The Metropolitan Water District of Southern California is a regional public agency responsible for developing, storing, treating and distributing supplemental water supply at wholesale rates to 27 member agencies for domestic and municipal uses, and provides surplus water for agriculture. It is important to assess the condition of the local groundwater supply, as any loss puts more pressure on the District's limited imported supplies, and basin contamination restricts conjunctive-use storage opportunities. This paper describes the comprehensive assessment that was made of groundwater quality conditions in the service area, covering 15 major groundwater basins in Southern California. An extensive groundwater database was developed, consisting of existing data collected from 46 public agencies, including the concentrations of 200 water quality constituents and production information for 3,500 municipal water supply wells. The analysis period, from 1976 to 1989, yielded regional groundwater quality and production conditions for 1.4 million acre-feet per year. A linear extrapolation was made from available data to project the impacts for the entire service area.