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1 '16 Fourth Quarter 2009 Monitoring and Sampling Report 2 7-Eleven, Inc Store No 17334 4501 North Pershing Avenue,Stockton CA December 13 2001 gradient of about 0 004 foot per foot Groundwater monitoring results for the quarter and historical monitoring results are presented in Tables 1 and 2, respectively (Attachment 2) Field data sheets are included in Attachment 3 Monitoring Well Sampling and Analytical Results Quarterly groundwater samples were collected after purging at least 4 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 1 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 8021B/modified 8015, and for oxygenates MTBE, TBA, DIPE, ETBE, and TAME using EPA method 8260B ' Analytical results for the quarterly sampling event are presented In Table 1, and historical laboratory analytical results are summarized In Table 2 Highest dissolved benzene concentrations were reported in well MW-5A at 4,000 micrograms per liter(ug/L) Dissolved MTBE concentrations were highest In wells MW-4 (3,100 uglL) and MW-5A (2,300 ug/L) Other monitoring wells contained less than 5 ug/L MTBE except well MW-5B which contained 11 ug/L Selected dissolved constituents are plotted on Figure 2 The laboratory analytical reports are included in Attachment 4 Field records indicate the presence of a possible e offsite contaminant source, a remediation system is present on the property located directly across North Pershing Avenue from the 7-Eleven site This finding may Indicate plume mixing in the area of well MW-4 with another hydrocarbon source (not 7-Eleven) Other quarterly data obtained during this sampling event are consistent with previous findings, indicating the onsite hydrocarbon source Is in the vicinity of the underground storage tanks Dissolved plume constituent concentrations in the deeper nested well MW-5B Indicate vertical plume delineation near the source area Prior to purging, dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels 1 indicate generally anearobic conditions (less than 0 9 milligrams per liter(mg/L)) (Table 1) As mentioned In previous quarterly reports, a well search database for public water supply wells located within one mile of the site indicates that there are no public water supply wells located within 4,000 feet downgradient from ' the site The nearest crossgradienVupgradient water supply well of unreported status is located more than 1,500 feet southeast of the site 17334Q14 QMR DOC