Geostatistical and geochemical classification of groundwaters considered in safety assessment of a deep geologic repository for high-level radioactive wastes in Japan

The general chemical characteristics of Japanese groundwaters at depths greater than 200 meters have been classified using geostatistical and geochemical modeling techniques applied to a dataset of more than 200 chemical analyses of groundwater samples from throughout the country. The objective of the evaluations is to identify representative, generic groundwater types that can be used to assess the long-term safety of a geologic repository for the permanent disposal of high-level radioactive wastes (HLW). Five such generic groundwater categories are defined based on observed trends in the concentrations of reactive solutes with increasing concentrations of the mobile tracer Cl-; FRHP (fresh-reducing-high pH), FRLP (fresh-reducing-low pH), SRHP (saline-reducing-high pH), SRLP (saline-reducing-low pH) and MRNP (mixing-reducing-neutral pH). Multivariate geostatistical methods and geochemical models of evolutionary processes controlling groundwater chemistry indicate, however, that only the FRHP and SRHP groundwaters are both representative of deep groundwater systems in Japan and relevant to subsurface conditions that could exist in a HLW repository. These groundwater types were therefore selected for detailed evaluation in the recent H12 safety assessment of the Japanese disposal concept for HLW.