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4. ENVIRONMENTAL ANALYSIS •r 4.8 HYDROLOGY AND WATER QUALITY possible under certain conditions,reintroduction of pollutants to the groundwater from the open excavation of a mine is less likely due to the seepage pressures and flow gradient of the groundwater entering the excavation. However, if high volume groundwater pumping were to occur near or adjacent to the mine excavation, contaminated surface water could be drawn from the pit to the extraction well. t' Impact Analysis Project mining would occur to an approximate depth of 70 to 120 feet below the surface. The proposed project is designed to limit all mining excavations to above the groundwater table at the time of mining. Proposed excavation depths are partly based on the depth of groundwater at the time the Applicant developed the proposed mining plan. The analysis performed for this EIR included a review of geologic exploratory logs and long-term California Department of Water Resources(DWR)groundwater records to evaluate the water table and the potential for groundwater fluctuations. The data indicates that the approximate groundwater elevation of the unconfined zone(water table)ranges from 57 to 85 feet above msl at the project site. DWR records obtained for well 4S/6E-05A001 (discussed above),indicate that " groundwater remains relatively stable over the long term but responds to seasonal fluctuation and groundwater pumping. However,water level measurements indicate that over the long term, water levels in the well are gradually rising. This can be verified by DWR records of two ' groundwater wells (4S/6E- and 15ROO1 gr ( ), installed within the unconfined aquifer and located within 6,000 feet from the proposed project site. Water level data in these wells also indicate that the depth to groundwater fluctuates due to season and well pumping. Groundwater levels in these wells would rise to its highest elevation during years of unusually high precipitation and drop below normal levels in dry years. However, over the long term, the 30 years of data suggest that average groundwater levels in the unconfined aquifer gradually increased from an elevation of approximately 60 feet in the late 1960s to near 90 feet in the late 1990s. This gradual increase since the 1960s could be attributable to reduced groundwater .. pumping from the unconfined aquifer,the use of additional surface water supplies,conservation, or increased infiltration and recharge. Groundwater levels in the unconfined aquifer could therefore gradually increase over the life of the project and eventually intersect the mining excavation. Considering the characteristics and depth of the unconfined aquifer and the fluctuating elevation of groundwater in that aquifer,groundwater could intersect the basal surface of the mine excavation during the life of the project. As discussed in this analysis,high groundwater could contribute to long term losses to the groundwater supply,could cause operational and flooding impacts prior to and after reclamation, and could result in groundwater and surface water quality impacts. A natural increase in the water table levels is unavoidable and counteracting or arresting such an increase is costly if not infeasible(i.e.,through pumping wells used to reduce .. groundwater levels). However,measures can be implemented to maintain an adequate separation between the basal mining surface and the upper water table surface. The measures are presented RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-21 ESA/203015 Administrative Daft E1R Mash 2004