Ambient Water Quality

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Water is a major pathway by which humans and all life-forms can be exposed to chemicals and pathogens. Several different sources of water pollution exist. "Point source" and "nonpoint source" are terms used to describe two such routes of entry.

Point-source pollutants, the most common violators of water quality standards, enter waterways at well-defined locations, such as a pipe or sewer outflow. These discharges are even and continuous. Industrial factories, sewage treatment plants, and storm-sewer outflows are common point sources of water pollution. A reduction of point-source pollutants has been accomplished during recent decades, because the products of these sources are easy to find and treat.

Unlike point sources, which have distinct entries, nonpoint-source pollutants run off or seep into waterways from broad areas of land. Nonpoint sources are the largest contributors to water pollution. It is estimated that 98 percent of bacterial contaminants and 73 percent of biological oxygen demand are due to nonpoint sources. Various human land-use practices cause nonpoint-source pollution, including agriculture, construction activities, urban street runoff, acid mine drainage, and fallout of airborne pollutants.

Agriculture is the main source of water pollution in the United States and is estimated to decrease the quality of 50 to 70 percent of U.S. surface waters. Soil is the main contaminant from farmland runoff. Soil increases the turbidity of water, which impacts agricultural productivity and damages aquatic life. Chemical fertilizers and pesticides are also detrimental to water quality since they can promote algal growth or destroy fish. Buffer zones (e.g., planting vegetation along waterways) can prevent runoff from farmlands.

Runoff from construction sites contributes the greatest amount of sediments. Approximately 5 percent of the United States' surface waters are affected by these contaminants. Mitigation strategies include mulches, vegetation, retention basins to detain runoff water and allow settlement of suspended sediments, and maintaining existing trees and shrubs.

Storms add large amounts of sediment and other pollutants into local waterways in urban areas. Urban runoff is the main contributor of oil to surface water. Frequent street cleaning, litter control, adding more vegetation and plants along waterways, and limited applications of salt and sand can reduce urban street runoff.

Byproducts of coal mining and mine drainage can also cause adverse effects when they enter bodies of water. Naturally occurring iron pyrite produces iron hydroxide and sulfuric acid. Iron hydroxide covers the bottom of streams and thus destroys life at the bottom. Sulfuric acid increases the acidity of water and destroys any species which cannot survive in such an environment. The lowered pH also causes certain metals to leach from soil and rocks and enter streams. Recent efforts such as regrading and revegetation have proven effective in counteracting the effects of acid mine drainage. Sealing openings to abandoned underground mines and creating engineered wetlands have also had positive results.

Airborne pollutant fallout is the leading source of PCBs and hydrocarbons in the oceans and waterways. Increasing the land's capacity to retain water and decreasing the overall amount of airborne pollutants and the amount of pollutants are two important ways of addressing water pollution from nonpoint sources.

Mark G. Robson

(see also: Acid Rain; Clean Water Act; Ecosystems; Pollution; Sewage System; Wastewater Treatment; Waterborne Diseases; Water Quality; Water Reuse; Water Treatment )


Morgan, M. T. (1997). Environmental Health, 2nd edition. Englewood, CO: Morton Publishing Company.

Nadakavukaren, A. (2000). Our Global Environment, 5th edition. Prospect Heights, IL: Waveland Press.