Anisotropic diffusion at the field scale in a 4-year multi-tracer diffusion and retention experiment – I: Insights from the experimental data

摘要

Claystones are consideredworldwide as barriermaterials for nuclearwaste repositories. In theMontTerri underground research laboratory (URL), a nearly 4-year diffusion and retention (DR) experiment has been performed in Opalinus Clay. It aimed at (1) obtaining data at larger space and time scales than inlaboratory experiments and (2) under relevant in situ conditionswithrespect to pore water chemistry andmechanical stress, (3) quantifying the anisotropy of in situ diffusion, and (4) exploring possible effects of a borehole-disturbed zone. The experiment included two tracer injection intervals in a borehole perpendicular to bedding, through which traced artificial pore water (APW) was circulated, and a pressure monitoring interval. The APW was spiked with neutral tracers (HTO,HDO,H_2O-18), anions (Br, I, SeO_4), and cations (Na-22,Ba-133, Sr-85, Cs-137,Co-60,Eu-152, stable Cs, and stable Eu).Most tracerswere added at the beginning, somewere added at a later stage. The hydraulic pressure in the injection intervalswas adjusted according to the measured value in the pressure monitoring interval to ensure transport by diffusion only. Concentration time-series in the APWwithin the borehole intervals were obtained, as well as 2Dconcentration distributions in the rock at the end of the experiment after overcoring and subsampling which resulted in ~250 samples and ~1300 analyses. As expected, HTO diffused the furthest into the rock, followed by the anions (Br, I, SeO_4) and by the cationic sorbing tracers (Na-22, Ba-133,Cs, Cs-137, Co-60, Eu-152). The diffusion of SeO_4 was slower than that of Br or I, approximately proportional to the ratio of their diffusion coefficients in water. Ba-133 diffused only into ~0.1 m during the ~4 a. Stable Cs, added at a higher concentration than Cs-137, diffused further into the rock thanCs-137, consistent with a non-linear sorption behavior. The rock properties (e.g.,water contents) were rather homogeneous at the centimeter scale, with no evidence of a borehole-disturbed zone. In situ anisotropy ratios for diffusion, derived for the first time directly fromfield data, are larger forHTOandNa-22 (~5) than for anions (~3–4 for Br and I). The lower ionic strength of the pore water at this location (~0.22 M) as compared to locations of earlier experiments in the Mont Terri URL (~0.39M) had no notable effect on the anion accessible pore fraction for Cl, Br, and I: the value of 0.55 is within the range of earlier data. Detailed transport simulations involving different codes will be presented in a companion paper.