Water pollution is caused when contaminants wash into streams, lakes, wetlands or infiltrate into groundwater. Water running off paved surfaces in urban areas often carry oil, heavy metals, pet waste, and toxic chemicals from industrial and residential areas into stormdrains, and eventually, water ways. Agricultural operations often apply fertilizers, pesticides, and animal wastes, the excess of may eventually leach into groundwater or enter waterways through irrigation return flows and surface runoff. These sources of "nonpoint" pollution are considered the primary source of water quality problems nation wide.

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Adapted from the Pollution Assessment and Prevention Theme page, http://www.usawaterquality.org/themes/pollution/default.html