Water Pollution

Water pollution occurs when harmful materials are introduced into the lakes, rivers, oceans, and groundwater. These harmful materials are referred to as pollutants. There are a variety of pollutants ranging from biological (e.g., bacteria and viruses) to chemical (e.g., metals, solvents, pesticides, and floating detergents and oils) forms, and from nutrients (e.g., phosphorus and nitrogen) to suspended sediment. They cause diverse problems in aquatic ecosystems. Pathogens serving as typical microbiological pollutants cause many illnesses ranging from typhoid and dysentery to minor respiratory and skin diseases. Chemical pollutants are poisonous to aquatic life, birds, animals, and people. Excess inputs of nutrients from agricultural management lead to eutrophication—the excessive growth of algae, periphyton, and weeds, which can deplete dissolved oxygen to the detriment of fish, reduce potability, and preclude recreational uses of water. Suspended sediment threatens aquatic ecosystems directly and indirectly. Reduction of light penetration due to the presence of suspended sediment directly reduces photosynthesis. In addition, sediment smothers bottom-dwelling organisms and clogs fish gills, prevents successful development of fish eggs and larvae, and modifies fish movement and migration. Indirectly, suspended sediment serves as a carrier for the nutrients, metals, and organic biocides that adsorb to the sediment.

Because of the complex interaction between groundwater and surface water, groundwater pollution, sometimes referred to as groundwater contamination, is not included here. Surface water pollution is generally classified by sources into point and nonpoint source pollutions. Point source pollution refers to the process by which harmful substances enter a waterway through a discrete conveyance, such as a pipe or a ditch. Examples of point sources include discharges from a sewage treatment plant or a factory, or a tributary that transports pollutant loads into the main stream. Nonpoint source pollution refers to diffuse contamination that does not originate from a single discrete source. It is often a cumulative effect of small amounts of contaminants gathered from a large area. Nutrient runoff in storm water from “sheet flow” over an agricultural field or metals and hydrocarbons from an area with highly impervious surfaces and vehicular traffic are sometimes cited as examples of nonpoint source pollution. Different types of nonpoint source pollutants derive from different types of sources. Suspended sediment is largely originated from cultivated agricultural lands, deforested areas, urban construction sites, dirt roads, and even eroded channel banks. Phosphorus inputs mainly come from fertilizer applied to agricultural soils and [Page 3067]manure supplied from concentrated livestock operations. Although, part of it (about 30% of the total input in the United States and Europe) can be sequestered by crops, the majority of applied phosphorus remains in the upper 10 centimeters of soils in the agricultural fields or exports to surface waters by erosion or leaching. In the United States and Europe, only 18% of the nitrogen input in fertilizer is removed from farms in produce. The surplus of nitrogen may accumulate in soils, erode, or leach to surface water and groundwater. Some may enter the atmosphere through volatilization of NH3 (ammonia) and microbial generation of N20 (nitrous oxide). Much of the nitrogen volatilized to the atmosphere is subsequently redeposited on land or water and eventually returns to aquatic ecosystems.

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