Diffuse sources of nitrogen and phosphorus are defined as any source of nitrogen and phosphorus that is not accounted for as a point source. Small, dispersed point discharges (e.g. from scattered dwellings or from point sources in agriculture, e.g. farmyards) should be dealt with as diffuse sources. Based on this definition, losses from scattered dwellings are included as diffuse sources (Borgvang, S-A. and Selvik, J.R, 2000.)

Diffuse losses of nutrients are being recognised as a major source of water quality problems in both surface and ground waters. Contrary to point source pollution, diffuse sources still remain a dominant source of losses of nutrients.