Effect of confining pressure on the diffusion of HTO, 36Cl- and 125I- in a layered argillaceous rock (Opalinus Clay):: diffusion perpendicular to the fabric

The through- and out-diffusion of HTO, Cl-36(-) and I-125(-) in Opalinus Clay, an argillaceous rock from the northern part of Switzerland, was studied under different confining pressures between 4 and 15 MPa. The direction of diffusion and the confining pressure were perpendicular to the bedding. Confining pressure had only a small effect on diffusion. An increase in pressure from 4 to 15 MPa resulted in a decrease of the effective diffusion coefficient of similar to20%. Diffusion accessible porosities were not measurably affected. The values of the effective diffusion coefficients, De, ranged between (5.6+/-0.4) x 10(-12) and (6.7+/-0.4) x 10(-12) m(2) s(-1) for HTO, (7.1+/-0.5) X 10(-13) and (9.1+/-0.6) x 10(-13) m(2) s(-1) for Cl-36(-) and (4.5+/-0.3)x 10(-13) and (6.6+/-0.4) x 10(-13) m(2) s(-1) for I-125(-). The rock capacity factors, a, measured were circa 0.14 for HTO, 0.040 for Cl-36(-) and 0.080 for I-125(-). Because of anion exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than the uncharged HTO. Unlike Cl-36(-), I-125(-) sorbs weakly on Opalinus Clay resulting in a larger rock capacity factor. The sorption coefficient, K-d, for I-125(-) is of the order of 1-2 x 10(-5) m(3) kg(-1). The effective diffusion coefficient for HTO is in good agreement with values measured in other sedimentary rocks and can be related to the porosity using Archie's Law with exponent m = 2.5. (C) 2003 Elsevier Ltd. All rights reserved.