Understanding the Influence of Spatially Varying Retardation Effect on the Solute Transport
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8QGHUVWDQGLQJWKH,QIOXHQFHRI6SDWLDOO\9DU\LQJ5HWDUGDWLRQ(IIHFW RQWKH6ROXWH7UDQVSRUW Yuichi Niibori, Satoko Shimoda, Kouichi Tanaka and Osamu Tochiyama Department of Quantum Science and Energy Engr., Graduate School of Engr., Tohoku University, Aobayama 01, Aoba-ku, Sendai, JAPAN 980-8579 $%675$&7 A two-dimensional approach to understanding the heterogeneity in retardation effect governing the solute transport was carried out. To express the simplified spatial distribution of retardation effect, this study prepared parallel flat boards packed with two kinds of solid materials, where these materials were not mixed, but arranged as two kinds of the layers in a direction parallel to the flow. The ratio of the width of the constituent layer to the total transport distance was assumed to be one of key parameters describing the solute release profile transported through the media. As a tracer, Eu3+ in HNO3 solution was used, and its breakthrough curve was monitored in the experiments. In order to determine the retardation coefficient of the each layer, the retardation coefficients was obtained from the separate column experiment. Moreover, the permeabilities of both layers were confirmed to be almost identical from the one-dimensional, column experiments. A mathematical model was constructed to express the two-dimensional advection and dispersion of the solute through the media with the distribution of retardation effect. The proposed mathematical model and the experimental results showed good agreements, and indicated that the solute release profile strongly depended on the ratio of the width of the constituent layers to the total transport distance and the standard deviation of the retardation. ,1752'8&7,21 In a geological disposal system for radioactive wastes, a natural rock formation contains various materials which can or cannot sorb solutes and retard their transport. For example, some kinds of secondary minerals are often observed on fracture surfaces [1,2]. In another occasion, the groundwater contacting with cement may attain pH values as high as 13 and cause continuous alteration in the physical and chemical properties [3,4]. Thus, as a solute transport medium, the retardation effect in the natural rock formation is not uniform in space and in time. So far, some approaches to understanding the heterogeneity affecting the solute transport have been conducted [5]. As for such a problem, however, two-dimensional examination has not been discussed sufficiently. In this study, in order to estimate the influence of the spatial distribution of retardation effect on the solute transport, column experiments have been carried out, where two kinds of solid materials were arranged to express the simplified spatial distribution of retardation effect and the release behavior of a tracer through the column was observed. A mathematical flow model considering the advection and dispersion of the solute in a two-dimensional space was constructed and confirmed to express the experimental result. (;3(5,0(17$/
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