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Third Quarter 2001 Monitoring and Sampling Report 2 7-Eleven,Inc Store No 20632,4627 Da Vinci Drive,Stockton,CA r 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 potentlometnc surface map (Figure 2) Based on the potenbometnc surface map,groundwater flow is toward the southwest at an approximate gradient of less than 0 001 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 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 Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency (EPA) methods 8021 Blmodified 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-3 at 12 micrograms per liter(ug/L) Dissolved MTBE was found In wells MWA (2 1 ug/L), MW-2 (2,800 ug/L) and MW-5 (26 ug/L), no detectable MTBE was found in well MW-3 Selected dissolved constituents are plotted on Figure 2 The laboratory analytical reports are included in Attachment 4 Prior to purging, dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels Indicate anearobic conditions (less than 1 milligram 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 1 mile downgradient from the site 20632013 QMR DOC