Estimation of palaeohydrochemical conditions using carbonate minerals

Amamiya, Hiroki; Mizuno, Takashi   ; Iwatsuki, Teruki  ; Yuguchi, Takashi; Murakami, Hiroaki  ; Kokubu, Yoko   

The long-term evolution of geochemical environment in deep underground is indispensable research subject for geological disposal of high-level radioactive waste. Many researchers have made efforts previously to elucidate the geochemical environment within the groundwater residence time based on the analysis of the actual groundwater. However, it is impossible to estimate the geochemical environment for the longer time scale than the groundwater residence time in this method. In this case, analysis of the chemical properties of secondary minerals are one of useful method to estimate the past hydrochemical characteristics (temperature, salinity, pH and redox potential). We reviewed the previous studies on carbonate minerals and geochemical conditions in deep underground and estimated the hydrochemical characteristics of past groundwater by using carbonate minerals in crystalline rock at Tono area, Japan. As a result, it is found that temperature and salinity of the groundwater during crystallization of carbonate minerals were evaluated quantitatively. On the other hand, pH and redox potential can only be understood qualitatively. However, it is suggested that the content of heavy metal elements such as manganese, iron and uranium, and rare earth elements in the carbonate minerals are useful indicators for estimating redox potential.