MAP SERIES NO. 47


UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY


FLORIDA DEPARTMENT OF NATURAL RESOURCES
published by BUREAU OF GEOLOGY


SEA-WATER INTRUSION

IN THE UPPER PART OF THE FLORIDAN AQUIFER

IN COASTAL PASCO COUNTY, FLORIDA, 1969


by R. C. Reichenbaugh


Prepared by
UNITED STATES GEOLOGICAL SURVEY
in cooperation with
FLORIDA BUREAU OF GEOLOGY,
SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT,
and
PASCO COUNTY COMMISSIONERS

TALLAHASSEE, FLORIDA
1972


INTRODUCTION

The population of western Pasco County has grown 266 percent in
the decade 1960-70. In a recent census (1970) the population of the
western half of the county was 43,000 persons. Accompanying this
rapid growth is the demand for more water from the Floridan aquifer,
the ground-water reservoir which supplies nearly all water users in
coastal Pasco County.
As the demand for ground water in the western area of the county
increases, the threat of sea-water intrusion into the Flondan aquifer
also increases. The chloride content of fresh ground water in the area is
less than 20 mg/I (milligrams per liter). Substantially higher chloride
content indicates sea-water intrusion. Salt water has been reported in
private wells in the New Port Richey area, and chloride increases were
noted in the city's well in 1949 (Black, Brown, and Pearce, 1953, p.
20). In 1969, five of the nine wells owned by the city of New Port
Richey yielded water whose chloride content was in excess of 250 mg/I
(water from one well had increased in chloride content more than 200
mg/I since 1949). The U. S. Public Health Service recommends that
chloride not exceed 250 mg/I in public water supplies in areas where
other more suitable water supplies can be made available (U. S.
Department of Health, Education, and Welfare, 1962, p. 7).
Because of the threat of sea-water intrusion, the Southwest Florida
Water Management District and Pasco County officials requested that
the U. S. Geological Survey furnish information concerning the extent
of intrusion in the coastal parts of the county (area shown on inset, fig.
1) to provide salt-water control. To supply the needed information, the
Geological Survey made an investigation there starting in July 1969, in
cooperation with the Southwest Florida Water Management District,
the Pasco County Commissioners, and the Florida Bureau of Geology.
This report defines the extent of the occurrence of salt water in the
Floridan aquifer in 1969 and its relationship to fresh water in the
aquifer. The report also depicts the potentiometric surface of the upper
part of the Floridan aquifer, the elevation of the top of the aquifer, and
the salinity of the lower reaches of the Anclote and Pithlachascotee
Rivers. This investigation was made under the general supervision of C.
S. Conover, Florida District Chief, and the immediate supervision of J.
S. Rosenshein and R. N. Cherry. The author acknowledges the generous
cooperation of personnel of the Florida Bureau of Geology, in assisting
in drilling and in collecting data. Appreciation is also extended to the
personnel of the Southwest Florida Water Management District and the
many city and county officials who generously provided information.

HYDROLOGY OF COASTAL PASCO COUNTY

In the coastal part of Pasco County, land surface ranges m elevation
from sea level to about 60 feet above sea level inland. In general, the
land surface ranges in elevation from 5 to 40 feet.
In west-central Florida, the Floridan aquifer is composed of a
sequence of limestone and dolomite more than 1,000 feet thick. The
geologic and hydrologic properties of the aquifer in west-central and
coastal Florida have been described by Wetterhall (1964), Hyde (1965),
Stringfield (1966), and Cherry and others (1970).
The top of the Floridan aquifer lies near land surface in the western
part of the County. The top of the aquifer ranges in elevation from near
sea level to about 50 feet below (fig. 1) and is overlain by a thin cover
of sand and mixed sand and clay. Elsewhere, the aquifer lies beneath as
much as several hundred feet of sediments. North of Port Richey, the
top of the Floridan aquifer generally lies within 20 feet of land surface.
South of Port Richey, the top of the aquifer is generally deeper than 20
feet below land surface.
Throughout much of coastal Pasco County the water in the loridan
aquifer is unconfined, so that the potentiometric surface is the water
table. The potentiometric surface (fig. 2) generally conforms to the
land surface. The term "potentiometric surface" is defined as the
imaginary surface to which water will rise in tightly cased wells that
penetrate the aquifer. The potentiometric surface is high in areas of
recharge and low in areas of discharge. The potentiometric surface also
is high in areas of lower permeability or in areas surrounded by zones of
lower permeability that prevent or retard the movement of water away
from an area of recharge.
Over much of the area, water percolates through the sandy soil and
through sinkholes and recharges the aquifer (Cherry and others. 1970,
p. 56). The fresh water recharging the Flondan aquifer moves from the
inland areas of higher water levels generally coastward toward areas of
lower water levels (fig. 2). Along the coast, the aquifer is discharged
either naturally or as a result of man's activities. Natural discharge
occurs as surface seepage and springs, where the potentiometric surface
intersects the earth's surface, and as evapotranspiration. Man-induced
discharge occurs as pumpage from wells tapping the aquifer or seepage
to drainage canals. These canals make possible the reclamation of
waterlogged areas, and they also reduce flooding during the wet season.

GENERAL ASPECTS OF SEA-WATER INTRUSION

Sea-water intrusion is the landward movement of sea water into
aquifers, where the sea water displaces the fresh water. Where seaward
movement of fresh water in the aquifer is indicated by a seaward
gradient on the potentiometric surface, that movement and the higher
fresh-water levels oppose sea-water intrusion.
The salt water-fresh water interface is the transition zone between
the salt water and fresh water in the aquifer. The two water types
merge gradationally through the processes of mechanical dispersion and
chemical diffusion. Chloride concentrations across the interface range
from less than 50 mg/I to greater than 15,000 mg/l. Because of the
difference in densities between fresh water and sea water, the interface
slopes downward and inland with the sea water intruding beneath the
less dense fresh water in the aquifer. The shape and movement of the
sloping interface are governed by the hydrodynamic balance between
the two water types and are subject to the influence of varying
fresh-water levels in the aquifer and tidal fluctuations.
The Ghyben-Herzberg principle expresses the relationship between
sea water and fresh water in a static condition in a coastal aquifer
(Cooper and others, 1964). This principle indicates that in a coastal
aquifer sea water of specific gravity 1.025 is theoretically depressed
about 40 feet below sea level for each 1 foot of fresh-water elevation
above sea level, as shown in figure 3. This concept of a 40:1 ratio, with
modifications to account for the dynamic nature of the moving water
involved, provides the following rule of thumb useful to estimate the
effect of depressed ground- water levels on the position of the salt
water-fresh water interface in the aquifer. For each foot that the
potentiometric surface is lowered in an aquifer, the salt water below has


the potential to rise as much as 40 feet over a period time, m the
absence of confining layers that would inhibit such rise. This upward
movement of salt water acts to reestablish a balance between the
fresh-and salt-water heads. Sea-water intrusion can be reversed if
ground-water levels near the coast are raised.
The Ghyben-Herzberg principle illustrates that the landward extent
of sea-water intrusion into the Floridan aquifer is directly influenced by
the elevation of the potentiometric surface, as reflected in ground-water
levels in wells penetrating the upper part of the aquifer. Generally, in
coastal areas where ground-water levels are sufficiently elevated, the
sloping salt water-fresh water interface intersects the earth's surface
either at or seaward of the coast, and sea-water intrusion is not a
problem. In coastal areas where ground-water levels have declined the
seaward gradient of the potentiometric surface may be significantly
reduced or even reversed. This allows the salt water-fresh water
interface to shift upward and landward and salt water to penetrate the
aquifer at increasingly shallow depths inland, thereby causing some
ground-water supplies to become salty.
The behavior of the water levels in the upper part of the Floridan
aquifer near the coast is an important indicator of the potential for
salt-water intrusion. Figures 3 and 4, when compared, show that,
theoretically, the effect of widespread lowering of the potentiometric
surface is to induce the salt water-fresh water interface to move
landward and upward, with the result that some wells will yield salty
water.
In coastal Pasco County, the potentiometric surface of the upper
part of the Floridan aquifer (fig. 2) slopes toward the coast at a
gradient of about 5 feet per mile. The contour indicating zero head, or
zero elevation above mean sea level, of the potentiometric surface is
inferred to be adjacent to or just inland from the coast and generally
parallel with it.
The location of the zero contour marks the seaward extent of fresh
water in the upper part of the aquifer; therefore, it marks the seaward
extent of fresh-water discharge from the aquifer. The position of the
zero contour somewhat inland from the coast in Pasco County indicates
that all discharge of fresh water occurs inland from the coast. Such
discharge occurs only in areas where the potentiometric surface is at or
above land surface. As an example of the volume discharged from
January 1964 to June 1966, aquifer discharge contributed about 15
percent of the average flow of the Pithlachascotee River, measured at
the mouth, and about 10 percent of the average flow of the Anclote
River measured 15 miles upstream from the mouth. Total seepage to
the two rivers from the Floridan aquifer during the period was about 18
cubic feet per second, or 11.6 million gallons per day. (Cherry and
others, 1970, p. 29.)
The potentiometric surface of the upper part of the Floridan aquifer
exhibits both seasonal and long-term fluctuations. Generally, water
levels in the upper part of the aquifer are highest in September and
October after the rainy season and lowest usually in May just preceding
the rainy season. Local variations from this pattern may be expected
near areas of substantial ground-water withdrawal. When recharge
(input from precipitation) exceeds discharge, water levels in the upper
part of the aquifer rise, the location of the contour of zero head on the
potentiometric surface shifts toward the Gulf, and the salt water-fresh
water interface is depressed. Conversely, when discharge exceeds
recharge, water levels decline, the zero contour shifts landward, and the
interface moves upward.
Tidal drainage canals and the tidal reaches of streams that have no
salt-water control structures influence the position of the salt
water-fresh water interface in the aquifer in two ways. First, the
lowered fresh-water head in the vicinity of such canals or streams allows
upward movement of salt water in the underlying aquifer, and the
canals and streams are avenues for sea-water intrusion. Second, during
certain times of the year when fresh water head in the aquifer is
reduced, high tides allow salt water in the canals and streams to
intrude downward and laterally into the aquifer (compare the idealized
block diagrams of figures 5 and 6).
In many coastal areas in Florida, including coastal Pasco County,
drainage canals enhance property values by draining waterlogged land in
some places creating waterfront conditions. However, the canals also
will facilitate sea-water intrusion of the aquifer, as illustrated in figures
5 and 6.
LOCATION OF SALT WATER-
FRESH WATER INTERFACE

To locate the salt water-fresh water interface in the upper part of the
iFordan aquifer in coastal Pasco County, 13 salinity-monitoring wells
were constructed at 11 sites along the coast to monitor salinity in the
aquifer. The wells were spaced I mile apart on each of four lines
extending inland from the coast, and data were collected during drilling
of the wells to show the variation of chloride concentration in water in
the aquifer with increasing depth. In addition, stratigraphic information
obtained during drilling was used to prepare a map depicting the
limestone surface of the aquifer.
The salinity-monitoring wells were drilled with hydraulic rotary
equipment. Water samples were collected from the bottom of the hole
as drilling progressed. These samples were analyzed to determine
chloride content.
A chloride concentration greater than 250 mg/I was used as the
criterion indicating that the well had contacted*the salt water-fresh
water interface zone.
Each well was then cased to within several feet of the bottom of the
hole with 4-inch polyvinyl chloride well pipe. The casing was sealed in
place by forcing cement grout up from the bottom around the outside
of the pipe. This grouting was intended to prevent waters from
shallower depths mixing in the annular space between the casing and
the well bore and entering the open end of the well. The grout thus
assured that samples withdrawn from the well for analysis represented
water from only that part of the aquifer open to the bottom few feet of
the well.
Data collected from the salinity-monitoring wells were supplemented
by analyses of samples from selected public-supply wells and private
wells near the coast. Only those wells were selected for which reliable
information on casing depth and drilled depth was available.
Determination of the exact depth range from which water samples were
withdrawn was important to the investigation; salinity typically varies
with depth in the aquifer. Water samples collected from the
salinity-monitoring wells were from a small depth range, usually only a
few feet. Samples from public-supply and other wells are mixtures of
water through a considerable depth range and include in the mixture
not only water from the deepest part of the range penetrated but also
the fresher water from the shallower part of the aquifer open to the
well bore. Hence, the salinity-monitoring wells were more useful in
detecting the position of the salt water-fresh water interface. Chloride
concentrations higher than those reported herein probably would have
been observed from some wells if it had been feasible to isolate and
withdraw water samples from only the bottom several feet of open
hole. In coastal Pasco County, the salt water-fresh water interface
slopes landward beneath the fresh water in the aquifer at a gradient of
about 200 feet per mile, a slope that is in apparent agreement with both
the Ghyben-Herzberg concept and the seaward gradient of about 5 feet
per mile on the potentiometric surface. The general slope of the
interface in parts of coastal Pasco County is shown on three sections
(fig. 7).


The position of the salt water-fresh water interface at a depth of 100
feet below mean sea level in the upper part of the Floridan aquifer in
coastal Pasco County is depicted in figure 8. The line represents that
part of the interface in which chloride concentrations of 250 mgl occur
100 feet below mean sea level, based on interpretation of data collected
before November 1969.
As stated earlier, the 250 mg/1I chloride concentration was chosen as
a criterion to indicate that a well had penetrated the salt water- fresh
water zone. The depth of 100 feet below land surface was chosen
arbitrarily because wells less than 100 feet deep generally are
considered to be shallow and those more than 100 feet to be deep.
Along the line shown in the map (fig. 8), which represents the position
of the interface at a depth of 100 feet, the fresh-water head
theoretically is 2.5 feet above mean sea level. Coastward from this line,
salty water occurs at depths less than 100 feet, and the fresh-water head
in tte aquifer is generally less than 2.5 feet, declining to zero at the
coast, where the interface approaches land surface. Inland from this
line, water in the upper part of the Floridan aquifer containing more
than 250 mg/I of chloride occurs generally at a depth greater than 100
feet.
Figure 9 defines the extent to which salty water migrates upstream
under varying tidal and streamflow conditions. Profiles of chloride
concentrations in the two rivers were constructed from samples
collected near the surface and near the bottom of each stream at high
and low tides during high streamflow conditions in 1969 (flows
expected to be equaled or exceeded only 10 to 20 percent of the time).
At each sampling site the chloride concentration was greater near the
bottom of the stream than at the surface. Water with a chloride
concentration of 250 mg/I or more was detected farther up each stream
during high tide than during low tide.
One such profile made in 1964 during low streamflow conditions
(flows that would be exceeded 80 to 90 percent of the time) showed
that water with a chloride concentration of 250 mg/I was detected in
the Pithlachascotee River as far as 4 miles upstream from the mouth
and in the Anclote as far as 8 miles upstream from the mouth. These
distances do not necessarily represent extremes of sea-water penetration
upstream, but do indicate a range of conditions likely to be common to
either river. Undoubtedly, sea water will migrate farther upstream at
those times when conditions of low fresh-water discharge coincide with
exceptionally high tides.

CONTROL OF SEA-WATER INTRUSION

The Ghyben-Herzberg principle indicates that sea-water intrusion
into the Floridan aquifer in coastal Pasco County is directly influenced
by the relation between sea-water and fresh-water levels. Maintaining
high ground-water levels inland from the coast would be a desirable step
in controlling intrusion of salt water in the aquifer because the high
fresh-water head would act to oppose both the lateral and vertical
movement of sea water into the Floridan aquifer. Over a long period of
time maintenance of elevated water levels would force the salt
water-ftesh water interface seaward, with the result that chloride
content in the aquifer would be reduced.
The erection of salinity-control structures on tidal streams and canals
would prevent the upstream movement of salt water and would raise
the fresh-water levels in and along the channels. Such controls have
been erected in Broward County and have successfully retarded
salt-vwater intrusion caused by heavy pumping of wells and the
widespread construction of uncontrolled tidal canals in some areas
(Sherwood and Grantham, 1966).
A program of close observation of chloride content of water
withdrawn from selected salinity-monitoring wells would allow the
detection of changes in the salt water-fresh water interface, as
development of coastal Pasco County continues. An expansion of this
network of wells along the coast would permit a more comprehensive
representation of the salt water-fresh water relationship in west central
Florida. With the inclusion of the entire coastline of Florida in the
network, information gained would provide the basis for evaluating the
effects of man's activities, as related to salt-water intrusion.

SUMMARY AND CONCLUSIONS

Sea water intrusion is evident in coastal Pasco County. Data collected
indicate that the salt water-fresh water interface (as defined by a
chloride concentration of 250 mg/I at 100 feet below mean sea level)
closely parallels the Gulf Coast I to 2 miles inland. Landward from the
interface, ground water containing chloride in concentrations greater
than 250 mg/I lies deeper than 100 feet below mean sea level, and the
elevation of the potentiometric surface exceeds 2.5 feet above mean sea
level.
Salt water encroaches upon the fresh-water supply in the upper part
of the Floridan aquifer in coastal Pasco County by three principal
avenues: (1) laterally through the permeable limestone of the aquifer,
where it is in contact with sea water offshore, (2) by way of the tidal
streams and canals in which salt water is free to move upstream and
inland, from which to intrude the aquifer, and (3) the upward
movement of salt water through the aquifer as a result of the lowering
of the fresh-water head in the coastal part of the aquifer.
Sea-water intrusion would be countered by placing salt-water control
structures on tidal streams and canals and by maintaining high fresh-
water head in the aquifer.
Construction of a network of salinity-monitoring wells would
provide a means of detecting changes in the location of the salt water-
fresh water interface in Florida.


SELECTED REFERENCES

Black, A. P.,
1953 (and Brown, Eugene, and Pearce, J. M.)Salt water intrusion
in Florida-1953: Fla. State Bd. of Conservation, Div. of
Water Survey and Research Paper 9.
Cherry, R. N.,
1970 (and Stewart, J. W. and Mann, J. A.)Generalhydrology of
the middle gulf area, Florida:Florida Bur. Geology Rept.
of Inv. 56.
Cooper, H. H., Jr.,
1964 (and Kohout, F. A., Henry, H. R., and Glover, R. E.)Sea
water in coastal aquifers: U. S. Geol. Survey Water-Supply
Paper 1613-C.
Hyde, L. W.,
1965 Principal aquifers in Florida:Florida GeoL Survey, Map
Series 16.
Sherwood, C. B.,
1966 (and Grantham, R. G.)Water control vs. sea-water
sion, Broward County, Florida: Florida Geol. Survey
Leaflet 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.
Wetterhall, W. S.,
1964 Geohydrologic reconnaissance of Pasco and southern
Hernando Counties, Florida:Florida GeoL Survey Rept. of
Inv. no. 34.


Published by the Department of Natural Resources, Division of Interior
Resources, Bureau of Geology.

This public document was promulgated at a cost of $1,357.50 or
$0.946 per copy for the purpose of disseminating Water Resources
data.


I

Figure 1. Generalized elevation of the upper surface of the Floridan aquifer underlying coastal Pasco County, with inset showing
location of coastal Pasco County.


Figure 4. Generalized section across a salt water-fresh water interface, showing the effect of
a substantially lowered potentiometsic surface (compare to fig. 3).
M w











OFT--7.,MrmaIVA r=


Figure 2. Elevation of the potentiometric surface of the upper part of the Floridan aquifer underlying coastal Pasco County,
November, 1969.


.'s.
S----SRc POTENTIOMETRIC
\ ---SURFACE
a --_MEAN SEA
Srs, LEVEL


U 1. J 1





%--















Figure 3. Generalized section across a salt water-fresh water interface, showing the
Ghyben-Herzberg principle.


Figure 5. Generalized block diagram showing salt water-fresh water relationship in
a coastal aquifer.


EXPLANATION

w P. i H-s eerri rodr 'ori I
TF, ;e-- -- --. .


Interface, dashed where fered. Chloride content
Sbelowinerfae exceeds250 ml.


B ,ue chloride content le than 250 ag/l; ared more
thun 250 rl.


Figure 8. Position of the salt water-fresh water interface at a depth of 100 feet below mean sea level in the Floridan aquifer
underlying coastal Pasco County, November 1969.


5,


ah'Blt.eLfll
-it-ill
ma -s*;fl-


c C


Ni


Figure 6. Generalized block diagram showing salt-water intrusion induced by
uncontrolled tidal stream or canals.


Figure 7. Three sections across the salt water-fresh water inter interface in the upper part of the
Floridan aquifer underlying coastal Pasco County, November 1969. See map, Fig. 8, for key
to section location.


82'45' 82*40'





Figure 9. Chloride concentrations at selected locations in the lower reaches of the Pithlachascotee and Anclote Rivers in
1964and1969. FLORIDA GEOLOGIC SURVEY MAP SERIES


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G 3931
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.I NO. 47


1969


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