Section 319: Nonpoint Source Program

Nonpoint source pollution (NPS) represents the most significant source of pollution overall in the country. According to states' 305(b) and 303(d) reports, more miles of rivers and acres of lakes are impaired by overland runoff from rowcrop farming, livestock pasturing, and other types of nonpoint sources than by industrial facilities, municipal sewage plants, and point source runoff from municipal storm sewer systems and storm water associated with industrial activity. The most recent set of 303(d) reports indicated that more than 40 percent of all impaired waters were affected solely by nonpoint sources, while only 10 percent of impairments were caused by point source discharges alone.

The CWA does not provide a detailed definition of nonpoint sources. Rather, they are defined by exclusion -- anything not considered a "point source" according to the Act and EPA regulations. All nonpoint sources of pollution are caused by runoff of precipitation (rain and/or snow) over or through the ground. However, as noted previously numerous types of precipitation-induced runoff are treated as point sources rather than as nonpoint sources under the CWA -- including stormwater associated with industrial activity, construction-related runoff, and discharges from municipal separate storm sewer systems (MS4s).

Atmospheric deposition is also a form of nonpoint source: pollutants discharged into the air and returned directly or indirectly to surface waters in rainfall and snow, as well as so-called dry deposition between precipitation events. (Of course, "smokestack industries" such as fossil-fueled electric generating plants could be considered "point sources of air pollution". But the diffuse deposition of pollutants emitted by such facilities is a form of nonpoint source in the context of water pollution.)

Pollutants commonly associated with NPS include nutrients (phosphorus and nitrogen), pathogens, clean sediments, oil and grease, salt, and pesticides.