Plutonium as an environmental tracer for erosion study

Abstract

Fallout radionuclides, including anthropogenic 137Cs and natural 210Pb and 7Be, have been used for many years as environmental tracers for erosion studies. The use of fallout plutonium (Pu) has not been exploited in the past due to the difficulties of detection using the conventional means of alpha-particle spectrometry. However, recent advances in the technique of accelerator mass spectrometry (AMS) allows Pu isotopes to be readily detectable at the femtogram (10-15 g) level with higher sensitivity and shorter analytical times than for 137Cs. These advances allow low-level Pu measurement in environmental samples and thereby facilitate its use as a tracer for detailed catchment-wide erosion assessment. Using the AMS technique of detection, this thesis explores the feasibility and practicality of fallout Pu as a tracer of soil transport and sediment deposition for assessment of erosion. The AMS measurements were carried out using the 14UD Pelletron accelerator operated by the Department of Nuclear Physics at the Australian National University. Using the methodology established in this thesis that includes an improved sample preparation procedure, fallout Pu is applied as an environmental tracer for the assessment of erosion at two case study sites, located within the Australian Capital Territory (Cotter River catchment) and New South Wales (Burra Creek catchment) respectively. The Cotter catchment is a highly disturbed environment that has been influenced by forestry operations and had recently experienced a catastrophic bush fire, whereas the Burra study area is under native vegetation and has been lightly impacted by land use since at least 1950, before the period of fallout from atmospheric nuclear testing. Plutonium inventories and depth profiles were measured at six study sites across the lower Cotter catchment, as well as at an 'undisturbed' reference site. Comparison of the measured inventories at the study sites with the reference inventory shows that, with the exception of one site, there has been minimal soil loss across much of the pine-forested area since the early 1960s. Locally a loss of ~6 cm of topsoil has been identified at the Pago site. The temporal variation of erosional activity in the Cotter catchment has also been explored based on the study of a 1.2 m deep sediment pit that was collected from the upper reaches of the Cotter dam lake. The Pu results together with a compilation of dates derived from Carbon-14 dating, indicates an average sedimentation rates of 1-4 cm/yr and 20 cm/yr over the pre-fire period (1990-2003) and the post-fire period (2003-2006) respectively. At the hillslope study sites in the Burra catchment, the extent of soil erosion ranges from mild to severe. The analysis showed a high erosion rate of 16 t/ha/yr at the vicinity of a steep hillslope, whereas the nearby gentle hillslope terrain is associated with a much lower erosion rate of 0.4 t/ha/yr. Show more