DETERMINING THE ORIGINS AND HISTORY OF SEDIMENTATION IN AN UNDERGROUND RIVER SYSTEM USING NATURAL AND FALLOUT RADIONUCLIDES

Natural and anthropogenic radionuclides were used to characterize the source of sediment reaching the underground river in the karst system at Jenolan, New South Wales, Australia. It was found that the relation ship between Ra-226 and Th-232 concentrations, in both spatially avera ged soil samples and transpored sediments, distinguished between two o f the most likely source subcatchments. Sedimentary sequences, collect ed from within the underground system, showed that contemporary deposi tion was made up of an equal mixture of sediment from these two source s. However, at least three earlier deposition periods were also identi fiable, each with different relative contributions from the source cat chments. The first appearance of Cs-137 in the sedimentary sequence an d correlations with historical information provided a chronology for t hese changes, and it is concluded that the contemporary phase was init iated in the early 1950s, most probably by forestry activities. This w ork demonstrates that natural radioactivity can be used as a tracer of the origins of sediment, providing an important new approach to probl ems in sediment supply, transport, and deposition.