Nonpoint source (NPS) pollution is water pollution affecting a water body from diffuse sources, such as polluted runoff from agricultural areas draining into a river, or wind-borne debris blowing out to sea. Nonpoint source pollution can be contrasted with point source pollution, where discharges occur to a water body at a single location, such as discharges from a chemical factory, urban runoff from a roadway storm drain, or from ships at sea.

NPS may derive from many different sources with no specific solution to rectify the problem, making it difficult to regulate. It is the leading cause of water pollution in the United States today, with polluted runoff from agriculture the primary cause.

Other significant sources of runoff include hydrological and habitat modification, and silviculture (forestry).

Contaminated stormwater washed off of parking lots, roads and highways, and lawns (often containing fertilizers and pesticides), is called urban runoff. This runoff is sometimes included under the category of NPS pollution, however, it is typically channeled into municipal storm drain systems and discharged through pipes to nearby surface waters, and is a point source.

Sediment

Sediment (loose soil) includes silt (fine particles) and suspended solids (larger particles). Sediment may enter surface waters from eroding stream banks, and from surface runoff due to improper plant cover on urban and rural land. Sediment creates turbidity (cloudiness) in water bodies, reducing the amount of light reaching lower depths, which can inhibit growth of submerged aquatic plants and consequently affect species which are dependent on them, such as fish and shellfish. High turbidity levels also inhibit drinking water purification systems. (Sediment can also be discharged from improperly managed construction sites, although these are point sources, which can be managed with erosion controls and sediment controls.)

Phosphorus

Phosphorus is a nutrient that occurs in many forms that are bioavailable. It is a key ingredient in many fertilizers used for agriculture as well as on residential and commercial properties, and may become a limiting nutrient in freshwater systems. Excess amounts of phosphorus in these systems lead to algae blooms and consequently hypoxia. This is also known as eutrophication. Phosphorus is most often transported to water bodies via soil erosion since the various forms of phosphorus tend to be adsorbed to soil particles.

Nitrogen

Nitrogen is the other key ingredient in fertilizers, and it becomes a pollutant in saltwater systems where nitrogen is a limiting nutrient. Excess amounts of bioavailable nitrogen in these systems lead to a boom of algae and diatoms. When the excessively large population of autotrophs reach the end of their life cycles, the process of decomposition consumes oxygen. The result is very suppressed levels of dissolved oxygen in the water, otherwise known as hypoxia.

Nitrogen is most often transported by water as nitrate (NO3). The nitrogen is usually added to a watershed as organic-N or ammonia (NH3), so nitrogen stays attached to the soil until oxidation converts it into nitrate. Since the nitrate is generally already incorporated into the soil, the water traveling through the soil (i.e., interflow and tile drainage) is the most likely to transport it, rather than surface runoff.

Pathogens

Pathogens may be present in nonpoint source runoff, and can be a source of disease if they enter drinking water supplies. Pathogens found in contaminated runoff may include:

Cryptosporidium parvum

Giardia lamblia

Salmonella

Novovirus and other viruses

Parasitic worms (helminths)

Coliform bacteria may also be detected in runoff. These bacteria are a commonly-used indicator of water pollution, but not an actual cause of disease.

Control Of Nonpoint Source Pollution