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November 12, 1996 Page 8 concentration"of benzene in:groun-dwater is 2 4 ppb (Well MW-9) Wells MW--4an-d-- MW-9 are located within and adjacent to the northeast of the former UST complex All wremaining groundwater monitoring wells reported no detectable concentrations of TPPH-g or benzene, with the exception of 110 ppb TPPH-g in Well MW-6 Therefore, minor petroleum hydrocarbons exist in groundwater locally in the vicinity of the north- eastern corner of the former UST complex The lateral extent of petroleum hydrocar- bons in groundwater have been delineated to non-detectable concentrations Groundwater monitoring well completions between November 1988 and January 1989 (Wells MW-1 through MW-8) were performed by screening across groundwater at approximately 36 feet bgs The wells vaned in total depth of screen from approximately 39 to 48 feet bgs Groundwater elevations then decreased resulting in all wells becoming dry by October 1991 through approximately October 1994 Groundwater monitoring well completions between July 1991 and January 1994 (MW-9 through MW-14, and MW-2R) were constructed by screening between approximately 40 r and 65 (to 75) feet bgs reflecting the decreased groundwater conditions With the exception of Well NI;W-12 (completed between 30 and 65 feet bgs), rising groundwater resulted in the screen intervals of these wells becoming submerged, but also resulted in gauging and sampling previously dry Wells MW-1, and MW-3 through MW-8 Groundwater samplmg results are deemed representative of site conditions with the 4 ! exception of Wells MW-9 and MW-13 Groundwater Monitoring Wells MW-10, MW-11, MW-14, and MW-2R all reported non-detectable concentrations ofpetroleuin hydrocarbons ingroundwater prior to the well screens becoming submerged Wells MW-9 and MW-13 reported detectable concentrations of petroleum hydrocarbons prior to, and after becoming submerged Well MW-4 is located proximal to submerged Well MW-9, and Wells MW-5 and MW-6 are located between the source area and submerged Well MW-13 Dunng the April and July 1996 sampling events, using groundwater was sampled in Wells MW-4 through MW-6, and is therefore representa- tive of site conditions The analysis of groundwater for methyl tert-butyl ether (MtBE) was initiated in the third quarter 1996 groundwater monitoring program (performed on July 30, 1996) Ground- water collected from Well MW-9 was the only well sampled to report detectable concentrations of MtBE (210 ppb) To confirm the presence of MtBE, subsequent sampling events will reanalyze the sample reporting the highest concentration detected by EPA Method 8260 Residual petroleum hydrocarbons in groundwater are attenuating In fact, the dissolved petroleum hydrocarbon plume is not only stable, but is decreasing in size The Chevron Research and Technology Company (CRTC) Memorandum (April 19, 1996) evaluated 3201337MCLOSURE2 5