Biodegradation of Water-Accommodated Fractions and Dispersed Oil in the Seawater Column

Abstract

Abstract During the project "Advances Management of Oil Spills (AMOS)" biodegradation of oil compounds in the water-accomodated fractions and dispersed oils have been determined after discharge to the seawater. Water-accomodated fractions (WAFs) in seawater were generated by careful stirring in flasks according to a procedure established by the "Chemical Response to Oil Spills: Ecological Effect Research Forum (CROSERF)", with subsequent separation from the oil phase. Mechanically dispersed oils were generated and maintained by vigorous agitation. The initial oil-seawater loading ratios were 1:10,000. Primary biodegradation of selected oil components were measured as transformation by GC-FID and GC-MS analysis in both WAFs and dispersed oils during a period of 7 days (WAFs) or 14 days (dispersed oils), and transformation rates were calculated after corrections for abiotic disappearance. Acute toxicity of WAF was determined by the Microtox assay. Biodegradation was determined for various oils, for fresh and weathered oil, and for oil treated with a chemical dispersant. The transformation rates for similar components differed moderately between the various oils, and strongly depended on the distribution between WAF and dispersed oil phases. Introduction of a chemical dispersant in the oil had only small influence on the transformation. Comparison of fresh and weathered oils showed only small differences in degradation for compounds present in the oils (WAF). During degradation the acute toxicity was reduced for the oils. In conclusion, biodegradation of the oil WAFs will strongly influence the effects on pelagic organisms due to the rapid microbiological component transformation, while components present in oil droplets will persist for a longer period of time. This has significant implications for effect studies. The degradation data will be included in the spill simulation model "Oil- Spill Contingency and Response" (OSCAR 2000) for computation of behaviour and fates of oil spills. Introduction During an oil spill a variety of processes will influence the fate and effects of the oil. For instance, the fate will be governed by a variety of physical-chemical processes and of the biodegradation of oil components available to the seawater microorganisms. To estimate the effects on natural resources in the water column the time-dependant fate of various oil-compounds should be included in spill simulation models. The Oil- Spill Contingency and Response (OSCAR) model uses a multi-component representation of crude oils and petroleum products to compute behaviour and fates of oil spills1. During the project "Advanced Management of Oil Spills" (AMOS) biological data, including degradation, acute toxicity and bioaccumulation in pelagic organisms, will be included in the OSCAR model to quantify the environmental consequences of various oil spill scenarios. The data and the model tool may be used for quantification of Net Environmental Benefit Analysis (NEBA)2 during surface oil spills, and used for decision-making during choice of combat strategies to reduce the overall impacts of surface oil spills. The fate of oil compounds in the seawater is strongly influenced by microbial biodegradation processes, especially for compounds with a significant dissolution potential in the water phase. During biodegradation in the seawater column the oil compounds are transformed primarily through aerobic oxidation, ending up as inorganic carbon by complete mineralisation. Compounds in the water-accomodated fraction (WAF) will be easily available to pelagic seawater organisms, including the biodegrading microbes, while the compounds with low dissolution potentials will mainly be accessible to organisms digesting oil droplets. Thus phase distribution will have strong implications both for the fate and effects of the oil compounds.