Testing different sub-models for the partition coefficient and the retention rate for radiocesium in lake ecosystem modelling

The partition coefficient (Kd) and the retention rate (RR) are fundamental components of dynamic, mass-balance models, not just for radionuclides in fresh waters but for most contaminants in most ecosystems. Kd may be regarded as an ‘entry gate’ and RR and ‘exist gate’. Uncertainties in Kd and RR cause uncertainties in model predictions. It also means that uncertainties in important rates for within-ecosystem processes, like sedimentation, diffusion, advection, biouptake and excretion, cannot be adequately evaluated when great uncertainties exist for Kd and RR. Empirical data show that Kd may vary two orders of magnitude with environmental factors, like pH. This is important since Kd regulates the amount of substances in dissolved and particulate phases and hence also pelagic and benthic transports. The pelagic transport is directly linked to the outflow and retention of substances in the water mass, and thus also to concentrations and ecological effects. There are many approaches for sub-models for Kd and RR. Which provide the best predictive power? The aim of this work is to test different approaches for Kd and RR, and to discuss benefits and drawbacks of these alternatives within the framework of a lake model for radiocesium (the VAMP model).