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4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALITY ` exceeds estimated annual safe yield by approximately 200,000 acre-feet(AF). Exceedence of the safe annual yield can lead to overdraft of a groundwater basin. All of the sub-basins within the San Joaquin River Region have experienced some overdraft(DWR, 1994). The network of water supply aqueducts in the Central Valley is sufficient to provide for over one- half of the water needed for irrigation in years of average or above-average rainfall. In dry years .. when rainfall is below normal,reliance on ground-water supplies is greater. During times of severe drought, groundwater could be removed from the regional aquifers in excess of recharge, as they were during the 1976-77 drought(USGS,2003). Although gains have been achieved by importing surface water from areas of surplus,projected water needs in the San Joaquin Valley may require tem�orar withdrawal of Around water ins=e �:::�lxirecirlte future The DWR estimates that by 2010, demand for water in the San Joaquin Valley,exclusive of the Tulare Basin, would be 8 percent greater than the groundwater demand in 1980. Without 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 ground water 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). Well Characteristics and Well Yields ++ The total depth of municipal and irrigation wells range from 60 feet to 1,020 feet and the average depth is 352 feet. Total depths of domestic wells range between 44 feet and 665 feet and average depth is 188 feet. Well yields and specific capacities for the domestic wells in the Tracy Sub- basin are not available. Well yields for Delta-Mendota Sub basin municipal and irrigation wells range from 20-to 5,000-gpm and average 800- to 2,000-gpm. Total depths for these wells range from 50 feet to 800 feet with an average depth of 400 feet to 600 feet. RMC Pacific Vernalis Quany Mining and Reda don Project 4.8-15 ESA/203015 Admin mdve Draft Eai Math 2004