Ionic strength effects in modelling radionuclide migration in environmental systems: Estimating the errors and uncertainties

Thermodynamic models are widely used to predict the chemical behaviour of radionuclides in the geosphere. The accuracy of such predictions depends upon the quality of the thermodynamic data used in the calculations. These data are affected by the ionic strength of the system. In most environmental situations modelled, the ionic strength is rather low: sea water is about 0.5 molal but many groundwaters are less than this (0.1–0.001 molal). By contrast, the great majority of experimental measurements of formation constants have been made at high ionic strength, between 1 and 10 molal, and in the majority of cases, measurements have been made at only one ionic strength. With the lack of thermodynamic data at ‘environmental’ ionic strengths, modellers find themselves having to extrapolate with only one data point! For instance, the formation constants of the actinides, of particular importance in radioactive waste disposal, have nearly all been measured at a single, high, ionic strength. There are a number of methods for extrapolating such data: the most widely used is the Davies equation. The magnitude of the errors that may be incurred by utilising this method, particularly with data at high ionic strength, has been evaluated, and found to be typically several orders of magnitude.