Rapid urbanization of the lower eastern coast and growth of agricultural areas pose additional threats of ground-water contamination by manmade liquid and solid wastes and by fertilizers and pesticides. Because of the benefits of seasonally heavy rainfall and dilution, however, the level of contamination probably has been excessive only in some areas. Ground water in the section of the Biscayne aquifer beneath the densely populated urban parts of southeastern Florida contains nutrients and coliform bacteria at shallow depths as a result of effluent from thousands of septic tanks and disposal wells and from seepage from polluted canals. These contaminants presumably have accumulated over the 60 years of urbanization. Although septic tanks operate effectively in the southeast, they provide only partial water treatment.
Septic tanks can become ineffective from the lack of periodic maintenance.
In such cases, raw wastewater seeps into the aquifer and causes local pockets of pollution. The recent constraints on the use of septic tanks and the construction of sewer systems to service large urban sectors will further curtail the use of septic tanks in many areas and thereby reduce a potential cause of contamination to the ground-water system. 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 southeastern Florida indicate that local pollution plumes exist downgradient from the dump sites but primarily in the shallow parts of the aquifer.