Nonpoint Source Pollution

Urban storm water pollution and most pollution in combined sewer overflows originates from nonpoint or diffuse sources. The processes controlling storm water quality are rather complex, as shown in Figure 9.20.1. In contrast to point source pollution, such as industrial and municipal treatment plant outfalls, these sources of pollution are numerous and their contributions are difficult to quantify. Diffuse pollution is a hydrologic process that closely follows the statistic character of rainfall, and must be evaluated similarly.

Urban nonpoint sources have been identified as a major cause of pollution of surface water bodies by the U.S. EPA (EPA 1984). In the 1988 Report to Congress (EPA 1990), the EPA stated that urban storm water runoff is the fourth most extensive cause of impaired water quality in the nation's rivers, and the third most extensive cause of impaired water quality in lakes. Combined sewer outflows (CSOs) are tenth on the list of significant sources of impairment for both surface-water bodies.

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