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■ SECTION 1.0 INTRODUCTION The following Additional Groundwater Study Report for the California Department of Transportation (Caltrans) Shop 10 Maintenance Station (Shop 10) in Stockton, California was prepared for Caltrans District 10 The report summarizes the results of the discrete vertical groundwater sampling, slug tests, petroleum hydrocarbon mass estimation in soil, and natural attenuation evaluation at Stockton Yard Caltrans contracted URS Greiner Woodward Clyde (URSGWC) under Contract 43A0019, Task Order No 1 to conduct these additional site investigation and data evaluation activities The results will be used in the human health risk assessment and the evaluation of remedial alternatives to be conducted for the Shop 10 site On February 10, 1999 a meeting was held with the Central Valley Regional Water Quality Control Board (CVRWQCB) and the San Joaquin County Public Health Services (SJCPHS) At that meeting, it was agreed that an additional groundwater investigation was needed to complete the delineation of petroleum hydrocarbons before preparing the Draft Human Health Risk Assessment and Groundwater Impact Analysis Report for Shop 10 The additional site investigation data and analysis that were requested included • A survey of public and private groundwater wells within 500 feet of Shop 10, • Vertical delineation of petroleum hydrocarbons in groundwater, a Estimation of hydraulic conductivity, • Estimation of petroleum hydrocarbon mass in soil and groundwater, and ' • Estimation of natural attenuation to CVRWQCB Basin Plan concentrations (i e , maximum contaminant levels for applicable contaminants) 1.1 SITE DESCRIPTION The Shop 10 Maintenance Station is an active facility located at 1603 South B Street in Stockton, California (Figure 1-1, USGS, 1968) Shop 10 has been operated as a state highway repair and maintenance yard since approximately the late 1940s The Caltrans District 10 Headquarters is located adjacent to the north of the site Adjacent to the east of the site across South B Street are the Caltrans Stockton Yard Maintenance Station (Stockton Yard) and residential properties including the Charter Mobile Home Park The San Joaquin County Fairgrounds is located adjacent to the west and southwest of Shop 10 (Figure 1-2) The nearest surface water body is Mormon Slough, located approximately 1,000 feet north of the site Duck Creek is located approximately 15 miles south of the site The site elevation is approximately 21 feet above mean sea level 51PROJ141440100041SHOPi015HOP105A.DOC 1-1 URS Greiner Woodward Clyde