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8820101-R COT-2 INTRODUCTION This report summarizes the activities conducted to date in association with the investigation of subsurface petroleum product contamination at the Thrifty Service Station 171 located at 1250 N. Wilson Way in Stockton, California. A site vicinity map is provided in Figure 1. The activities consisted of the installation and development of three groundwater monitoring wells, the acquisition and analyses of associated soil and groundwater samples and the monitoring of fluid levels in the wells. Included in this report are descriptions of the above activities, analytical results, boring logs, well construction details and a discussion of the findings and recommendations for subsequent actions to be taken at the site. BACKGROUND 5 In July of 1988, Thrifty Oil Co. retained Woodward-Clyde Consultants to conduct an assessment of subsurface petroleum product contamination at Station 171 following the guidelines given in the request for proposal (RFP) dated May 23, 1988. The RFP specified the installation of three 4-inch-diameter monitoring wells to 20 feet beneath first groundwater or a maximum of 50 feet below grade. Soil samples were to be collected at 5-foot intervals, and each well developed and groundwater samples collected. Samples were to be submitted to a certified laboratory for a variety of analyses based, in part, on the level of contamination. A copy of the RFP is provided in Appendix C. This assessment was in response to soil contamination discovered during a previous investigation conducted by Hydrotech Consultants, Inc. Details of ' that investigation are provided in their report to Thrifty Oil dated January 7, 1988. In summary, Hydrotech installed four soil borings to a depth of 20 feet around the underground storage tank cluster, collecting samples at 5-foot intervals. Locations of the borings are shown in I