The diffusion of SO42− in Opalinus Clay: Measurements of effective diffusion coefficients and evaluation of their importance in view of microbial mediated reactions in the near field of radioactive waste repositories

Abstract Through-diffusion experiments with 36 Cl − , 35 SO 4 2− and HTO in Opalinus Clay (OPA) samples from a deep borehole in North-East Switzerland (Benken; BE) have been performed. The effect of burial depth on the experimental results has been investigated. It could be shown that the effective diffusion coefficients decrease with sample depth for all three tracers. Moreover, there was a good correlation with the texture of the samples. The diffusion coefficients for HTO are the largest (D e  = 5.4–8.8 × 10 −12  m 2  s −1 ), followed by those for 36 Cl − (D e  = 0.7–1.9 × 10 −12  m 2  s −1 ), and finally 35 SO 4 2− (D e  = 0.2–0.6 × 10 −12  m 2  s −1 ). 36 Cl − was partially excluded from the total porosity resulting in an accessible porosity smaller than the total porosity (e Cl  = 0.041–0.064). 35 SO 4 2− , on the other hand, showed interaction with OPA resulting in a capacity factor (α) larger than the total porosity (e tot  = 0.13–0.16). Using extended Archie's law the accessible porosity for 35 SO 4 2− was estimated between 0.013 and 0.030. This enabled to evaluate the sorption coefficient of 35 SO 4 2− from the measured capacity factor, resulting in values of K d between 6 × 10 −5 and 9 × 10 −5  m 3  kg −1 .