Upscaling of Se(IV) sorption coefficients with hierarchical mineral characterization in multi-scale fractured granite

The radionuclides sorption on fractured rocks is greatly influenced by the constituent minerals and their spatial variations of the fracture matrix. The mineral heterogeneity distributions can lead to potential scale-dependent sorption coefficients in fractured rocks. The indicator geostatistics-based upscaling (IU) method is one promising upscaling approach but has a gap in linking parameters from different sources in real applications. Taking Se(IV) sorption on fractured granite as an example, this study developed a sorption coefficient transition function between individual minerals and mineral assemblages and validated the IU method for estimating the sorption coefficients under a hierarchical framework of reactive mineral facies. The sorption coefficients of the reactive mineral assemblage at the mesoform scale were estimated based on the spatial distribution of minerals on the fracture surface and multimodal mineral sorption coefficients at the microform scale. Compared to the parameters derived from our fracture transport experiment and traditional component additivity method, the IU method accurately estimates the sorption coefficients of mineral assemblage and delineates the variations of sorption coefficients at different scales. This study successfully bridges the gap between laboratory-scale measurements and field-scale simulations and provides important information for field-scale predictive modeling.