Salt Water Encroachment in Florida*
GARALD G. PARKERt
It seems desirable at the outset to define the sub-
ject of this paper. Salt-water encroachment, or in-
trusion, has been defined previously in many ways,
usually in connection with the incursion of ocean water
into freshwater domains. This usage of the term, how-
ever, is considered too narrow for our purposes, for
saline water, moving into a former fresh-water domain,
may be damaging no matter what the source or origin.
Therefore, in this paper, salt-water encroachment is
considered to be the intrusion into fresh-water do-
mains of any saline water in concentrations and vol-
umes large enough to be deleterious.
Salt-water encroachment may stem from a variety
of different sources. In oil-field areas it may arise
from improper handling or disposal of the brines that
so commonly occur in volumes greater than those of
the oil produced; in certain industrial areas it may
arise from inadequate or improper disposal of waste
liquors bitternss) that are by-products of the manu-
facturing of such commodities as table salt or mag-
nesium from brines; in arid agricultural regions it
may arise as a result of accumulations of salts of sod-
ium, calcium, magnesium, potassium, or other bases,
concurrent with the rising of the water table and water-
logging of the fields; in certain areas, as the Gulf Coast
of Texas and Louisiana, it may arise as a result of
groundwater circulation past salt domes; in other areas,
as Nevada, California, Utah, and even here in Florida,
it may stem from salt-spring discharge; in still other
places streams flow through "salt-banks," as does the
Salt River in Arizona, and they pick up heavy salt
loads to contaminate downstream areas. Salt-water
encroachment may stem naturally from a rising sea
level, or from regional downwarping of coastal areas;
hurricane tides and seiches may carry ocean water
into fresh-water areas in a very short length of time
but with comparatively long-lasting results. But most
common of all causes in Florida are the manmade ones
stemming from overdrainage as in southeastern Flor-
ida, particularly Dade County; from pumping of wells
that are too near bodies of ocean water, as at Fort
Pierce; from pumping of wells overlying a zone of
salty ground water, as at Fort Lauderdale; and from
*Publication authorized by the Director, U. S. Geological
tPrincipal Geologist, Ground Water Branch, U. S. Geological
Survey, Washington, D. C.
local overdevelopment of the artesian aquifer adjacent
to sea water, as in Pinellas County.
We now have a fairly well-defined picture of the
salt-water encroachment situation in Florida, largely
because of the enlightened interest of the responsible
citizens of this state in learning about their water re-
sources. A very large amount of basic data has been
gathered, organized, studied, and much of it published
as a result of cooperative ground-water studies between
the U. S. Geological Survey and the Florida Geological
Survey beginning about 1930; and with several other
state and Federal agencies and with a number of coun-
ties and municipalities particularly since 1939, when
the comprehensive southeastern Florida investigation
began in cooperation with Dade County and the cities
of Miami, Miami Beach, and Coral Gables; and, more
lately, with the Central and Southern Florida Flood
As a result of the cooperative work mentioned, and
a significant amount of study by consulting engineers
and chemists and by other interested Federal and
state organizations, a large body of published informa-
tion is available and perhaps an equally large or
larger amount of data is available in unpublished form
in the open files of the U. S. Geological Survey and
the Florida Geological Survey, where it may be in-
spected by interested persons.
Because of the large number of publications deal-
ing with the groundwater geology and with problems
of salt-water encroachment in the state, it is not deemed
advisable to repeat again at this time what has been
so well described and explained before. Less than two
years ago one of the finest reports of its kind was pub-
lished in quarto size by the Florida State Board of
Conservation as Water Survey and Research Paper
No. 9. Written by A. P. Black, Eugene Brown, and
J. M. Pearce, it bears the title "Salt water intrusion in
Florida-1953"; it contains 38 pages, 15 figures, 23
tables, and a bibliography of 68 references to all the
important research work in Florida on the subject. In
the year and a half since its publication no really im-
portant changes have taken place in the salt-water
encroachment picture-the status of the encroachment
is essentially the same now as then. Only a few aspects
of the situation require additional discussion-par-
ticularly the implications of the field research on the
relations of salt water to fresh water now being done by
Francis A. Kohout and Nevin D. Hoy, geologists of
the Miami Office of the U. S. Geological Survey.
The recent comprehensive investigation of the
geology and water resources of southeastern Florida
included what was perhaps the most intensive research
on the relation of salt water to fresh water in coastal
aquifers that had been done to date. Gross relation-
ships had been investigated, particularly in Holland,
Germany, and Belgium, by W. Badon Ghyben, Baurat
Herzberg, and J. M. K. Penninck, working inde-
pendently about the turn of the century. Out of this
work came the well-known and widely applied "Ghy-
ben-Herzberg principle,"-that of the 1-to-40 ratio
for predicting depth of salt water in coastal areas of
freely permeable materials where salt water and fresh
water are in contact with each other.
A goodly number of investigators in the United
States and its island territories also have worked on
the gross aspects of the relationships between salt
water and fresh water in coastal aquifers. In general
their work has confirmed that of the Europeans.
Among the first of these later workers was John S.
Brown (1922). Brown, in addition to reporting his
own limited findings, gives an excellent review of the
work of the earlier European investigators. Later
workers have included Stearns and others (1935, 1940,
1942); Wentworth (1942, 1947, 1951a, 1951b); Barks-
dale (1940); Thompson (1926, 1932, 1933); Winslow
and Doyel (1954); Turner and Foster (1934); and
Poland, Piper, and others (1953). In Florida, Brown
and Parker (1945) and Parker (1945, 1951), were
enabled to examine the salt-water-fresh-water rela-
tionship in greater detail than had previous workers
in this country, but even so they did not have the
opportunity to do the intensive kind of research now
being done so well by Kohout and Hoy.
On the basis of the earlier work involving the
Biscayne aquifer in the Miami area, the following
general conclusions were reached: (1) the gross salt-
water-fresh-water conditions described by the Euro-
peans and the workers in Hawaii were essentially the
Same as those at Miami; (2) salt water is encroaching
in the Biscayne aquifer as a result of upset equilibrium
conditions due to lowering the water table; (3) equi-
librium in accordance with the Ghyben-Herzberg
principle is not established over the whole area, and
the salt front is destined to occupy a final position
farther inland; (4) the fact that the salt front can
fluctuate seasonally both inland and seaward is of con-
siderable but undetermined significance; _(5) the
changes in the position of the salt-water front reflect
long-term trends rather than overnight variations;
(6) the eventual position to be occupied by the salt
front will be in accordance with the Ghyben-Herzberg
principle unless it is somewhat modified by a depres-
sive force acting upon the salt-water wedge as the fresh
ground water flows up over it; and, finally, (7) the
fresh-water resources could be protected by placing
water-control structures in the tidal canals as far
downstream as possible. These would: (a) prevent
salt water from flowing inland in the canals in sig-
nificant quantities beyond the controls; (b) conserve
fresh water by preventing its uncontrolled wastage to
the sea; and (c) raise water levels somewhat in the
canals and aquifer upstream from the controls, thus
creating greater freshwater head to push against en-
croaching salt water at depth in the aquifer.
After the writer left the Miami area in 1948 only
routine observations of the salt-water situation were
continued, and it was not until Francis A. Kohout
was transferred to the Miami project that intensive
field work began again, late in 1952. Since that time
Kohout and Hoy, with able help from other staff
members, have made a remarkable advance in both
detailed and overall knowledge of the relation between
salt water and fresh water in that area, and, by in-
ference, of the relationships in coastal aquifers any-
where. Although their research is far from complete
and the results of their work are still unpublished, a
preliminary open-file report entitled "Research on
salt-water encroachment in the Miami area, Fla.,"
was prepared in November 1953. In that report Ko-
hout and Hoy show that the 1-to-40 ratio holds only
in a gross manner, and that the observed departures
of the actual salt front at depth in the aquifer from
the theoretical position (in accordance with the Ghy-
ben-Herzberg principle) is due to powerful dynamic
conditions not taken into account in Ghyben-Herzberg
calculations, which are based on a simple, static U-tube
Kohout and Hoy, after careful inquiry into the
nature of the contact zone between fresh and salt water,
(1) Molecular diffusion is a much more potent force
in the interface zone than has been previously con-
sidered, and of itself diffusion tends to dissipate the
encroaching salt-water wedge.
(2) Alternating tidal thrust and pull in the shore
zone is a powerful mixing force at depth in the aquifer,
and it greatly widens and thickens the zone between
wholly fresh and wholly salt water.
(3) Fresh-water flow over the salt wedge exerts
only a slight downward pressure and does not account
of itself for the disparity between theoretical and ob-
served depths to salt water. Instead, the seaward-flow-
ing fresh water is a highly effective "eroding" force,
sweeping seaward the diffused and tidally mixed salt
It is expected that the results of the research by
Kohout and Hoy will be available in the near future
in the scientific press.
- 3Crr. I 1 111
In the meantime, the Corps of Engineers has been
proceeding with model studies simulating, I under-
stand, aquifer-ocean relationships in the Miami area.
It is not known at this time what conclusions the Corps
may have reached as a result of its research.
In California, salt-water encroachment has become
a major problem in 13 coastal basins, and as a means
of attacking the problem, research using geohydrologic
models was begun. This work, which produced ex-
cellent results, was done by T. R. Simpson, F. L.
Hotes, James A. Harder, and Leung-ku Lau of the
Sanitary Engineering Research Laboratory of the Uni-
versity of California at Richmond, Calif., under a
cooperative agreement between the University and
the State Water Resources Board. The results were
published by the University under the title "Final
report on sea water intrusion" in September 1953.
The principal conclusions drawn from the experi-
mental data are:
"1. In order to prevent sea water from entering an aquifer
which has direct access to the sea, the fresh water piezo-
metric surface must be held above sea level a distance equal
to (S-1) times the distance below sea level to the lowest
pumping zone which must be protected, where S is the
specific gravity of the ocean or inland bay water.
"2. For aquifers of finite thickness, the maintaining of a fresh
water surface above sea level will result in a seaward leak-
age of fresh water in the upper portions of the aquifer.
A sea water wedge will form in the lower portion, extending
inland from the ocean outlet a distance which is inversely
proportional to the fresh water flow rate. For a uniform
aquifer this seaward leakage may be determined from
formulas given in this report; for non-uniform aquifers
methods are given for its estimation.
"3. The relationship between the equilibrium wedge length
and the rate of seaward flow of fresh water is independent
of the distance of the aquifer below sea level.
"4. There is no marked change in the shape of the fresh water-
sea water interface at the beginning of an overdraft period.
From any initial position the interface moves inland at
a rate determined by the rate of fresh and salt water move-
ment. In a uniform aquifer, the wedge tends to flatten out
and the toe tends to move somewhat faster than the inter-
face as a whole due to the greater density of the salt water.
In most prototype aquifers, however, the uncertainties of
nonuniformity within the aquifer make it impossible to
generalize on the rate of intrusion, since the sea water may
enter the more permeable portions and travel relatively
rapidly within them.
"5. If fresh water can be injected into the aquifer at a sufficient
rate, the piezometric surface can be maintained at the re-
quired height above sea level in a region along the coast.
The spacing of the wells is of little importance, except that
the toe of the intruded wedge should be held at least half
a well spacing seaward from the centerline of the wells.
The seaward fresh water leakage will be related to the
length of the sea water wedge in the same way as before.
However, unless the inland demand for fresh water is re-
duced, the injection rate must equal not only the leakage
rate, but also the entire overdraft rate which has originally
caused the intrusion.
"6. If fresh water is injected on top of the wedge at a rate
sufficient to halt intrusion, the portion of the wedge ex-
tending inland from the wells will be cut off. It continues
to move inland depending on the hydraulic gradient exist-
ing there, but its rate of travel will be no greater, and
actually will be somewhat less, than the rate of travel of
the entire wedge under the same overdraft conditions.
"7. Unless there is a pronounced impediment to vertical flow
within the aquifer the injection of water near the bottom
of the aquifer provides no benefits in the form of a reduced
"8. Intruding sea water can be intercepted by a line of pump-
ing wells, which would form a "pumping trough" near the
coast. There would be no recharge benefits from this plan,
but under proper operation the seaward leakage of fresh
water need be no greater than in the injection process."
Thus, with the newer knowledge of the forces con-
trolling the relationship of fresh water and sea water
in coastal aquifers, we are in a position to safeguard
our priceless fresh-water supplies against ruination by
salt-water encroachment-if we are willing to pay the
necessary cost. We are certain, for example, that the
1-to-40 ratio of the Ghyben-Herzberg principle is a
"safe" factor to apply in the development or conserva-
tion of water supplies in coastal aquifers, for seldom
will equilibrium ever be reached, even close to the sea,
at this limited ratio. Rather, the depth to salt water
will usually be greater than that predicted on a 1-to-40
ratio, and, likewise, the amount of inland encroach-
ment of an intruding wedge of salt water will be some-
But even with this information, the control by man
of salt-water encroachment demands rather complete
knowledge of all the factors involved. These are the
most important: (1) The geology of the aquifer and
associated aquicludes, if present; (2) the constant
hydrologic properties of the aquifer, including the
coefficients of transmissibility and storage; (3) the
changing hydrologic factors, such as changes in storage,
head, and migration of the salt-water-fresh-water in-
Thus, our duties as citizens of Florida or as the
representatives of the Federal Government who are
interested in the evaluation or in the conservation and
effective use of our natural resources-water in this
particular instance-seem quite clear. We must care-
fully chart and map our aquifers and aquicludes; and
we must maintain an accounting system of "input and
outgo" so that changes in storage can be known and,
under given expected conditions of rainfall, runoff,
recharge, evapotranspiration, and other related factors,
predictions can be made of future availability of water
for development and use. Such an evaluation and
accounting are important anywhere, but in seacoast
areas or inland areas where saline sources exist that
are potential sources of encroachment, it is manda-
_.____ ___ls~ ____~
tory that we use all our skills and every precaution to
safeguard our water supplies.
We may think, on the basis of past experience, that
there is little to worry about. We have faced problems
of salt-water encroachment before and have solved
them without too great loss or damage. However, it
would be most unwise to think that conditions will
always be as easily handled, and our duties as respon-
sible citizens would not be fulfilled if a proper note
of warning were not sounded and precautionary mea-
sures taken in adequate time.
Florida's population is increasing by leaps and
bounds, and its use of water is increasing proportion-
ately. Already many municipalities and industries
are engaged in enlarging their water-supply facilities,
usually by increasing ground-water supplies. It is
believed that future expansion will be at a rate even
greater than that of the present, and as larger and
still larger quantities of water are required there will
develop many serious ground-water problems, espec-
ially in areas where salt-water encroachment is a factor.
In the past, small supplies of water generally have
been easy to develop, but in some places difficulty has
been experienced owing to the low permeability of
the materials comprising the aquifer or to the existence
of salty water even at very shallow depths. In some
parts of the state salty ground water both underlies
and overlies the uppermost fresh-water aquifers, thus
endangering supplies developed in them.
Fortunately, there are few areas in Florida where
a plentiful supply of good ground water cannot be
obtained if the development is wisely done. Likewise,
there are few areas where industrial wastes, overpump-
ing, or overdrainage are seriously dangerous at present.
Where these dangers exist or develop potentially, they
can usually be corrected or prevented if they are in-
telligently studied and suitable regulatory measures
are taken. If, through ignorance or carelessness, the
proper development and management of our aquifers
are neglected, our water supplies in many areas of the
state could be ruined and lost needlessly.
Barksdale, Henry C., 1940, "The contamination of ground water
by salt water at Parlin, N. J."; Am. Geophys. Union Trans.,
Black, A. P., Brown, Eugene, and Pearce, J. M., 1953, "Salt water
intrusion in Florida-1953"; Florida State Board of Conserva-
tion, Water Survey and Research Paper No. 9, May.
Brown, John S., 1922, "Relation of sea water to ground water
along coasts"; Am. Jour. Sci., 5th ser., v. 4, p. 274-294.
Brown, R. H., and Parker, Garald G., 1945, "Salt-water encroach-
ment in limestone at Silver Bluff, Miami, Fla."; Econ. Geology,
v. 40, no. 4, p. 235-262, June-July.
Parker, Garald G., 1945, "Salt-water encroachment in southern
Florida"; Am. Water Works Assoc. Jour., v. 37, no. 6, p.
Piper, A. M., Garrett, A. A., and others, 1953, "Native and con-
taminated ground waters in the Long Beach- Santa Ana Area,
Calif."; U. S. Geol. Survey Water-Supply Paper 1136.
Simpson, T. R., Hotes, F. L., Harder, James A., and Lau, Leung-
ku, 1953, "Final report on sea-water intrusion"; Univ. Cali-
fornia, Sanitary Eng. Research Lab. Rept., September.
Stearns, H. T., and Macdonald, G. A., 1940, "Geology and ground-
water resources of the islands of Lanai and Kahoolawe,
Hawaii"; Hawaii Dept. Pub. Lands, Div. Hydrography Bull.
6, p. 75-95.
1942, "Geology and ground-water resources of the island
of Maui, Hawaii"; Hawaii Dept. Pub. Lands, Div. Hydro-
graphy Bull. 7, p. 1-222.
Stearns, H. T., and Vaksvik, K. N., 1935, "Geology and ground-
water resources of the island of Oahu, Hawaii"; Hawaii Dept.
Pub. Lands, Div. Hydrography Bull. 1, p. 215-278.
Thompson, D. G., 1926, "Ground-water problems on the barrier
beaches of New Jersey"; Geol. Soc. America Bull., v. 37, p.
1932, "Ground-water supplies of the Camden area, N. J.";
New Jersey Dept. Conserv. and Devel. Bull. 39.
1933, "Salt water in seacoast islands and wells"; Civil Eng.,
v. 3, no. 10, p. 579-580.
Turner, S. F., and Foster, M. D., 1934, "A study of salt-water
encroachment in the Galveston area, Texas"; Am. Geophys.
Union Trans., p. 432-436, June.
Wentworth, C. K., 1942, "Storage consequences of the Ghyben-
Herzberg theory"; Am. Geophys. Union Trans., p. 683-693.
1947, "Factors in the behavior of a Ghyben-Herzberg sys-
tem"; Pacific Sci., v. 1, no. 3, p. 172-184.
Wentworth, C. K., 1951a, "The problem of safe yield in insular
Ghyben-Herzberg systems"; Am. Geophys. Union Trans., v.
32, no. 5, p. 739-742, October.
1951b, "Geology and ground-water resources of the Hono-
lulu-Pearl Harbor area, Oahu, Hawaii"; Honolulu Board of
Water Supply, p. 90-95.
Winslow, A. G., and Doyel, W. W., 1954, "Salt water and its
relation to fresh ground water in Harris County, Tex."; Texas
Board of Water Engineers Bull. 5409.