publications > water resources investigations > report 90-4108
US Department of the Interior
Hydrogeology of the Surficial Aquifer System, Dade County, Florida
Water-Resources Investigations Report 90-4108
By Johnnie E. Fish and Mark Stewart
Prepared in cooperation with the South Florida Water Management District
The surficial aquifer system, in which an unconfined ground-water flow system exists, is composed of the sediments from land surface downward to the top of a regionally extensive zone of sediments of low permeability called the intermediate confining unit. The aquifer system units, which vary in composition from clay-size sediments to cavernous limestone, are hydro stratigraphically divided into the Biscayne aquifer at the top; an intervening semiconfining unit that consists principally of clayey sand; a predominantly gray limestone aquifer in the Tamiami Formation in western and west-central Dade County; and sand or clayey sand near the base of the surficial aquifer system. The base of the surficial aquifer system ranges from a depth of about 175 to 210 feet below land surface in westernmost Dade County to greater than 270 feet in northeastern Dade County. Test drilling and aquifer-test data indicate a complex hydraulic conductivity distribution. Hydraulic conductivities of the very highly permeable zone of the Biscayne aquifer commonly exceed 10,000 feet per day; in the gray limestone aquifer, they range from 210 to 780 feet per day.
Transmissivities of the surficial aquifer system vary locally but have a recognizable areal trend. Estimated values generally are about 300,000 feet squared per day or greater in nearly all of central and eastern Dade County. Transmissivity is lower to the west, decreasing to less than 75,000 feet squared per day in western Dade County. High transmissivity usually is associated with thick sections of the Fort Thompson Formation within the Biscayne aquifer. The gray limestone aquifer of the Tamiami Formation has transmissivities that range from 5,800 to 39,000 feet squared per day in western Dade County. The transition from high transmissivity to relatively low transmissivity is often only a few miles wide and coincides with the decrease in thickness of the very highly permeable Fort Thompson Formation, which marks the western boundary of the Biscayne aquifer.
More effective drainage as a result of extensive canal
systems and large-scale pumping from municipal well fields has greatly altered
the predevelopment flow system in eastern Dade County by: (1) eliminating or
greatly reducing a seasonal and coastal ground-water ridge; (2) reducing deep
circulation; (3) reducing or eliminating seasonal westward movement of ground
water; (4) causing accelerated stormwater runoff and short ground-water flow
paths; and (5) generally lowering the water table and inducing saltwater
intrusion. Under predevelopment conditions in western Dade County, water
entered the gray limestone aquifer by lateral movement from Broward and Collier
Counties, and by downward seepage from The Everglades and the Biscayne aquifer,
and moved southward and southeastward into Dade County to coastal discharge
areas. Circulation in the Biscayne aquifer inland also was primarily to the
south and southeast. In eastern Dade County, the seasonal ground-water ridge
that formed under predevelopment conditions supported both easterly and
westerly ground-water flow away from the ridge axis. This seasonal flow created
a zone of lower dissolved solids.
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Conversion Factors, Vertical Datum, and Abbreviated Water-Quality Units
Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 -- a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.
Abbreviated water-quality units used in report:
Hydrogeology, Aquifer Characteristics, and Ground-Water Flow of the Surficial Aquifer System, Broward County, Florida (Water Resources Investigations Report 87-4034)
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 15 January, 2013 @ 12:44 PM (KP)