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r workplan for Additional Offsite Assessment, Fate and Transport Evaluation Addendum, 6 ' Sensitive Receptor Survey, and Fourth Quarter 2002 Monitoring And Sampling 7-Eleven Store No 14117,2725 Country Club Blvd ,Stockton,CA January 30, 2003 70 GROUNDWATER MONITORING AND SAMPLING 71 Groundwater Monitoring Data Before monitoring, the Gaps well opened to allow the water to each well to stabilize to atmospheric p were p pressure The DTW was measured to the nearest 0 01 foot from the top of the well casing using an rinterface probe ' A potentlometrlc surface map generated using the quarterly groundwater elevation data is shown in Figure 2 Based on the groundwater elevation data, the estimated groundwater flow is to the northeast at a gradient of about 0 002 foot per foot Quarterly groundwater elevations and historical data are ' presented in Tables 1 and 2, respectively Field data sheets are Included in Appendix D 72 Monitoring Well Sampling Before sampling, at least four well-casing volumes of water were removed from each well The groundwater samples were collected using disposable bailers Groundwater samples were placed in 40-milliliter glass vials, which were acidified with hydrochloric acid to preserve the samples The samples were then sealed using Teflon septum caps, labeled with the lob 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 The groundwater samples were analyzed for TPH-G, BTEX, and MTBE using Environmental Protection Agency (EPA) ' methods 8021B/modified 8015, and oxygenates MTBE, TBA, DIPS, ETBE, and TAME using EPA method 8260B The SJCPHS has requested resumption of sampling for 1,2-dichloroethane (1,2-DCA) and ethylene dibromide (EDB) at revised reporting levels Sampling for 1,2-DCA(at an EPA MCL of ' 0 005 pg/L) and EDB (at an MCL of 0 00005 µg1L) are proposed to resume during the first quarter 2003 Samples were also collected (post-purging) and analyzed for heterotrophic plate count(HPC) using analytical method SM 9215 ' 7.3 Results of Groundwater Sample Analysis ' 731 Gasoline The plume is currently delineated to the south (MW-6), the west(MW-5), northwest(MW-2), east- southeast(Exxon MW-7) by non-detectable levels of benzene and TPH-G, and less than 1 gglL MTBE Dissolved TBA levels ranged between non-detectable (MW-6, MW-5) and 15 µglL (Exxon MW-7) Quarterly and historical analytical results for groundwater samples are summarized in Tables 1 and 2, ' respectively Laboratory analytical reports for the groundwater samples are included in Appendix E ' 14117024_SAR doe r