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4.Environmental Analysis Impact Significance After Mitigation: Less than Significant. Mitigation developed for this impact provides measures to avoid a hydrologic connection between the unconfined groundwater and the mining 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 r intersects the groundwater would not occur and therefore,the impacts would be less than significant. Impact 4.8.2: Accidental spill,leaks,or improper disposal of petroleum fuel would increase the potential for degradation of groundwater quality over the life of the project. This would be a potentially significant impact. The project requires heavy equipment and the use of other vehicles associated with normal ` mining operations. Onsite equipment also includes the stationary processing plant and associated conveyors. The vehicles and stationary equipment require fuels and lubricants that would be ` stored in various quantities on the project site (see Section 4.11 Hazards and Hazardous Materials). Accidental discharge to the ground of these liquid petroleum fuels and lubrication products could contaminate shallow soils and eventually migrate to the groundwater. According to the Significance Criteria stated above,this impact would be significant if the activities associated with the project would significantly degrade groundwater quality. L The release of low viscosity fuels such as gasoline and diesel fuel would represent the greatest threat to soil and groundwater especially if stored in bulk such as in above-ground storage tanks. Engine oil is less of a potential threat due to its low viscosity and inability to migrate readily through soils. The clayey overburden above the aggregate would hinder downward migration of discharged fuels and oils because liquid petroleum plumes in less permeable, fine-grained, unsaturated soils will migrate laterally as well as vertically. The underlying aggregate, however, �- transmits liquid contaminants at higher rates because of its higher porosity(more and larger pore spaces for liquids to flow through) and rather than migrating laterally,the contaminants in the aggregate material tend to flow primarily downward.The potential for groundwater quality degradation could be higher if discharge of these fuels occurred on the active surface of the mining phase because of the minimal vertical separation between the surface and groundwater (see discussion for Impact 4.8.2). With implementation of the proposed project,petroleum products stored and used to support _ normal mining operations could enter the subsurface soils and eventually migrate to the groundwater if they are inadvertently or purposely discharged to an unpaved,permeable surface. Large spills, such as those generated from ruptured above ground fuel storage tanks, would ` represent the greatest threat because of the larger quantity and ability for the fuel to migrate more rapidly through the subsurface. Mitigation of impacts associated with potential groundwater degradation from petroleum typically includes the reduction of potential sources and immediate clean-up response in the event of an inadvertent release. Most releases in an active industrial quarry operation are likely to be small quantity, incidental spills resulting from poor RMC Pack Vernalis Quarry Mining and Reclamation Project 4.8-24 ESA/203015 Draft Environmental Impact Report May 2006 r