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Calculated dissolved benzene concentrations-present in-MW-11=and --- - MW-13 . The following equation provides an estimate of the maximum dissolved benzene concentration expected in downgradient wells MW- 11 and -13. K = 2 . 303/t log(a/a-x) where: a = 10, 000 ppb - MW-4, April 13 , 1989 a-x = calculated reduced concentration in MW-11 and MW-13 t = d/v = 75 ft/0. 02 ft/day = 3750 days K = 0. 001 days-1 a-x = calculated maximum benzene concentration = 229 ppb, after a migration time of 3750 days With the exception of the July 1993 groundwater analytical data, all previous samples collected from MW-11 contained dissolved benzene concentrations in excess of the estimated concentration of 229 ppb. Concentration values from May 1992 , July 1992 and October 1992 were 1500 ppb, 620 ppb and 1300 ppb, respectively. The possibility exists that the elevated dissolved BTEX concentrations observed in MW-11 may in part be derived from a secondary source area (eg. the smaller product tanks removed in 1984) . Further analyses to determine the possibility of a secondary hydrocarbon source is 3ustif ied. MW-13 has only been sampled during the July 23 , 1993 monitoring event . The July 1993 sample contained total toluene/ethylbenzene/xylene concentrations less than 10 ppb, no detectable benzene, and a TPH-G value of 730 ppb. Due to the 15 foot recorded decline in local depth to groundwater, it is possible that the dissolved BTEX concentrations represented in the July 1993 sampling event do not represent the- maximum values which have migrated, or will migrate, through the groundwater sampled by MW- 13 . 5) Estimated 10 ppb benzene concentration distance from MW-11, at time "t". The following estimation assumes the same dissolved benzene degradation/attenuation rate calculated for MW-3 , MW-4 and MW-5, above. (a) K = (2 . 303/t) log(a/a-x) 5