Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones (Meuse/Haute–Marne, France)

Abstract

Abstract Diffusive parameters of tritiated water (HTO) and several anionic tracers (36Cl−, 125I−, 35 SO 4 2 - and 75 SeO 3 2 - ) were determined in the Callovo-Oxfordian argillite formation (Meuse/Haute–Marne, France), and also in the Oxfordian limestone formation, located just above. Twenty two drillcore samples, from depths of 150–480 m and from almost each lithofacies along the same borehole EST205, were tested. Three distinct and complementary experimental set-ups were used (batch, column filled with crushed argillite and through-diffusion experiments) associated with a thorough characterisation of the rock, care being taken to maintain redox conditions. Tritiated water behaviour is similar to water which diffuses in the total porosity of the rock. Tritiated water and anionic tracers displayed analogous diffusive behaviour in calcareous samples with ea(anion)/ea(HTO) = 1 and De(anion)/De(HTO) close to 0.85, i.e. the ratio of their diffusion coefficients in free water. Lower and scattered values were observed in the lower part of the Oxfordian limestones which is characterized by the presence of mineralogical heterogeneities and dolomitic diagenesis. In the Callovo-Oxfordian argillite formation, diffusive fluxes of 36Cl−, 125I− and 35 SO 4 2 - are similar but lower than that of HTO. The diffusive behaviour analogy between I− and Cl− was verified, as De(halide)/De(HTO) and ea(halide)/ea(HTO) ratios were always lower than 1. This reduction in diffusivity is a consequence of the anionic exclusion, which limits the diffusion-accessible porosity to a value lower than the total porosity for anions. A weak sorption of I− and SO 4 2 - on argillite samples was always observed whatever the technique used. In the case of I−, this sorption appears to be partly reversible and kinetically controlled. Very low values of diffusive parameters were obtained for 75 SeO 3 2 - and are still not well explained. Last, diffusive parameters of each radioactive tracer tested can be plotted according to Archie’s law, which therefore allows an estimation of De(non-reactive species) from the knowledge of both the sample total porosity and the corresponding exponent m = 2 for argillite samples.