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4.8 Hydrology and Water Quality <br /> Impact 4.8.7: Mining operation would alter the natural, preferential flow pathways of — <br /> surface water resulting in areas of localized onsite flooding.This is a less-than-significant <br /> impact. <br /> During large storm events, storm water in the unmined phases of the proposed project site would <br /> infiltrate,pond, and flow as it does under existing conditions. Areas that currently undergo <br /> temporary inundation by storm water flows would remain and be managed as they are under <br /> existing conditions. The proposed project would not result in onsite,temporary storm water <br /> flooding in unmined phases surface water project. <br /> In the active mining areas and the processing plant area, stormwater flows would either be <br /> directed to silt ponds or contained within the mining excavations.This stormwater drainage <br /> approach would be independent of offsite drainage and be contained within the boundaries of the <br /> site. Although there may be periods of heavy rain that inundate localized areas and causes delays <br /> in mining operations,the stormwater would be managed onsite and directed to either the silt <br /> ponds or active excavations. Onsite conveyance systems including berms, ditches,pipes, would <br /> be constructed as needed to avoid water ponding or excessive erosion. Stormwater entering the <br /> silt ponds would be recycled or would eventually evaporate and stormwater directed into the <br /> mining excavations would then infiltrate into the ground. Upon final reclamation, site drainage — <br /> would drain towards the northeast and southeast corners where it would first pond in two separate <br /> sumps then percolate into the ground. <br /> According to the Significance Criteria presented in this chapter, the project would result in <br /> significant impacts if it creates or contribute runoff water which would cause flooding on or <br /> offsite. As discussed above,the drainage system would be contained within the project site and <br /> stormwater would be directed into onsite silt ponds and excavations.The changes in stormwater <br /> flows and quantities generated by the proposed project would not adversely impact offsite areas <br /> and therefore is not a significant impact. Although temporary onsite flooding would be an <br /> operational nuisance, it would be managed onsite and does not constitute a significant impact. <br /> Mitigation: No mitigation is required. <br /> RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-33 ESA 1203015 <br /> Draft Environmental Impact Report May 2006 <br />