Non-point source pollution is pollution that enters water from many different sites, rather than from just one site. Examples of non-point source pollution are contaminated rain falling from the sky, polluted melting snow, runoff (water flow on land) of polluted water, and impure water draining down into the groundwater from many different sites on the surface. In contrast, an example of point source pollution is a polluted river flowing into a lake.

Because non-point source pollutants enter a water body such as a stream, river, or lake at different locations, the control and prevention of non-point pollution can be much more difficult than when the contaminants are entering at a single site. As the water runs over the land or through the ground on its way to the body of water, it can pick up a variety of pollutants. These chemicals and...