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III. ENVIRONMENTAL SETTING,IMPACTS AND MITIGATION MEASURES F. HYDROLOGY AND WATER QUALITY The container storage area west of Pershing Avenue could also provide increased pollutant levels to surface runoff if additional bins and containers,potentially containing residual waste,are stored on the site. Collection bins are emptied at the facility, then stored in the open. Currently bins containing putrescible waste, including all front-end loader bins, are rinsed prior to storage in the container yard and rolloff bins used for inert materials at construction sites are stored without any cleansing or rinsing of residual waste; the same practices would be followed under the project(Maclsaac,2000b). If the number of bins stored in this area were to increase under the project,failure to ensure that containers with residual waste are appropriately rinsed or free of residual debris prior to storage could expose an increased level of residual waste within the bins to precipitation. Precipitation,in turn, could carry these pollutants(including additional sediment)to surface runoff that could potentially be transported off-site. Off-site transport of pollutants and sediment from the container storage area has not been an issue in the past due to the limited use of the storage area and the relatively flat topography and gravel surface, which limited any runoff from the site. Surfacing the bin storage area with an impervious,all-weather surface, as proposed under the project, would generate additional surface runoff that could require collection and treatment under the site's NPDES permit. If the stormwater runoff from this location is entraining pollutants that would degrade the water quality of the off-site receiving water,this runoff would need to be contained and treated prior to discharge from the project site. Currently,the collection basin located east of Pershing Avenue on the main facility site can adequately hold approximately 193,000 gallons or about 25,800 cubic feet of stormwater for treatment and discharge during and following storm events. Following treatment,collected stormwater is discharged through a 2.5-inch pipe to the drainage ditch along Pershing Avenue. Under the current configuration of the project area,the collection basin receives runoff from an area approximately 2.15 acres in size. With implementation of the proposed project, a considerably larger area(approximately 5.4 acres) would be paved and generate additional surface runoff. The collection basin could be required to receive surface runoff from approximately 7.5 acres.2 This acreage includes the 3.6-acre site west of Pershing Avenue used for container storage and additional acreage at the main facility site, including the proposed green waste pad,the storage yard north of the green waste pad, and a small area of unpaved surface surrounding the detention basin (shown as a shaded area in Figure II-2)that would be paved under the project. Assuming that all surfaces would be concrete or a similar impervious surface, runoff volumes for the a continuous 10-year,48-hour storm event(3.12 total inches of rainfall) and a 100-year,24- hour storm event(3.5 total inches of rainfall) under pre- and post-project conditions were calculated. Conservatively assuming collection of 90 percent of all rainfall in the approximated 2 These area estimates were used to calculate existing and potential runoff volumes in order to provide a preliminary analysis of the capabilities of the collection basin to adequately contain surface runoff during different storm events. All calculations are preliminary and should not be used as design criteria for installation of additions to the existing drainage system. Area estimates were calculated based on pre-and post-project site plans and are not meant to indicate firm acreage amounts required for stormwater collection and/or treatment under NPDES requirements. Stockton Scavenger Transfer Station Expansion III.F.7 ESA/990190