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�. f '�'i':�{i"�."'s'4�a.. S "rrra�iF�v�s"�k'S�'r"4 ,.,rJ�xx-n.F r.,o..�.�—..«w......r"<�m• rte. w .n .Y`. moi:sF: tas. ro xn i^�" .nX F' Mr. Kyle Christie November 8, 1988 Page 8 The groundwater elevation in monitoring well W-213 is consistently lower than the groundwater elevation in monitoring well W-2 (Exhibit B). Monitoring well W-213 is screened across a deeper interval than monitoring well W-2 (see Figure 3). The water level measured in monitoring well W-2B was measured at 0.56 feet lower than the water level measured in monitoring well W-2 on July 1, 1988. This difference has decreased to "-' 0.18 feet on September 14, 1988. The decreasing difference may be due to a faster respoi,se in monitoring well W-2 to the dropping water table. The lower groundwater elevation in monitoring well W-213 may indicate a downward component of the hydraulic " gradient beneath the site. A downward component of the hydraulic gradient may be responsible for the BTX concentrations identified in the groundwater sample collected from monitoring well W-213. June 1988 BTX concentrations in well W-213 indicate that dissolved hydrocarbon constituents are present in groundwater in the 74- to 88460 depth interval. Conclusions ? Sufficient data points exist to adequately characterize the lateral extent of dissolved hydrocarbon contamination in the groundwater. Dissolved hydrocarbon constituents appear to be pooled beneath Station 434 and do not appear to have migrated significantly off site. BTX concentradons in wells W-2 and W-3 have decreased while BTX concentrations in well W-1 have increased. While several factors might contribute to an overall decreased - BTX concentration, the factor that might contribute to the decreased concentrations in wells W-2 and W-3 but nos W-1, is the removal of free product from wells W-2 and W-3. It ty appea.s that the dominant factor in hydrocarbon occurrence in groundwater beneath Station 434 is that hydrocarbons within the vadose zone continue intermittently to migrate downward and enter the groundwater. Laboratory analyses of groundwater samples from monitoring well W-2B indicate dissolved "y BTX exists in the groundwater beneath Station 434 to at least 88 feet below ground +�v surface. Recommendations BC recommends drilling and soli sampling of additional boreholes on site in order to determine the lateral and vertical extent of hydrocarbon contamination in the vadose zone. Characterization of the extent of hydrocarbon contamination in soil in the vadose zone is ` necessary to confirm that there are significant hydrocarbon concentrations in the vadose zone. A complete characterization of the extent of hydrocarbon contamination :n the vadose zone is also necessary for designing a soil remediation system. BC recommends quarterly sampling of all on- and off-site wells to track the dissolved BTX trends in a systematic manner. This sampling should begin immediately. As only one } = grundwater sample has been collected from monitoring wells W-2A and W-213 additional -r-_ ^" --_ _BTXanalysesof groundwater--from these_monitoring wellswill also be helpful to: __ r (1) confirm the presence of hydrocarbons in the 74- to 88-foot depth interval accessible so---—- BLOWN AND CALDWELL ..0 123SSTREET 3ACRAMENTO.MIFORNW9561/-7092.1916141/-0129 ..