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2905 West Benjamin Holt Drive, Stockton, California March 16, 1994 <br /> Chevron U.S.A. Products Company Page 5 <br /> in three of the five groundwater monitoring wells. Seasonalatterns in dissolved benzene P e e concentrat,on <br /> are not well established (Figures 6, 7 and 8). Four (MWs-1, 2, 5, and 6) of the eight groundwater <br /> monitoring wells have contained detectable concentrations of benzene since their installation. <br /> Monitoring wells MW-3 and MW-4 have contained low concentrations of benzene on some sampling <br /> dates, but are <MDL for benzene most of the time. MW-7 has been <MDL for BTEX and?PH-G since <br /> Its Installation in 1992. It should be noted that MW-8, installed in January 1994 for downgradient plume <br /> definition, has been sampled only once. <br /> ti <br /> As seen in Figure 9, the downgradient boundary of the dissolved benzene plume has been defined by <br /> dissolved benzene concentrations <MDL in MW-8. The dissolved benzene plume is defined to the west <br /> (upgradient, MW-7), east (MW-8) and north (cross gradient, MWs-3 and 4). As a result of impacted <br /> groundwater emanating from ARCO (Brown and Caldwell, 1993), southern boundaries of the dissolved <br /> benzene plume cannot be inferred from the data. The dissolved TPH-G plume is defined to the north <br /> and west (Figure 10). Low concentrations (51 ppb) of TPH-G were detected in MW-8. An eastern <br /> boundary of dissolved TPH-G can be inferred near MW-8; based on the concentration gradient in the <br /> downgradient direction. <br /> 5073PA R.RPT <br /> ��i►! GROUNDWATER <br /> !� TECHNOLOGY <br />