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f <br /> Ms Lori Duncan <br /> October 10, 2003 <br /> Page 2 <br /> Groundwater Groundwater <br /> WeIIIDs Monitoring Sampling <br /> Fr uenc Frequency <br /> ARCO Service Station <br /> E-2, E-8 through E-12, PR 1 Quarterly Quarterly <br /> BHD-1, MW-4 and MW 5 <br /> E-3 through E-7, PR-2, BHD-2 Quarterly Semiannually <br /> and MW-1 through MW 3 <br /> Chevron Service Station <br /> MW 1 through MW-9,City of Quarterly Quarterly <br /> Stockton Well#10 <br /> Samples collected from the groundwater monitoring wells at the ARCO Service Station were <br /> analyzed for the following parameters through the second quarter 2003 <br /> o Total petroleum hydrocarbons as gasoline (TPHg) by EPA Method 8015M, and <br /> Benzene, toluene, ethylbenzene, xylenes (BTEX) and methyl tertiary butyl ether (MtBE) by <br /> EPA Method 80216 <br /> • As requested by the SJCEHD In the September 11, 2003 letter, SECOR has implemented the <br /> revised analytical scope of work beginning during the third quarter 2003 Groundwater samples were <br /> collected on September 24 and 25, 2003, and analyzed for the presence of TPHg, BTEX, fuel <br /> oxygenates including MtBE, tertiary amyl methyl ether (TAME), di-Isopropyl ether (DIPE), ethyl <br /> tertiary butyl ether (EtBE), tertiary butyl alcohol (TBA), 1,2-dichloroethane (1,2-DCA), and ethylene <br /> dibromide (EDB) by EPA Method 82608 <br /> Samples collected from groundwater monitoring wells at the Chevron Service Station and from the <br /> City of Stockton, Municipal Well #10 are analyzed for the presence of TPHg by the Northern <br /> California Leaking underground Fuel Tank (LUFT) Method, and BTEX, fuel oxygenates (MtBE, <br /> TAME, DIPE, EtBE, TBA), 1,2-DCA, and EDB by EPA Method 8260B <br /> PROPOSED MODIFICATIONS TO GROUNDWATER SAMPLING PROGRAM <br /> Due to its (1) hydraulically downgradient location of the ARCO Service Station, (2) the predominant <br /> north/northeastern direction of groundwater flow, (3) the continued presence of petroleum <br /> S' <br /> —north/northeastern <br /> and MtBE in site wells, and (4) the close proximity of City of Stockton Municipal Well <br /> #10, SECOR does not propose any changes to the groundwater monitoring and sampling program <br /> currently implemented at the Chevron Service Station However, based on historically low to non- <br /> detectable concentrations of petroleum hydrocarbons and MtBE in several groundwater monitoring <br /> wells at the ARCO Service Station, which is located hydraulically upgradient (south) of the Chevron <br /> Service Station, SECOR proposes to modify the sampling frequency for selected wells The <br /> proposed changes in the groundwater sampling program at the ARCO Service Station are presented <br /> in bold type in the table below <br /> 11ARC0121331Misc LetterslSHCEHD RspLir&M&S Redx Req doc <br />