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Runoff is water from rain, agricultural or landscape irrigation, or from other sources that flows over the land surface. Pollutants present in urban runoff are generated from both on-site and off-site sources. Examples of pollutants commonly generated in the Santa Clara Valley include: sediment from construction sites; products of internal combustion engine operation such as hydrocarbons from automobiles; heavy metals, such as copper from automobile brake pad wear and zinc from tire wear; dioxin as a product of combustion; mercury resulting from atmospheric deposition; and natural-occurring minerals from local geology.
These pollutants, which can be harmful to humans and aquatic ecosystems, may be deposited on impervious surfaces1, such as paved roadways, parking areas, walkways, patios, or roofs. The pollutants can then flow into local creeks either directly or indirectly through the City's storm drains (also referred to as the "municipal separate storm sewer system" or "MS4") during rainstorms or other activities that generate the flow of water, thus creating urban runoff. Polluted runoff to local creeks may result in impairment of both the creeks and downstream water bodies, including rivers, lakes, and ultimately, San Francisco Bay.
New development may increase the amount of impervious surface area within a watershed. In addition to conveying pollutants, impervious surfaces may also affect local waterways by increasing the volume and intensity of runoff. Flooding, excessive bank erosion, and channel modification may occur as a result of increases in runoff flows.
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