Assumptions and challenges in the use of fallout beryllium-7 as a soil and sediment tracer in river basins

This contribution reviews research surrounding the assumptions that underpin the use of beryllium-7 (7Be) as a soil and sediment tracer in river basins. As a cosmogenic radionuclide, the constant natural production of 7Be and fallout via precipitation, coupled with its ability to bind to soil and sediment particles provides the basis for its application as a conservative soil and sediment tracer. Consequently, 7Be has been extensively employed as a tracer across a range of spatial and temporal (event to seasonal) scales. The short half-life of 7Be (53.3 days) lends itself to tracing sediment dynamics over short time periods, thus, providing complementary data to medium-term estimates derived from longer lived radionuclides such as caesium-137 (137Cs). This short half-life could provide a major advantage when considering the potential for 7Be to document recent effects of climate or land use change upon soil redistribution, with the latter having particular relevance for assessing the effectiveness of mitigating strategies within a catchment-wide approach to management. Although 7Be has been widely applied as a tracer to date, application is still in its infancy and there remains a lack of knowledge in relation to the assumptions for its use as a tracer. Specifically, our findings suggest that there are crucial information gaps with regard to 7Be application as an erosion tracer. Of key importance is the potential for fallout uniformity and rapid tracer sorption to be compromised under certain conditions. The assumption of irreversible sorption is likely to hold for common hillslope conditions but literature identifies the potential for tracer mobility with changing environmental parameters in the wider catchment. Further research is required to determine the likelihood of 7Be sorption being affected by conditions found at relevant field sites and the impact of this upon tracer applications at the catchment-scale.