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III. ENVIRONMENTAL SETTING IMPACTS AND MITIGATION MEASURES F. HYDROLOGY AND WATER QUALITY SETTING The project site is located at 1240 Navy Drive,approximately one-half mile west of Interstate-5, just north of Charter Way and east of Fresno Avenue. The site is located in a heavily urbanized area dominated by industrial and commercial uses. Surface runoff is collected in storm drain lines and roadside ditches and discharged to major watercourses in the area. Due to the flat topography of the area, substantial amounts of precipitation do pond and infiltrate where open spaces exist. Stockton has a typical mediterranean climate with wet, cold winters, and warm,dry summers. Most of the rainfall occurs between November and April with an average annual rainfall of 13.7 inches. The 10-year,48-hour estimated precipitations amount for the City of Stockton is 3.12 inches(San Joaquin County, 1973) and the 100-year,24-hour estimated precipitation is 3.5 inches (Western Regional Climate Center, 1999). HYDROLOGY The portion of the site east of Pershing Avenue(5.8 acres), currently used as a small-volume transfer station, includes a small office building, maintenance building, maintenance and storage sheds, vehicle cleaning islands,parking area,and a stormwater detention basin. Nearly all stormwater runoff from this portion of the site drains to the existing stormwater detention basin (a minor portion drains the inlet road out to the street). This area is graded so runoff drains to collection points where underground piping routes stormwater runoff to the collection basin in the southeast corner of the main facility portion of the site.. Stormwater collected in the basin is allowed to settle and then passes through a slant-rib coalescing oil/water separator, prior to discharge through a 2.5-inch diameter pipeline to the drainage ditch along Pershing Avenue. A float-activated pump initiates the discharge from the basin when a certain surface water elevation is reached(Maclsaac, 2000a). The portion of the site west of Pershing Avenue(3.6 acres) is unpaved and currently used for rolloff container and bin storage. No stormwater controls are currently in place for this parcel. The flat topography and aggregate cover of the site produces only minimal surface ponding. Stormwater runoff in the general vicinity of the project site is collected in the streets,gutters,and roadway drainages and conveyed through closed-conduit storm drain systems to major watercourses in this area which include the Stockton Deep Water Channel and the San Joaquin River(City of Stockton, 1992). The project area and surrounding vicinity experience only minimal and localized flooding due to flood control levees along all major watercourses (City of Stockton, 1992). According to the Federal Emergency Management Agency (FEMA)Flood Insurance Rate Map for City Stockton, California(Community-Panel Numbers 060302 0020 D, revised February 4, 1988),the project I The 10-year.48-hour precipitation estimate refers to the approximate amount of rainfall that is expected to fall over a 48-hour period during a 10-year storm event or an event that has a 10 percent probability of occurring during a during a normal year. Stockton Scavenger Transfer Station Expansion III.F.1 ESA 1990190