Saline Water
Intrusion Related to Well Construction in Lee County, Florida
Water-Resources Investigations Report 77-33
By D.H. Boggess, T. M. Missimer,
and T.H. O'Donnell
ABSTRACT
Ground water is
the principle source of water supply in Lee County, Florida where an estimated
30,000 wells have been drilled since 1990. These wells ranges in depth from
about 10 to 1,240 feet and tap the water table aquifer or one or more of
the artesian water-bearing units or zones in the Tamiami Formation, the
upper part of the Hawthorn Formation, the lower part of the Hawthorn Formation
and the Tampa Limestone and the Suwannee Limestone. Before 1968, nearly
all wells were constructed with galvanized or black iron pipe. Many of these
wells are sources of saline-water intrusion into freshwater-bearing zones.
The water-bearing
zones in the lower part of the Hawthorn Formation, Tampa Limestone, and
Suwannee Limestone are artesian-they have higher water levels and usually
contain water with a higher concentration of dissolved solids than do the
aquifers occurring at shallower depths. The water from these deeper aquifers
generally range in dissolved solids concentration from about 1,500 to 2,400
mg/L, and in chloride from about 500 to 1,00 mg/L. A maximum chloride concentration
of 15,200 mg/L has been determined. Few of the 3,00 wells estimated to have
been drilled to these zones contain sufficient casing to prevent upward
flow into overlaying water-bearing zones. Because of water-level differentials,
upward movement and lateral intrusion of saline water occurs principally
into the upper part of the Hawthorn Formation where the chloride concentrations
in water unaffected by saline-water intrusion ranges from about 80 to 150
mg/L. Where intrusion from deep artesian zones has occurred, the chloride
concentration in water from the upper part of the Hawthorn Formation ranges
from about 300 to more than 2,100 mg/L
Surface discharges
of the saline water from wells tapping the lower part of the Hawthorn Formation
and the Suwannee Limestone also had affected the water-table aquifer which
normally contains water with 10 to 50 mg/L of chloride. In one area, the
chloride concentration in water from the water table aquifer ranged from
200 to 590 mg/L as a result of intrusion.
In areas adjacent
to tidal-water bodies, the water table aquifer contains water that is very
saline, Where the wells in such areas have been constructed with metal casings,
the metal corrodes when exposed to the saline water, and many ultimately
develop holes. This permits saline water to leak into the well where the
water level in the well is lower than the water table. The intrusion of
saline water from the water-table aquifer into the upper part of the Hawthorn
Formation is a major problem in parts of Cape Coral. Withdrawal of water
from the upper part of the Hawthorn Formation has caused water levels to
decline below the lowest annual position of the water table, so that downward
leakage is perennial. In some coastal areas, wells that tap the upper part
of the Hawthorn Formation contain water whose chloride concentration is
as much as 9,500 mg/L.
Upward leakage of
saline water from the deep artesian aquifers and downward leakage of saline
water from the water-table aquifer can be prevented by proper well construction.
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