Estimating residence times of fine sediment in river channels using fallout 210Pb

The residence time of fine-grained sediment (<10 μm diameter) within river channels is often assumed to be short, and has been rarely quantified. Here we investigate a new approach to estimate residence times of such material. Fine-grained sediments collected from along the Brisbane River, Queensland, Australia, have higher excess 210 Pb activities and 210 Pb/ 137 Cs ratios than sediment eroded from uncultivated, cultivated and channel bank sources in the catchment. We explore the possibility that these higher 210 Pb activities are related to residence of the sediment within the channel and propose a method for using this data to make a residence time calculation. The method gave a range of residence times of 0-21 years for the Brisbane sediments. Data from the adjacent Logan River system provide a range of 0-9 years, although both ranges were dependent on a number of assumptions regarding sediment thickness and 210 Pb fallout rates and amounts.