Non-point source (NPS) pollution comes from diffuse sources rather than a single identifiable discharge point. It is the leading cause of water quality impairment in the United States.
Non-point source pollution originates from widespread, diffuse sources across the landscape rather than from a specific, identifiable discharge pipe or outfall. Common NPS pollutants include sediment from erosion, nutrients from fertilizers, pesticides, bacteria from animal waste, oil and grease from urban surfaces, and salt from road de-icing. Agricultural activities are the largest contributor to NPS pollution, followed by urban and suburban stormwater runoff, forestry operations, and atmospheric deposition. Unlike point source pollution, NPS pollution is not regulated under the National Pollutant Discharge Elimination System (NPDES) permit program, making it more difficult to control through traditional regulatory approaches. The Clean Water Act Section 319 program provides grants to states and tribes to implement NPS management programs including best management practices (BMPs) such as buffer strips, cover crops, constructed wetlands, and low-impact development. NPS pollution is typically most severe during and immediately after rainfall events, when runoff mobilizes and transports pollutants from the land surface to receiving waters. Addressing NPS pollution requires integrated watershed-scale approaches that combine land use planning, agricultural conservation practices, and public education.