The concept is based on the fact that the distribution of radionuclides or chemical pollutants between a liquid phase and a mineral surface (sorption) is dependent on geochemical properties. On the one hand, the mineral surfaces (type and amount/area) and on the other hand, the geochemical conditions of the solution play key roles. Here, these geochemical properties of the solid-solution interface are described with environmental parameters. For the considered hydrogeological systems (System) the most relevant environmental parameters were identified as:
- ionic strength (IS),
- concentrations of calcium [Ca], aluminum [Al] and sulfate [SO42-],
- concentration of dissolved inorganic carbon [DIC],
- and concentration of the radionuclides themselves.
Distribution coefficients for radionuclides on specific sediment surfaces are calculated in dependence of these parameters using mechanistic sorption models (namely smart Kd-values). A sediment is defined here as a mixture of selected minerals and sorption is described by the “component additivity approach”, in which the contribution of the sorption of one element is considered on every single mineral fraction of the sediment, also including relevant competitive effects. The calculation produces a multidimensional matrix of Kd-values (Multidimensional Matrices) which serves as a lookup table and is accessed by the transport code in each time step for each node. A detailed description of the overall methodology is published in Stockmann et al. (2017).
The transport and specific treatment of the environmental parameters are implemented into the transport code d³f++. A crucial aspect concerning the implementation of the environmental factors into the transport code is that the pH-value is affected not only by proton transport but also by chemical reactions with the dissolved ions (preferably DIC, Ca and Al) and mineral phases of the sediments. According to the results from site investigation, the main reactions in the Gorleben aquifers are calcite dissolution (mainly in the upper aquifer), microbial degradation of sedimentary organic carbon causing enhanced CO2 partial pressures (DIC source), and dissolution of feldspars. At present, dissolution and precipitation of calcite is considered as the most relevant reaction affecting the pH-value: the pH is increased by calcite dissolution and decreased by calcite precipitation. In case calcite is not or no more available in the system the pH is determined by equilibrium with a generic gibbsite mineral. Details describing the transport and treatment of the environmental parameters in the transport code can be found here (Implementation of Environmental Parameters and Transport of Relevant Components).
Stockmann, M., Schikora, J., Becker, D.-A., Flügge, J., Noseck, U., Brendler, V., Smart Kd-values, their uncertainties and sensitivities - Applying a new approach for realistic distribution coefficients in geochemical modeling of complex systems. Chemosphere 187 (2017), 277-285 p.