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4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALITY i 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 (i.e., once the — physical compaction and subsidence occur, they are essentially permanent). 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 and public-supply wells from 1979 through 1984 and since 1977. California has since — outlawed its use. At least 50 other pesticides, including 1,2—dichloropropane and ethylene dibromide had been detected in ground water in the Central Valley by 1984(USGS,2003). REGULATORY FRAMEWORK Water Quality Regulations The major federal legislation governing the water quality aspects of the project is the CWA as amended by the Water Quality Act of 1987. The objective of the CWA is "to restore and maintain the chemical, physical, and biological integrity of the Nation's waters"(U.S. Government Printing Office, 1988). The State of California's Porter-Cologne Water Quality _ Control Act(Division 7 of the California Water Code)provides the basis for water quality regulation within California. The State Water Resources Control Board(SWRCB)administers water rights, water pollution control, and water quality functions throughout the state, while the — RWQCB conducts planning,permitting,and enforcement activities. The project would be subject to review under provisions of the RWQCB. The project area lies within the jurisdiction of the CVRWQCB. The California Department of Fish and Game has jurisdiction over any RMC Pacific Vernalis Qaany Mining and Reclamation Project 4.8-16 ESA/203015 Adminis¢ativ Dnfl IDt March 2004