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Degradation of Water QualityWater quality has been degraded by human activities in large areas of south Florida. Water in urban and agricultural canals commonly has high concentrations of nutrients and toxic compounds compared to water in marshes that are remote from canals. Figure 36 illustrates the wide variability in the concentration of the nutrient, phosphorus, in south Florida waters. Drainage of nutrient and toxic-ladened water from urban and agricultural lands has degraded lakes, streams, canals, estuaries, and bays of the region.
A degradation of water quality has been noticeable in recent years in the urban-industrial areas along the eastern coast. Most degradation has been in urban canals, especially during periods of low flow when many of these canals are covered with algae and scum and are choked with aquatic weeds. These conditions have been brought about through the discharge of nutrient-laden sewage and stormwater runoff into canals. Runoff also carries bacteria, viruses, oil and grease, toxic metals, and pesticides into urban canals from which they can be discharged into coastal waters or can enter the ground-water system and the public water supply (Klein and others, 1975).
An ample supply of dissolved oxygen is most important for water of good quality, especially in urban areas where much of the oxygen is used in the decomposition of sewage. In contaminated canals with a luxuriant growth of plants, the dissolved-oxygen content is high during daylight. During the night, however, the dissolved-oxygen content may approach zero as the oxygen is depleted to oxidize sewage. Thus, many urban canals lack popular sport fish, such as bass, and are inhabited instead by gar and mullet, which are able to tolerate low levels of dissolved oxygen (Klein and others, 1975).
Chlorine chemicals, such as polychlorinated biphenyls (PCB's), dioxin, and furans, which are generated and used primarily in urban and industrial areas, pose serious health hazards to fish, wildlife, and human populations (Colborn and others, 1993). PCB's are a diverse family of compounds with a wide variety of industrial applications, which include use in transformers, plasticizers, rubber, adhesives, inks, sealants, caulking compounds, and other products. Although most uses of PCB's have been banned since the late 1970's, these persistent chemicals are still found in the environment. Dioxins and furans are byproducts of chlorinated industrial processes, which include incineration of hazardous, medical, and solid wastes; chemical manufacturing; plastic production; and chlorine bleaching of pulp and paper. Chlorine chemicals have long been known to have toxic effects on fish and wildlife, but it has been recently reported that these chemicals can disrupt the endocrine systems of fish and wildlife, which cause such problems as reproductive failure, birth deformities, demasculinization, defeminization and immune system disorders (Colborn and others, 1993).
PCB's are widely distributed in the south Florida environment (Klein and others, 1975). The wide distribution is probably a result of volatization and transport by aerosols and fallout with dust or rain (Pfeuffer, 1991). Concentrations, are probably highest in industrial urban areas. For example, fish from a canal at Miami International Airport contained PCB's in a concentration of 1,000 mg/Kg (Freiberger and McPherson, 1972). The high concentrations of these chemicals in industrial urban areas could pose a hazard for the drinking water supply by entering the shallow Biscayne aquifer (Klein and others, 1975).
Generally, ground water is less susceptible to pollution than surface water because it is filtered as it moves through sediments. However, because contamination in ground water is less readily detected than in surface water, and because of the difficulties associated with working below land surface, the contaminants can become widely dispersed and quite expensive to remediate by the time the magnitude of the problem is recognized. In past years, the prime threat to ground-water quality in south Florida has been that of seawater intrusion in coastal areas and near heavily pumped municipal wells. In recent years, however, seawater intrusion generally has been controlled by providing sufficient freshwater so that adequate high-water levels are maintained near the coast.
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. Contaminated 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 and organics also have been found in the shallow ground water beneath the dumps. Because of the high permeability of the aquifer, contaminants can move readily to wells and estuaries in downgradient areas.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:03 PM(KP)