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Third Quarter 2002 Monitoring, Sampling,and Remediation Progress Report 5 7-Eleven Inc, Store#19976, 1399 North Main Street, Manteca CA October 31 2002 ' Two potentlometrlc surface maps were prepared using the quarterly groundwater elevation data Based on the groundwater elevation data, groundwater flow In both water-bearing zones Is toward the northwest at a gradient of 0 003 foot per foot (Figures 2 and 3, Attachment 1) Quarterly and historical groundwater ' monitoring results are summarized In Tables 1 and 1A, respectively (Attachment 2) Field data sheets are Included In Attachment 3 ' Monitoring Well Sampling and Analytical Results ' Groundwater samples were collected using disposable bailers Groundwater samples were placed In analysis-specific containers with analysis-specific preservative The samples were then sealed using Teflon septum caps, labeled with the job Identification number, the sample number, date, time, and type of laboratory analysis required The samples were stored on Ice and transported with a chain-of-custody record to a state-certified laboratory for analysis for BTEX and TPH-G using Environmental Protection Agency (EPA) method 8020/modified 8015 and for oxygenates TBA, MTBE, RIPE, ETBE and TAME using ' EPA method 8260 ' Dissolved concentrations of up to 2,700 pg/L benzene, 36,000 pg/L TPH-G, and 1,800 pg/L MTBE were detected In the shallower water-bearing zone Highest benzene, TPH-G, and MTBE levels were found In well MW-2 located Immediately adjacent to and downgradlent from the hydrocarbon/MTBE source area (the UST/pump Island area) Historically, downgradlent attenuation In the shallower water-bearing zone has been evidenced by much lower concentrations of hydrocarbons/oxygenates In well MW-5 relative to source area well MW-2 These conditions remain demonstrated by 550 pg/L MTBE In well MW-5a-sof the ' July 2002 sampling date Furthest downgradlent well MW-7 usually contains less than 10 pg/L MTBE concentrations, documenting lateral plume delineation In the shallow water-bearing zone These conditions are verified by the current data (3 8 pg/L MTBE In well MW-7 as of July 2002) — MTBE concentrations In well MW-2 have decreased by 84% since the second quarter of 2002 (from ' 11,000 pg/L to 1,800 pg/L) Quarterly and historical analytical results for groundwater samples are summarized In Tables 1 and 1A, respectively (Attachment 2) Laboratory reports are Included In Attachment 4 8. vmll 3 In the deeper water-bearing zone, hydrocarbon levels ranged up to 0 g/L benzene and low level (14 pg/L) MTBE In near-source well MW-1A Downgradlent concentrations attenuate as observed by non-detectable levels of benzene and TPH-G and relatively low levels of MTBE (2 5 pg/L) In well MW-5A ' Dissolved oxygen (DO) levels ranged from 1 0 milligrams per liter(mg1L) In most of the wells to 1 6 mg/L In well MW-1A, Indicating generally anaerobic conditions ' 19976Q23 ooc