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4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALITY below as mitigation measures developed to ensure that impacts associated with an increased groundwater surface remain less-than-significant. Mitigation 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 basal mining surface. The vertical separation shall be determined appropriate by a California-certified hydrogeologist or civil engineer but be no less than five feet. Mitigation Measure 4.8.1b: Maintaining Excavation/Groundwater Separation In the event that the water table level increases unexpectedly over a short period of time or an increase occurs that is not identified through monitoring(see Mitigation Measure 4.8.1c)and the water table rises to an elevation less than the required separation distance, the applicant shall _ cease vertical excavation in the affected area and place surface fill on the basal mining surface to establish the required separation. Fill placement shall be supervised by a California-certified engineering geologist and be engineered by a California-registered civil engineer to adequately _ support heavy equipment. Mitigation Measure 4.8.1c: 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 5 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 bi-monthly basis and within a week following each large rainstorm from October through April, and on a monthly basis during the remainder of the year. Reclaimed phases shall be monitored once monthly. Monitoring T technique shall be based on recommendations and approval by a California-certified hydrogeologist. Significance after Mitigation: Less-than-Significant. Mitigation developed for this impact provides measures to avoid a hydrologic connection between the unconfined groundwater and the basal surface of the excavation. The mitigation establishes a proactive monitoring mechanism to ensure that the water levels are at an adequate vertical separation distance between the lowest mining surface and the upper level of groundwater. Potential adverse effects that could occur if the mining operation intersects the groundwater would not occur and therefore, the impacts would remain less-than-significant. RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-22 ESA 1203015 Administrative Draft Ent March 2004