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4.8 Hydrology and Water Quality in the soil borings are consistent with historical DWR water level measurements in the unconfined well 4S/6E-5A01. The hydrograph for well 4S/6E-5AOI is shown in Figure 4.8-4. DWR records obtained for well 4S/6E-05A01 indicate that groundwater remains relatively stable over the long term but responds to seasonal fluctuation and groundwater pumping. The depth to groundwater in this well has fluctuated between 85 feet and 110 feet above msl during approximately 40 years of recorded data, and based on the hydrograph,the 40-year average depth to water is approximately 97 feet below ground surface. As discussed in the setting above,the water levels could rise from the currently observed average levels due to years of high precipitation,due to reduced regional groundwater pumping, or due to regional aquifer recharge. However,based on the recorded groundwater fluctuations over the±40-year historic trend that represents extreme drought(1977)and years of excessive precipitation, the increase in groundwater levels could reach but may not significantly exceed the maximum historic recorded _ water level depth of 85 feet below msl. Considering the characteristics and depth of the unconfined aquifer and the fluctuating elevation — of groundwater in that aquifer, groundwater could approach the active surface of the mine excavation during the life of the project. As discussed above,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. These conditions would be considered significant impacts according to the significance criteria presented above. 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 mining surface and the upper water table surface. These measures are presented — below as mitigation measures developed to ensure that impacts associated with an increased groundwater surface remain less than significant. Mitigation Measures Measure 4.8.1a: Excavation/Groundwater Separation. The project applicant, shall, at no — time,extend the depth of excavation below the depth of the water table and shall maintain a vertical separation between the water table and the active mining surface. Determination of the water table depth shall be based on historic regional groundwater levels for the — unconfined aquifer. The vertical separation shall be determined appropriate by a California-certified hydrogeologist or civil engineer but shall not be less than five feet. Measure 4.8.1b: Groundwater Elevation Monitoring Schedule. The applicant shall monitor groundwater to ensure that the vertical separation between the nearest groundwater surface and the lowest point of the active excavation and reclaimed phases is maintained as required by the California-certified hydrogeologist or at no less than five — feet. The groundwater levels shall be monitored on-site prior to and during all phases of excavation,prior to the initiation of excavation for each phase, and in the reclaimed phases. Groundwater shall be continually monitored in the active mining area on a — bimonthly basis, within a week following each large rainstorm from October through April, and monthly during the remainder of the year. Reclaimed phases shall be monitored once monthly. Monitoring technique shall be based on recommendations and approval by _ a California-certified hydrogeologist. RMC Pacific vernalis Quarry Mining and Reclamation Project 4.8-23 ESA/203015 Draft Environmental Impact Report May 2006