Sources of pollution are categorized as either point sources or nonpoint sources. Point sources are those that are typically permitted or regulated under state National Pollutant Discharge Elimination System (NPDES) programs. Examples of point sources include human waste and industrial discharges to surface and ground water. The law defines point sources as entities such as pipes, ditches, channels, tunnels, wells, containers, concentrated animal feeding operations, and any vessels from which pollutants are or may be discharged. Irrigated agriculture and agricultural storm water runoff are not considered point sources, though permitted stormwater discharges from many urban areas are categorized as point sources.
A nonpoint source is a discharge that is not regulated (at least not regulated by the Clean Water Act). Nonpoint sources are diffuse, that is, they do not have a single point of origin or are not introduced into a receiving stream from a specific outlet. Examples of nonpoint source pollution include agriculture, construction activities, and urban runoff. Nitrogen and phosphorus from both point and nonpoint sources contribute to the problem of nitrogen and phosphorus pollution, but the extent to which they contribute to water quality degradation varies by watershed and surrounding land uses.