Mineralogical analysis of a long-term groundwater system in Tono and Horonobe area, Japan

Abstract

Teruki Iwatsuki Japan Atomic Energy Agency, Horonobe Underground Research Center, Hokushin, Horonobe-cho, Teshio-gun, Hokkaido, JapanTakashi Mizuno & Katsuhiro Hama Japan Atomic Energy Agency, Mizunami Underground Research Laboratory, Yamanouchi, Akeyo-cho, Mizunami, Gifu, JapanTakanori Kunimaru Japan Atomic Energy Agency, Horonobe Underground Research Center, Hokushin, Horonobe-cho, Teshio-gun, Hokkaido, JapanABSTRACT: A methodology to demonstrate the long-term hydrochemical evolution of deep groundwater is indispensable in the geological isolation of high level radioactive waste (HLW). A key component is to extrapolate the future changes in hydrochemical conditions based on the analogue of past geological events and their impact on hydrochemical conditions, and their relationships within the space of a geological system. This study conducted hydrochemical research using secondary minerals to evaluate the long-term changes in the groundwater systems in: (i) crystalline rock in the Tono area (Honshu, Japan), and (ii) sedimentary rock in the Horonobe area (Hokkaido, Japan). The spatial distribution of fracture-filling carbonate minerals provides an indication of the depths to which long-term weathering and rock-water interaction by infiltration of fresh recharge water have affected the geological sequence at each site. The petrographic characteristics (including morphology, multiple generations) and isotopic/chemical signatures of the carbonate mineralization suggest that the groundwaters have been displaced several times during their history in the Tono area. In contrast, the deep system in the Horonobe area appears to have preserved ancient groundwaters. Hydrochemical interpretations based on mineralogical information are useful to construct hydrogeological models of both areas, in particular providing insights into groundwater residence time.