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REPORT ON ADDITIONAL SITE ASSESSMENT AND RECOMMENDATIONS 6 7-Eleven Store No 14117,2725 Country Club Blvd,Stockton,CA December 12,2001 43 Fourth Quarter 2001 Groundwater Monitoring Analytical Results Quarterly groundwater mondonng and analytical data for newly-installed and pre-existing monitoring wells are summarized In Table 1 and shown on Figure 2 Historical data are Included In Table 1A Dissolved hydrocarbon concentrations ranged up to 280 micrograms per liter(ug/L) benzene, 5,800 ug/L TPH-G, 9,600 ug/L MTBE, and 98,000 ug/L TBA The highest concentrations ' were observed in wells MW-1 and MW-2, located northeast of and In the existing underground storage tank pd, respectively Copies of the analytical reports and chain-of-custody forms are 1 included In Appendix D ' Non-detectable analytical concentrations In the newly-Installed monitoring wells Indicate that the dissolved benzene, TPH-G, MTBE, and TBA plumes are delineated to the west, south, and northwest of the existing and former tank areas ' 50 RECOMMENDATIONS A 2000-foot door-to-door sensitive receptor evaluation Is recommended to be completed during the first quarter 2002, along with a feasibility study to determine the best means of remediation of dissolved MTBE and TBA The suggested means of feasibility testing would be an 8-hour dual-phase high-vacuum (DPH1/)test ' using wells MW-1, MW-2, and possibly MW-4 as extraction points,while monitoring for Induced vacuum response in all nearby wells using Magnehelic gauges Results of the test will indicate the ' radius of vapor extraction Influence and accessible gasoline mass in the vadose zone Groundwater generated during the test Is proposed to be drummed pending disposal by a licensed waste-hauling ' company ' A file review of nearby service stations is also recommended to be completed In conjunction with feasibility testing and sensitive receptor evaluation, as the SJCPHS has indicated that gradient Information from the service station east of the 7-Eleven site conflicts with gradient information from ' the 7-Eleven site Research Is needed to determine the source of the discrepancy Wellhead elevation data and surveying methods/benchmark references should be compared between the two ' sites If necessary, re-surveying of all monitoring wells at the two sites by the same surveyor maybe required to ensure data conformity The results of these findings will be used to complete the comprehensive site model requested to be completed by June, 2002, by the SJCPHS