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4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALITY Impact 4.8.7: Mining operation would alter the natural,preferential flow pathways of surface water resulting in areas of localized onsite flooding. This is a less-than-significant impact. During large storm events, storm water in the unmined phases of the proposed project site would infiltrate,pond,and flow as it does under existing conditions. Areas that currently undergo temporary inundation by storm water flows would remain and be managed as they are under existing conditions. The proposed project would not result in onsite,temporary storm water flooding in umnined phases surface water project. In the active mining areas and the processing plant area, stormwater flows would either be directed to silt ponds or contained within the mining excavations. This stormwater drainage approach would be independent of offsite drainage and be contained within the boundaries of the site. Although there may be periods of heavy rain that inundate localized areas and causes delays in mining operations,the stormwater would be managed onsite and directed to either the silt .. ponds or active excavations. Onsite conveyance systems including berms, ditches,pipes,would be constructed as needed to avoid water ponding or excessive erosion. Stormwater entering the silt ponds would be recycled or would eventually evaporate and stormwater directed into the mining excavations would then infiltrate into the ground. Upon final reclamation,site drainage would drain towards the northeast and southeast comers where it would first pond in two separate sumps then percolate into the ground. According to the Significance Criteria presented in this chapter,the project would result in significant impacts if it creates or contribute runoff water which would cause flooding on or offsite. As discussed above,the drainage system would be contained within the project site and stormwater would be directed into onsite silt ponds and excavations. The changes in stormwater flows and quantities generated by the proposed project would not adversely impact offsite areas and therefore is not a significant impact. Although temporary onsite flooding would be an operational nuisance, it would be managed onsite and does not constitute a significant impact. .. Mitigation Measure: No mitigation is required. RMC Pacific Vemalis Quarry Mining and Reclamation Project 4.8-35 ESA 1203015 Adminisuauve Draft Eat March 2004