7/9/79 v" < ^ ^
UJhile Floridians are united in their fear of dwindling gasoline
supplies, there looms in the Sunshine State's future the possibility
of another kind of shortage, one so potentially devastating as to make
a lack of gas only a minor annoyance.
LThe e t -t y is that fresh groundwater supplies, essential to
Growth, development and" the maintenance of existing populations,
are already imperilled and may soon become contaminated by salt water
throughout much of the State.
LScientists, using elementary principles of hydrology, are able to
fashion a doomsday scenario which unfolds as follows:
L--Complete contamination by salt water of all the fresh water wells
in the Pinellas Peninsula, upon which St. Petersburg is located, and
the Inter-Bay Peninsula, which is tipped by the City of Tampa.
CBoth the cities, among Florida's largest, have experienced major
problems with salt water contamination of drinking water wells for
the past fifty years. Both had to develop extensive inland well
fields to satisfy the needs of their growing coastal populations.
---The gradual intrusion by salt water into the fresh water wells
throughout the entire Floridan Peninsula (draw an east to west line
across Florida as"Orlando and everything south of it is the Peninsula)
until all drilling for fresh water in the region will be a waste of
CFresh water for home, industrial, and agricultural use will have
ceased to exist by the time the intrusion process has spread from
the coasts to the middle of the Peninsula.
L-The social and economic effects of such a catastrophe are difficult
to imagine. Growth and development would obviously cease and
existing populations would choose between mass exodus and paying
dearly for alternative water supplies.
LIs such a catastrophe possible? The likelihood of complete
elimination of Peni"sula Florida's fresh groundwater supply by salt
water contamination is, of course, remote. But even Scientists
' who predict & little chance of disaster ahead acknowledge that because
J of its unique hydrological characteristics, the Floridan Peninsula
provides a fragile and challenging water source, deserving of much
study and of even greater respect.
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LFor many years, --".twater intrusion has been-" fact of everyday life
in almost all coastal communities on the Floridan Peninsula. Most
dramatically affected are high growth counties, such as Sarasota, which
feature a rapidly increasing demand and a geography generally un-
suitable to development of water storage facilities. Another is
Pasco County, where coastal groundwater supplies were unable to with-
stand the demands a burgeoning local population in addition to
requirements of Tampa and St. Petersburg to the south.
When an acquifer --a water bearing formation -- is exposed below
sea level along the coastline of a body of land, the upper part of
this formation usually contains freshwater and the lower is saturated
with saltwater. In effect, the freshwater virtually floats on top
of the sea water within the acquifer.
Fresh and salt waters have different densities Because the fresh
water table is above sea level, the boundary zone between the two
waters is maintained in a hydraulic balance.
According to the Ghyben-Herzberg principle, a formula often used
by hydrologists to predict the depth and abundance of groundwater
supplies, fresh after extends to a depth of about 40 times the height
that the water table is found above mean sea level.
(jf freshater is to exist to any appreciable depth, the water table
must always be above sea level. Under such circumstances, there
exists a freshwater gradient ffom the land(iowhard:the sea. Following
this gradient, fresh water discharges into the sea, the natural out-
flow zone for that part of the acquifer which is exposed below sea
_There exists, then, a hydrodynamic balance between the fresh and
salt waters -- known as the freshwater-saltwater interface. However,
when freshwater is taken from coastal regions by pumping from wells,
this balance between fresh and saltwater often changes. The outflow
of freshwater from the land toward the sea is reduced and the water
table becomes lower. Consequently, the salt water body is able to
move farther inland. The result: intrusion into freshwater wells
by saltwater. -- Cc'k S c iunb%' UWC- % A r
wcU J LeQi OJrt C evQ.
IThe bottom line in this situation, according to the scientists,
appears to be that the more freshwater wells are drilled in coastal
areas the greater the danger of sea water intruding upon these wells.
LHow does one tell if the situation in a given area is becoming
alarming? One aid to .making this determination is a concept called
potentiometric surfaceA A imaginary hydrolof c surfacesconsisting of
points which represent the level or elevation water would rise in
tightly cased wells tapping a confined acquifer. This surface may be
above or below the land surface. Contour maps showing potentiometric
surfaces at various points within a given geographical area can be
used to determine where areas of most concentrated well drilling
have taken place. In most cases, such maps can be used to predict
those areas where saltwater intrusion can be expected to become a
problem. The potentiometric values are highest in high-recharge
relatively l'w-use ?ress, such as the Green Swamn area in Polk and
Lake Counties and very low in such densely populated high-usage areas
such as Pinellas County, Coastal Hillsborough County, Manatee County
and "asota County.
The two enclosed maps represent changes within the potentiometric
services in the area surrounding Tampa Bay from 1969 to 1975.
The "doomsday scenario under which sea water would intrude into
all of the Fl p)dan Peninsula's fresh ground water sources, is based
upon a considerable simplification of the very complicated hydro-
geology of the Floridan Peninsula. For example, under the Ghyben-
Herzberg principle, it could be assumed in Florida, where the fresh
ground water level averages a foot above sea level, that where an
average water table altitude is two feet above sea level, the depth
to saltwater would be eighty feet.
In the Green Swamp High, where the potentiometric surface is a
120-130 feet above sea level, the measured depth to the base of the
freshwater supply is about 2,000 feet below sea level. If the
Ghyben-Herzburg formula operated, the fresh water could be expected
to extend to 4800 feet below mean sea level.
There also exists within the Floridan Peninsula underground saltater
from sources other than the traditional freshwater-saltwater interface.
Saltwater that underlies the Everglades, Big Cypress Swamp, Lake
Okeechobee and the St. Johns River Valley, for example, is probably
a remnant of the late Pleistocene age, when sea levels in Florida
were averaged twenty-five and forty-two feet above the present levels.
undergroundd rock formations and clay-like materials have in some
parts of the Floridan peninsula trapped and held in place sea water
which has been in existence for sixty to seventy million years.
SThere exist several methods for reducing the likelihood of
Lhe most important, in terms of overall and long-term effectiveness
is modification of pumping, exactly what Florida's water management'
districts are charged by statute with handling through the con-
sumptive use permitting process. Greatly simplified, that process
allows the management districts,1g, in effectTissue licenses to
persons who wish to extract significant amounts'of fresh water from
the acquifer. Unreasonable, unjustifiable requests are denied.
Another method, also in use in Florida, is artificial recharging-
putting back into the acquifer enough w te to prevent intrusion
after the danger has already been e by pumping. The problem?
The method can be expensive if recharge is by deep well injection,
or some form of recharge well, which requires that water supplies be
brought from afar and injected into the acquifer.
Let another method is the pumping trough -- a line of wells sunk
adjacent to and paal ing the coast, so as to form troughs in the
ground water level. -hile there is considerable technical merit
to the proposal, scientists feel it to be too expensive.
L further alternative, a pressure ridge, is the exact opposite of
the pumping trough. Instead of a series of pumping wells paralleling
the coast, a series of recharge wells would be installed. The
hoped-for result would be an increase aog- t- at ; of potentiometric
surfaces a Tt.4A t\ A
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