[HCG28-07] Investigation on evaluation method for diffusion field -a case study in the Horonobe area-
Keywords:fossil seawater, diffusion dominant, isotope fractionation
Under stagnant groundwater condition, solutes are transported by diffusion. Diffusion is the slowest transport phenomena, and diffusion dominant condition is called as diffusion field, which is suitable condition for radioactive waste disposal. Diffusion field could be formed in low permeable formation where fossil seawater is remained.
Separation of solute and fractionation of isotopes will be promising to evaluate diffusion field. Fossil seawater has high Cl concentration and high δD compared to meteoric water. Cl and δD will be separated by diffusion because diffusion coefficients of Cl and δD are different. Cl mainly consists of 35Cl and 37Cl, and their diffusion coefficients are different due to mass weight. Thus, isotope fractionation between 35Cl and 37Cl is caused by diffusion.
In the Horonobe area, the separation of Cl and δD and isotope fractionation of Cl accompanied by change of δ37Cl were observed. Therefore, diffusion is seemed to be dominant in the Horonobe area. From theoretical approach, diffusion could occur after a part of fossil seawater was flushed with meteoric water. Thus, the historical change of geological condition should be considered to evaluate the distribution of δD, Cl and δ37Cl.
This study entitled “FY2019 Development and Improvement on groundwater flow evaluation technic in rock” supported by the Japanese Ministry of Economy, Trade and Industry. A part of data is supplied from Japan Atomic Energy Agency.
Separation of solute and fractionation of isotopes will be promising to evaluate diffusion field. Fossil seawater has high Cl concentration and high δD compared to meteoric water. Cl and δD will be separated by diffusion because diffusion coefficients of Cl and δD are different. Cl mainly consists of 35Cl and 37Cl, and their diffusion coefficients are different due to mass weight. Thus, isotope fractionation between 35Cl and 37Cl is caused by diffusion.
In the Horonobe area, the separation of Cl and δD and isotope fractionation of Cl accompanied by change of δ37Cl were observed. Therefore, diffusion is seemed to be dominant in the Horonobe area. From theoretical approach, diffusion could occur after a part of fossil seawater was flushed with meteoric water. Thus, the historical change of geological condition should be considered to evaluate the distribution of δD, Cl and δ37Cl.
This study entitled “FY2019 Development and Improvement on groundwater flow evaluation technic in rock” supported by the Japanese Ministry of Economy, Trade and Industry. A part of data is supplied from Japan Atomic Energy Agency.