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GeaNydrologlc Consultants, Inc. 1.0 INTRODUCTION. On behalf of Best California Gas, Ltd. (Best), GeoHydrologic Consultants, Inc. (GHC) has prepared this workplan for conducting a soil vapor survey and vapor risk assessment at for Best Station No. 171 located at 1250 North Wilson Way, Stockton, California("the Site'; Figure 1). The purpose of the proposed soil vapor survey and health risk assessment is to further investigate the potential vapor risk posed by benzene and MTBE beneath the Site. The San Joaquin County Environmental Health Department (SJCEHD) is the oversight agency for this Site. 1.1 SITE DESCRIPTION The Site is owned by Best and is currently leased and operated as a self-service gasoline sales facility by ARCO Products Company since May 1997. It is located on the southeast corner of Wilson Way and Harding Way in Stockton, California(Figure 1).The Site is generally square and about 12,300 square feet in size. The Site features a cashier's booth, three product dispenser islands, and three double-walled fiberglass underground tanks (Figure 2).These tanks were installed in November 1994 and are located near the northeastern corner of the Site. The newer tanks replaced five steel USTs that were located in the southwest portion of the Site. There is an equipment enclosure located in the southeast corner of the Site which currently houses the thermal oxidizer being used at the Site to perform ongoing soil remediation through vapor extraction. The Site has been operated as a service station for at least the last 30 years and there are no plans for alternate use. A mixed commercial-residential area surrounds the Site. To the north of the Site across East Harding Way is City Tune Up. To the northwest of the Site,on the corner of North Wilson Way and East Harding Way, are single-family residences and a vacant parking lot for the former Olde Hoosier Inn. Immediately east of the Site and beyond are single- family residences. To the west of the Site across North Wilson Way is a Kragen Auto Parts store. South of the Site is the Doz Roses Flower Shop. 2.0 GEOLOGY/HYDROGEOLOGY The Site is located in the San Joaquin Valley of the Central Valley geomorphic province. Recently published geologic maps indicate that Holocene age flood-basin deposits extend from ground surface to a maximum depth of approximately 100 feet ground surface (bgs). Oligocene to Holocene age alluvial deposits lie below the younger flood-basin deposits to a maximum depth of approximately 3,500 feet bgs. Continental and marine rocks of pre-Tertiary to Oligocene age underlie the alluvial deposits,and are comprised of shale, sandstone, and conglomerate. Crystalline bedrock underlies the sedimentary formations. Best 171 Workplan for SVS-HRA Page 1