Rapid and irreversible sorption behavior of ⁷Be assessed to evaluate its use as a catchment sediment tracer

Beryllium-7 (⁷Be) has been used as a sediment tracer to evaluate soil redistribution rates at hillslopes and as a tool to estimate sediment residence time in river systems. A key assumption for the use of ⁷Be as a sediment tracer is the rapid and irreversible sorption of ⁷Be upon contact with the soil particles. However, recent studies have raised questions about the validity of these assumptions. Seven soil types were selected to assess the adsorption rate of ⁷Be on the soil particles, subsequently an extraction experiment was performed to assess the rate of desorption. Next, different treatments were applied to assess the impact of soil pH, fertilizer, humic acid and organic matter on the adsorption of Be. Finally, the influence of regularly occurring cations present on the soil complex on the adsorption of Be on pure clay minerals was evaluated. The adsorption rate experiment showed a rapid and nearly complete sorption of Be for Luvisols and Cambisols under agriculture. For a temperate climate Stagnosol under forest and two highly weathered tropical Ferralsols sorption of Be was less rapid and less complete. This may result in an incomplete adsorption of ⁷Be on these three soils when runoff initiates, which could lead to an overestimation of erosion rates and sediment residence time. Additional observations were made during the extraction experiment, showing a significant loss of Be from the forest Stagnosol and a stable binding of Be to the arable soils. Of the different treatments applied, only pH showed to be of influence. Finally, Ca²⁺ and NH₄⁺ on the soil complex had only a limited effect on the adsorption of Be, while Al³⁺ in combination with a low pH inhibits the adsorption of Be on the exchange complex of the pure clay minerals. All these findings more rigorously support the use of ⁷Be as a soil redistribution tracer in arable soils in a temperate climate at a hillslope scale. The use of ⁷Be in highly weathered Ferralsols or forest rich environments should be limited to avoid overestimations of erosion rates. The spatially extended use of ⁷Be to evaluate residence times of sediments should be avoided in catchments with rapid changing environmental parameters as they might influence the sorption behavior of ⁷Be.