An integrated assessment of natural attenuation of hydrocarbons in drill cuttings on the North Sea seabed
Oil production in the North Sea sector pre-1990 used predominantly oil-based drilling muds. The drill cuttings still contained significant amounts of muds and associated hydrocarbons despite cleaning prior to disposal onto the seabed. The cuttings accumulating on the seabed caused physical smothering and toxic effects of the hydrocarbons or other constituents of the muds. Although decreases in hydrocarbon levels have been observed over 10 years, it has been difficult to distinguish whether this 'decrease' is due to natural attenuation or whether it is an artefact of the sampling methods. This study aimed to answer this question by determining whether the biological potential for hydrocarbon-degradation was in place by using integrated molecular, microbiological and analytical chemical and biochemical techniques. Several bacterial isolates were retrieved from drill cuttings material. The aerobic isolates belonged to the genus Halomonas or were closely related to the genera Marinomonas or Pseudoalteromonas and were capable of degradation of the main constituents of oil-based drilling muds. The anaerobic isolates were sulphate-reducing Desulfovibrio species for which indications of hydrocarbon-degrading capacity were found. The isolates were shown to be active in cuttings pore water as well as at the in situ temperature. ATP measurements and molecular techniques (DGGE/PLFA) indicated the presence of a highly active community with a population structure different to the surrounding clean sediment and suggested strong presence of metal- and sulphate-reducing bacteria. Anaerobic microcosm experiments with 14C-labelled hydrocarbons were used to determine the extent of degradation and the involvement of functional bacterial groups in the degradation process. Extensive mineralisation of n-hexadecane was found, but mineralisation of n-octacosane and naphthalene was slow or not significant. The influence of sulphate reduction and methanogenesis was shown. An integrated approach for the determination of natural attenuation, as selected in this study, can verify the process where individual findings are of limited value.