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 to be approximately 16,000 acre feet. Groundwater recharge can be augmented through releases to Mission Creek and through injection capability at various production wells.
Perennial yield is the amount of water that can be pumped from the basins on a continual basis without causing overdraft. The portion of the perennial yield used by the City from all Storage Unit 1 and the Foothill Basins is approximately 1,300 acre feet per year. Storage Unit 3 is not normally used. Long-term average annual pumping has been estimated at 1,083 acre feet per year under the 2011 Long Term Water Supply Plan.
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 5,500 acre feet per year, but poor water quality and threat of seawater intrusion significantly reduces this capacity. Replacement wells are planned and a pumping capacity of at least 4,125 acre feet per year is the target for meeting long-term supply requirements.