SFPUC 2001 Alameda Watershed Management Plan

III. ENVIRONMENTAL SETTING AND IMPACTSD. HYDROLOGY AND WATER QUALITYwould not bring about any new water quality impacts beyond those disclosed and mitigated in theEIR prepared for SMP-32. Alameda County’s conditions of approval for SMP-32 includecontrols for drainage, erosion, and sedimentation that mitigate proposed related mining impactsto a less than significant level.Extending the area of mining south of I-680 could affect both surface water and the groundwatersystem. A larger pit would require redirecting the drainage around the expanded perimeter andwould require construction of associated drainage controls for the runoff that would eventuallyflow to Alameda Creek. There would be a negligible decrease in the volume of runoff from theperimeter of the pit to the creek, with the associated slight increase directly entering the miningpit. Similar to existing conditions, runoff directly entering the mining pit would likely either bedirected for mining process water or discharged to Alameda Creek in compliance with anydischarge permits. On a program level, continued implementation of required drainage, erosion,and sedimentation controls, as required by the conditions of approval for SMP-30 and SMP-24,as well as compliance with regulatory discharge permits, would reduce any impact associatedwith runoff draining to Alameda Creek to a less than significant level.• Increasing the area of mining south of I-680 would also require installation of bentonite cutoffwalls in the upper 50 feet of the expanded perimeter of the mining pits, where appropriate, incompliance with existing permit and lease conditions. In some locations, the fault trace alongCalaveras Road acts as an impermeable barrier to groundwater, precluding the need for a cutoffwall along the eastern limit of the mining pits in that area. Bentonite cutoff walls at the MissionValley Rock Company SMP-24 area have been effective in diverting groundwater flow aroundthe pits and in maintaining the overall flow of groundwater to the Alameda Creek channel.Similar to the SMP-24 mining pits, the installation of bentonite cutoff walls along the north,west, and south sides of the expanded pits could be expected to prevent the flow of shallowgroundwater into the pits and protect the groundwater system.•If groundwater were present, extending the depth of mining in existing mining pits(Actions sun2a and sun2b) could further alter groundwater flow patterns within the SunolValley. However, groundwater sampling in the valley has indicated limited groundwater below50 to 60 feet, and the Management Plan would allow for extending mining from 140 to 200 feet.Thus, at these depths, groundwater flows should not be affected. On a program level, theexpanded pits would not be expected to affect Alameda Creek flow and the groundwater system,based on studies conducted to date. However, a comprehensive groundwater and hydrologicstudy has not been conducted for all of the proposed expanded mining areas south of I-680.Therefore, potentially significant groundwater impacts from expansion of mining pits south ofI-680, and subsequent impacts to Alameda Creek and associated resources, cannot be ruled out.The top portion of Table III.D-5 lists those policies and management actions related to gravelmining operations that could result in significant water quality impacts, while the bottom portionof the table lists the full range of policies and management actions that could be required toreduce the potential impacts. Not every action would be necessary to mitigate the effects of theassociated potential impact-causing management action. Because implementation information isNOP 96.223E: Alameda Watershed Management Plan III.D-28 ESA / 930385January 2001