Runoff from rain in urban areas is a major source of nonpoint source pollution. Much of the urban environment is paved with asphalt or concrete. These surfaces are usually impervious, meaning that water runs off of them without being absorbed into the soil. Impervious surfaces make it easier for storm water to pick up, absorb, and carry pollutants. For example, water will flow across a parking lot and pick up oil left by cars driving and parking on the asphalt. This runoff then runs over the edge of the parking lot, and most likely, it eventually empties into a stream. The water flows downstream into a larger stream, and then to a lake, river, or ocean.

In addition to runoff from urban areas, agricultural operations account for a large percentage of nonpoint source pollution in the United States. In agriculture, large tracts of land are typically plowed to grow crops. Plowing the land exposes the soil, making it more vulnerable to erosion during rainstorms. This increases the runoff that carries fertilizers and pesticides away from the farm and into nearby waters.

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