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The initial groundwater sample result was the basis for the subsequent installation of groundwater monitoring wells MW-1 through MW-4 During 10 quarterly groundwater monitoring events between September 2000 and December 2002, the only gasoline compounds detected in the four ' monitoring wells have been MTBE in well MW-3, at a maximum concentration of 2 4 µg/l in September 2001, and a single detection of 0 55 µg/l of xylenes in monitoring well MW-4 in December 2001 No gasoline compounds were detected in any of the monitoring wells during the four consecutive sampling events completed during 2002 ' Estimate of hydrocarbon mass remainin�c As outlined above, any remaining gasoline compounds in sod beneath the western dispenser island and in groundwater beneath the site appear to be negligible The mass of gasoline hydrocarbons remaining beneath the north end of the former UST (Tank 2) is defined as follows ' • A 10-foot radius circle as shown on Figure 4 represents the estimated area of soil impact • The constituents of concern are present between 14 and 25 feet bgs, based on the non- detection of TPH-as-gasoline, benzene, and MTBE in the soil sample collected at 25 feet from SB-1 • The concentrations of the constituents of concern are assumed to be distributed uniformly throughout the volume of impacted soil and are an average of the maximum concentrations detected in soil sample SBI-15, and zero at limit of impacted soil ' • Soil density = 110 pound (lb)/cubic foot, and • 1 part per million (ppm) = 1 pound contaminant per 1 million pounds of soil ' Based on these assumptions, the remaining mass is calculated as follows r TPHgmass= f(3 14){102)(11)#t3�e+11(1101�-s )(2,733 Ib TPHg) = 1,034 Ib TPH-as-gasoline ' (ese+l) (10" lb sell) 1 Benzenemass f(3 14)(102)(1I)#e-94l(1101)-�)(1 73 Ib benzene) = 0 65 Ib Benzene (€ -so 4) (106 Ib sell) ' MTBEmass= r(3 14)(102)(11)ft3-sG41(110Ib-5e4)(0 21 Ib MTBE)_ = 0 08 Ib MTBE ^94)4) (1 Os Ib sell) 14 0