URANIUM IN SEDIMENTS FROM THE MAGELA CREEK CATCHMENT, NORTHERN-TERRITORY, AUSTRALIA

The 'fine silt and clay' fraction of sediments from various tropical b illabongs (permanent waterbodies) in the Magela Creek catchment, North ern Australia, were analysed for uranium by delayed neutron activation analysis. Sediment samples were from a range of billabong types: back flow (4 samples), channel (2 samples) and floodplain (5 samples) and w ere sampled and prepared for analysis as described by Thomas and Hart (1). Uranium concentration data from this study are compared with prev iously reported uranium concentrations and organic carbon concentratio n in Magela Creek sediments. Uranium concentration tends to be higher in 'fine silt and clay' fractions of sediments (<20 mum; mean 6.1 mug g-1 uranium) compared with that in the total sediment (mean 2.7 mug g- 1 uranium). It is also higher in backflow billabongs (mean 7 mug g-1 u ranium) and floodplain sediments (mean 3.0 mug g-1 uranium) compared w ith creek/channel sediments (mean 2.5 mug g-1). Low uranium concentrat ion in sediment was correlated with low organic carbon concentration. The transfer of uranium in sediments from upstream billabongs to the f loodplain probably takes place over one wet season, since there is lit tle evidence of storage of uranium in the sandy sediments that predomi nate in the creek channel.