More than 40 percent of California's drinking water is from groundwater. To protect this vital resource, the State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State's groundwater quality and increases public access to groundwater-quality information. The San Diego Drainages Hydrogeologic Province (hereinafter referred to as San Diego) is one of the study units being evaluated. The San Diego study unit is approximately 3,900 square miles and consists of the Temecula Valley, Warner Valley, and 12 other alluvial basins (California Department of Water Resources, 2003). The study unit also consists of all areas outside defined groundwater basins that are within 3 kilometers of a public-supply well. The study unit was separated, based primarily on hydrogeologic settings, into four study areas: Temecula Valley, Warner Valley, Alluvial Basins, and Hard Rock (Wright and others, 2005). The sampling density for the Hard Rock study area, which consists of areas outside of groundwater basins, was much lower than for the other study areas. Consequently, aquifer proportions for the Hard Rock study area are not used to calculate the aquifer proportions shown by the pie charts. An assessment of groundwater quality for the Hard Rock study area can be found in Wright and Belitz, 2011. The temperatures in the coastal part of the study unit are mild with dry summers, moist winters, and an average annual rainfall of about 10 inches. The temperatures in the mountainous eastern part of the study unit are cooler than in the coastal part, with an annual precipitation of about 45 inches that occurs mostly in the winter. The primary aquifers consist of Quaternary-age alluvium and weathered bedrock in the Temecula Valley, Warner Valley, and Alluvial Basins study areas, whereas in the Hard Rock study area the primary aquifers consist mainly of fractured and decomposed granite of Mesozoic age. The primary aquifers are defined as those parts of the aquifers corresponding to the perforated intervals of wells listed in the California Department of Public Health (CDPH) database. Public-supply wells typically are drilled to depths between 200 and 700 feet, consist of solid casing from the land surface to a depth of about 60 to 170 feet, and are perforated, or consist of an open hole, below the solid casing. Water quality in the shallow and deep parts of the aquifer system may differ from water quality in the primary aquifers. Municipal water use accounts for approximately 70 percent of water used in the study unit; the majority of the remainder is used for agriculture, industry, and commerce. Groundwater accounts for approximately 8 percent of the municipal supply, and surface water, the majority of which is imported, accounts for the rest. Recharge to groundwater occurs through stream-channel infiltration from rivers and their tributaries, infiltration in engineered recharge basins, and infiltration of water from precipitation and irrigation. The primary source of discharge is water pumped from wells.