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4. ENVIRONMFMAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALITY destruction shall be designed to create a sanitary seal that would not conduct vertical water flow in the event that the well is truncated or damaged during mining operations. Significance after Mitigation: Less-than-Significant. Impact 4.8A: Groundwater extraction associated with the proposed project could lower groundwater levels in areas beyond the boundaries of the project site resulting in groundwater drawdown through the life of the project. Excessive groundwater level decline could deplete available groundwater resources by lowering local or regional groundwater levels below the current pumping depths of existing private or municipal groundwater supply wells. This would be a significant impact without mitigation prescribed as part of this EHL The proposed project would require water for aggregate processing,dust control,and landscaping. Under normal operations,the project applicant would generate the majority of its water supply from surface water decanted and recycled within a series of proposed silt settlement ponds. Groundwater would supplement the surface water and under normal operations,account for approximately 25 percent of their required process water supply. When surface water from settlement ponds is not available,groundwater would be the sole source of process water supply. Groundwater would be used exclusively,for example, during the initial phases of mining when aggregate extraction and processing is underway simultaneously with the establishment of the initial,temporary silt pond.10 During the life of the project, the applicant's operations may require that groundwater may be the only water supply source. However,such situations would be infrequent because the proposed project phasing and silt pond construction are expected to provide adequate available surface water storage for recycling. The estimated water demand for the proposed aggregate mining operations would range between 3.8 and 8.9 million gallons of water per day (mgd)depending on production quantities. This equates to approximately 2,600 to 6,200 gallons per minute(gpm). The two existing onsite agricultural wells and the one existing domestic well would provide the required 25 percent of this demand(between 950,000 and 2.2 million gallons) and provide the entire supply when recycled surface water is not available. The amount of groundwater extracted to supply this quantity could result in an adverse aquifer response including,but not necessarily limited to, extended periods of groundwater drawdown to the extent that lower groundwater levels would restrict other local water users from groundwater pumping. The city of Tracy,as well as other local municipal, agricultural, and industrial operations relies on groundwater as either a primary water supply or to supplement surface water entitlements 10 Once a silt pond is excavated,permeable aggregate material would be exposed on the bottom of the pond. Because of the high permeability of the underlying aggregate material,water entering the pond would seep vertically into the underlying gravels. Ile seepage would continue until the fine-grained silts discharged into the pond begin to settle into the pore spaces of the aggregate. As the fine-grained sediments accumulate,the pond bottom becomes less permeable and the infiltration volumes and rates reduce. RMC PadOc V.1i.Quarry Mining and Reclamation Project 4.8-25 PSA/703015 Administrative unit EQt Mamh 2004