Simulations of Radionuclide Transport as Affected by Variable Charge Soil Chemical Properties.

Accurate modeling of radionuclide transport in the geosphere is a major challenge because of the presence of many interrelated physical and geochemical processes potentially affecting transport rates. A case study was considered on the fate of radioactive strontium (⁹⁰Sr) in a soil having a pH-dependent cation exchange capacity. Two consecutive modeling approaches were implemented. In a first approach the potential exchange capacity of the soil was described using a variable charge model representing the pH dependent particle charges on the basis of protonation/dissociation reactions of hydrogen from several types of surface hydroxyl groups. Parameter values for the variable charge model were determined from an alkali titration experiment using the PHREEQC geochemical data base. The second modeling approach involved the development of a reactive transport model for predicting ⁹⁰Sr migration from a hypothetical surface repository, subject to transport plumes having different pH levels. The model was run using the HP1 program, which couples the HYDRUS-1D hydrological and PHREEQC geochemical codes. Strontium adsorption in this second approach was characterized by user-defined surface complexation reactions with parameters obtained by the initially developed variable charge model. Finally, an analysis was made of strontium solute concentration pathways as affected by temporal variations in pH dependent geochemical conditions.