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III. ENVIRONMENTAL SETTING IMPACTS AND MITIGATION MEASURES F. HYDROLOGY AND WATER QUALITY area (assuming a runoff coefficient of 0.90 for concrete3 with no other losses),the pre-project runoff volume collected during the 10-year,48-hour storm event would equal approximately 21,915 cubic feet of stormwater, while the runoff volume collected during the 100-year, 24-hour storm event would equal approximately 24,583 cubic feet of stormwater. The post-project runoff volume for the 7.5-acre area during a 10-year,48-hour storm event would equal 76,448 cubic feet, and runoff volume collected during a 100-year, 24-hour storm event would equal 85,758 cubic feet of stormwater. These values indicate that the increase in collection area would exceed the capacity of the existing collection basin and likely overwhelm the treatment system.4 Expansion of the collection basin, additional basins, or additional treatment methods would be required under the proposed project to meet NPDES requirements and is discussed below under the following mitigation. Mitigation Measures Proposed as Part of the Project None. Mitigation Measures Identified in This Report Mitigation Measure F.2: The NPDES-required Storm Water Pollution Prevention Plan currently in use at the site will be updated to reflect changes to the facility. Changes made to the SWPPP will be implemented accordingly and may include, but will not be limited to,the following changes to the stormwater collection and treatment system: ® For the area east of Pershing Avenue, size the collection system(drains and pipes) and treatment system to capably handle an increase in flows and collection volumes for compliance with the existing industrial NPDES permit. The objective of the collection and treatment system is to limit the discharge of polluted runoff from the industrial area. The project sponsor has multiple options for either reducing industrial areas exposed to stormwater runoff or collection and treatment of polluted runoff. Not all stormwater from the industrial activity areas has to be collected and treated if it can be shown that stormwater from certain areas would not degrade the quality of off-site receiving waters according to RWQCB Basin Plan standards. The project sponsor also has the option of collecting, treating, and discharging stormwater from other locations than the existing collection basin if expansion of the existing basin is not feasible. ® For the 3.6-acre parcel west of Pershing Avenue, install appropriate collection and treatment facilities based on the results of NPDES monitoring. In amending the SWPPP,adopt best management practices(BMPs)to improve runoff quality from this area. Options are available to the project sponsor for collecting and treating the runoff on this parcel or potentially routing the runoff to the existing collection basin opposite of Pershing Avenue. Significance After Mitigation: Less than Significant. 3 This coefficient is based on runoff coefficients recommended by the American Society of Civil Engineers and Water Pollution Control Federation in Handbook of Hydrology(Maidment, 1993). For"asphaltic and concrete" pavement surfaces a range of runoff coefficients from 0.70 to 0.95 is given. 4 Runoff volume estimates are considered conservative due to the use of the 100-year,24-hour and 10-year,48-hour precipitation amounts with the collection of 90 percent of all rainfall across the estimated industrial areas. Stockton Scavenger Transfer Station E.rpansion III.F.8 ESA 1990190