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Eastern San Joaquin Groundwater Basin Groundwater Management Plan Northeastern San Joaquin County Section 2 Groundwater Banking Authority 71 Hydrogeology Table 2-3 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 Notes Source: San Joaquin County Water Management Plan Volume I 2.3.7 Saline Groundwater Intrusion 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 2-3 and 2-4. 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 2-9 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 2-9 also shows 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. Section 4 discusses further the current groundwater monitoring program and future actions to be undertaken by the Authority and its member agencies.