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Third Quarter 2001 Monitonng and Sampling Report 2 7-Eleven Inc Store No 17334 4501 North Pershing Avenue,Stockton CA October 1 2001 ' Groundwater Monitoring Data ' Before monitoring,the well caps were opened to allow the water 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, and the resulting data was used to prepare a potentiometric surface map (Figure 2) Based on the potent4ometnc surface map, groundwater flow is toward the north at an approximate gradient of about 0 02 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 ' Quarterlygroundwater well- casing g r 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 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 ' 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 3,200 micrograms per liter(ug/L) Dissolved MTBE concentrations were highest in wells ' MW-4 (4,800 ug/L) and MW-5A(1,300 ug/L) Other monitoring wells contained less than 5 ug/L MTBE, except well MW-5B which contained 15 ug/L Selected dissolved constituents are plotted on Figure 2 The laboratory analytical reports are included in Attachment 4 ' Field records indicate thea resence of o p possible 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 ' Pnor to purging, dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels indicate generally anearobic conditions (less than 2 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 17334013 QMR DOC 1