The Distribution of Caesium-137 in Rangeland Soils at Three Sites in Western Australia.

Abstract

Soil sampling for the fallout isotope caesium-137 (137Cs) was canied out on three pastoral properties in Western Australia to assess its suitability for estimating soil erosion status. The sites were situated east and north-east of Geraldton in a region receiving an annual rainfall of approximately 200 rnm. It was hypothesised that 137Cs levels would be lower outside Department of Agriculture exclosures, established in the early 1950s, because of higher rates of soil erosion due to pastoral activities. The exclosures are areas of fenced-off rangeland which have the purpose of excludin'g in11 herbivores. It was further hypothesised that 137Cs levels would be related to microtopography in this rangeland-plains landscape. Soil erosion and deposition have given rise to scalds and soil mounds under shrubs, respectively, and amounts of 137Cs at these sites could reflect these processes. The Mann-Whitney U test showed that there were no significant differences in 137Cs levels between samples collected inside and outside the exclosures. At one property there was a significant relationship between 137Cs and microtopography (U test: sig. 0.025 level), suggesting that soil erosion was more severe on open and scald sites than under shrubs. No detectable 137Cs was found at 23% of all sites, but there was evidence that localized deposition of sediments had occurred. Because the total number of samples used in this survey was small, further work will be required to confirm the utility of the 13'Cs technique for measuring erosion status in arid Australia.