Characterizing the chemical containment properties of the deep geosphere: water-rock interactions in relation to fracture systems within deep crystalline rock in the Tono area, Japan

Abstract

Abstract A water/rock interaction study has been integrated with interpretations of geological structures, in order to understand the geochemical conditions in deep granitic rock at the Tono study site, central Japan. Geological investigations show that fracture systems in the granitic rock can be classified into: intact zones; moderately fractured zones; and intensely fractured zones. This classification is based upon the frequency and width of fractures and fractured zones. Isotopic analyses of the groundwaters indicate that the groundwater at SL 80m (SL: sea level) has recharged within the last 30 years. Petrological and mineralogical observations were undertaken, and coupled with theoretical calculations using the PHREEQE geochemical computer code. The results of this approach suggest that the chemical evolution of the groundwater is generally controlled by water-rock interactions involving plagioclase, clay minerals and ferric hydroxide in the intact zones and moderately fractured zones. The geochemical condition of the groundwater is correlated closely with the nature of the fracture systems, mineral compositions and water-rock interaction processes. Different fractures contain chemically distinct groundwaters, which have differing potential capacities to mobilize radionuclides. The investigation method, based on a comparison between geological structures and groundwater chemistry, can be applied to develop a realistic hydrogeochemical model for deep, fractured granitic rocks. Such an approach is necessary if the chemical containment potential of such lithologies is to be evaluated adequately. The work demonstrates that structural controls on chemical heterogeneities in groundwaters should be understood, before the disposal of redox-sensitive wastes can be undertaken safely in fractured crystalline rocks.