Water pollution control laws have greatly reduced the worst water pollution in most industrialized countries; in many developing countries, water pollution is getting worse, but access to safe drinking water is improving slowly, especially in urban areas.
Water pollution is any physical, biological, or chemical change in water quality that adversely affects organisms or makes it unsuitable for other uses. Pollution is classed as point source, originating at a specific location such as a drain pipe, ditch, or sewer outfall, or nonpoint source, which is widespread runoff from fields, streets, or cities. Nonpoint sources are usually more irregular and harder to control than point sources. Sediment, untreated human waste, and nutrient enrichment are the most widespread pollutants.
Major types of water pollution include infectious agents; oxygen-demanding wastes; nutrients; inorganic salts, metals, acids and bases; organic chemicals; sediment; and thermal pollution. In developing countries, infectious agents in drinking water are a primary cause of disease and debilitation.
Nutrient enrichment and warming cause eutrophication, which involves rapid plant and algae growth, and often results in rapid decomposition and oxygen depletion in aquatic systems. Oligotrophic lakes and streams, by contrast, are cold, and oxygen-rich, but have low biological productivity. In marine environments, excess nutrients can produce dead zones and toxic tides—concentrations of poisonous microorganisms.
The U.S. Clean Water Act requires that the EPA monitor water quality and regulate discharge. Total maximum daily loads (TMDLs) are set for major pollutants. Only 10 percent of water pollution in the United States now comes from point sources. Nonpoint sources, including feedlots, manure lagoons, farm fields, construction sites, and cities, remain important problems.
Groundwater can be contaminated by waste in recharge zones, through abandoned wells, and by buried waste. Leaking underground storage tanks release contaminants, such as MTBE, into groundwater.
Ocean pollution comes from many sources, including garbage, sewage, nutrients, and oil spills. About half of oil releases come from routine releases and tank cleaning.
The cheapest and most effective way to reduce water pollution is source reduction. Often, industrial wastes can be recycled or reclaimed rather than released. Agricultural practices can reduce field runoff, and sediment barriers at construction sites can reduce sediment releases. Land-use planning can greatly reduce pollution production.
Sewage treatment is one of the most important steps in maintaining clean drinking water. Primary treatment removes, strains, and settles out solids. Secondary treatment, including aeration, digestion, and chlorination, removes pathogens and organic material. Tertiary treatment removes inorganic nutrients and oxidizes remaining organics. A variety of low-cost methods, such as constructed wetlands, can be used to purify water.
Water legislation is credited with radically improving water quality. Legislation, including the Clean Water Act, remains controversial: costs can be high and are largely borne by producers; it can be difficult to identify the best, most affordable, or best practicable technology; and proponents of greater controls object that stricter rules are needed and that enforcement is too often lax.
