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�. <br /> A :ssXX <br />µ <br /> Mr. Kyle Christie <br /> December 9, 1988 <br /> Page 6 <br /> 1988 groundwater sample analyses are in Exhibit A. Benzene concentrations in groundwater <br /> in all on-site monitoring wells samples are greater than California Department of llcaltlt <br /> Services' (D]IS) action levels. DHS action levels for BTX are 0.7 pans per billion (ppb), <br /> 100 ppb, and 620 ppb, respectively. The concentration of toluene in groundwater is greater <br /> than the DUS action level in the sample collected from monitoring well W-2. Concentrations <br /> of toluene and xylene in groundwater samples collected from ail other on-site monitoring wells <br /> are below DHS action levels. No BTX was identified in groundwater samples collected during <br /> September/October 1988 from any off-site monitoring well. <br /> BTX were identified in September/October 1988 samples collected from monitoring well <br /> W-2B. Monitoring well W-2B is the deepest monitoring well on site and isaccessible to <br /> 1 groundwater in the 74- to 88-font depth interval (see Figure 2). Concentrations of toluene and <br /> xylene in groundwater in this depth interval are less than the DHS action levels. The <br /> concentration of benzene in groundwater in this depth interval is slightly greater than the DHS <br /> action level. <br /> BTX concentrations in groundwater from monitoring well W-2 are considerably greater than <br /> from monitoring well W-2A. These two wells are 5 feet apart (see Figure 1) and are <br /> s accessible to groundwater near the surface of the aquifer(see Figure 2). The screened interval <br /> of monitoring well W-2 extends about 15 feet above and 5 feet below the screened interval <br /> in monitoring well W-2A. <br /> Conclusions <br /> P <br /> Benzene concentrations in groundwater in on-site monitoring wells W-2, W-2A, W-2B, and <br /> { W-3 have decreased since June 1988. Factors that may have contributed to the decreased <br /> t, <br /> s benzene concentrations include the dropping water table (leaving hydrocarbons adsorbed onto <br /> soils in the vadose zone above the water table), vertical and lateral dissemination, and natural <br /> degradation processes. No September/October 1988 benzene data were obtained from <br /> monitoring well W-1. <br /> im <br /> BTX in September/October 1988 were not identified in groundwater from any off-site well. <br /> Dissolved hydrocarbon constituents appear to remain pooled beneath the site and have not <br /> migrated.significantly off site. <br /> s September/October 1988 laboratory results confirm the presence of dissolved hydrocarbons in <br /> u groundwater in the 74- to 88-foot depth interval (monitoring,well W-2B). BTX concentrations <br /> in groundwater from monitoring well W-2B, however, .are significantly less than those <br /> : identified in June 1988. <br /> The fact that BTX concentrations were considerably greater in monitoring well W-2 than in <br /> monitoring well W-2A suggests that hydrocarbons have more ready access into monitoring well <br /> W-2. .This access is probably from the vadose zone accessible to the screened interval of <br /> monitoring well W-2, which is above the screened interval of monitoring well W-2A, <br /> as <br /> i <br /> BROWN AND CALDWELL <br /> t27SSMWT SACf"EHtq CALIFOFtNu95914'?M-(9161441-0123 <br /> - k <br />