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San Joaquin County Community Environmental Setting,Impacts,and Mitigation Measures <br /> Development Department Groundwater and Groundwater Quality <br /> Impact GW-3: Reductions in Groundwater Levels Caused <br /> by Dewatering (Less than Significant with Mitigation) <br /> Dry Pit <br /> The mining operation would excavate to depths that are below the local <br /> groundwater table. Groundwater would flow into the excavation pits unless <br /> blocked and would be removed. Thus,dewatering would occur to remove <br /> groundwater that seeps into the excavation pits. <br /> Pumped groundwater would be used as process washwater and for dust control. <br /> Excess water would be pumped into either the South Pit or the Stage I North Pit <br /> (depending on the mining operation stage) and then would infiltrate into the <br /> ground, serving as an on-site recharge system. Although the excess water would <br /> infiltrate back into the ground, dewatering could result in a lowering of <br /> groundwater levels on a local or regional basis. Groundwater levels at nearby <br /> well pumps range from 30 feet bgs to 187 feet bgs(San Joaquin County Public <br /> Health Services 2005). Impacts related to water level drawdown may include <br /> reduced production from wells nearest the dewatering areas,increased pumping <br /> w costs for wells that show significant water level drawdown, and changes in <br /> groundwater gradients and flow directions. These impacts could be potentially <br /> significant. Implementation of mitigation measure GW-3 would reduce these <br /> .. impacts to a less-than-significant level. <br /> Wet Pit <br /> Dewatering would not occur as part of the Wet Pit mining operations. Therefore, <br /> there would be no significant reductions in groundwater levels as a result of <br /> dewatering. Some loss of groundwater would occur through evaporation from <br /> r the pits' water surface. There would be no significant impact and no <br /> mitigation is required. <br /> Mitigation Measure GW-3: Monitor and Report Groundwater <br /> Levels; Implement Measures to Maintain Groundwater Levels <br /> as Necessary <br /> A network of on-site sentinel and off-site monitoring wells would be <br /> used to monitor baseline,or background,water level characteristics to <br /> detect any groundwater level impacts caused by the Project. The sentinel <br /> wells would be located on the project parcel and would be designed to <br /> .. detect impacts before they spread beyond the parcel boundaries. Off-site <br /> monitoring wells,which may include existing water supply wells,would <br /> be used to provide an ongoing,continuous baseline record to <br /> complement the monitoring at the sentinel sites. <br /> If the groundwater monitoring resulted in a determination that the Project <br /> was adversely affecting groundwater levels,by reducing them such that <br /> + local well owners were affected by a reduction in well production, the <br /> following measures would be implemented,listed in general order of <br /> implementation: <br /> DeSilva Gates Quarry Project <br /> Draft Environmental Impact Report 3.H-9 <br /> J85 05105.05 <br /> 1 <br />