IONIC-STRENGTH EFFECTS IN MODELING 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 d epends upon the quality of the thermodynamic data used in the calculat ions. These data are affected by the ionic strength of the system. In most environmental situations modelled, the ionic strength is rather l ow. sea water is about 0.5 molal but many groundwaters are less than t his (0.1-0.001 molal). By contrast, the great majority of experimental measurements of formation constants have been made at high ionic stre ngth, between 1 and 10 molal, and in the majority of cases, measuremen ts have been made at only one ionic strength. With the lack of thermod ynamic data at 'environmental' ionic strengths, modellers find themsel ves having to extrapolate with only one data point! For instance, the formation constants of the actinides, of particular importance in radi oactive waste disposal, have nearly all been measured at a single, hig h, ionic strength. There are a number of methods for extrapolating suc h 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.