Groundwater is produced primarily from two groundwater basins: Storage Unit 1 (in the general vicinity of downtown), and the Foothill Basin (upper State Street area). Storage Unit 3 (located generally in the Westside area) has the potential for limited production, though water quality is relatively poor. The United States Geological Survey currently estimates the usable storage volume in the Foothill and Storage Unit 1 basins to be approximately 24,000 acre feet. In the summer of 2022 the City will be piloting a project to recharge the Foothill groundwater basin through injection at one of the City's production wells.
Groundwater is generally banked during wet years and primarily used as a supply during drought as surface water supplies decrease. During periods of ample surface supplies, groundwater is allowed to recharge naturally and by means of artificial recharge. Artificial recharge takes place only when space is available in the basin and as long as it is not precluded by pumping associated with the clean-up of groundwater contamination. Maximum pumping capacity is approximately 3,500 acre feet per year, but poor water quality and threat of seawater intrusion significantly reduces this capacity on a year to year basis.