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Microbial degradation in soil microcosms of fuel oil hydrocarbons from drilling cuttings.

The biodegradation of the fuel oil hydrocarbons contained in drilling cuttings was studied in soil microcosms during a 270-day experiment. Concentration and chemical composition of residual hydrocarbons were periodically monitored by quantitative capillary gas chromatography. The decrease in hydrocarbon concentration was logarithmic with time. At the end of the experiment, the fuel oil was 75% degraded. In the saturated fraction, normal and branched alkanes were almost totally eliminated in 16 days ; 22% of the cycloalkanes were not assimilated. The aromatic fraction was 71% degraded ; some polycyclic aromatics were persistent. The resin fraction (10% of the initial weight) was completely refractory to biodegradation. The inorganic part of drilling cuttings had no influence on the biodegradation rates of hydrocarbons. Biogenic hydrocarbons and traces of degradable fuel oil hydrocarbons were protected from microbial activity by the soil and cuttings matrix. Enumerations of total heterotrophic bacteria and hydrocarbon-utilizing bacteria showed a strong stimulation in both populations. Hydrocarbon-degrading strains of bacteria and fungi were isolated and identified at the generic or specific level.