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3.1 Evaluation of Chemical Concentration Trends Groundwater sampled from monitoring well MW-2 located closest to the former tank pit has historically indicated the highest concentrations of gasoline hydrocarbons and presents the most I complete data set for evaluation of chemical trends Figure 3 presents a water level hydrograph combined with the detectable concentrations of benzene in well MW-2 The graph shows a ' significant decline in benzene concentrations from the December, 1991 sampling event to non- detectable in the June 1992 sampling event Following the June 1992 event, detectable benzene concentrations returned to a high of 440 ug/L in December 1992 then showed a steady decline again to non-detectable concentrations in August, 1993 Benzene has not been detected in any on-site or off-site wells since it was last detected in monitoring well MW-2 during the June, 1993 sampling ' event at a concentration of 1 3 ug/L The reduction of benzene concentration in MW-2 to non-detectable in the June 1992 sampling event corresponds with the lowering of the water level in wells at the Site (See Figure 3) The measured June 1992 water level elevation of-60 feet MSL is consistent with the base of the sandy zone that exists from -42 to -60 feet MSL (See Figure 2) As the water level elevation increased in later months, the detectable benzene concentrations also increased and then showed a steady decline again to non-detectable This correlation suggests that petroleum hydrocarbon impacts to groundwater were limited to the sandy zone which represents the major permeability transport pathway at the Site As groundwater \ / levels lowered below this zone hydrocarbon concentrations similarly lowered, as expected, the v concentrations re-occurred upon the return of the seasonal water level rise However, this phenomenon cannot be used to explain the subsequent decrease of detectable hydrocarbons in ' wells to non-detectable, as the water level has since remained within the sandy zone-r-A preliminary assessment of microbial activity in selected samples from deposits indicates that significant degredation of petroleum hydrocarbon compounds may be occurring in the Site subsurface SE4COR' N0026.001 01 ROF 8 August 11994