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Second Quarter 2002 Monitoring,Sampling and Interim Remediation Report 5 16 7-Eleven, Inc Store#19976, 1399 North Main Street, Manteca, CA July 30,2002 ' Two potentlometric 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 ranging about 0 001 to 0 002 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, DIPE, ETBE and TAME using ' EPA method 8260 ' Dissolved concentrations of up to 5,000 pg/L benzene, 58,000 pg/L TPM-G, and 11,000 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 downgradient from the hydrocarbon/MTBE source area (the UST/pump island area) Historically, downgradient attenuation in the shallower water-bearing zone has been evidenced by sharply lower concentrations of hydrocarbons/oxygenates in well MW-5 relative to the source well MW-2 These conditions remain demonstrated by 570 pg/L MTBE In well MW-5 as of the ' April sampling date Furthest downgradient 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 (8 6 pg/L MTBE in well MW-7 as of April 2002) MTBE concentrations in well MW-2 have risen since last quarter(from 170 NgIL to 11,000 pg/L), but TBA concentrations in this well have also risen to the same level, possibly indicating natural biodegradation of MTBE 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 ' In the deeper water-bearing zone, hydrocarbon levels ranged up to 0 58 pg/L benzene and low level (6 9 pg/L) MTBE in near-source well MW-1A Downgradient concentrations attenuate as observed by non-detectable levels of benzene, TPH-G, and MTBE in wells MW-5A ' Dissolved oxygen DO levels ranged from 0 9 milligrams per liter m IL in wells MW-1 and MW-5A to Yg { } g g p ( 9 } � ' 2 1 mg1L in well MW-1A, indicating generally anaerobic conditions 1 r19976022 DOC