Fuel spills have occurred at three sites on Macquarie Island, resulting in soil and groundwater contamination with petroleum hydrocarbons from Special Antarctic Blend, diesel and heavy lube oils. The Risk and Remediation group at the Australian Antarctic Division aim to clean up ('remediate') these sites by 2014 using a number of low-risk, low-cost, on-site remediation techniques. These techniques harness the natural microbes in the soil to break down petroleum hydrocarbons into carbon dioxide and water.

Between 2003 and 2008 the location and extent of fuel contamination at the three sites were precisely defined using soil sampling test pits, piezometers (sampling tubes) and mini-piezometers. The test pits enabled soil to be taken for laboratory analysis of metals, nutrients and total petroleum hydrocarbons. Piezometers allowed sampling of groundwater (for hydrocarbon contamination) and oxygen levels at different depths throughout the soil profile.

Laboratory studies by the team have shown that maximum microbial activity, and therefore biodegradation of fuel contaminants in the soil, occurs with the addition of nitrogen to a concentration of 600–1200 mg of nitrogen per kilogram of soil water. The addition of 10% oxygen to Macquarie Island’s relatively waterlogged soil also enhances hydrocarbon degradation.

In the 2008–09 Antarctic season the team began aerating and adding nutrients to contaminated soil at all sites — two areas around the Main Power House, and the eastern side of the Fuel Farm. The initial focus of the work is on the areas of highest hydrocarbon concentration (>4000 mg/kg soil), to reduce both the concentration of hydrocarbons and further movement through the soil.

The bioremediation process is being monitored and optimised where necessary, using oxygen sensors, piezometers and mini-piezometers (to sample water), through soil sampling, by measuring carbon dioxide and volatile organic compounds, and by examining changes in soil microbial communities and chemical processes in the soil.

As there are no locally derived national guidelines for the concentration of petroleum hydrocarbons at which management intervention should begin, the team has also been developing risk assessment guidelines and target hydrocarbon levels, for remediation. In most countries the presence of fuel or oil floating on surface water is a trigger for remediation. However, the Risk and Remediation team is using modelling to identify precautionary hydrocarbon target levels for different soil types found on Macquarie Island, which will prevent the occurrence of visible, free-floating hydrocarbon. An experiment has also been set up to examine the effect of hydrocarbons on native soil microbes and invertebrates, to identify toxicity thresholds for these organisms.

WENDY PYPER

Corporate Communications, AAD

What microbes eat oil?

Soil microbes are important in breaking down and recycling nutrients such as carbon, nitrogen, sulphur and phosphorous, and making these nutrients available to plants. As petroleum hydrocarbons can only be used as an energy source by certain microbial groups, fuel or oil contamination leads to a change in the composition of the microbial community – an increase in hydrocarbon degrading organisms such as Rhodococcus, Sphingomonas and Pseudomonas species.

It is likely that on Macquarie Island the microbes work together as a community to degrade hydrocarbons, in a process with many steps. Some use oxygen to do this and, when the oxygen is low, other species use nitrate or ammonia instead. Other microbes known as ‘autotrophs', use the carbon dioxide produced by the hydrocarbon-degrading microbes as an energy source.

Similar types of microbes are found in ordinary garden soil, where they are involved in the process of decomposition. The ability to degrade petroleum hydrocarbons is surprisingly widespread in the environment and may be related to the fact that some plants and algae produce similar complex compounds.

SHANE POWELL

Environmental Protection and Change program, AAD