Comparison betweenin situand laboratory diffusion studies of HTO and halides in Opalinus Clay from the Mont Terri

Abstract

A long-term single-borehole diffusion experiment (DI) using tritiated water (HTO) and stable iodide ( 127 I − ) was carried out In the Opalinus Clay of the Mont Terri Underground Rock Laboratory (URL). Diffusion coefficients D L and accessible porosity for HTO, 36 Cl − and 125 I − were also measured on centimetric Opalinus clay samples using the through diffusion technique. The evolution of tritium and iodide concentration in the injection system over time and in situ profiles were interpreted with a 3-D numerical simulation. A detailed analysis of the results pointed out the effect of a disturbed zone around the borehole with higher diffusion coefficients. The best estimate values for HTO and iodide in the undisturbed rock are D L = 5 × 10 −11 m 2 /s and D L = 1.5 × 10 −11 m 2 /s respectively. For the laboratory tests, D L values for HTO are in the range of 5 × 10 −11 m 2 /s to 8.5 × 10 −11 m 2 /s. For 125 I − and 36 Cl − the measured values are D L = 1.4 × 10 −11 and D L = 1.6 × 10 −11 m 2 /s respectively.All HTO results obtained with a through diffusion technique are within the same range as those obtained in the in situ tests. The D L values obtained in diffusion cells with 125 I − and 36 Cl − and the value drawn from the interpretation of stable 127 I − concentration profiles from the in situ tests are very close. In fact, some significant uncertainties could be identified (i.e. a likely chemical retention of iodide on argillites, effect of the disturbed zone).

Exploring diffusion and sorption processes at the Mont Terri rock laboratory (Switzerland): lessons learned from 20 years of field research

“…This difference is fully explained by the smaller concentration gradient due to the higher background concentration of Brin the pore-water. Note also that, for DI, the derived diffusivities and porosities show a large uncertainty, which is related to the BdZ created during borehole construction and (2014) installation (Tevissen et al 2004). The smaller effective diffusion coefficients and diffusion-accessible porosities of anions compared to water tracers and cations is explained by repulsion of anions from the negatively charged clay surfaces, which leads to a deficit of anions relative to cations in the electric double layers (EDLs).…”

“…Conceptually, the models for the earlier experiments considered single tracer diffusion only (e.g. Palut et al 2003;Tevissen et al 2004;Van Loon et al 2004b;Yllera et al 2004;Wersin et al 2004;Soler et al 2008;Wersin et al 2008). That is, transport of a tracer (neutral, anion and/or cation) was assumed to be independent of that of any other solute.…”

“…6 in Bossart et al 2017) for tritiated water HTO, I -and 22 Na ? (Palut et al 2003;Gómez-Hernández and Guardiola-Albert 2004;Tevissen et al 2004;Van Loon et al 2004b;Wersin et al 2008;Soler et al 2008). Most of the field experiments in Opalinus Clay used relatively long injection intervals (*1 m) and were thus dominated by diffusion parallel to the bedding plane of the clay formation.…”

In-situ diffusion of HTO, 22Na+, Cs+ and I- in Opalinus Clay at the Mont Terri underground rock laboratory

“…It is common practice to study diffusion on small-scale samples in the laboratory [4][5][6]. However, to what extent the results of such studies can be extrapolated to field situations is unclear, and therefore it is also important to study diffusion in-situ and to compare the results with lab diffusion data.…”

Exploring diffusion and sorption processes at the Mont Terri rock laboratory (Switzerland): lessons learned from 20 years of field research

“…This difference is fully explained by the smaller concentration gradient due to the higher background concentration of Br -in the pore-water. Note also that, for DI, the derived diffusivities and porosities show a large uncertainty, which is related to the BdZ created during borehole construction and (2014) installation (Tevissen et al 2004). The smaller effective diffusion coefficients and diffusion-accessible porosities of anions compared to water tracers and cations is explained by repulsion of anions from the negatively charged clay surfaces, which leads to a deficit of anions relative to cations in the electric double layers (EDLs).…”