Fate of 14C-labeled pyrene in a creosote- and octadecane in an oil-contaminated soil

The disappearance of dichloromethane-extractable organic C (DEO-C) from hydrocarbon-contaminated soil is seldom fully accounted for by the evolution of CO2. An oil-contaminated soil was spiked with 14C n-octadecane and a creosote-contaminated soil with 14C[4,5,9,10]pyrene to test the hypothesis that contaminant C is redistributed among soil C fractions during bioremediation. The soils were incubated for 12 months and destructively analyzed at one week, two weeks, one month, and thereafter monthly. The soil fractions examined were biomass-C (BC), humic-C (HC), and water soluble-C (WSC). Humin-C (HNC) was calculated as 14Ctotal minus the sum of direct measurements of the other C fractions. Labeled CO2 and volatile organic C (VOC) were collected and measured. The data provide a series of ‘snapshots’ of the transformations of contaminant-C in each soil. VOC was insignificant and for both soils 47–50% of the applied 14C was recovered in the evolved CO2 only after three months incubation. The primary fate of C from the labeled organic compounds in both soils was into the humic-C and humin-C fractions; in some cases as much as 50% of the applied 14C was recovered from these fractions. Neither BC nor WSC were significant fates of the 14C during this incubation, and in combination did not exceed 12% of the total 14C applied in each soil. Enrichment of the humic-C and humin-C with contaminant-derived C poses interesting questions regarding the end point of bioremediation, traditionally total removal of the contaminant. Metabolism of the C to other fractions is clearly an alternate possibility. Furthermore, the fractions enriched may actually be those considered beneficial to soil quality, such as humic and humin-C.