Predicting Areas of 137Cs Loss and Accumulation in Upland Catchments

Anthropogenic radioisotopes with long physicalhalf-lives derived from atmospheric fallout remain inthe environment for decades after deposition. Process- and field-based studies within uplandcatchments show that radiocaesium is transported insolution as well as in particulate form. Catchmenthydrology is therefore an important control onradiocaesium transport and natural processes of soildevelopment. The topographic index, from thehydrological model TOPMODEL, has been used as a basisfor the development of a simple model for predictingradiocaesium redistribution in temperate uplandcatchments. The model is particularly suited topredicting ¹³⁷Cs redistribution within uplandenvironments as it is based on topography, which isreadily calculated from digital terrain models withingeographical information systems. A conceptual modelof radiocaesium losses on hillslopes and re-depositionon the valley floor was calibrated with atmosphericweapons testing ¹³⁷Cs inventories from soil coredata for the Raithburn catchment, Renfrewshire, U.K. The model fitted the observations well and showed thatin this catchment a topographic index value of about5.0, for 10 m grid cells, forms the threshold between¹³⁷Cs loss and accumulation. The resultsindicated that about 20% of the total atmosphericweapons testing ¹³⁷Cs deposited in the catchmenthad been transported out of the catchment over theca. 30 yr period since deposition.