Validation of models of radionuclide wash-off from contaminated watershedsusing Chernobyl data

Based on data from the Chernobyl accident, the 'Wash-off' scenario was deve loped to provide an opportunity to test models intended to simulate the mov ement of trace contaminants from terrestrial sources to bodies of water. Th e specific objective of the test was to take into account chemical speciati on, its effect on the transfer of contamination from soil to water, and the geochemical and geophysical processes that affect such transfer. Modellers were provided with descriptions of two experimental plots near the Chernob yl Nuclear Power Plant (NPP), one using simulated heavy rain (HR) and one u sing snowmelt (SM). They were requested to estimate the vertical distributi on of total Cs-137 and Sr-90 and their specific forms in the soil prior to the experiments, concentrations of each radionuclide in surface runoff (sep arately for particulate and dissolved forms) and the total amount of each r adionuclide lost from plot HR during the experiment. All predictions were t o be provided as best estimates with 95% subjective confidence intervals ab out the best estimates. In this paper, a brief description of the modelling results is provided, together with discussions of the performance of indiv idual models in comparison with the actual measurements and of the sources of uncertainty in the model predictions. Our conclusion is that the predict ive accuracy of the mathematical models could be improved by (1) improvemen t of model structure to include all relevant mechanisms; and (2) further us e and improvement of methods for estimation of parameter values for the sit uation being modelled. (C) 1998 Published by Elsevier Science Ltd. All righ ts reserved.