Land and Water Link

Issue No. 11, December 2001

To curb the pollution of soils and associated risks to human health and ecosystems, many countries have introduced regulatory guidelines based on risk assessment of individual contaminants. But as researchers in CSIRO Land and Water’s Remediation of Contaminated Environments Program are finding, the mixture of contaminants in the soil may be even more critical than the concentration levels of a particular chemical.

For Drs Mallavarapu Megharaj, Rai Kookana, Ravi Naidu and their team, the starting point is the reality that soil is rarely contaminated with single pollutants.

Dr Megharaj observes, ‘In soil environments, especially in agricultural ecosystems, organisms are often exposed to various combinations of pesticides, applied together or in tandem. Yet ecotoxicity tests are generally restricted to individual contaminants.’

Driven by the need to find better ways of assessing the real impact of complex mixtures of soil contaminants, the CSIRO scientific team has been investigating the interplay between toxicity and bioavailability, with some surprising results.

Trying to determine the toxicity of a mix of contaminants from a polluted site is far from easy. One of the more vexing variables appears as parent chemicals break down and are then found together with their degradation products.

As CSIRO researchers found when they analysed the impact of long-term DDT pollution of soil, the presence and interaction of DDT residues such as DDE and DDD can exert a more noxious effect than the parent compound itself.

In another experiment to evaluate the toxicity of total petroleum hydrocarbons at a long-term contaminated site, the team discovered that some soil samples with medium levels of contaminants proved more toxic to microbial activity than other samples of highly contaminated soil.

As Dr Megharaj explains ‘This demonstrates the critical importance of bioavailability in risk management and remediation. Bioassays must be used in conjunction with chemical analysis’, he says, ‘because they go beyond reporting on just the presence of a chemical, to reveal its bioavailability and biological effects. And these are the factors that matter in terms of risk assessment and remediation.’

Dr Megharaj also warns ‘Because different pollutants affect living organisms at different levels and in many ways, we now recognise that no single bioassay is adequate. Given that bioavailability is species dependent, any monitoring program will need a battery of bioassays entailing a range of biological tests.’

In this quest to determine the impacts of contaminants, scientists have found an auspicious lead. Algae, and especially algal diversity, may prove to be the ‘litmus test’ that signals contaminant pollution in soils.

‘Algae make ideal indicators because they are remarkably sensitive to a variety of toxicants, both organics and heavy metals’, explains Dr Megharaj.

‘For test purposes, they are easy to sample, simple to handle and quick to culture – making them relatively cheap and cost effective. Best of all, algae are abundant across a range of soil types and environments. In agricultural soils, one gram of soil may contain up to 300,000 algae.

‘Being the primary producers at the bottom of the food chain, these beneficial soil organisms play a vital ecological role in sustaining a healthy environment.’

Ironically, as well as being remarkably sensitive indicators for monitoring soil pollution, algae also show promise as agents for remediation. The CSIRO research team is currently trialling their use in degrading highly persistent pesticides and contaminants.

By broadening our understanding of bioavailability and the effects of contaminant mixtures, especially in the context of the Australian environment, this work is contributing to more reliable risk assessment protocols and remediation options.

For further information:

Refer to the Bioavailability Workshop website

Or contact

Dr Mallavarapu Megharaj
Ph: (08) 8303 8703
Megharaj.Mallavarap@csiro.au