The Long-Term Alteration Rate of Na-Smectite to Ca-Form in the Bentonite Deposite Evaluated by a Cation Diffusion-Convection Model

The depth of alteration zone in the bentonite deposite in Kuroishi mine located in north Japan was estimated by measuring the vertical element profiles of Na, K, Ca, and Mg in the drilled core samples. The ammonium acetate extraction method was employed to measure the exchangeable ion content in smectite and the results indicated Na depleted from the ground surface to 20m in depth. The data also indicated the total loss of Na coincided with the accumulated amount of Ca and Mg. The contribution of K in the reaction was however negligible. This suggests the Na depletion was mainly caused by the ion exchange reactions with Ca ang Mg. The equilibrium constants of the ion exchange reactions were calculated by using the ionic compositions of the underground water analyzed by the in-situ sampling system and the results indicated ca. 103 g/L for both Ca and Mg. By using these constants, the one dimensional depth profile in the bentonite layer were calculated by a diffusion-convenction equation coupled with the ion exchange and calcite dissolution equilibria. The results indicated the time required for the development of the observed element profiles were estimated as 0.4 million years and the resulting average alteration rate was 4 cm/1000years. This value is equivalent to that independently obtained by geomorphological studies.