Non-point sources refer to situations in which there are numerous, relatively small sources of emission of gases, metals, pesticides, nutrients, or other substances into the environment. Collectively, these many sources can comprise a regional emission of large quantities of pollutants into the environment. However, this is different from point sources, in which there is a single, discrete source, such as a large smokestack or a sewage outfall.
For example, if the many buildings in a city are heated using individual furnaces that burn coal or oil, then each chimney will be a discrete source of emissions of sulfur dioxide, oxides of nitrogen, and particulates to the atmosphere. The urban air quality will also be influenced by numerous other relatively small sources of emissions, for example, those from automobiles, trucks, and buses. Although each chimney and tailpipe is, strictly speaking, a point source, their emissions rapidly coalesce to form a regional air pollution that affects the atmospheric environment throughout the city and its surroundings.
These diverse, non-point emissions can contribute to the occurrence of episodes of regionally degraded air quality, known as smog. So-called reducing smogs are characterized by large concentrations of sulfur dioxide, particulates, and soot, and are common where large quantities of coal are burned in industry and used to heat homes and other buildings. Oxidizing smogs occur in sunny regions where there are large non-point emissions of hydrocarbons and oxides of nitrogen, chemicals that are precursors for the secondary formation of ozone. Non-point sources also contribute to the formation of acid rain, another regional environmental problem associated with emissions of sulfur dioxide and oxides of nitrogen.
Inputs of nutrients can affect aquatic ecosystems through a syndrome characterized by algal blooms, deoxygenation, and other symptoms of degraded water quality. The nutrient inputs can occur through large point sources such as municipal or industrial sewer pipes, or by more diverse non-point sources associated with agricultural fields and sewage disposal by individual homes.
Often, environmental degradation over large areas is caused by a combination of emissions of pollutants from both point-sources and non-point sources.
See also Point source.
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