Sediment transport and deposition

Soil depth profile comparing Cs-137 and Plutonium concentrations

Sampling sites and Land Class uses in the Herbert River catchment

Radio-isotopes have been used for many years as tracers of soil erosion processes. In particular, erosion and sediment transport studies have made significant use of ⁷Be, ¹³⁷Cs and ²¹⁰Pb, by counting the decay radiation. These isotopes bind with soil particles, providing a marker which can trace the movement of the soil. The 3 isotopes provide complementary data because their different chemical properties make their binding properties slightly different and this can be exploited to elucidate different physical processes in the moving soil. Berylium-7 and ²¹⁰Pb are produced naturally and continuously in the environment and are best suited to probe the top few centimeters of the soil profile, but the ¹³⁷Cs, which probes to approximately 30 centimeters, is fallout from atmospheric weapons tests, and hence is not being replenished. The ¹³⁷Cs half life is 30 years and its sensitivity as a tracer is therefore decreasing.

AMS measurements of plutonium isotopes provide an alternative to using ¹³⁷Cs, and may well prove to be a superior tracer for such studies. The techniques for AMS Pu measurements were pioneered at ANU, and routine measurements of the two isotopes indicate that plutonium provides similar quantitative information to Caesium but has better sensitivity. A soil depth profile comparing the two elements is shown in the figure.

A study of sediment movement through the Herbert River catchment, Queensland, Australia, comparing ¹³⁷Cs and ^{239, 240}Pu measurements is presently underway. This study, which is partially funded by an ARC discovery grant, aims to trace sediment movement from the catchment to the Great Barrier Reef. Caesium readily desorbs from the sediment particles in saline environments. The relative extent to which plutonium desorbs is currently under investigation, and initial studies indicate that the plutonium binds much more tightly to the particles than does the caesium. The image show the initial sampling sites where material has been collected to date.

Contact

Keith Fifield

(02) 612 52095