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SECTION 1.0 . INTRODUCTION The following Additional Groundwater Stud Report for Y p the California Department of Transportation (Caltrans) Stockton Yard Maintenance Station (Stockton Yard) 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 43AO019 Task Order 1 to conduct these additional site investigation and data evaluation activities The results will be used in the human health risk assessment and evaluation of remedial alternatives to be conducted for the Stockton Yard 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 a Human Health Risk Assessment and Groundwater Impact Analysis Report for Stockton Yard The additional site investigation data and analysis that were requested included 0 A survey of public and private groundwater wells within 500 feet of Stockton Yard, • Vertical delineation of petroleum hydrocarbons in groundwater, • Estimation of hydraulic conductivity, • Estimation of petroleum hydrocarbon mass in soil and groundwater, and • Estimation of natural attenuation to CVRWQCB Basin Plan concentrationsi e ( , maximum contaminant levels for applicable contaminants) ' 1.1 SITE DESCRIPTION ' The Stockton Yard is an active facility located at 1604 South B Street in Stockton, California (Figure I-1) The Stockton Yard covers approximately 4 acres and is part of the Caltrans District ' 10 Headquarters Complex (Figure 1-2) The Caltrans Construction Division building is located to the north of the site To the west of the site and across South B Street are the Caltrans Shop 10 and the San Joaquin County Fairgrounds The other surrounding properties are primarily ' residential and commercial The nearest surface water body is Mormon Slough, located approximately 1,000 feet north of the site Duck Creek is located approximately 1 5 miles south ' of the site The site elevation is approximately 21 feet above mean sea level • The Stockton Yard was built in 1955 A fueling station is located within the northwest portion of the site The Stockton Yard is operated as an equipment maintenance and storage facility The ' s IPROJW1040100o ISYYARDkMAJ TYARDSA.00c l_I URS Gremer Woodward Clyde