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publications > paper > PP 1011 > south florida's hydrologic systems > water quality > ground-water quality
South Florida's hydrologic systems
Rapid urbanization of the lower east coast and growth of agricultural areas pose additional threats of ground-water pollution - those of contamination by manmade liquid and solid wastes and by fertilizers and pesticides. Ground water in the section of the Biscayne aquifer beneath the densely populated urban parts of southeast Florida contains nutrients and coliform bacteria at shallow depth as a result of effluent from thousands of septic tanks and seepage from polluted canals. These pollutants presumably have built up over the 60 years of Urbanization. Although septic tanks operate adequately in the southeast, they provide only partial water treatment. Because of the benefits of seasonal heavy rainfall and dilution, however, the level of pollution probably has been excessive only in some local areas. Septic tanks become inoperative because of lack of periodic maintenance. In such cases raw waste water seeps into the aquifer and causes local temporary pockets of pollution. The recent constraints on the use of septic tanks and the construction of sewage systems to service large urban sectors will further curtail the use of septic tanks in many areas and thereby reduce the potential pollution of the ground-water system. These sewage systems will utilize ocean outfalls rather than outfalls to inland canals, as was the case for many years. Furthermore, within the past 2 years, the deep part of the Floridan (artesian) aquifer, at a depth of about 900 m (3,000 ft), has been locally used as a reservoir for wastewater discharge.
Solid-waste disposal in dumps poses another but less widespread source of pollution of shallow aquifers by contributing leachates from garbage and trash. Many materials in dumps are toxic and nonbiodegradable. Preliminary data from widely scattered study sites in southeast Florida indicate that local pollution plumes exist downgradient from the dump sites but primarily in the shallow parts of the aquifer. Polluted ground water near dumps also contains coliform bacteria and nutrients because sludge from many septic tanks and small sewage-treatment plants is disposed at some dumps. Traces of toxic metals have also been found in the shallow ground water beneath the dumps. Because of the high permeability of the aquifer, contaminants can move readily to wells in downgradient areas.
Numerous deep wells under high artesian pressure, drilled to the Floridan aquifer many years ago, have been a source of brackish-water contamination of shallow aquifers in Lee and Hendry Counties and to a lesser extent in Collier County. The wells were not cased deeply enough or the casings have corroded, and brackish water under pressure from the Floridan aquifer moves upward along open well bores or through corroded sections and seeps outward to contaminate large parts of shallow aquifers (Klein and others, 1964; Sproul and others, 1972). Such contamination can be controlled by a program of plugging of the leaking wells. Other areas of brackish-water contamination are inland from Naples (McCoy, 1972) and south of Lake Okeechobee (Parker and others, 1955). These sources of contamination are the natural upward leakage of artesian water from the Floridan aquifer through partly permeable confining layers or open well bores or incompletely flushed residual saltwater remaining from ancient inundations by the sea.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:04 PM (KP)
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