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4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALITY t of recorded data. The depth to groundwater appears to correlate with climatic conditions such as the drought of the late 1970s and increased precipitation amounts in the mid-1980s. Current conditions over the past five years show relatively stable water levels at depths between 95 and 100 feet for the upper confined aquifer. Based on the historical trends of groundwater level measurements, it is likely that the observed groundwater levels are influenced by seasonal groundwater fluctuations,drought, and local groundwater pumping. The water levels could rise from the currently observed levels in years of high precipitation, if pumping in the area is Lreduced, or if recharge is augmented 7. l Groundwater levels in the lower confined aquifer were reviewed for this EIR based on two i groundwater wells located at the project site(4S/6E-04N001-Costa property well and 4S/6E- 0414001-Barnes property well). The hydrographs for these wells are presented in Figures 4.8-5 and 4.8-6. The groundwater elevation in the Costa well has fluctuated between 10 and 85 feet Labove msl during the 40 years of recorded data(1960 to present)or equivalently, 112 feet and 184 feet below ground level. The groundwater elevation in the Barnes well has fluctuated between 17 and 76 feet below msl (87 feet and 138 feet deep)during the 50 years of recorded ` data(1946 to 1998). iAs with the unconfined aquifer,the elevations of groundwater in the lower confined zones yr appear to correlate with climatic conditions such as the drought of 1977 and increased precipitation amounts in the mid-1980s. The groundwater elevation in the confined aquifer 1 reached its lowest recorded level during the 1950's and gradually increased through the mid L1970's. Groundwater levels decreased in response to the drought conditions of the late 1970s and the rose to stable levels in the 1980s. The hydrographs for the wells in confined zones also indicate that water levels have remained stable or have gradually declined since the early 1980's. Groundwater levels in the Barnes and Costa well rose during the 1990s. In contrast,water levels in the Costa well have decreased since 1999 and are currently at an elevation of approximately 20 feet above msl(173 feet below ground level). ti Groundwater Storage �+ Infiltration and percolation of rainfall, stream flow, and water from canals,ditches, and irrigation water provide recharge to the groundwater basin. The deep percolation of irrigation water is a major source of groundwater recharge in the San Joaquin Valley. The primary source of recharge to the lower,confined aquifer system is subsurface flows from the east side of the San Joaquin Valley. Rivers, such as the Tuolumne River and Stanislaus River,contribute to the ` recharge of this zone originate in the Sierra Nevada and flow westward to the San Joaquin River. M 7 Water levels in unconfined wells represent the actual elevation of the water table. In confined aquifers,the groundwater is under pressure and the groundwater levels represent the level to which the water would rise in a well tapping the aquifer. This may be higher(closer to the ground surface)than the top of the confined water body, which is at the interface with the overlying(confining)clay layer. RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-11 ESA/203015 Administrative Draft EIR March 2004 L