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1 ' Report on Additional Site Assessment and First Quarter 2000 Monitoring and Sampling Report 7 • 7-Eleven inc Store#19976 1399N Main St Manteca California February 4 2000 1 43 Results of Groundwater Monitoring/Analyses ' Table 2 and Figures 2 and 3 summarize the laboratory analytical results of groundwater samples collected from existing monitoring wells and newly-installed well MW-6 on January 24, 2000 Depth to ' groundwater was about 17 - 18 feet below grade Dissolved hydrocarbon concentrations ranged up to 6,990 pg/L benzene, 36,200 pg/L TPH-G, and 753 pg/L MTBE, with the highest concentrations observed in source area well MW-2 Shallow downgradient wells MW-5 and MW-6 contained concentrations of up to 4 21 pg/L benzene, 1,010 pg/L TPH-G, and ' 83 pg/L MTBE The deeper downgradlent monitoring well, MW-5A, did not contain any dissolved hydrocarbons or oxygenates above the reporting limits Copies of the analytical reports and chain-of-custody forms are included in Appendix D The dissolved hydrocarbon plume is delineated to lie within the upper part of the saturated zone (at ' depths of less than 50 feet below grade) by continuing non-detectable to very low levels in well MW-5A 60 CLOSURE ABSTRACT Soil and groundwater hydrocarbon plume delineation is proposed as complete, pending changes in groundwater concentrations over time which may indicate additional assessment is necessary Sensitive receptor survey and regional hydrogeologic setting evaluation data indicate that hydrocarbons at the site pose a relatively low risk to nearby land and groundwater users Remediation requirements, if any, have ' yet to be defined by the SJCPHS Further offsite assessment will likely be infeasible due to inability to obtain encroachment permits from the City of Manteca, and/or access permission from offsite private land owners • 19976 3$A