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Eastern San Joaquin Groundwater Basin Groundwater Management Plan Northeastern San Joaquin County Executive Summary Groundwater Banking Authority 9 Table ES-5 Simplified Groundwater Balance for Eastern San Joaquin County Groundwater Flow Component Average Value Explanation Inflows (af) Deep Percolation/Recharge 608,400 Net infiltration from rainfall, irrigation, canal leakage etc. Gain from Streams 198,170 Net inflow from streams to groundwater system Lateral Inflow 98,000 Net of subsurface inflows and outflows. Total Inflows 904,577 Outflows (af) Groundwater Pumping 867,600 Net agricultural, municipal and industrial pumping Loss to Streams 108,898 Net outflow from groundwater system to streams Lateral Outflow 35,300 Subsurface Outflows Total Outflows 1,011,815 Groundwater Overdraft (af) Mined Aquifer Storage 107,238 Total Inflows minus Total Outflows Estimated Saline Intrusion 42,000 Lateral Saline Intrusion into the Stockton Area Total Estimated Overdraft 150,700 Sum of Mined Aquifer Storage and Saline Intrusion Source: San Joaquin County Water Management Plan Volume I Groundwater flow in the Basin now converges on the depression with relatively steep groundwater gradients eastward from the Delta toward the cone of depression as depicted in Figures ES-2 and ES-3. The eastward flow from the Delta area is significant because of the typically poorer quality water now moving eastward in the Stockton area. Increased lateral inflow from the west is undesirable, as this water is typically higher in TDS and chloride levels and causes the degradation of water quality in the Basin. Figure ES-4 illustrates the approximate location of the 300 mg/L isochlor as measured in 2000. Projections indicate that the rate of eastward migration of the saline front is approximately 150 to 250 feet per year. Figure ES-4 also depicts the projected 2030 location of the 300 mg/L isochlor under no-action conditions. Degradation of water quality due to TDS or chloride contamination threatens the long-term sustainability of a very important water resource for San Joaquin County, since water high in TDS and/or chloride is unusable for either urban drinking water needs or for irrigating crops. Damage to the aquifer system could for all practical purposes be irreversible due to saline water intrusion, withdrawal of groundwater from storage, and potentially subsidence and aquifer consolidation. The saline intrusion problem is not well understood by the Authority. Further studies and monitoring methods are necessary to ensure the problem is addressed and monitored adequately. The Plan further defines the groundwater science and monitoring investigations geared towards both saline intrusion and general Basin understanding. A no-action or baseline simulation was conducted to predict how current groundwater and surface management practices would impact the groundwater basin in 2030. Groundwater modeling has shown that unless there is a change in how groundwater is used or managed,