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<br />
<br />M
<br />
<br />Point and Nonpoint
<br />Sources of Contaminants
<br />
<br />Contaminants may be present in water or in air as
<br />a result of natural processes or through mechanisms of
<br />displacement and dispersal related to human activities.
<br />Contaminants from point sources discharge either into ground
<br />water or surface water through an area that is small relative to
<br />the area or volume of the receiving water body. Examples of
<br />point sources include discharge from sewage-treatment
<br />plants, leakage from gasoline storage tanks, and seepage
<br />from landfills (Figure M-l).
<br />Nonpoint sources of contaminants introduce
<br />contaminants to the environment across areas that are
<br />large compared to point sources, or nonpoint sources may
<br />consist of multiple, closely spaced point sources. A non point
<br />source of contamination that can be present anywhere, and
<br />affect large areas, is deposition from the atmosphere, both
<br />by precipitation (wet deposition) or by dry fallout (dry deposi-
<br />tion). Agricultural fields, in aggregate, represent large areas
<br />through which fertilizers and pesticides can be released to the
<br />environment.
<br />
<br />Figure M-1. The transport of contamination from a point
<br />source by ground water can cause contamination of surface
<br />water, as welf as extensive contamination of ground water.
<br />
<br />The differentiation between point and nonpoint sources
<br />of contamination is arbitrary to some extent and may depend
<br />in part on the scale at which a problem is considered. For
<br />example. emissions from a single smokestack is a point
<br />source, but these emissions may be meaningless in a regional
<br />analysis of air pollution. However, a fairly even distribution of
<br />tens or hundreds of smokestacks might be considered as a
<br />non point source. As another example, houses in suburban
<br />areas that do not have a combined sewer system have indi-
<br />vidual septic tanks. At the local scale, each septic tank may
<br />be considered as point source of contamination to shallow
<br />ground water. At the regional scale, however, the combined
<br />contamination of ground water from all the septic tanks in
<br />a suburban area may be considered a nonpoint source of
<br />contamination to a surface-water body.
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<br />
<br />Contaminant
<br />plume
<br />
<br />56
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