Water pollution originating from diffuse sources across the landscape rather than from a single identifiable discharge point. Agricultural runoff, urban stormwater, and atmospheric deposition are major non-point sources of water contamination.
Non-point source (NPS) pollution refers to contamination of water bodies from diffuse sources that cannot be attributed to a single, identifiable discharge point, in contrast to point source pollution from pipes, outfalls, or ditches. NPS pollution is the leading cause of water quality impairment in the United States, affecting 53% of assessed river miles and 67% of assessed lake acres. Major NPS categories include agricultural runoff (carrying sediment, nutrients, pesticides, and pathogens from crop fields and livestock operations), urban stormwater runoff (transporting oil, heavy metals, trash, and chemicals from impervious surfaces), atmospheric deposition (acid rain, mercury), forestry operations (sediment from logging roads and clear-cuts), and septic system leachate. The intermittent and spatially variable nature of NPS pollution makes it inherently more difficult to monitor, regulate, and control than point source pollution. The Clean Water Act Section 319 program provides funding for NPS management, while Total Maximum Daily Load (TMDL) programs set maximum allowable pollutant levels for impaired water bodies. Best management practices (BMPs) for NPS control include riparian buffers, cover crops, conservation tillage, constructed wetlands, bioretention cells, and permeable pavement. Despite decades of effort, NPS pollution remains the dominant water quality challenge facing the United States and much of the world, particularly nutrient pollution (nitrogen and phosphorus) driving eutrophication, harmful algal blooms, and hypoxic dead zones.