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4.8 Hydrology and Water Quality • Even though the well depths would be shorter in the unconfined zone and energy _ demand to extract the water may be less, the potential yield of the shallow unconfined zone is lower than the confined zone and pumps would have to operate for longer periods of time to provide the same amount of water. _ • Whether groundwater is pumped from the deep,confined zone, or the shallow unconfined zones,assessing feasibility would require appropriate pump tests. • Groundwater extraction in the unconfined zone could draw down water enough to maintain the necessary separation between the groundwater surface and the bottom of the mine. This would prevent groundwater seepage into the mine. In order to draw _ down the water surface sufficiently, two wells would likely be inadequate. Additional wells and long-term pumping schedule would be developed to maintain lower water levels. • The urban and agricultural beneficial uses of the confined aquifer would be protected regardless of whether groundwater pumping occurred in the unconfined and confined aquifer. Surface water quality degradation may occur if the groundwater from the — unconfined unit is extracted and is allowed to enter surface water features. • Localized subsidence may occur in some areas due to pumping in the unconfined zone. • Increased continued pumping in the unconfined zone could eventually pull nitrates and pesticides deeper in the soil horizon, thereby increasing the potential for — localized, shallow groundwater contamination. To consider this scenario in greater detail, additional data research and groundwater pump testing of the unconfined zone would be necessary. In contrast, the feasibility of pumping the deeper confined zone aquifer is already known based on performance and historic pumping of the existing onsite wells. The preliminary analysis demonstrates that the alternative groundwater pumping scenario could result in potentially significant water quality impacts. In contrast, pumping from the deeper,confined aquifer would result in less than significant water quality impacts prior to mitigation. Given the potentially significant water quality effects of the _ alternative pumping scenario, it is not considered a feasible alternative. Mitigation: No mitigation is required. Impact 4.8.5: Reuse of excavated overburden containing clay and silts combined with equipment compaction of the reclaimed mine areas could decrease infiltration of surface — water and reduce groundwater recharge from the project site throughout the life of the project and after reclamation.This would be a less-than-significant impact. Under current conditions,the surface soils and underlying aggregate deposits provide a natural recharge system for the water table. The surface water is transmitted downward to the water table, driven by gravity, through pore spaces in the overburden material and then through the more _ porous,underlying aggregate unit. The infiltration capacity of the soil and rates that water travels RMC Pacific Vernalis Quarry Mining and Reciamation Project 4.8-29 ESA/203015 Draft Environmental Impact Report May 2006