Bentonite is adopted as an impervious material for prevention of groundwater from seepage. A focus has been placed upon long-term stability of the bentonite in terms of the impervious capacity. In order to investigate actual conditions of degradation development in natural bentonite layers, field data were taken at three bentonite mines in Japan: Kuroishi, Kawasaki and Dobuyama. Tests were made on all the samples which were extracted from drilled cores of 30-50 m depth. The measurement includes 1) methylene blue capacity, 2) swelling property and 3) exchangeable cation extracted by ammonium acetate solution.
Measures of both the swelling property and concentration of the extracted cations such as Na, K, Ca and Mg were normalized in accordance with measures of the methylene blue capacity, which represent heterogeneity of smectite contents between the samples. A result of the swelling property suggests the degradation spread from the surface toward the depth in the bentonite layers. Degradation regions reached a depth of 10-20 m. While concentration of extracted Na is less in the degradation region than that in a virgin bentonite layer, the concentrations of extracted Ca and Mg are greater. The degradation development in the bentonite layers would be ascribed to ion exchange of Ca and Mg for Na, which are exchangeable cations in smectite, due to the groundwater percolating from surface layers.
On some assumptions of commencement of the degradation in Kuroishi mine, where detailed data was obtained, the development rate is estimated at some 10 mm/1000 years of the degradation in terms of the ion exchange.
