Public water systems are required to prepare Consumer Confidence Reports (CCRs) for their clients. Many (but not all) CCRs for California water systems are available on the US EPA's website: http://yosemite.epa.gov/ogwdw/ccr.nsf/California?OpenView
Annual Compliance Reports for Public Water Systems are available from the California Department of Public Health (CDPH) at http://www.cdph.ca.gov/certlic/drinkingwater/Pages/Publications.aspx
Water quality data is also available on the National Environmental Public Health Tracking Program Portal: http://www.cdc.gov/ephtracking