Diffuse source pollution is one of the most intricate environmental problems with extensive impacts on surface and groundwater quality. It is a major factor impacting the quality of water supply, and the rate at which diffuse source pollutants are generated and delivered to water resources is greatly affected by anthropogenic activities as well as natural processes. The main hydrologic component transporting these pollutants to surface water bodies is runoff, which results from precipitation or snowmelt (Leeds et al., 1993). Stormwater is part of a natural hydrologic process; however, human activities, especially urban development and agriculture, cause significant changes in patterns of stormwater flow from land into receiving waters. Urban runoff can be or is often a significant source of water pollution, causing decline in fisheries, swimming, and other beneficial attributes of water resources (U.S. EPA, 1993). Urban stormwater runoff includes all flows discharged from urban land uses into stormwater conveyance systems and receiving waters; in this context, urban runoff includes both dry-weather non-stormwater sources (e.g., runoff from landscape irrigation, dewatering, and water line and hydrant flushing) and wet-weather stormwater runoff. Water quality can also be affected when runoff carries sediment and other pollutants into streams, wetlands, lakes, estuarine and marine waters, or groundwater.