961). This project would incorporate
Lake Thonotosassa and a part of Baker Creek within a plan to reduce
flooding in the lower Hillsborough River.
To meet the foregoing objectives, selected water-quality parameters
were periodically measured in the lake and its tributaries, and
streamflow was determined at selected points in the basin. Lake depths
were measured in May 1971 and were used to contour the lake bottom,
as shown on the aerial photograph (base map-photo). Observation wells
were drilled to provide details of geology and water levels in the shallow
aquifer in the vicinity of the lake.
The Lake Thonotosassa drainage basin (fig. 1) is an area of low relief.
The elevation of the land surface ranges from 20 feet above msl (mean
sea level) near the Hillsborough River to 140 above msl near Plant City,
at the head of the basin. The basin contains many intermittent streams
and drainage ditches, which are tributary to Mill, Pemberton,
Sparkman, and Baker Creeks. Baker Creek flows into the southeast
corner of Lake Thonotosassa through a dredged channel. Outflow from
Lake Thonotosassa is through Flint Creek, which joins the Hillsborough
River about 2 miles north of the lake. Campbell Branch drains a small
area east of the lake and is tributary to Flint Creek.
The major land-use features of the basin are citrus groves, vegetable
crops, and rural residential areas. The principal industry in the basin is
food processing, most of which takes place near Plant City.
The drainage basin is underlain by a sequence of undifferentiated
sand and clay strata that is 60 to 100 feet thick and that overlies the
confined Floridan (limestone) aquifer. The shallow aquifer is contained
within this sand and clay and is as much as 40 feet thick near
the lake (table 1). Along the west side of the lake the surficial sand is
very fine to medium grained, light colored, permeable, and well
drained. Along the east side of the lake and elsewhere in much of the
basin, the surficial sand is very fine to medium grained, clayey, silty,
dark colored, semipermeable, poorly drained, and contains much
organic matter (Stewart and Hanan, 1970). The shallow aquifer is
hydraulically separated from the underlying Floridan aquifer by a
confining clay layer. The separation of the two aquifers is probably
general throughout the basin except where sinkholes breach the
relatively impermeable clay beds, or where the impermeable clay beds
Contrary to local understanding, Lake Thonotosassa is not fed by
springs from the underlying Floridan aquifer. The surface elevation of
the lake in 1971 closely corresponded to the water-table elevation in
the surrounding shallow sand aquifer and was about 10 feet higher than
water levels in nearby tightly cased wells penetrating the Floridan
aquifer. Thus, if the impermeable layer separating the two aquifers were
breached, water from the shallow aquifer or the lake would move
downward into the Floridan aquifer.
Annual rainfall in the Lake Thonotosassa vicinity is about 56 inches.
It is unevenly distributed throughout the year. About 59 percent of the
rain falls from June to September, and 16 percent from November to
February (U.S. Dept. of Commerce, Environmental Data Service,
The fluctuation of the lake level has been generally less than 2 feet
from 1956 to 1971 (fig. 2). The lake surface probably had an elevation
of about 40 feet in the recent geologic past, (see inferred location of
the paleoshoreline on the aerial photograph). The lake level is
controlled at the outlet by a concrete structure with removable boards.
County engineers report that most of the control operations are at the
request of owners of land adjacent to the lake.
The depth-to-bottom contours (aerial photograph) are based on
sonic-depth soundings recorded in May, 1971, when the lake stage was
34.9 feet above msl. The bottom of Lake Thonotosassa is nearly flat in
the deeper west-central part. The lake bottom slopes gradually
downward from the east shore and more steeply from the west shore. A
borrow channel had been dredged adjacent to the seawall along the east
shore of the lake. Reported sinkholes or springs in the lake were not
confirmed by the depth survey. The maximum depth recorded was 14
feet at several places in the west-central part of the lake, and repeated
attempts to locate reported holes deeper than 14 feet proved futile. The
absence of recognizable sinkholes and similar irregularities in the
uniformly flat lake bottom indicates that the lake occupies an original
depression on the rolling land surface, although the shoreline
irregularity on the northwest corner of the lake may indicate a
land-collapse feature in the lake near that spot.
Biologists of the Florida Game and Fresh Water Fish Commission
report that in 1969 the bottom of the lake was generally sandy above
the 10-foot contour of Kenner (1964) and that muck covered the
bottom where the water was more than 10 feet deep; the muck was
reported to be an organic nutrient-rich ooze resulting from fallout of
phytoplankton in the lake and organic wastes carried into the lake
through Baker Creek. (Buntz, Jon, Regional Fisheries Biologist, Florida
Game and Fresh Water Commission, oral commun., 1971). The location
of Kenner's 10-foot contour is similar to the 10-foot contour shown in
the aerial photograph in this report, with the slight differences
attributable to stage differences at the times of mapping.
Water quality in the Lake Thonotosassa drainage system has been
affected by runoff from agricultural lands, undeveloped marsh lands,
and the municipal-industrial effluents from Plant City and vicinity. The
most significant of these in the degradation of Lake Thonotosassa water
quality has been the municipal-industrial effluents.
For many years the domestic sewage and some industrial wastes from
Plant City were treated in a biofilter treatment plant constructed in
1951 and modified in 1961. The waste-treatment facility was not
designed to treat all the industrial (principally food-processing) waste
waters, which, typically, were high in suspended carbonaceous material
with a high BOD (biochemical oxygen demand) and a high nutrient
content (nitrogen and phosphorus). Most of the effluents were
discharged, untreated, to ditches near the plants and eventually reached
Mill, Pemberton, and Baker Creeks, and Lake Thonotosassa.
Preliminary plans for a combined domestic and industrial waste
treatment plant were drawn in December 1967, and Plant City began
operation of the new modified activated-sludge treatment plant in April
1970. The facility was designed to yield an effluent to Mill Creek that
would not exceed 30 mg/I (milligrams per liter) BOD and 30 mg/I
Water samples were collected at several sites in the basin in March
1970, just before the treatment plant was placed into operation to
document existing water-quality conditions. Data from these samples
and from others collected subsequently are shown in table 2, and the
sites where the samples were collected are shown on figure 1.
Comparison of streamflow and water-quality data collected in March
1970 from site 2 below the food processing waste outfalls and from site
1 above the outfalls reveals that temperature and streamflow increased
below the outfalls, total coliform concentration increased, DO
decreased, and organic nitrogen and total nitrogen loads (ammonium,
nitrite, and nitrate) increased, as did the total phosphorus load.
Comparisons of data from samples collected at that time at site 3
below the waste-treatment plant and site 2, above the plant, indicates
that the temperature decreased, streamflow increased, total coliform
numbers increased, DO decreased, total nitrogen and total phosphorus
loads increased, and organic nitrogen load decreased.
The principal tributaries to Pemberton Creek downstream from
sampling site 3 are Sparkman Branch (site 4) and the Baker Creek
Tributary Canal (site 7). Analysis of samples collected at these two sites
in March 1970 showed the total nitrogen and phosphorus loads to be
much lower than loads in Pemberton Creek. At that time, the natural
processes of biodegradation, nutrient uptake, chemical alteration, and
particulate fallout had diminished the total nitrogen and phosphorus
loads in the mainstream somewhat, though samples collected on March
18, 1970, at the mouth of Baker Creek (site 8) showed about 650
pounds per day total nitrogen and about 320 pounds per day total
phosphorus were in transport into Lake Thonotosassa. The stream had
a very high total coliform concentration and a DO less than 4 mg/l, too
low to support fish life adequately.
Analyses of the water samples collected in Lake Thonotosassa in
March 1970 indicate that the lake waters were reasonably well mixed.
At that time, concentrations of organic nitrogen tended to be slightly
higher in samples collected from near the bottom at most sites,
probably because the near-bottom samples contained more particulate
matter. The highest concentrations of total coliform in the lake (based
on membrane-filter determinations) were in samples from sites 13, 14,
and 15 near the mouth of Baker Creek. At these sites, coliform
concentrations were over 25,000 colonies per 100 mis. Concentrations
of DO measured at several sites in the lake (during the daytime) ranged
from 6.8 to 9.5 mg/1.
Water-quality data collected after the new treatment plant was
placed in operation and compared with previously collected data in
March 1970 (table 2) indicate a reduction in nutrient loads at the
mouth of Baker Creek. For example, on September 24, 1970, the total
nitrogen load was 111 pounds per day, and the total phosphorus load
was 98 pounds per day. On April 20, 1971, the total nitrogen load was
203 pounds per day, and total phosphorus load was 32.4 pounds per
day. The nutrient load of March, 1970 represents to a large extent the
effects of effluent that now passes through the new treatment plant.
The average daily discharge of the plant is about 3.4 cfs or 2.2 mgd.
Although the new sewage-treatment plant is designed principally to
reduce BOD and suspended solids, indications are that some nutrient
removal is effected. Analyses of waste-water samples collected within
the treatment plant m May 1971 indicated a one-third reduction in
total nitrogen and a three-fourths reduction in orthophosphate. The
reduction is probably attributable to solids removal and nutrient uptake
by vegetation in the polishing pond (Larson, Ronald L., Public Works
Director, City of Plant City, Florida, oral common., 1971).
Comparison of the effluent from the new treatment plant to that of
the former facility reflects the degree of improved treatment given the
wastes. The former facility treated predominantly domestic wastes to
60 to 80 percent BOD removal and yielded an effluent that reportedly
varied from 30 to 130 mg/I BOD. The new plant treats the combined
waste waters to better than 98 percent BOD removal and yields an
effluent that usually contains less than 10 mg/1 BOD (Larson, oral
Records indicate that in the first 16 months of operation of the new
waste-treatment plant the industrial BOD load varied seasonally as did
food-processing activities. Industrial BOD load averaged 67 percent of
the total monthly plant load and peaked at 82 percent of plant load in
December 1970; the minimum was 54 percent of plant load in April
1971. During this period, the average total (domestic and industrial)
BOD load to the plant was about 250,000 pounds per month.
EFFECTS ON BIOTA
Lake Thonotosassa was established as a fish-management area by the
Florida Game and Fresh Water Fish Commission in 1964. Under the
fish-management program, frequent investigations are made by the
Commission to determine the capability of the fishery to maintain good
fishing. Fish-population studies made in 1964, resulted in chemical
treatment to kill about 50,000 pounds of undesirable fish. After that,
in early 1965, desirable fish species constituted about 74 percent, by
weight, of the total fish population sampled, and the lake was judged to
be a good fishery.
By 1968 the lake had changed drastically. Elodea, which once
covered all the shoreline and grew in water as deep as 12 feet, had died
back, covered only about 30 percent of shoreline, and grew only in
water less than 4 feet deep. Water clarity had decreased considerably,
indicating increased phytoplankton growth. A sizeable area in the lake
near the mouth of Baker Creek was unsuitable for fish life because of
DO depletion. Extensive mats of pollution-tolerant worms and sewage
fungus were reported in Pemberton Creek, and floating aquatic plants
were growing in the lake. The game-fish population was generally lower
than in 1965, and undesirable fish species constituted over 50 percent
by weight of samples collected (Florida Game and Fresh Water Fish
In January 1969, Lake Thonotosassa was the site of the largest
documented fish kill in the Nation that year. Investigations were made
at the time by the Hillsborough County Health Department (1969) and
Florida Game and Fresh Water Fish Commission (Buntz, 1969). They
attributed the deaths of over 90 percent of the fish in the lake (26.5
million fish) primarily to asphyxiation, resulting from the depletion of
oxygen in the water by oxygen-demanding organic wastes flowing into
the lake from Mill, Pemberton and Baker Creeks.
During February and March 1971 DO was depleted in the streams
and a part of Lake Thonotosassa, resulting in a minor fish kill. The kill
was concurrent with a mechanical breakdown in the newly constructed
Plant City sewage-treatment plant. During the breakdown, treatment
efficiency was sufficiently impaired that BOD in the plant effluent
increased to about 300 mg/l (Larson, oral common., 1971).
Fish were observed dying in the lake again in May 1971.The kill was
the consequence of oxygen depletion caused by an algal bloom. During
algal blooms in shallow lakes, daytime DO measurements typically
indicate high oxygen content as a result of plant photosynthesis;
nightime DO measurements usually indicate oxygen depletion resulting
from respiration by these same organisms and the decomposition of
organic matter (Welch, 1952). Because the lake is enriched, and
analyses of waters from Baker Creek indicate the continued though
reduced input of nutrients, algal blooms are likely to occur again, and
the resulting low DO content will probably cause periods of fish distress
LAKE THONOTOSASSA AND REGIONAL WATER MANAGEMENT
The Four River Basin project proposal by the U.S. Army Corps of
Engineers (1961) includes a series of short-term and long-term
detention reservoirs on the Hillsborough River basin as part of an
overall objective of the project to reduce flood-water problems and to
provide surface-water storage in four river basins in north-central
Florida. As proposed, the lower Hillsborough detention area would
receive controlled discharges from upstream structures and tributaries
and would provide short-term storage and controlled releases of excess
streamflow down both the Hillsborough River and the Tampa Bypass
Canal (fig. 3). The Tampa Bypass Canal, already under construction, is
designed to carry flood water directly to Tampa Bay, bypassing the
lower Hillsborough River and the City of Tampa.
Lake Thonotosassa has a single outlet, Flint Creek, to the
Hillsborough River. As presently proposed (U.S. Army Corps of
Engineers, 1965) a new control structure would be built on Flint Creek
and the present channel dredged. A canal and control structure would
connect Baker Creek with the Tampa Bypass Canal. Normal operations
would permit flow from Baker Creek through the lake and Flint Creek
to the lower Hillsborough River. During flood, the structures would
route flows from the lake and Baker Creek directly to the Tampa
In addition to the Four River Basins project, a work plan for the
Pemberton Creek watershed in Hillsborough County has been prepared
by the Hillsborough Soil and Water Conservation District, the
Hillsborough County Board of County Commissioners, assisted by the
U.S. Department of Agriculture Soil Conservation Service and Forest
Service (Hillsborough Soil and Water Conservation District 1971). The
objective outlined in that report is to provide adequate
water-management capability of sufficient flexibility to meet the needs
of different crops and hundreds of landowners with small acreages. The
objective is to be achieved by providing for flood-water removal, profile
drainage for citrus, and capabilities for water conservation.
Inflow to Lake Thonotosassa is primarily rural-area runoff combined
with treated municipal and industrial waste water. The lake bottom is
uniform and is covered with muck in deeper parts of the lake. Wells
near the lake show that the water level in the shallow sand aquifer and
the level of the lake are similar, and both are at a higher elevation than
the potentiometric surface in the confined limestone aquifer. The lake
level has varied little in recent years.
Lake Thonotosassa had received inadequately treated municipal and
industrial (food-processing) wastes for many years. The input of these
wastes has enriched the lake. The clarity of the lake water has
decreased, and the population of undesirable fish has increased.
Periodically, fish kills in the streams and lake have resulted from
oxygen depletion which is attributable to both biochemical oxygen
demand of inadequately treated wastes and oxygen depletion during
Plant City began improved waste treatment in April 1970, when a
modified activated-sludge treatment plant was placed into operation to
treat the combined municipal and industrial wastes. The
oxygen-demanding waste load reaching the lake has been reduced since
that time, and the nutrient load has been somewhat reduced. Because
the lake is presently enriched and the nutrient input continues, algal
blooms in the lake are likely to continue to cause periods of low DO
content and fish distress or death.
Lake Thonotosassa and Pemberton Creek are incorporated into two
water-management proposals. The lake may have a role in the proposed
Four River Basins project, which calls for channel improvements and a
second outlet from the lake to be constructed using the present channel
of Baker Creek. The Pemberton Creek work plan contains proposals for
floodwater routing and land drainage in the basin to assist crops and
1969 Lake Thonotosassa fish kill report: Florida Game and
Fresh Water Fish Comm.
Florida Game and Fresh Water Fish Commission
1968 ann. repts.
Hillsborough County Health Department
1969 Report of fish kill Lake Thonotosassa, January 27,
1969: Hillsborough County Pollution Control Comm.,
Div. Environmental Eng.
Hillsborough Soil and Water Conservation District
1971 Watershed work plan Pemberton Creek watershed,
Hillsborough County, Florida: Hillsborough County
Board of County Commissioners.
1964 Maps showing depths of selected lakes in Florida: Florida
Geol. Survey Circ. 40.
1970 (and Hanan, Robert V.) Hydrologic factors affecting the
utilization of land for sanitary landfills in northern
Hillsborough County, Florida: Florida Dept. Nat.
Resources, Bur. Geology, Map Set. 39.
U.S. Army Corps of Engineers
1961 Comprehensive report on four river basins, Florida: U.S.
Army Engineer District, Jacksonville.
U.S. Army Corps of Engineers
1965 Lower Hillsborough general design memorandum, pt. 1,
supp. 1: U.S. Army Engineer District, Jacksonville.
U.S. Department of Commerce, Environmental Science Services
1970 Climatological data, ann. summ., Florida: v. 74, no. 13.
U.S. Department of Interior, Federal Water Quality Administration
1970 1969 Fish kills caused by pollution:
Watson and Company
1967 Hillsborough River basin study: report to Southwest
Florida Water Management District, Commission No.
Welch, Paul S.
1952 Limnology: New York; McGraw-Hill, 538 p.
Figure 1. The Lake Thonotosassa drainage basin and selected water-quality sampling sites.
F 1 uI I I2M m- Ime I I d pI I I I T o Iat In I I I m
Figure 2. Month-end stage hydrograph, Lake Thonotosassa at Thonotosassa, Florida.
Figure 3. Structures proposed for the lower Hillsborough River Basin part of the Four River Basins project.
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A Gaging station
O 1 Observation well and number
'''''' Approximate location of dredged borrow-channel 4 to 10
feet deep (not to scale)
Depth-to-bottom contour, May, 1971. Datum is lake stage of
34.9 feet above mean sea level Contour interval is 2 feet for
depths of 4 feet and greater.
Land elevation contour. Datum
interval is 20 feet.
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I'FLORIDA GEOLOGIC SURVEY MAP SEke-..
MAP SERIES NO. 48
4 47 (-7 1)
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