Nonpoint source pollution is pollution that reaches the environment from many, diffuse sources. Each of these diffuse sources may be very small and contribute only a tiny portion of the overall volume of pollution. However, the cumulative impacts of many, diffuse sources of pollution can have a very real, significant impact.
Before the Federal Clean Water Act of 1972, the majority of water pollution entering rivers, streams, and lakes, was attributable to large, identifiable sources contributing high volumes of polluted effluents. Many of these sources were industrial. Today, these sources are highly controlled using the National Pollutant Discharge Elimination (NPDES) system permits. As much as 70% of all water pollution today is caused by the discharge of storm water to lakes and streams. As storm water runoff travels across land, it accumulates pollutants such as fertilizers, pesticides, eroded soil, oil and grease, road salt, and trace metals-- the stuff of urban landscapes. Because of the high percentage of impervious surfaces in the watershed (parking lots and rooftops that prevent water from percolating into the soil), this water reaches rivers, streams, and lakes without the benefit of natural cleaning provided by wetlands and soil percolation. Because the source of this pollution does not derive from a single place, we call it non point source pollution.
An added problem of impervious surfaces is the increased speed with which storm water is delivered to rivers and streams. Because the storm water is not attenuated (slowed), it is rapidly delivered to rivers and streams, contributing to flow instability. This results in frequent and increasingly severe flooding, which erodes stream banks and increases the sediment load of the water. The net effect of this is diminishing aquatic habitat, regardless of water quality.