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Fourth Quarter 2002 Monitoring and Sampling Report 4 7-Eleven Inc Store No 17334,4501 North Pershing Avenue,Stockton,CA January 13,2003 ' A potentiometric surface map prepared using the quarterly groundwater elevation data is shown on Attachment 1, Figure 2 Based on the groundwater elevation data, groundwater flow is toward the northeast at an approximate gradient of about 0 005 foot per foot, consistent with historic measurements Groundwater ' monitoring results for the quarter and historical monitoring results are presented in Tables 1 and 1A, respectively (Attachment 2) Field data sheets are included in Attachment 3 ' Monitoring Well Sampling and Analytical Results ' Prior to purging, dissolved oxygen (DO) levels were recorded in each well Quarterly groundwater samples were collected after purging at least three well-casing volumes of groundwater 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 analysis-specific preservative The samples were then sealed using Teflon septum caps, labeled with the fob 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 Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency (EPA) methods 8021 B/modified 8015, and for oxygenates MTBE, TBA, DIPS, ETBE, and TAME using EPA method 82608 Dissolved benzene concentrations were detected at 1,300 pg/L in well MW-5A Dissolved MTBE concentrations were highest in well MW4 (1,100 gg/L) Well MW-5A contained 160 pg/L MTBE, while in well MW 56, MTBE was detected at 1 2 µg/L Quarterly and historical analytical results for the groundwater samples are presented in Tables 1 and 1A, respectively (Attachment 2) The laboratory analytical reports are included in Attachment 4 DO levels were at or near 1 milligram per liter(mg/L) in all wells, indicating generally anaerobic conditions Purgewater Disposal tAll purgewater generated during this quarterly monitoring and sampling event was stored in 55-gallon Department of Transportation (DOT)-approved drums, pending proper transport and disposal at a licensed recycling facility Waste disposal documentation will be forwarded if requested Conclusion and Recommendations ' As reported previously, field observations indicate the presence of a remediation system at the property P P Y located directly across North Pershing Avenue from the 7-Eleven site This indicates the potential for plume mixing in the area of well MW4 with an offsite source of dissolved constituents Other data obtained during the most recent sampling event are consistent with previous findings Non-detectable levels of dissolved plume constituents in the deeper nested well MW-5B verify vertical plume delineation i 17334024 QMR DOC