Source water protection is an important issue for all of California. Large water utilities are required by the California Department of Health Services to complete a Watershed Sanitary Survey every five years to examine possible sources of drinking water contamination. Metropolitan's most recent surveys were conducted in 2001 and include suggestions for how to better protect its source waters. Updated versions of these surveys are expected to be available later this year.

Source waters used by Metropolitan - the Colorado River and State Water Project - each have different water quality challenges. Both are exposed to activities that bring down the quality of water such as storm-water runoff, recreational activities, wastewater discharges, wildlife and fires. There are many other factors that negatively affect the quality of source waters. Treatment to remove specific contaminants can be quite costly and more expensive than measures to protect water at the source, which is why Metropolitan and other water agencies invest resources to support improved watershed protection programs.

Similar to the state's Watershed Sanitary Survey, EPA requires utilities to complete a Source Water Assessment that utilizes information collected in the sanitary surveys. Metropolitan completed its SWA in December 2002. The SWA is used to evaluate the vulnerability of water sources to contamination and also helps determine whether more protective measures are needed.

Water from the Colorado River is considered to be most vulnerable to contamination from recreation, urban/storm-water runoff, increasing urbanization in the watershed, and wastewater. Water supplies from Northern California are most vulnerable to contamination from urban/storm-water runoff, wildlife, agriculture, recreation, and wastewater.