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MSot? where: a = initial concentration = ;L-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(15oo/10) t -- L7197 ' days to degrade the initial dissolved benzene concentration (1500 ppb) to 10 ppb. (b) Vbm=z = distance/time {9. 7�� 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) . Vb,,=, = 0. 02 ft/day. Vb==, = 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 150. The revised migrational distance estimate is therefore 165 feet from MW--11. r Summary The solution to the above equations _t =_ 739�-days, indicates that with a calculated degradation decay rate -of 0 . 001 days-', the dissolved benzene concentrations at the leadi-ng 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. I ` 6