Often, however, a change that adversely affects one organism may be advantageous to another. Conversely, antibiotic designed for use at one site, might pose a pollution threat to non-target or beneficial downstream microorganisms and ultimately other life forms.
The United States Environmental Protection Agency (EPA) oversees National Primary Drinking Water Regulations (NPDWRs or primary standards) that are legally enforceable standards regarding water contained in public water systems. Primary standards are intended to promote and protect public health by setting limits for levels of contaminants in drinking water. Agents of pollution or contaminants are divided into categories of microorganisms, disinfectants, disinfection byproducts, organic chemicals, inorganic chemicals, and radionuclides.
Nutrients that stimulate growth of bacteria and other oxygen-consuming decomposers in a river or lake, for example, are good for the bacteria but can be lethal to game fish populations. Similarly, warming of waters by industrial discharges may be deadly for some species but may create optimal conditions for others. Whether the quality of the water has suffered depends on your perspective. There are natural sources of water contamination, such as arsenic springs, oil seeps, and sedimentation from desert erosion, but most environmental scientists restrict their focus on water pollution to factors caused by human actions and that detract from conditions and uses that humans consider desirable.
Water pollution control regulations usually distinguish between point and nonpoint pollution sources. Factories, power plants, sewage treatment facilities, underground mines and oil wells, for example, are classified as point sources because they release pollution from specific locations, such as drain pipes, ditches, or sewer outfalls. These individual, easily identifiable sources are relatively easy to monitor and regulate. Their unwanted contents can be diverted and treated before discharge. In contrast, nonpoint pollution sources are scattered or diffuse, having no specific location where they originate or discharge into water bodies. Some nonpoint sources include runoff from farm fields, feedlots, lawns, gardens, golf courses, construction sites, logging areas, roads, streets, and parking lots. Whereas point sources often are fairly uniform and predictable, nonpoint runoff often is highly irregular. The first heavy rainfall after a dry period, for example, may flush high concentrations of oil, gasoline, rubber, and trash off city streets, while subsequent runoff may have much lower levels of these contaminants. The irregular timing of these events, as well as their multiple sources, scattered location, and lack of specific ownership make them much more difficult to monitor, regulate, and treat than point sources.
Among the most important categories of water pollutants are sediment, infectious agents, toxins, oxygen demanding wastes, plant nutrients, and thermal changes. Sediment (dirt, soil, insoluble solids) and trash make up the largest volume and most visible type of water pollution in most rivers and lakes. Rivers have always carried silt, sand, and gravel down to the oceans but human-caused
erosion now probably rivals the effects of geologic forces. Worldwide, erosion from croplands, forests, grazing lands, and construction sites is estimated to add some 75 billion tons of sediment each year to rivers and lakes. This sediment smothers gravel beds in which fish lay their eggs. It fills lakes and reservoirs, obstructs shipping channels, clogs hydroelectric turbines, and makes drinking water purification more costly. The most serious water pollutant in terms of human health worldwide is pathogenic (disease-causing) organisms. Among the most deadly waterborne diseases are cholera, dysentery, polio, infectious hepatitis, and schistosomiasis. Together, these diseases probably cause at least two billion new cases of disease each year and kill somewhere between six and eight million people. The largest source of infectious agents in water is untreated or insufficiently treated human and animal waste. The United Nations estimates that about half the world's population has inadequate sanitation and that at least one billion people lack access to clean drinking water.
Toxins are poisonous chemicals that interfere with basic cellular metabolism (the enzyme reactions that make life possible). Among some important toxins found in water are metals (lead, mercury, cadmium, nickel), inorganic elements (selenium, arsenic), acids, salts, and organic chemicals such as pesticides, solvents, and industrial wastes. Some of these materials are so toxic that exposure to extremely low levels (perhaps even parts per billion) can be dangerous. Others, while not usually found in toxic concentrations in most water bodies, can be taken up by living organisms, altered into more toxic forms, stored, and concentrated to dangerous levels through food chains. For example, fish in lakes and rivers in many parts of the United States have accumulated mercury (released mainly by power plants, waste disposal, and industrial processes) to levels that are considered a threat to human health for those who eat fish on a regular basis.
The United States continues to work toward a goal of making all surface waters "fishable and swimmable." Investments in sewage treatment, regulation of toxic waste disposal and factory effluents, and other forms of pollution control have resulted in significant water quality increases many areas. Nearly 90% of all the river miles and lake acres that are assessed for water quality in the United States fully or partly support their designed uses. Lake Erie, for instance, which was widely described in the 1970s as being "dead," now has much cleaner water and more healthy fish populations than would ever have been thought possible 25 years ago. Unfortunately, surface waters in developing countries have not experienced similar progress in pollution control. In most developing countries, only a tiny fraction of human wastes are treated before being dumped into rivers, lakes, or the ocean. In consequence, water pollution levels often are appalling. In India, for example, two-thirds of all surface waters are considered dangerous to human health.
See also Waste, toxic.