Natural Radionuclide Behaviour in the Fluvial Environment

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Abstract
Variable concentrations of uranium and thorium series nuclides and 7Be have been measured in soils and sediments. Strong correlations between 226Ra and thorium series nuclides were found in sediments but not in soils. Laboratory measurements suggest the correlations arise from particle size and density dependent transport, and transport-related abrasion of iron oxide coatings. These correlations are characteristic of the sampled location, and provide a method for identifying the source areas which dominate the fluvial nuclide flux, and by implication, the associated sediment flux. Cosmogenic 7Be (half-life 53 d) also contributes to nuclide fluxes. Over an 18 month period, individual rainstorms increased the 7Be soil inventory by 10% on average. Dry precipitation contributed less than 10% to the total. Most 7Be was retained within the top few millimetres of soil. It is deduced that 7Be presence in fluvial sediments indicates a significant surface source contribution to the overall nuclide and sediment flux.

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