Cleaning up pollution - naturally

Research by scientists at the CRC for Contamination Assessment and Remediation of the Environment (CRC CARE) has shown that the microbes naturally present in Australian soils can usually be relied on to break down toxic substances (such as hydrocarbons left behind from old fuel dumps, leaky service stations or refineries) that pollute groundwater.

Research at two contaminated sites by the CRC's Professor Megh Mallavarapu has demonstrated that natural breakdown (or 'attenuation') of contaminants may be the cheapest and most efficient way to remove lingering traces of pollution — and stop potentially cancer-causing substances getting into Australia's water supplies.

"Natural attenuation is now gaining recognition as a potentially cost-effective technique for cleaning up hydrocarbon contamination," Prof Mallavarapu said.

"However, there is no consensus in Australia among our various regulators about whether natural attenuation is an acceptable remediation method.

"Our research provides convincing scientific evidence that natural attenuation will work in most oil spill situations. It can be used more widely, and even enhanced, provided environmental conditions are optimised for microbes."

The CRC CARE team looked at two groundwater 'plumes' downstream from the sites of old fuel dumps, where hydrocarbons had leaked into the soil and groundwater over many years.

They confirmed the plumes were potentially toxic — but they allso found that the toxins gradually disappeared with increasing duration of exposure, the result of being attacked and degraded by soil microbes.

Further detailed investigation by the research team provided several lines of evidence demonstrating a gradual degradation of the hydrocarbons in the groundwater.

According to Prof Mallavarapu, this shows the need to monitor the levels of the toxins, signs the right bacteria are present and are active, and various other indicators that show that degradation of the contaminants is actually taking place.

"Provided appropriate scientific monitoring is in place, we feel it is possible for Australian environmental authorities to have much greater confidence in this practical, low-cost approach to cleaning up old oil spills — and to encourage industry and governments to use it more widely," Prof Mallavarapu said.

"This is a particularly important issue in the many parts of Australia where groundwater is used for drinking or for irrigation, where there is a risk that contamination from old oil and fuel spills that leaches into it can reach the public. It shows that what is out of sight isn't necessarily harmless — and we need to keep a strict eye on it.

"However, provided the right sort of microbes are present, they will do a remarkably good job of breaking the toxins down and making the water safe to drink. The important thing is to have multiple scientific lines of monitoring, so we can be sure."

Prof Mallavarapu said the research had also revealed new kinds of microbes which were particularly efficient at breaking down the worst sorts of hydrocarbon pollution, including substances such as benzene and toluene which cause cancer.

The managing director of CRC CARE, Prof Ravi Naidu, said oil spills were one of the most common sources of soil and groundwater pollution.

"Almost every suburb has or had a service station in it, and many fuel tanks have leaked. Then there are the sites of fuel dumps, refineries, drycleaners, vehicle service depots and other sites where hydrocarbon products were widely used — and spilled," Naidu said.

"This work of Prof Mallavarapu is important because it means that environmental authorities in all states can develop clear guidelines to ensure these sites are cleaned up, using natural processes that are much cheaper and easier than other forms of remediation.

"For industry, it means that yesterday's pollution need no longer be tomorrow's headache — and the cost of cleaning it up is a lot lower than many feared."

In consultation with industries and regulators, CRC CARE is currently developing a national guidance document on natural attenuation.