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IWOill013=01_1 Mr. Kyle Christie February 23, 1989 Page 7 Toluene was identified in the January 1989 sample collected from on-site monitoring well W-2B, at a concentration of 2.0 ppb. Monitoring, well W-213 is the deepest monitoring well Oil site and is accessible to groundwater in the 74- to 88-foot depth interval (see Figure 2), Benzene, ethylbenzene, and xylene were not detected in groundwater in this depth interval. t Benzene was identified at a concentration of 4.2 ppb in off-site monitoring well W-5. This concentration exceeds the DHS action level. Toluene and xylene were identified in -- groundwater in off-site monitoring well W-6 at concentrations of 2.6 ppb and 0.5 ppb, respectively. These concentrations are below the DHS action level. No BETX were identified in groundwater in any other off-site monitoring well. r a BETX concentrations in groundwater from monitoring well W-2 are considerably greater than from monitoring well W-2A. These two wells are 5 feet apart (see Figure 1) and are accessible to groundwater near the surface of the aquifer(see Figure 2). The screened interval M of monitoring well W-2 extends about 15 feet above and 5 feet below the screened interval in monitoring well W-2A. i 1 t» Conclusions BETX concentrations in groundwater in on-site monitoring wells screened across the water '1 table have increased since the September/October 1988 sampling. The approximate 2-foot rise in the water table since September/October 1988,has probably enabled hydrocarbons adsorbed onto soils in the capillary fringe zone to reenter the groundwater. BETX concentrations in the 74- to 88-foot depth interval continue to decrease. Factors that may have contributed to this decrease are dispersion and natural degradation. The benzene, toluene, and xylene identified in downgradient off-site monitoring wells in ri.N January 1989 were at significantly lower concentrations than in on-site monitoring wells. ""' The fact that BETX concentrations were considerably greater in monitoring well W-2 than in monitoring well W-2A suggests that hydrocarbons have more ready access into monitoring well, W-2. This access is probably from the vadose zone accessible to the screened interval of Y monitoring well W-2, which is above the screened interval of monitoring well W-2A. fkA P" The majority of hydrocarbons appear to remain pooled beneath the site near the water table and in the vadose and capillary fringe zones. The rate of vertical and laterad dispersion in groundwater appears to be slow. i BROWN AND CALDWELL. 723SSTREET. SWR MENMCAWFORNIAgsai4.7J42•f816F<16-0129