Modelling of radiocesium in lakes — the VAMP model

The objective of this work is to present a new, state-of-the-art model for predicting radiocesium in lakes. The target variables to model and predict are Cs concentrations in lake water and in predatory fish. These results emanate from IAEA ʹs VAMP project, initiated after the Chernobyl accident in 1986. The sites included in this work cover a wide range of lake and catchment characteristics. This new VAMP model has nine specific components (see below), which are meant to increase the predictive accuracy of the model and make it easy to apply for lakes in general. (1) A seasonal variability moderator for water discharge. (2) A dimensionless moderator for the water retention rate. (3) A seasonal variability moderator for epilimnetic and hypolimnetic temperatures, which influence lake stratification and redox-induced internal loading of radiocesium from sediments. (4) A transfer coefficient to calculate relationships between biomasses. (5) An outflow rate function for the transport of cesium from the catchment to the lake. (6) A dimensionless moderator for planktonic uptake of radiocesium, i.e. for the transfer of cesium in dissolved phase in the lake water to phytoplankton. (7) An algorithm for the lake partition coefficient (K.d). (8) A sub-model to express biological half-lives. (9) An approach to quantify internal loading. Empirical validations, and critical sensitivity tests, have shown that the VAMP model in many lakes yields just as good predictions as parallel sets of empirical data, and this is as good as any model can yield. It should be possible to improve the VAMP model further if and when more field data become available, for both the VAMP lakes and other sites.