English: Nonpoint source pollution is caused when precipitation (1) carries pollutants from the ground such as nitrogen (N) and phosphorus (P) pollutants which come from fertilizers used on farm lands (2) or urban areas (3). These nutrients can cause eutrophication (4).

Nonpoint source pollution comes from many different places. It is caused by rainfall or snowmelt that travels through the ground and picks up pollutants along the way, eventually depositing those pollutants into rivers or lakes. The nitrogen and phosphorus pollutants can come from pesticides and fertilizers used on farm lands or from toxic chemicals in oils used in urban areas. The pollutants can also come from construction sites, pet wastes, and faulty septic systems.The excess chemical nutrients that the rainfall has washed into the water causes eutrophication. Nonpoint source pollution is the leading cause of water quality problems and can have harmful effects on drinking water supplies, recreation, fisheries and wildlife.

Source: https://www.epa.gov/nps/what-nonpoint-source