Researchers from Australia and the UK tracing soil in one of the areas affected in the 'Black Saturday' bushfires in Victoria have shown how nuclear science can explain the environmental impact of soil erosion.
Severe bushfires, such as Australia's 200
Researchers from Australia and the UK tracing soil in one of the areas affected in the 'Black Saturday' bushfires in Victoria have shown how nuclear science can explain the environmental impact of soil erosion.
Severe bushfires, such as Australia's 2009 Victorian 'Black Saturday' bushfires, expose the soil in bushland.
Subsequent high intensity storms – possibly once-in-a-hundred year events – can then cause significant erosion with massive debris flows. This can happen even in small creek systems.
This study used fallout radionuclides (radioactive atoms) as tracers to better understand the processes that influence the evolution of landscapes experiencing these events.
The fate of the forest soil is particularly important because it can tell us whether or not existing conditions are sustainable. For example, if severe bushfires followed by severe storms causes soil depletion, we know there will be damage to the soil and subsequently, the forest, rather than a sustainable equilibrium.
Fallout plutonium isotopes, measured at trace levels using the ANTARES accelerator, were used for the first time in this kind of study.
The upper part of Myrtle Creek, in the Ovens River basin of North East Victoria, forms a small 23 hectare catchment, covered in dry open eucalypt forest.
This area was completely burnt by one of the numerous high severity bushfires which swept Victoria in February 2009. In the first few days of March 2009, several storm cells passed over the catchment, producing short duration, high intensity rainfall (around 50 mm per hour rainfall rate).
Even in a small catchment such as Myrtle Creek, the water pouring off the hillsides into the creek generated significant debris flows, moving hundreds of tons of material including gravel, sediments, tree trunks and small boulders.
Fallout radionuclides as tracers
Fallout from atmospheric nuclear weapons tests, carried out mainly in the late 1950s and early 1960s, distributed radionuclides all around the world. Certain longer lived radionuclides remain where they fell, in soil, sediments and elsewhere in the environment.
These radionuclides act as markers in the soil or sediment and can then be used to trace the movements of such material. Of these, 137Cs fallout has been used widely in erosion studies, as it tends to be firmly attached to the fine particles in soil or sediment.
137Cs has a half-life of 30 years, so this marker is diminishing as the years go by. Plutonium, in particular 239Pu and 240Pu, also forms a major component of fallout and, like caesium, binds well to fine particles.
With half-lives of several thousand years, the fallout signal from Pu isotopes provides a long-term marker in soils and sediments. Plutonium has only recently been used in this kind of work, as a result of the recent development of accelerator mass spectrometry for actinides. In the present study, both caesium and plutonium were used.
Sampling and measurements
137Cs can be measured by gamma spectrometry in large samples. Pu is not detectable in this way, but can be measured with high sensitivity, even in small samples, by accelerator m