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San Joaquin County Community Environmental Setting, Impacts,and Mitigation Measures Development Department Groundwater and Groundwater Quality impact is considered potentially significant. The proposed Project includes a spill prevention,control,and countermeasures plan(see the`Environmental Commitments"section of Chapter 2 of this EIR)to minimize the potential for accidental spills,which would ensure consistency with the water quality policies in the San Joaquin County General Plan. The project also includes a _ commitment to restrict access to the excavation and open bodies of water using fences and locked gates. Implementation of the above environmental commitments,coupled with the following mitigation measures would reduce + this impact to a less-than-significant level. Wet Pit Because the proposed Project involves mining within the aquifer,Project-related contaminants could degrade groundwater. Contaminants that could reach the aquifer during wet pit operations would be similar to those described for dry pit operations. Because the pits during the wet pit operations would not be i dewatered and would therefore provide more open water bodies with direct connectivity to the aquifer than the dry pit operations,the potential groundwater quality impacts of the wet pit operations would be greater. These open water .,, bodies would have increased exposure to potential contaminants and provide a vector for contamination of the aquifer.Implementation of the environmental commitments described in Chapter 2, "Project Description" and Mitigation .. Measures GW-la and GW-lb would reduce this impact to a less-than- significant level. Mitigation Measure GW-1a: Monitor Groundwater Quality; Implement Measures to Maintain Groundwater Levels as Necessary A network of on-site sentinel and off-site monitoring wells will be used to monitor baseline,or background, groundwater quality characteristics to detect any groundwater quality impacts caused by the Project. The project proponent will prepare a groundwater quality monitoring plan .. that will identify constituents to be analyzed, sampling frequency,and sampling locations. The plan must be approved by San Joaquin County prior to Project implementation,which would ensure consistency with r, the San Joaquin County General Plan. Monitoring may include weekly monitoring of groundwater temperatures,pH,and dissolved oxygen and quarterly monitoring of other contaminants(such as gasoline and oils). If monitoring results in a determination that Project dewatering activities have adversely affected groundwater quality, all dewatering activities will cease,and a detailed analysis will be performed by a licensed engineer in conjunction with a certified hydrogeologist to identify the likely cause of contamination. This analysis will conform to American Society for Testing and Materials (ASTM)standards,and will include recommendations for reducing or eliminating the source or mechanisms of contamination. Based on this analysis,the Operator will select and implement measures to control contamination,with a performance ti standard that groundwater quality must be returned to baseline ~' DeSilva Gates Quarry Project Draft Environmental Impact Report 3.1-1-7 aas 05105.05 s