Summary
A common technique for comparing the in situ microbial potential for mineralisation of petroleum hydrocarbons in different environments or under different remediation strategies, is by use of radiolabelled tracer studies. Soil microcosms are prepared with the addition of a 14C-labelled molecule such as 14C-octadecane and the 14CO2 produced by catabolism of the molecule is measured. Microcosms are particularly useful as the mineralisation of the compound is unambiguously measured, even when little mineralisation occurs. With the cost and technical difficulty involved in the remediation of hydrocarbon contamination in extreme environments, such as Antarctica, radiometric microcosm experiments offer a cost effective replicated laboratory-based estimate of biodegradation potential. However, by their nature, tracer studies are artefactual and require validation with more extensive geochemical profiling, to comprehensively describe the fate of the complex array of molecules in hydrocarbon contaminated sites. A radiometric experiment was conducted to determine the effects of temperature on hydrocarbon mineralisation in contaminated sediment from Old Casey Station, Antarctica (Ferguson et al., Submitted). The sediment is a sandy aggregate contaminated with well-sorbed Special Antarctic Blend (SAB) fuel. SAB is a highly refined petroleum product, which does not contain hopanes, and was designed for use in extremely cold climates. Microcosms for geochemical comparison were prepared in a similar manner as the radiometric experiment, and both sets of microcosms were incubated in triplicate at 4, 10, 28 and 42°C for 40 days. Microbially-inhibited controls were incubated at 4 and 42°C. Squalane was added as an internal conservative tracer. The microcosms were subsampled on days 0, 5, 8, 20, 30 and 40 for GC-FID analysis. The degree of both physical and biological contaminant removal was determined by monitoring the changes in selected weathering indicis. These indicis included heptadecane: pristane and octadecane: phytane for biological processes and the lighter isoprenoids (i13, i14 and i15) to pristane for physical processes such as evaporation. The radiometric results showed that 14C-octadecane mineralisation was positively correlated with temperature, with the fastest rate of mineralisation occurring at the highest incubation temperature (42°C). Geochemical validation of this experiment supported the radiometric result, with the highest measured degradation of SAB occurring at 42°C. Biodegradation indices, such as heptadecane to pristane, decreased with increasing temperature, with significant decreases at and above 28°C. The correlation between the radiometric method and GC for determining the degradation of the individual hydrocarbon molecules varied with molecular weight and chemical structure. While the radiometric method provided a convenient measure of biodegradation potential of the radiolabeled molecule, extrapolation to the full range of molecules in SAB requires caution. Ferguson, S.H., Franzmann, P.D., Snape, I., Revill, A.T., Trefry, M.G. and Zappia, L.R. (Submitted). Environ. Sci. Technol.