Remediation of petroleum-contaminated Antarctic soil

Remediation of petroleum hydrocarbons in polar environments is more costly and logistically and technically more difficult than corresponding temperate and tropical contaminated sites. Bioremediation and in-situ chemical oxidation (ICO) are possible strategies which may overcome the financial and technical challenges associated with polar-region site remediation. ICO involves introducing reactive chemicals to contaminated soils so that organic contaminants such as petroleum hydrocarbons are oxidised to environmentally innocuous compounds, while bioremediation relies on microbial activity to achieve this. At Old Casey Station, East Antarctica (66°17'S, 110°32'E) more than 20 000 L of Special Antarctic Blend (SAB) diesel fuel was spilt over 15 years ago. Concentrations in the spill zone are still about 20 000 ppm and the rates of natural attenuation are relatively slow. The application of oxidative chemicals to the site did not significantly reduce petroleum hydrocarbon concentrations and would likely hinder biodegradation through the destruction of the subsurface microbial communities to below the level of detection for over 2 years. Bioremediation is considered the only likely viable alternative to natural attenuation or dig-and-haul procedures. The factors which were suspected of limiting microbial degradation of petroleum contaminants were temperature, nutrients and water availability. Their potential limitations were investigated with a series of radiometric treatability (microcosm) studies. A positive correlation between temperatures (between -2 and 42°C) and the rate of 14C-octadecane mineralisation was found. The high rate of mineralisation at 37 and 42°C was surprising, as most continental Antarctic microorganisms have an optimal temperature between 20 and 30°C and a maximal growth temperature of less than 37°C. 14C-octadecane mineralisation at nine different inorganic nitrogen concentrations (ranging from 85 to over 27 000 mg N kg-soil-H20 -1 ) was monitored. Total mineralisation increased with increasing nutrient concentration peaking in the range 1000-1600 mg N kg-soil-H20-1 . Higher N concentrations reduced the rate of mineralisation, highlighting the importance of avoiding over-fertilisation. Gas chromatographic analysis of the aliphatic components of the SAB diesel in the contaminated soil showed good agreement with the radiometric microcosm outcomes. Ratios of n-C17: pristane and n-C18: phytane indicated that low nutrient concentrations rather than water were the main limiting factor for biodegradation of hydrocarbons in the soil collected from Old Casey Station when incubated at 10°C. The high rate of mineralisation at 42°C and the microbial population dynamics were also investigated in a series of non-radiometric microcosm studies. Denaturing gradient gel electrophoresis of nutrient-amended contaminated soil after 40 days incubation at 4, 10 and 42°C indicated significant differences between the microbial communities at each of the incubation temperatures. 16S rRNA gene sequences and fatty acid methyl ester analysis indicate that the dominant hydrocarbon degrading bacteria at 4 and 10°C are Pseudomonas spp., while Paenibacillus spp. are likely to be the dominant hydrocarbon degrading bacteria at 42°C. The main implication for bioremediation in Antarctica from this study is that a high-temperature treatment would yield the most rapid biodegradation of the contaminant. In situ biodegradation using nutrients and other amendments is still possible at soil temperatures that occur naturally during summer in Antarctica. However, because the soils from this site are characterised by low water holding capacities, it would be difficult to maintain optimal nutrient concentrations during full scale treatment, and thus the use of a controlled release nutrient should be considered for full scale remediation of petroleum contaminated soil in Antarctica.