Presentation Information
[T9-O-3]Properties and Estimated Erosion Rate of a Bentonite Layer in a River Flow Environment
*Ryosuke Kikuchi1, Tsutomu Sato1, W. Russell Alexander2, Masakazu Ito3, Heini M. Reijonen4, Shingo Yokoyama5 (1. Hokkaido Univ., 2. Bedrock Geosciences, 3. Kunimine Industries, 4. Geological Survey of Finland, 5. Central Research Institute of Electric Power Industry)
Keywords:
Bentonite,Erosion,River,Buffer material,Geological disposal
Bentonite is widely used as a sealing material because of its high swelling capacity. However, long-term interaction with dilute surface water or groundwater may induce erosion and swelling, resulting in density reduction and degradation of sealing performance. To investigate the long-term behavior of bentonite in a natural surface-water environment, vertical borehole cores were obtained from a bentonite layer exposed on the riverbed of the Tsukinuno River, Yamagata Prefecture. This study characterizes the obtained bentonite samples and estimates the erosion rate of the bentonite layer using multiple independent approaches.
In the study area in Oe Town, Yamagata Prefecture, Miocene sedimentary rocks are widely distributed, and bentonite layers several centimeters to several meters thick occur as intercalations. Two borehole cores were collected from a bentonite layer exposed on the riverbed and intercalated within the Sachio Pyroclastic Rock Member of the Mizusawa Formation; the core lengths from the riverbed surface were 5.45 and 5.15 m. River-water chemistry was analyzed by field measurements, ICP-AES, and ion chromatography. After core dismantling, bentonite samples from different depths were analyzed for density, water content, degree of saturation, methylene blue adsorption and exchangeable cation composition. Bulk chemical composition and mineral composition were determined by XRF and X-ray diffraction, respectively.
The river-water chemistry was comparable to that of dilute glacial groundwater. The total cation charge concentration was 1.2 mM, indicating that, even allowing for seasonal variation, the water chemistry falls within a range in which swelling or extrusion of bentonite may occur (<8 mM). Both cores consisted entirely of bentonite from the surface to the deepest sampled intervals. Little vertical variation in density was observed, suggesting that density reduction associated with swelling had not occurred. In contrast, the shallow intervals were enriched in exchangeable Ca and Mg, whereas the deeper intervals were relatively enriched in Na, indicating ion exchange between river water and the bentonite layer. Field observations provided no significant sign for preferential erosion of the bentonite layer relative to the surrounding sedimentary rocks, such as siltstone and sandy tuff, suggesting comparable erosion rates among these lithologies. The erosion rate was evaluated from three independent approaches: (1) altitudinal dispersion around the study area, (2) the relative height and formation age of river terraces, and (3) diffusion profiles of river-water-derived ions in the bentonite. These approaches consistently yielded an estimated erosion rate on the order of 0.1–1 mm/yr. The absence of preferential erosion of the bentonite layer may be attributed to two factors: (1) conversion of the surface layer to Ca- and Mg-type bentonite through interaction with river water, which may suppress free swelling relative to Na-type bentonite; and (2) cementation which retains interparticle bonding among montmorillonite in natural bentonite rock, unlike to industrial bentonite products. Laboratory erosion tests are currently being conducted to evaluate the these possible factors to erosion suppression.
In the study area in Oe Town, Yamagata Prefecture, Miocene sedimentary rocks are widely distributed, and bentonite layers several centimeters to several meters thick occur as intercalations. Two borehole cores were collected from a bentonite layer exposed on the riverbed and intercalated within the Sachio Pyroclastic Rock Member of the Mizusawa Formation; the core lengths from the riverbed surface were 5.45 and 5.15 m. River-water chemistry was analyzed by field measurements, ICP-AES, and ion chromatography. After core dismantling, bentonite samples from different depths were analyzed for density, water content, degree of saturation, methylene blue adsorption and exchangeable cation composition. Bulk chemical composition and mineral composition were determined by XRF and X-ray diffraction, respectively.
The river-water chemistry was comparable to that of dilute glacial groundwater. The total cation charge concentration was 1.2 mM, indicating that, even allowing for seasonal variation, the water chemistry falls within a range in which swelling or extrusion of bentonite may occur (<8 mM). Both cores consisted entirely of bentonite from the surface to the deepest sampled intervals. Little vertical variation in density was observed, suggesting that density reduction associated with swelling had not occurred. In contrast, the shallow intervals were enriched in exchangeable Ca and Mg, whereas the deeper intervals were relatively enriched in Na, indicating ion exchange between river water and the bentonite layer. Field observations provided no significant sign for preferential erosion of the bentonite layer relative to the surrounding sedimentary rocks, such as siltstone and sandy tuff, suggesting comparable erosion rates among these lithologies. The erosion rate was evaluated from three independent approaches: (1) altitudinal dispersion around the study area, (2) the relative height and formation age of river terraces, and (3) diffusion profiles of river-water-derived ions in the bentonite. These approaches consistently yielded an estimated erosion rate on the order of 0.1–1 mm/yr. The absence of preferential erosion of the bentonite layer may be attributed to two factors: (1) conversion of the surface layer to Ca- and Mg-type bentonite through interaction with river water, which may suppress free swelling relative to Na-type bentonite; and (2) cementation which retains interparticle bonding among montmorillonite in natural bentonite rock, unlike to industrial bentonite products. Laboratory erosion tests are currently being conducted to evaluate the these possible factors to erosion suppression.
Comment
To browse or post comments, you must log in.Log in