Glaus, M. A., Van Loon, L. R., and Wüst, R. A.J. (2024) Diffusion of HTO, 36 Cl and 22 Na in the Mesozoic rocks of northern Switzerland. II: Data interpretation in terms of an electrical double layer model. Applied Geochemistry, 162. 105842.

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Abstract

It is widely recognised that diffusive transport of charged species in charged clay media must be consistently described for ions with different charge signs and numbers, as well as consistently with pore diffusion schemes for inert solutes that do not undergo interactions with the clay surface. This study presents a modelling approach that meets these requirements for the diffusion of tritiated water, 22Na+ and 36Cl− tracer in heterogeneous Mesozoic sediment sequences of multiple deep-boreholes in northern Switzerland. The model uses an electrical double layer (EDL) approach and a discrete Donnan potential description of cation enrichment and concomitant anion depletion in the Donnan layer close to the basal (planar) clay surfaces to predict effective diffusion coefficients and capacity factors of the charged tracers. It incorporates diffusion data of tritiated water to determine the parameters related to the functional relationships between the accessible porosity and geometry factors with the total clay content as the main input variable. It also calculates concentration enrichment or depletion factors for mobile cationic and anionic species using relationships with the total clay content, such as the average density of surface sites or the thickness of the Donnan layer and the related volume fraction of free pore water. These factors are subsequently used to derive effective diffusion coefficients that include surface diffusion of cations and anion exclusion effects and the related capacity factors. Despite the various lithologies of the rock samples, where significant variability between carbonates and siliciclastic contents are observed, the model shows good agreement with experimental data. Discrepancies between experimental and modelled data are mainly due to inadequacies in the geometric description of diffusion pathways, rather than by potential bias in the calculation of concentration enhancement or depletion factors. However, the model approach assumes that diffusion in clay rock occurs via the pore space of clay minerals and treats all related parameters as simple average values, neglecting heterogeneity of mineral constituents and the unknown statistical distribution of the parameter values. The model has identified the chemical composition of the equilibrium pore water as a significant influencing variable for prediction concentration enhancement or depletion factors and related effective diffusion coefficients. Experimental data obtained for different synthetic pore water compositions support this finding. The robustness of the model predictions across various types of lithologies and formations, all of which exhibit diffusion as the dominant transport mode, is crucial for the generic application of the EDL diffusion model to predict diffusion parameters for elements lacking experimental data.

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