Geochemical modeling of cation exchange reactions in Horonobe mudstone from Northern Hokkaido, Japan

Abe, Takeyasu  ; Iida, Yoshihisa ; Sasamoto, Hiroshi  ; Ishii, Eiichi  

Cation exchange is one of important elementary reactions among water-rock interactions in sedimentary rocks. In order to model non-constant Na-K-Ca-Mg-H exchange selectivity of sedimentary rocks, exchange isotherms and pH dependence on cation exchange capacity of smectite were analyzed following active fraction model. As the result of comparison of exchangeable cationic composition between measured and calculated, selectivity coefficients determined in this study were preferred to those presented by previous studies. Using the coefficients and groundwater hydrochemistry reported previously, expected compositions of exchangeable cations were calculated. The results suggested that pH and total Na-K-Ca-Mg-H concentration of groundwater are important factors to interpret observed compositional variation in exchangeable base cations.