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III. ENVIRONMENTAL SETTING,IMPACTS AND MITIGATION MEASURES F. HYDROLOGY AND WATER QUALITY site is labeled Zone X or"Areas of 500-year flood; areas of 100-year flood with average y y depths of less than 1 foot or with drainage areas less than 1 square mile; and areas protected by levees from 100-year flood." No flooding problems have been apparent on the project site under its current configuration and operation (Maclsaac, 1999). The street drainage system surrounding the project is known to experience localized ponding during peak stormflows (MacIsaac, 1999). The entire project site is underlain by Jacktone-Urban land complex soil (SCS, 1988). This soil is moderately deep and was formed in alluvium derived from mixed rock sources. Permeability is slow with a moderate water capacity and a high shrink-swell potential. The soil has a hydrologic group rating of D or soils that have a high runoff potential(slow infiltration rate) when thoroughly wet(SCS, 1988). However,this soil rarely floods and has a depth to the high water table typically greater than five feet. The area is not considered one of the main recharge areas for the underlying regional aquifer(City of Stockton, 1990). WATERQ UALITY No substantial water resources are located within the general vicinity of the site. The area is dominated by the urban storm drain system, which carries surface runoff and urban pollutants to the major watercourses, which include the Stockton Deep Water Channel to the north and the San Joaquin River to the west. Contaminants available from industrial sites and general urban areas can be carried by stormwater runoff to surface watercourses or absorbed into the ground, where pollutants may enter an underlying aquifer. If not controlled (e.g.,through best management practices or other permit compliance measures),the generation,use, and storage of industrial materials and waste can pollute ground and surface waters through contact with surface runoff during storm events and also from non-storm discharges such as wash water. During periods of wet weather,rain carries pollutants and soil particles from all parts of a watershed into surface water bodies such as storm drains, streams,rivers,reservoirs,or marshes. In an urban setting, natural drainage patterns have been altered and stormwater runoff, as well as non-storm discharge (irrigation water,accidental spills, washdown water,etc.),picks up soil particles and contaminants from land surfaces,and transports these pollutants into surface and ground water. These diffuse sources of pollutants range from parking lots, industrial lots,bare earth at construction sites,agricultural and landscape sites, and a host of other sources. The total amount of pollutants entering aquatic systems from these diffuse,nonpoint sources is now generally considered to be greater than that from any other source, such as pipe discharges (point source). Typical of an industrial facility, waste pollutants do accumulate on the transfer stationg round surface, thereby requiring collection and treatment of stormwater runoff to remove entrained pollutants. Drainage from the industrial work area of the 5.8-acre area east of Pershing Avenue is directed to the stormwater detention basin for storage and treatment through a coalescing oil/water separator. Following treatment,the runoff is tested for pollutant levels then discharged to an off-site City of Stockton stormwater drainage ditch along Pershing Avenue(EMCON, Storkton Sravenger Transfer Station Expansion III.F.2 ESA 1990190