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Eastern San Joaquin Groundwater Basin Groundwater Management Plan Northeastern San Joaquin County Section 2 Groundwater Banking Authority 69 Groundwater Management Plan consumptive use, evapotranspiration, runoff, and unsaturated zone retention. Average rainfall within the study area is 14-16 inches per year. Figure 2-24 illustrates total annual rainfall for the Lodi Station. Within the Basin the estimated net deep percolation based on the modeling results is 590,000 af. Figure 2-25 illustrates the deep percolation for eastern San Joaquin County. 2.3.4.4 Lateral Inflow Lateral inflow into the study area occurs primarily across the northern, western and southern boundaries. Under predevelopment conditions a net outflow existed, however due to the changed hydraulic conditions in eastern San Joaquin area there is now a net groundwater inflow. The groundwater model estimates net lateral inflow to be 120,000 af for the 1970 to 1993 period. 2.3.5 Surface Water Interaction A large number of streams and rivers dissect the study area. The rivers that have a regional impact on the hydrogeology are Cosumnes River, Mokelumne River, Dry Creek, Calaveras River, Stanislaus River, Tuolumne River, and San Joaquin River. Based on modeling results for the five-year period from 1989 to 1993, the Tuolumne and the upstream reaches of the Mokelumne and San Joaquin Rivers were gaining rivers – that is groundwater discharged into the rivers. The Calaveras, Dry Creek, Stanislaus, and the downstream reaches of the Mokelumne and San Joaquin Rivers were all losing rivers – i.e. surface water recharged the groundwater. On average from 1970 to 1993, there was a groundwater gain from streams of 140,000 af and a groundwater loss to streams of 100,000 af. The net gain to the groundwater system was 40,000 af. 2.3.6 Groundwater Balance Current and historical groundwater pumping rates exceed the sustainable yield of the underlying groundwater basin on an average annual basis. Based on a simplified groundwater balance, as shown in Table 2-3, the net groundwater overdraft over the historic hydrologic record is estimated to be approximately 150,000 to 160,000 af/yr. The net groundwater overdraft is defined as the difference between total basin outflow and inflow plus the estimated accretions from the San Joaquin River and lateral basin inflow in west Stockton. Because much is unknown about the source and rate of migration of the saline front, the conceptual groundwater model assumes that all basin inflow in west Stockton is saline. The result of long-term groundwater overdraft is two fold: significant decline in groundwater levels and increased accretions from area waterways. Although increased accretions to the groundwater basin from high quality surface water sources are desirable, accretions in the western fringes of the Basin and the Lower San Joaquin River are undesirable due to elevated salinity levels. Saline groundwater intrusion has forced the closure of several wells in the Calwater service area.