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

Abstract

The through- and out-diffusion of HTO, 36Cl − and 125I − 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 ∼20%. Diffusion accessible porosities were not measurably affected. The values of the effective diffusion coefficients, D e, ranged between (5.6±0.4)×10 −12 and (6.7±0.4)×10 −12 m 2 s −1 for HTO, (7.1±0.5)×10 −13 and (9.1±0.6)×10 −13 m 2 s −1 for 36Cl − and (4.5±0.3)×10 −13 and (6.6±0.4)×10 −13 m 2 s −1 for 125I −. The rock capacity factors, α, measured were circa 0.14 for HTO, 0.040 for 36Cl − and 0.080 for 125I −. Because of anion exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than the uncharged HTO. Unlike 36Cl −, 125I − sorbs weakly on Opalinus Clay resulting in a larger rock capacity factor. The sorption coefficient, K d, for 125I − is of the order of 1–2×10 −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.