Identifying sediment sources in a gullied catchment using natural and anthropogenic radioactivity

Abstract

The ratio of the lithogenic nuclides 226Ra and 232Th indicates that the major source of sediment in transport in a gullied catchment, located in the head waters of Whiteheads Creek, New South Wales, Australia, is the gully itself. This observation is supported by 7Be and 137Cs concentrations which indicate that the sediment is derived from erosion of either gully floor or gully wall material. While subcatchment 7Be and 137Cs data show sheet, rill, and gully erosion are occurring in the upstream catchments, sediment from these sources is either being trapped before entering the gully, or the volume of sediment is small compared to that derived from the gully itself. The differences in the 226Ra to 232Th ratio between the suspended solids collected on the rising stage and samples of sediment deposited during the falling stage at the outlet of the catchment, indicate a change in the source of sediment during storm events. The results presented here demonstrate the applicability of radionuclide tracers in providing quantitative estimates of the relative contributions of individual catchments to the sediment moving within a drainage network, and in determining whether or not the sediments are mainly derived from subsoil or topsoil sources.