ARCHIVED REPORTS XR0004308 - Laserfiche WebLink

Home Browse Search

16 Fourth Quarter 2401 Monitoring and Sampling Report 2 Remediation Well Installation/Interim Remediation Report 7-Eleven Inc Store*19976,1399 North Main Street Manteca CA January 31 2002 Groundwater Monitoring Data Before monitoring,each well cap was opened to allow the water level In each well to stabilize to atmospheric pressure The DTW was measured to the nearest 0 01 foot from the top of the well casing using an Interface probe I Two potentrometric 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 about 0 002 ft/ft(Figures 2 and 3) Groundwater monitoring results are presented in Table 1 Field data sheets are Included to Attachment 3 ' Monitoring Well Sampling And Analytical Results ' Before sampling, at least four well-casing volumes of groundwater were removed from each monitoring well unless the well was purged dry The groundwater samples were collected using disposable bailers Groundwater samples were placed in analysis-specific containers with analysts-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 analysts 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 ' Laboratory analytical results for quarterly groundwater samples are included to Table 1, and historical results are summarized in Table 2 Analytical results for the shallower part of the water-bearing zone indicated dissolved concentrations of up to 4,000 micrograms per Ider(ug/L) benzene,46,000 ug/L TPH-G, ' and 1,300 ug/L MTBE Highest benzene and TPH-G levels were found in well MW-2, located Immediately adjacent to and downgradient from the hydrocarbon/MTBE source area (the underground storage ' tanks/pump island area) Well MW-1, located crossgradient near to the MTBE source area had the highest MTBE concentrations Historically,downgradient attenuation in the shallower water-bearing zone has been evidenced by sharply lower concentrations of benzene and TPH-G in well MW-5 relatively to the source ' well MW-2 Furthest downgradient well MW-7 contains no detectable MTBE concentrations, documenting lateral plume delineation in the shallow water-bearing zone ' In the deeper water-bearing zone, hydrocarbon levels ranged up to 3 6 ug/L benzene and no detectable MTBE in near-source well MW-1A Down/crossgradient concentrations attenuate as observed by low to non-detectable benzene,TPH-G, and MTBE In wells MW-5A, MW-4/4A, MW-6, and MW-7 ' As mentioned in previous quarterly reports, there are no active public water supply wells within 2,000 feet downgradient from the site, although there are two wells of unknown status within 2,000 feet ' upgradient/crossgradient of the site ' 1997601dDpC 1