Health impacts of large releases of radionuclides. Transport and processes in freshwater ecosystems.

The partition coefficient (Kd) and the water retention rate (RR) are fundamental components of dynamic, mass-balance models, not just for radionuclides in fresh water but also for contaminants in all aquatic ecosystems. Kd may be regarded as an 'entry gate' and RR an 'exit gate'. Uncertainties in Kd and RR cause uncertainties in model predictions. Uncertainties in important rates for processes within ecosystems (such as sedimentation, diffusion, advection, bio-uptake and excretion) cannot be adequately evaluated when uncertainties exist for Kd and RR. Empirical data show that there may be a variation in Kd of two orders of magnitude with environmental factors such as pH. This is important because Kd regulates the amount of radionuclides in dissolved and particulate phases, and hence also pelagic and benthic transport. 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 of Kd and RR. Which provide the best predictive power? This chapter gives a brief overview and discussion of the benefits and drawbacks of different alternatives for Kd and RR within the framework of a lake model for radiocaesium.