Predicting areas of Cs-137 loss and accumulation in upland catchments

Anthropogenic radioisotopes with long physical half-lives derived from atmospheric fallout remain in the environment for decades after deposition. Process- and field-based studies within upland catchments show that radiocaesium is transported in solution as well as in particulate form. Catchment hydrology is therefore an important control on radiocaesium transport and natural processes of soil development. The topographic index, from the hydrological model TOPMODEL, has been used as a basis for the development of a simple model for predicting radiocaesium redistribution in temperate upland catchments. The model is particularly suited to predicting 137Cs redistribution within upland environments as it is based on topography, which is readily calculated from digital terrain models within geographical information systems. A conceptual model of radiocaesium losses on hillslopes and re-deposition on the valley floor was calibrated with atmospheric weapons testing 137Cs inventories from soil core data for the Raithburn catchment, Renfrewshire, U.K. The model fitted the observations well and showed that in this catchment a topographic index value of about 5.0, for 10 m grid cells, forms the threshold between 137Cs loss and accumulation. The results indicated that about 20% of the total atmospheric weapons testing 137Cs deposited in the catchment had been transported out of the catchment over the ca. 30 yr period since deposition