A change in the chemical, physical, biological, and radiological quality of water that is injurious to its existing, intended, or potential uses. (for example, boating, waterskiing, swimming, the consumption of fish, and the health of aquatic organisms and ecosystems). The term “water pollution” generally refers to human-induced (anthropogenic) changes to water quality. Thus, the discharge of toxic chemicals from a pipe or the release of livestock waste into a nearby water body is considered pollution. Conversely, nutrients that originate from animals in the wild (for example, a herd of elk) or toxins that originate from natural processes (for example, red tides) are not considered pollution.
The contamination of ground water, rivers, lakes, wetlands, estuaries, and oceans can threaten the health of humans and aquatic life. Sources of water pollution are generally divided into two categories (Table 1). The first is point-source pollution, in which contaminants are discharged from a discrete location. Sewage outfalls and the 1989 Exxon Valdez oil spill are examples of point-source pollution. The second category is non-point-source or diffuse pollution, referring to all of the other discharges that deliver contaminants to water bodies. Acid rain and unconfined runoff from agricultural or urban areas are examples of non-point-source pollution. The principal contaminants of water are shown in (Table 2). These include toxic chemicals, nutrients and biodegradable organics, and bacterial and viral pathogens.
Since the passage of the Clean Water Act in 1972, there has been considerable progress in reducing the amount of pollution originating from municipal and industrial point sources. Most wastewater treatment plants are designed to accomplish the removal of suspended solids, biodegradable organics, and pathogenic organisms. The impact of the discharge of biodegradable organics can be measured in terms of the buildup of sludge deposits and depletion of the dissolved oxygen resources of water bodies. This situation led to requirements for secondary treatment of wastewaters. Similarly, concern over the toxicity caused by the discharge of heavy metals in treated effluents led to the development of effective pretreatment programs.
Reducing contamination from nonpoint sources has been considerably more difficult, in part because these inputs are widely distributed and highly variable. To better understand the relationship between water quality and chemical use, land use, climate geology, topography, and soils, Congress appropriated funds for the National Water-Quality Assessment (NAWQA) Program in 1991. The NAWQA Program is an ongoing investigation by the U.S. Geological Survey in river basins and aquifers around the country.
Transport and transformation processes
Contaminants discharged to water bodies are subject to a variety of transport and transformation processes and operations that can alter their composition. The physical, chemical, and biological processes that control the fate of the contaminants discharged to water bodies are numerous and varied. It is convenient to divide them into transport processes that affect all water-quality parameters in the same way and fate and transformation processes which are constituent-specific.
After initial dilution, contaminants discharged to a water body are transported by two basic processes, advection and dispersion. Advection refers to the transport of a constituent resulting from the flow of the water in which the constituent is dissolved or suspended. Turbulent velocity fluctuations, in conjunction with concentration gradients and molecular diffusion, lead to a mass transport phenomenon called dispersion.
The principal fate and transformation processes that affect contaminants discharged to the environment are operative in most water bodies (Table 3). The relative importance of individual fate and transformation processes will be site-specific and will depend on the water-quality parameter under evaluation. For example, deoxygenation brought about by bacterial activity, surface reaeration, sediment oxygen demand, and photosynthesis and respiration are of major importance in assessing the oxygen resources of a stream.
Harmful effects on human health
Water pollution can threaten human health when pollutants enter the body via skin exposure or through the direct consumption of contaminated food or drinking water. For example, many states have issued fish consumption advisories following the detection of mercury in fish tissues. Other priority pollutants, including dichlorodiphenyl trichloroethane (DDT) and polychlorinated biphenyls (PCBs), persist in the natural environment and bioaccumulate in the tissues of aquatic organisms. These persistent organic pollutants are transferred up the food chain (in a process called biomagnification), and they can reach levels of concern in fish species that are eaten by humans. Finally, bacteria and viral pathogens can pose a public health risk for those who drink contaminated water or eat raw shellfish from polluted water bodies. See also: Environmental toxicology; Food web
Harmful effects on aquatic species
Contaminants have a significant impact on aquatic ecosystems. for example, enrichment of water bodies with nutrients (principally nitrogen and phosphorus) can result in the growth of algae and other aquatic plants that shade or clog streams. If wastewater containing biodegradable organic matter is discharged into a stream with inadequate dissolved oxygen, the water downstream of the point of discharge (typically an outfall) will become anaerobic and will be turbid and dark. Settleable solids, if present, will be deposited on the streambed, and anaerobic decomposition will occur. Over the reach of stream where the dissolved-oxygen concentration is zero, a zone of putrefaction will occur with the production of hydrogen sulfide, ammonia, and other odorous gases. Because many fish species require a minimum of 4–5 mg of dissolved oxygen per liter of water, they will be unable to survive in this portion of the stream. In addition to reductions in dissolved oxygen, aquatic species are sensitive to changes in other physical habitat factors, including pH, temperature, and suspended solids.
Direct exposures to toxic chemicals is also a health concern for individual aquatic plants and animals. For example, pesticides are used to kill undesirable or nuisance organisms in many urban and agricultural areas. These chemicals are frequently transported to lakes and rivers via runoff, and they can have unintended and harmful effects on aquatic life. Obvious signs of contaminant exposure in fish from polluted environments include lesions, tumors, and skeletal deformities. Toxic chemicals have also been shown to reduce the growth, survival, reproductive output, and disease resistance of exposed organisms. These effects, while subtle, can have important consequences for the viability of aquatic populations and communities. See also: Insecticide
Wastewater discharges are most commonly controlled through effluent standards and discharge permits. In the
Water-quality standards and criteria
Water-quality standards are sets of qualitative and quantitative criteria designed to maintain or enhance the quality of receiving waters. In the
For toxic compounds, chemical-specific or whole-effluent toxicity studies are used to develop standards and criteria. In the chemical-specific approach, individual criteria are used for each toxic chemical detected in the wastewater. Criteria can be developed to protect aquatic life against acute and chronic effects and to safeguard humans against deleterious health effects, including cancer. The chemical-specific approach, however, does not consider the possible additive, antagonistic, or synergistic effects of multiple chemicals. The biological availability of the compound, which depends on its form in the wastewater, is also not considered in this approach.
The whole-effluent toxicity approach can be used to overcome the shortcomings of the chemical-specific approach. In the whole-effluent approach, toxicity or bioassay tests are used to determine the concentration at which the wastewater induces acute or chronic toxicity effects. In bioassay testing, selected organisms are exposed to effluent diluted in various ratios with samples of receiving water. At various points during the test, the organisms affected by various effects, such as lower reproduction rates, reduced growth, or death, are quantified. To protect aquatic life discharge limits are established based on the results of the tests. See also: Hazardous waste; Sewage treatment