Sorption behaviour of beryllium-7 and implications for its use as a sediment tracer

The cosmogenic fallout radionuclide beryllium-7 (t1/2 = 53.3 days) can be employed as a tracer to estimate soil redistribution rates, residence times and relative contributions of surface material to fluvial systems. As such, 7Be-based sediment tracing can contribute to the construction of sediment budgets with the potential to become a valuable decision support tool in the context of catchment management. A key assumption for the use of 7Be as a tracer is that following fallout sorption to particles is rapid and effectively irreversible over the timescales of interest. To date, little attention has been given to the sorption behaviour of 7Be with regard to sorption rates and geochemical partitioning. Experiments have been carried out, using four representative soils, to assess the geochemical mobility of 7Be under various chemical conditions and the rate and extent of sorption of stable Be. Time-dependent uptake of stable Be over 10 days was complete (i.e. > 90% removal) within 0.1 h. No detectable 7Be was found in artificial rainwater solutions following a 24‐hour extraction period. The optimised BCR three-step sequential extraction procedure showed that soil‐sorbed 7Be was predominantly associated (42–62%) with the reducible fraction of the soil. The exchangeable fraction held 27–37% and < 21% was associated with the oxidisable fraction. Incubation of the soils over 53 days indicated that Be was associated with stronger binding sites within the soil matrix as the proportion of exchangeable Be decreased and the proportion of reducible and oxidisable Be increased. The results imply that 7Be is held on binding sites of differing energy and that stronger binding occurs with time of contact. Findings support the assumption of rapid sorption of 7Be upon fallout in unsaturated environments. Stability in natural soil solutions provides confidence in the use of 7Be as a tracer under common field conditions. Sequential extractions identified potential for greater mobility under changing environmental conditions in the wider catchment. Further research is required to quantify the effects of changing chemical parameters upon 7Be signatures and subsequent tracer estimates.