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• Water, Contaminant Removed C = 12,200 gal x 0 000042 ppm = 0 51 gal TPHG C = 12,200 gal x 0 000003600 ppb = 0 043 gal Benzene C = 12,200 gal x 0 000000058 ppb = 0 0007 gal MTBE Discussion- Excavated and Residual Contaminant Mass The calculated estimates indicate that very low quantities of residual contaminants are present both in the capillary fringe and in area of former tank pit/well MW-1 The range of concentrations suggests that these residuals, as well as those in excavated soil, were absorbed onto the soil Column experiments have demonstrated that soil absorption may hold from about 9% to 25% of the total spill hydrocarbon mass (Wilson and other, 1988) The lack of hydrocarbons at well MW-4 supports this assumption Current working models of MTBE suggest more rapid solubility and movement than BTEX fuel components However, only one occurrence of MTBE was observed in well MW-2, hence MTBE presence and movement is interpreted as negligible The observed soil sample and monitoring data above show that BTEX contaminants (the most prevalent on-site) are effectively immobile Well MW-4, located on the most conservative . flowline (and historically most consistent flow trend) has shown only three separated occurrences of low concentrations of TPHG In our opinion, this further substantiates the conclusion of absorbed and immobile contaminants are minimal Rationale for Residual Remaining Contaminants Fate The site assessment showed that a small dissolved plume of TPHG and BTEX arose from the former tanks and pit The underground storage tanks were removed in 1991 Contaminants absorbed in the soil posed a residual contaminant source, which was removed in 1999 A deep boring (B-14) sampled soil and groundwater at depth intervals below the former tank pit These soil samples showed that contaminants were not observed in soil samples B-14 groundwater samples collected at the 16 to 20-foot depth interval and 28 to 32-foot depth interval showed that both TPHG and Benzene substantially declined, and MTBE was not observed in either water sample The 1999 soil excavation removed additional contaminated soil and groundwater Perimeter soil samples revealed residual contaminants at the capillary fringe and base of excavation (see Figure 4) MTBE was observed in the base of excavation samples, however it was not present in the adjacent deeper samples of B-14 This contaminant distribution indicates these contaminants are residual absorbed onto soil Minor residual contaminants may remain just under East 1 pth Street, however, the excavation could not enter the street A calculated estimate of TPHG and Benzene show that very low quantities of both contaminants remain in soil and capillary water It is anticipated that these residual soil and water contaminants will degrade with time through in- Page 7