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4.8 Hydrology and Water Quality increased surface-water imports, the San Joaquin Valley might require groundwater withdrawals of 150,000 AF per year in excess of recharge to meet agricultural demand until at least 2010. Those estimates probably underestimate additional increased demand that would result from sustained drought periods. Occasional large withdrawals from an aquifer are a viable solution to the problem of reduced surface-water supplies in dry periods, provided the aquifer is replenished during wet years. However,continual withdrawal of groundwater in excess of recharge can increase the cost of pumping,reduce water availability, and, in certain hydrogeologic settings, — can lead to overdraft and possibly cause land subsidence(USGS, 2003). A detailed groundwater budget is not available for the Tracy or Delta-Mendota Sub-basin. — However,groundwater demand for the Delta-Mendota Sub-basin is estimated by the DWR, based on land and water use. DWR estimates for the Delta-Mendota Sub-basin considered overall applied water demands, agricultural groundwater pumping,urban pumping demand, and other extraction data.Natural recharge to the groundwater aquifers is estimated at 8,000 AF,and applied water recharge is approximately 74,000 AF. Annual urban and agricultural groundwater — extraction is approximately 17,000 AF and 491,000 AF,respectively. Overall, the difference between aquifer recharge and rates of extraction from that aquifer results in a net negative water budget of 440,000 acre feet per year within the Tracy-Mendota Sub-basin(DWR, 2003a). — Groundwater Extraction and Land Subsidence Significant land subsidence in the San Joaquin Valley has occurred in areas where the Corcoran Clay confines the lower aquifer. In these areas, pressure changes caused by groundwater pumping promote greater compressive stress than in the unconfined zone(DWR, 1977).The maximum _ land subsidence levels recorded in the Central Valley occurred in the 2,600-square-mile Los Banos-Kettleman City area. Because of the slow drainage of the fine-grained deposits, subsidence at a particular time is more closely related to past water-level change than to current change. For example,in the San Joaquin Valley, groundwater withdrawals increased greatly until large imports of surface water through various canals occurred; but even though water levels in the area started to rise, the rate of subsidence began to decrease three years later. The rate of ground — subsidence can be reduced or arrested by a rise in the artesian head and water levels, but restoration of water levels has not caused any restoration of land surface heights. Such subsidence and compaction permanently reduces the storage capacity of the aquifer. Significant land subsidence has been detected in the San Joaquin Valley due to increased groundwater pumping associated with the drought of 1987-1992(USGS,2003).The areas of subsidence are primarily south of the Merced River and no subsidence has been reported in the Vernalis Area(Williamson and others, 1989). Groundwater Quality On a regional scale, the water quality in the upper, unconfined aquifer is,for the most part, relatively poor and of variable quality.Water within the lower,confined aquifer is of better — quality and is more commonly used as a water supply source. The agricultural pesticide dibromochloro-propane (DBCP) is a potential carcinogen found in — groundwater throughout San Joaquin Valley. DBCP has been detected in 30 percent of the private RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-15 ESA 1203015 Draft Environmental Impact Report May 2006