Non-point sources of pollution include hundreds of other ways water can be contaminated from less concentrated sources. These range from things such as runoff into streams of chemicals from city parking lots, home lawns and cropland, to infiltration into groundwater of substances such as fertilizer, herbicides and natural organic materials in the soil. For example, with a field next to a stream, the erosion into the stream is a non-point source. If you have 30 cattle grazing above a stream, there's a potential pollution source. But how those cattle are managed determines the impact.
Federal regulations require states to control non-point sources of pollution.
Because of this, cities with a population above 50,000 are required to treat runoff from rain. But often individual landowners and property owners are the sources of problems. This is difficult to monitor and police, especially if the pollution occurs at the time of rainfall. The way to do a lot of it was by education and by the use of different management practices. In many cases, Best Management Practices are more a case of identifying and integrating techniques.