UNITED STATES DEPARTMENT OF THE INTERIOR
MAP SERIES NO. 49 GEOLOGICAL SURVEY


FLORIDA DEPARTMENT OF NATURAL RESOURCES
published by BUREAU OF GEOLOGY


A HYDROLOGIC DESCRIPTION OF LAKE MAGDALENE NEAR

TAMPA, FLORIDA
By
J.D. Hunn and R.C. Reichenbaugh

Prepared by
UNITED STATES GEOLOGICAL SURVEY
in cooperation with
FLORIDA DEPARTMENT OF NATURAL RESOURCES
DIVISION OF INTERIOR RESOURCES
BUREAU OF GEOLOGY
and
SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

Tallahasee, Florida

1972


INTRODUCTION

Urban growth, the value of lakeside property for homesites, the
intended use of some lakes as reservoirs for public water supplies and
the desire to have controlled lake levels with water suitable for fishing
and recreation have brought about an awareness of the need to better
understand the hydrology of lakes. Dredging of lake bottoms, drainage
of swamps, pumping from aquifers, diversion of water for agricultural
use, and inflow containing nutrients and pollutants can, under some
circumstances, upset the natural equilibrium of a lake. The results of
such an upset can be varied and undesirable.
Lake atlases are intended to document the hydrologic and geologic
setting of Florida lakes and to determine the existing or impending
problems and, when possible, to determine the causes of the problems.
This atlas, prepared by the U.S. Geological Survey in cooperation
with the Southwest Florida Water Management District, describes the
hydrologic and geologic setting of Lake Magdalene. The lake, about 1/
miles north of Tampa (inset on aerial photograph) is one of many lakes
in northern Hillsborough County. The lake is used for fishing,
swimming, and boating. Residential areas are replacing citrus groves
that formerly surrounded the lake.
The level of Lake Magdalene has been below average much of the
time since 1965, with a low for the period of record occurring in 1968.
The Southwest Florida Water Management District requested the
Geological Survey to determine the hydrologic conditions in the lake
basin and, if possible, to determine whether the low lake levels were
caused by a precipitation deficiency, by drainage of adjacent land, by
dredging of the lake bottom, or by large withdrawals from wells in
nearby areas.
Data collected during the investigation include: hydrologic and
geologic information from test borings, altitude of land surface at test
borings and selected ponds, water-level measurements in wells, and lake
depth from 63 traverses with an echo sounder and check soundings.
The field work was done in the period March-September 1971.

HYDROLOGY OF LAKE MAGDALENE BASIN

Lake Magdalene has a surface area of 0.4 square mile at a stage of 47
feet above mean sea level and a drainage basin area of 1.7 square miles
(U.S. Geological Survey, 1971). The lake basin consists of several
coalescing natural depressions, most of which probably result from
collapse of solution cavities in limestone. The configuration of the lake
bottom has been modified in some places by dredging, which, according
to local residents, was done mostly during the late 1950's. The lake
bottom is sandy along the shores, except for the cove at the south end
of the lake. Beginning at a depth of about 4 feet below the lake surface,
the sandy bottom grades laterally into black organic clay and silt of
unknown thickness. This deposit covers most of the lake bottom that is
less than 40 feet above sea level.
Lake Magdalene is a water-table lake hydraulically connected on all
sides with a shallow aquifer (fig. 1) that consists of silty fine to medium
sand. The slope of the water table in this aquifer near the lake (fig. 2) is
from the northwest to the southeast, indicating the general direction of
ground-water flow. Immediately southeast of the lake, because of some
ground-water mounds, the direction of flow is toward the lake.
The shallow aquifer is underlain by as much as 15 feet of gray to
green clay. This clay was found in all test holes drilled during the course
of investigation, including one drilled in a sinkhole. Locally, in the lake
basin, and where present in sinkholes outside the basin, the clay occurs
at altitudes below nearby bedrock (limestone) highs.
The clay, in turn, is underlain by the Floridan aquifer, a thick
sequence of limestone and dolomite formations. In the Lake Magdalene
area, the contact between the clay and the uppermost limestone of the
Floridan aquifer is gradational. Water in the Floridan aquifer is
confined chiefly by the clay stratum and in part by the weathered
upper few feet of the limestone.
During the investigation, the potentiometric surface in the Floridan
aquifer was 7 to 10 feet lower than the water table in the shallow
aquifer, indicating that no direct hydraulic connection exists between
the two aquifers. However, because of the vertical hydraulic gradient,
some water moves from the shallow aquifer into the Floridan aquifer.
The shallow aquifer supplies water to many lakes and ponds, some of
which are used as sources of water for agriculture. The Floridan aquifer
is extensively developed with many irrigation and domestic wells in the
vicinity of the lake. Six wells in St. Petersburg's well field, about 2
miles northwest of the lake beyond the area covered by the aerial
photograph, supplied about 18 mgd (million gallons per day) of water
for municipal use in 1971. Production from the well field began in
1963.

RAINFALL AND LAKE STAGE FLUCTUATION

Rainfall records are available for three stations in the general vicinity
of Lake Magdalene. The record from none of these stations extends
through the same time span as the stage record for the lake. The record
at Bay Lake extends from 1947 to 1962, and during this period annual
rainfall ranged from 36.1 to 81.6 inches and averaged 55.6 inches.
Annual rainfall at the Tampa Airport ranged from 28.9 to 76.6 inches
and has averaged 51.6 inches from 1951 to 1970. Annual rainfall at
Starvation Lake ranged from 44.3 to 57.9 inches, 1966-71. The rainfall
records at Bay Lake and Starvation Lake are apparently more
representative of rainfall at Lake Magdalene than that at Tampa
Airport.
Since May 1946 the stage of Lake Magdalene has fluctuated between
42.5 and 50.6 feet above sea level and has been at or above 48 feet
more than 50 percent of the time (fig. 3 and 4). During most years, the
stage rises about midyear then declines late in the year, a fluctuation
pattern that follows the seasonal rainfall patterns. The rainy season
ordinarily is from June through September. In most years, rainfall on
the lake surface is slightly greater than evaporation (Visher and Hughes,
1969).
The close relation between lake stage and rainfall is demonstrated on
figure 3: lake stage recovered sharply in 1957 during 1 year of
above-normal rainfall and also in 1969-70 during 2 years of near-normal
rainfall at Starvation Lake. Since 1966, lake levels have been below 48
feet much of the time, and the rainfall at Starvation Lake-and at
Tampa-has been below normal most of the time. Hence, the
lower-than-normal lake stage is consistent with the rainfall deficiency.
Above-normal rainfall has caused the lake stage to rise above an
altitude of 50 feet in 6 of the years since 1946 (fig. 3). In these years,
the annual rainfall has exceeded 62 inches and has been as much as 81.6
inches. Above-normal rainfall in excess of or nearly equal to 62 inches
can be expected to recur in the future. If drainage from new urban
areas is diverted to the lake, an annual rainfall somewhat less than 62
inches can be expected to cause the lake level to rise above an altitude
of 50 feet. For these reasons, buildings constructed on land whose
altitude is below about 51 feet will continue to be subject to flooding.

LAKE INFLOW AND OUTFLOW

Lake Magdalene does not have a large perennial inflow and outflow
of surface water. A ditch on the northwest side of the lake drains a field
and a small pond into the lake. Another ditch leads westward from the
lake to Bay Lake (fig. 2). A board dam in this ditch just upstream from
Bay Lake was intended to prevent flow from Lake Magdalene to Bay
Lake when the stage of Lake Magdalene declines below an altitude of
about 48 feet. This ditch has been partly blocked by slump and by
growth of brush between Lake Magdalene and the board dam. During
the investigation, in the area of the blockage, water stood about 2V feet
higher in the ditch than in Lake Magdalene, and water in the ditch was
flowing into Lake Magdalene.


Inflow to the lake is primarily from the shallow aquifer. Most of this
inflow enters the lake from the west and northwest (fig. 2). Some
ground-water inflow comes from the isolated water-table mounds east
and southeast of the lake. These isolated mounds may be drained after
prolonged droughts thereby decreasing ground-water seepage to the
lake.
Ditching of the land before 1965 for improved drainage and flood
control has lowered the water table between Platt Lake (fig. 2) and the
Section 21 well field. The ditch probably has decreased the water-level
gradient toward Lake Magdalene, and as a consequence, ground-water
inflow to the lake has been decreased somewhat.
Outflow from the lake occurs as lateral flow into the shallow aquifer
to the northeast and southeast and as vertical seepage into the Floridan
aquifer. Appreciable seepage could occur only if the surficial aquifer
and the lake have a better hydraulic connection with the Floridan
aquifer beneath the lake than the two aquifers have in adjacent areas. In
order for a good connection to exist the clay layer must be either very
thin, extensively breached or absent beneath the lake.
The vertical leakage was not sufficient to capture the lateral outflow
from the lake near the end of the 1970-71 drought. Therefore, the
vertical leakage out of the lake is controlled chiefly by the rate of
leakage that occurs as a result of movement of water through the
confining layer at the base of the shallow aquifer.
Hydraulic properties of the Floridan aquifer and the confimng layer
in the vicinity of Lake Magdalene have been interpreted from aquifer
tests run at St. Petersburg's Section 21 well field (Bredehoeft and
others, 1965, and Stewart, 1968). Drawdown in a well about 500 feet
northwest of Lake Magdalene was used in the analysis of one of the
tests. Based on the interpretation of the tests, a continuous pumping
rate of 18 million.gallons per day at the well field could cause less than
1 foot of drawdown of the water table per year 2 miles away from the
well field and a small annual decline of lake levels-somewhat less than 1
foot per year. Such a fluctuation would be overshadowed by the much
greater fluctuations caused annually by rainfall variations.
The difficulty involved in attempting to separate a presumed minor
fluctuation in stage from a major fluctuation is demonstrated by an
analysis of segments of the record of stage on figure 3. Lakestage
declines of equal duration, of similar magnitude, during the same time
of year, and from about the same initial altitude occurred in 1948-49
(before intensive pumping from the Floridan aquifer) and in 1966-67
(after 3 years of intensive pumping from the Floridan at the Section 21
well field, the only place where appreciable development of the
Floridan had occurred between the two date spans). In 1948-49 the
stage declined 3.5 feet in 8 months; rainfall at Bay Lake was 13.58
inches during the decline. In 1966-67 the stage declined 4.0 feet in 8
months; rainfall at Starvation Lake was 12.43 inches during this time.
The additional half a foot decline may be attributed to the smaller
rainfall, but the difference is too small to definitely relate to cause.
Further, were the pumping at the Section 21 field to produce a
continued drawdown of, say, 0.5 foot per year, the net decline from
1963 to 1970 would be 3.5 feet, yet in 1969-70 the seasonal high stage
was about 49.5 feet, within about 1 foot of the all-time month-end high
stage of mid-1960.

WATER QUALITY

The Florida Air and Water Pollution Control Commission has set
criteria for Class III waters, which are those to be used for recreation
purposes, including swimming and water skiing, and for the
maintenance of a well-balanced fish and wildlife population. Data from
the analyses of water samples of Lake Magdalene (table 1) are within
established limits for Class III waters. The data also reveal that the
dissolved-mineral content of the water is low and the principal
constituents are calcium, sodium, sulfate, and chloride.
Organic carbon is one measure of the organic content of a water.
Sparse organic carbon data collected in 1970 and 1971 indicate that the
total organic carbon content was fairly low and ranged from 7 to 19
mg/l. Bacteriological samples collected during the summer of 1971
show that the water of Lake Magdalene did not exceed established
bacteriological limits for Class III waters (H.A. Cheatwood,
Hillsborough County Health Department, oral commun., 1972).
The nutrients that support growth of aquatic vegetation and other
aspects of biological productivity in lakes originate in the surrounding
drainage basin and enter the lake chiefly with surface runoff. The
enrichment of a lake is known to result in levels of biological
productivity that may adversely affect its use. Algal blooms (an
overabundance of phytoplankton) are one indication of nutrient
enrichment resulting in high biological productivity. Of the essential
nutrient elements, carbon, nitrogen, and phosphorus are considered the
key elements for phytoplankton production, though the minimum
requirements for these and many other nutrients are difficult to
determine. In many lakes, when there is an abundance of nitrogen and
phosphorus, algal blooms may occur. In Lake Magdalene the
concentrations of nitrogen and phosphorus are low, and algal blooms
have not been reported.
Samples of sediment were collected from two sites in the lake. (See
aerial photograph.) These samples contained dieldrin, chlordane, DDD,
and DDE in measurable quantities (table 2). In addition, the analyses
revealed the presence of polychlorinated biphenyls (PCB's), a class of
environmentally hazardous compounds which have been used for a
variety of purposes, such as dielectrics, heat exchangers, and additives
to pesticides. Though safe upper limits for all these compounds are not
well established for lake sediments, their presence indicates that
pesticides have reached the lake in runoff from adjacent land or by
atmospheric fallout. These compounds are known to persist in
sediments for many years. Consequently, they represent a continuing
source to the fauna, and may be concentrated in the systems of living
creatures.

SELECTED REFERENCES

Bredehoeft, J.D.
1965 (and Papadopulos, I.S., and Stewart, J. W.) Hydrologic
effects of ground-water pumping in northwest
Hillsborough County, Florida: U.S. Geol. Survey open-file
report.
Cherry, R.N.
1970 (and Stewart, J.W. and Mann, J.A.) General hydrology of
the Middle Gulf area, Florida: Florida Dept. Nat.
Resources, Bur. Geology Rept. Inv. 56.
Federal Water Pollution Control Administration
1968 Water quality criteria: Rept of the Nail. Tech. Advisory
Comm to the Secretary of the Interior.
Greeson, Phillip E.
1969 Lake eutrophication--a natural process: Am. Water
Resources Assoc. Bull. Vol 5, no. 4, p. 16-30.
Kenner, W.E.
1961 Stage characteristics of Florida lakes: Florida Geol. Survey
Inf. Circ. 31.
Stewart, J.W.
1968 Hydrologic effects of pumping from the Floridan aquifer
in northwest Hillsborough, northeast Pinellas, and
southwest Pasco Counties, Florida: U.S. Geol. Survey
open-file report.
U.S. Geological Survey
1971 Water resources data for Florida, Part 1, Surface-water
records, vol. 3:Lakes, 1969.
Visher, F.N.
1969 (and Hughes, G.H.) The difference between rainfall and
potential evaporation in Florida: Florida Dept. Nat.
Resources, Bur. Geology, Map Series No. 32.


i0



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1
I



a


.0


0 400 800 FEET
Vertical Exaggeration
40X




EXPLANATION

E Lake Magdalene

D Sand, very fine to fine, silty

Clay

* Limestone (Floridan Aquifer)

sru Clay and silt, black, organic,
thickness unknown


Figure 1. Cross sections showing the relationship of the lake to rock units.


28005 0':,1


Figure 3. Lake stage compared with rainfall and pumpage from the Section 21 well field.
The rain gages at Tampa airport and Starvation Lake near the Section 21 well field, are
outside the map area.


5------ --------------------------------------
5 -
4








0 10 20 30 40 50 60 70 80 90 10

PERCENT OF TIME GIVEN STAGE

WAS EQUALED OR EXCEEDED

Figure 4. Stage-duration curve for Lake Magdalene, 1947-1970, based on weekly
staff gage readings.


TABLE 1. CHEMICAL ANALYSES OF WATER IN LAKE MAGDALENE

Water samples taken near staff gage. (See aerial photograph.)
Results in milligrams per liter except as indicated. E .B
U U


00
IU a l o I I { :
Cs Z E I : sao s0 e t Q s0 a, n.




5-16-67 45.2 0.0 6.6 3.5 9.6 3.0 0.02 4 24 19 0.2 95 69 6.8 145 5.9 28
5-21-68 43.0 .3 8.8 4.1 11.0 3.3 .00 5 33 20 .2 0.01 83 8.1 168 5.5 29
5-14-69 44.9 8.3 3.8 31 16 7.8 141 24
5-20-70 48.1 .3 6.4 2.6 7.7 2.2 .03 4 17 19 .2 .019 .023 76 58 102 6.2 28
10- 8-70 46.8 .1 6.2 2.7 7.4 2.3 .02 5 19 14 .2 .096 .023 90 54 104 6.4 26
4-22-71 44.8 .2 7.0 3.3 8.5 2.7 .05 3 26 16 .1 .022 .016 91 67 7.7 6.1 25


TABLE 2. PESTICIDE AND PCB ANALYSES, AUGUST 9, 1971

Bottom Sediments

Results in micrograms per kilogram


Location
(see aerial
photo)


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82 29'30"


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82'29'


82028'40"


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1. 4.


FLETCHER AVENUE


0 400 800 1200 FEET

T i .EXPLANATION

Areas 2 feet or less above lake stage of 46.4 feet

"- 0 Lake depth below stage of 44.4 feet. Contour interval 5 feet.
[,loe A [J / eu e(Isolated depth measurements represented by numerals.)

SA B Line of cross section (Fig. 1)


Department of Natural Resources
Bureau of Geology
This public document was promulgated at
a cost of $ 1352.00 or a per copy cost
of $0.90 for the purpose of disseminating
water resource data.


4 c .
.5 u u>


Site QW 1 0.0 7.9 16 0.0 0.8 0.0 0.0 0.0 80 20

Site QW 2 0.0 12 9.7 0.0 0.0 0.0 0.0 0.0 20 10


FL- I 751 0 IIIECROII.175 LIRVI-.y'MASERIF'S1 ~/7 >


EXPLANATION
Sx46 V
Contour shows atude of Locon of test hole, By Lake
waer table in feet abov-e water -table pond, weather

where Iferred from topo- level elevItI on o:
graphy .are.t fuet
0 2000 4000 FEET

Figure 2. Map showing contours on the water table, August 1-10, 1971.


0 Test boring

A Staff gage _5',

QW I [] Sampling site for pesticide analysis. .

On November 7, 1970, when photograph was made, lake -
stage was 46.4 feet; from May 5 to May 11, 1971 when lake :
soundings were made, lake stage was 44.4 feet. All stage
heights are referred to mean sea level.


No. 4~9

1597


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