What is stormwater runoff and why is it a concern?
Stormwater runoff is rainwater that flows over the land surface and through collection pipes. In vegetated areas such as forests, fields and wetlands, rainwater seeps slowly into the ground, limiting runoff. However, when rain falls on paved concrete and other hard (impervious) surfaces such as those found in most of San Francisco, it runs off quickly and is conveyed by pipes and other drainage features. Though starting as relatively pure rainwater, stormwater runoff collects pollutants as it flows over impervious surfaces. For example, runoff from parking lots picks up oil and grease from leaking engines, copper from worn brake linings, and zinc from tires. Although most runoff in San Francisco flows into the combined sewer system and receives treatment at the city’s two sewage treatment plants, there are a few areas in the city that discharge directly into San Francisco Bay or other surface water such as Lake Merced without receiving any treatment. These polluted stormwater flows can
