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III. ENVIRONMENTAL SETTING,IMPACTS AND MITIGATION MEASURES F. HYDROLOGY AND WATER QUALITY Impact F.3: The proposed project would increase impervious surfaces and could alter drainage patterns across the site,thereby increasing and potentially rerouting surface runoff,which could produce flood hazards or impact the regional drainage system. (Less than Significant) The main facility site(east of Pershing)is nearly completely paved with only minimal additional pavement called for as part of the project. Drainage improvements to the main facility site are proposed as part of the project to adequately collect surface runoff and contain it within the stormwater detention basin according to standards of the City's Municipal Utilities Department, and is also addressed under Mitigation Measure F.2 for NPDES runoff requirements. Project- proposed drainage improvements would also include replacing the existing surface storm drainage ditch that runs along the to-be-abandoned portion of Pershing Avenue with an adequately sized underground drain consistent with City standards. The existing drainage ditch is currently used to convey the treated stormwater from the detention basin to the regional drainage system. Improvements made to Pershing Avenue would not induce additional flood hazards or substantial increases in surface runoff. Installation of curbs,gutters, and an appropriately sized underground drain would enhance a previously uncontrolled drainage area and provide better connection witi,the regional drainage system. There would be a substantial increase in surface runoff at the container storage area west of Pershing due to installation of an all-weather surface that would be impervious in nature. An engineering analysis will be needed to determine whether the increased flows can be handled by the existing storm drain system. Depending on the results of NPDES-required monitoring, runoff generated on this portion of the site may need to be collected and treated prior to discharge under the requirements of the industrial NPDES permit, as discussed under Mitigation Measure F.2. Mitigation Measures Proposed as Part of the Project Mitigation Measure F.3: Develop and implement a drainage plan for the area west of Pershing Avenue that allows for collection,treatment, and discharge of runoff from this area separate from that for the rest of the site, in accordance with the capacity of the off- site stormwater system. Mitigation Measures Identified in This Report None required. REFERENCES —Hydrology and Water Quality City of Stockton, City of Stockton General Plan Background Report. Adopted January 22, 1990. City of Stockton, City of Stockton 1992 General Plan Revision, Draft Environmental Impact Report. Prepared by Michael Paoli and Associates for the City of Stockton. August 1992. City of Stockton,Stormwater Division,Model Storm Water Pollution Prevention Plan for Construction Activities. May 15, 1997. Stockton Sravenger Transfer Station Erpansion III.F.9 ESA/990/90