Smart Kd-values as realistic distribution coefficients

One important natural process retarding the transport of contaminants is sorption onto mineral surfaces. A respective process understanding and realistic geochemical modelling of sorption is thus of high relevance in safety assessments of radioactive waste repositories. Further application areas are groundwater protection, environmental remediation or e.g. disposal of chemotoxic hazardous waste. Most often conventional concepts with constant distribution coefficients (Kd-values) are applied in reactive transport simulations, with the advantage to be simple and computationally fast, but not reflecting changes in geochemical conditions. Here, the smart Kd concept (www.smartkd-concept.de), a mechanistic approach mainly based on surface complexation models, is applied in geochemical modelling and has been further developed to calculate more realistic distribution coefficients for a wide range of important environmental parameters, e.g. pH, ionic strenght, competing cations and complexing ligands [1, 2] using PHREEQC, UCODE and RepoSUN/SimLab [3, 4, 5]. The philosophy behind this approach is to compute a-priori multidimensional smart Kd matrices which are available for subsequent transport simulations. We could demonstrate that constant Kd-values (e.g. for U(VI) [6], see Fig. 1) used so far are too crude an assumption but they rather range over several orders of magnitude. For considering worst case scenarios much smaller Kd-values have to be use than in conventional concepts. Similar results will be presented for Am and Np.