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�SoD -_ - where: a=- -initial- concentration = ,1,6 ppb (5/19/92) - analytical data) . a-x = 10 ppb. t = time to degrade the initial concentration, 1500 ppb, to a concentration of 10 ppb. K = 0. 001 days-' = (2 . 303/t) log(1500/10) t - L7197' days to degrade the initial dissolved benzene concentration (1500 ppb) to 10 ppb. (b) VbCo=C = distance/time where: d = estimated distance dissolved benzene will migrate before concentrations will be degraded to 1 ppb. t = 7197 days = estimated time to degrade initial concentration of dissolved benzene, 10 ppb, to 1 ppb (calculated above) . Vbm=, = 0. 02 ft/day. Vben=L = 0. 02 ft/day = distance/7197 days Distance to 10 ppb concentration = 144 feet The above analytical model does not include the hydraulic parameter", of dispersivity. To compensate for omitted parameter, the estimated migrational distance is increased by a factor of 15%. The revised migrational distance estimate is therefore 165 feet from MW-11. Summary The solution to the above equations, t = 7i97-'days, indicates that with a calculated degradation decay rate f 0. 001 days-', the dissolved benzene concentrations at the leading edge will exceed 10 ppb for 7197 days following the initiation of migration. The dissolved benzene is also estimated to migrate 165 feet from the presumed "origin" (vicinity of MW-11) in the 7197 days needed to degrade the benzene concentration from 1500 ppb to 10 ppb. The 1-dimensional contaminant fate calculations presented above assume steady state site conditions, including minimal groundwater fluctuations, no additional hydrocarbon mass added and a constant degradation/attenuation rate. The fifteen foot decline .in groundwater elevation recorded from 1988 to the present violates one of the key assumptions of the model; the resulting migrational estimates should therefore be viewed with some reservation. 6