by Joseph L. Haberfeld, Professional Geologist,
Florida Department of Environmental Protection,
Tallahassee, Florida
The Floridan aquifer is a major water supply source in the
southeastern United States, but in south Florida it is
generally too
saline for use as a water supply. Because of unique geologic
circumstances, the Floridan aquifer is used for disposal of
wastewater in south Florida. The injection zones are
dolomite, and
have secondary porosity development through the occurrence
of
extensive fracturing. Permeability of the injection zones is
extremely high (approximately 100 darcies), with
transmissivity
values often 1 million gallons per day per foot and higher.
Overlying these injection intervals are confining units
comprised
principally of limestone, with permeabilities and
transmissivities
several orders of magnitude lower than those of the
injection zones.
The formation of the south Florida injection zones (commonly
referred to as "Boulder Zones" because of the
break-up of fractured
dolomite into boulders when encountered by the drill bit) is
not
fully understood by geologists. Dolomitization and secondary
porosity development may have occurred during deposition of
carbonate sediments, after subsequent burial by overlying
carbonates, or as a result of some combination of the two
processes.
The movement of ground water through the Floridan aquifer
certainly
played a critical role in the formation of permeable zones.
Stratigraphic occurrence of these zones grades from the Avon
Park
Formation (middle Eocene) in the Tampa region to the
underlying
Oldsmar Formation (early Eocene) in the Fort Myers and Miami
regions.
Thus, the injection zones are not at the same stratigraphic
level
throughout the state.
The bulk of the data on the Floridan aquifer of south
Florida is
derived from drilling of injection wells. The data includes
lithologic information (cuttings and cores), geophysical
logs, video
surveys, drill stem tests, driller's logs, injection and
pumpout
tests, and long term monitoring of overlying aquifers. There
are
approximately 95 sites with 175 deep injection wells in
Florida which dispose of secondary treated municipal
wastewater and,
to a lesser extent, industrial wastewater (including
concentrate
left over from desalinization processes). In addition to
these 95
sites, there are a smaller number of injection sites
disposing of
oil and gas production related fluids.