The National Oceanic and Atmospheric Administration divides the causes of water pollution into two categories: point source pollution and non-point source pollution. Point source pollution comes from a single, identifiable source that discharges pollutants directly into a body of water or after undergoing treatment processes. Non-point source pollution comes from multiple locations that are hard to distinguish and travels some distance before entering a water body.
According to NOAA, the most common point source polluters are factories and sewage treatment plants that discharge industrial by-products or human waste. Ships, transport pipes and ditches, and underground storage tanks are also considered point sources. The National Pollutant Discharge Elimination System regulates the types and amounts of pollution that may be discharged into water bodies, and industrial polluters must receive a permit from their state and the Environmental Protection Agency prior to releasing pollutants. The pollutants must also be treated with advanced technology to reduce the potential for environmental harm.
NOAA explains that non-point source pollution is typically the result of runoff. Non-point source pollution is more likely to occur in urban and suburban areas, where runoff flow over parking lots, asphalt roads and sidewalks picks up the pollutants that are lying on the ground. These events can also occur in agricultural settings where runoff waters pick up animal waste and herbicide and pesticide residues. These polluted waters ultimately empty into some type of waterway, such as a stream or storm drain.