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South Florida's hydrologic systems


History of the Study
Regional System
Hydrologic Systems
- Importance of Water
> Aquifers
- Water Balance
- Hydrologic Changes
- Quantity Problems
- Resource Limits
- Quality
Final Word
PDF version
Nearly all municipal water supplies of south Florida are withdrawn from shallow aquifers, generally from wells less than 75 m (250 ft) deep. The most productive and widespread of the shallow aquifers are the Biscayne aquifer of southeast Florida and the shallow aquifer of southwest Florida (fig. 20).

Important also, but of lower yield, are the coastal aquifer, which extends northward from Palm Beach County, and local aquifers scattered through the remaining area, particularly those in Lee County that supply potable water to coastal urban areas (fig. 21). The Floridan aquifer, at greater depth, is capable of large yields of brackish and saline water by artesian flow, but because the water is not potable, the Floridan aquifer is not in general use as a source of water supply in south Florida.

The Biscayne aquifer, a highly permeable, unconfined, shallow aquifer more than 60 m (200 ft) thick in east Broward County, wedges out 56 to 64 km (35 to 40 mi) to the west in the Everglades (Klein and others, 1973). The principal water-yielding beds in the aquifer are solution-riddled limestones. The permeability of the aquifer is highest in south Dade County and decreases to the north and inland. Coastal parts of the aquifer are affected by seawater intrusion. Superimposed upon the aquifer is the system of levees, canals, and conservation areas (fig. 1) of the Central and Southern Florida Flood Control District (FCD) utilized for water control and water management. The responsibilities of the FCD include operating the system to prevent flooding in developed areas during the rainy season, insuring that adequate supplies of good quality water are available to meet increasing domestic and agricultural needs and providing adequate water supply to the eastern part of Everglades National Park.

illustration of generalized subsurface section showing aquifers of south Florida
FIGURE 20. (above) Generalized subsurface section showing aquifers of south Florida. [larger image]

map showing principal freshwater aquifers of south Florida
FIGURE 21. (above) Principal freshwater aquifers of south Florida. [larger image]

The shallow aquifer of southwest Florida is as important to the urban lower Gulf coast as the Biscayne aquifer is to southeast Florida (Klein and others, 1973). The shallow aquifer also is unconfined and is thickest, about 39 m (130 ft), along the west coast. It thins to about 24 m (80 ft) in central Collier County and wedges out near the east edge of the county. The area of greatest potential for water supplies is central and west-central Collier County where the limestone beds in the aquifer are thickest, most extensive, and highly permeable. Permeability of the aquifer decreases within 16 km (10 mi) of the Gulf because of increasing content of fine sand and marl. Canals dug during the 1960's for urban development have accelerated freshwater runoff in the west part of the county (McCoy, 1972). Southward sheet flow occurs in the east half of the county during most of rainy season and for a few months after the rainy season. Major losses from the aquifer are by evapotranspiration and by canal flow in the west part. Municipal and irrigation uses are minimal.

Except for West Palm Beach, all coastal communities north of the vicinity of Boca Raton obtain water supplies from an unconfined shallow aquifer. The aquifer is about 75 m (250 ft) thick near the coast and becomes thinner to the west. The aquifer probably is hydraulically connected with the Biscayne aquifer to the south, but its hydrogeologic properties differ in that its permeability is less than that of the Biscayne aquifer and that it is composed predominantly of sand rather than limestone.

About 60 percent of the annual rainfall in south Florida is lost by evapotranspiration; the remainder infiltrates shallow aquifers, runs off as surface flow, or is used by man. During the wet season and for several weeks afterward, much of the surface flow is in the form of sheet flow in the Everglades and Big Cypress Swamp. Because the land surface is virtually flat, sheet flow is slow, leaving extensive areas inundated for long periods. Canals and levees for flood control and water management have greatly altered surface flows in southeast and southwest Florida coastal areas, so that part of the water that originally flowed southward through the interior is now being diverted eastward and westward. Even during extremely dry years, the quantities of water discharged to the ocean greatly exceed the quantities currently pumped for municipal purposes (fig. 22).

illustration showing dry-year outflows of freshwater in south Florida

FIGURE 22. Dry-year outflows of freshwater in south Florida. [larger image]

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