The DI-B in situ diffusion experiment at Mont Terri: Results and modeling

Abstract

[Abstract:] The diffusion experiment “DI-B” aimed at studying the transport and retention properties of selected non radioactive tracers in the Opalinus Clay (shaly facies) at the Mont Terri URL. Selected tracers were: 2H (water), I− (anionic species), 6Li+ (non- or weakly-sorbing cation) and 87Rb+ (strongly-sorbing cation). The experiment was performed as a single point dilution test by injecting tracers into a packed-off section at the bottom of a vertical borehole. The experiment, which lasted slightly over one year, was designed so that the length of the injection interval (0.6 m) was larger than tracer transport distances. A rock overcoring around the injection interval after the experiment allowed the measurement of tracer distribution profiles. Two and three-dimensional modeling has been performed using the CRUNCH and CORE3D reactive transport codes, respectively. The experimental setup favored, in principle, a 2D interpretation of the results, since diffusion occurs mainly along bedding planes. Conservative tracers from 2D modeling gave: De|| = 5.0 × 10−11 m2/s, φ = 0.16 (HDO) and De|| = 1.2 × 10−11 m2/s, φ = 0.09 (I−). Results from 3D modeling are: De|| = 4.0 × 10−11 m2/s, De||/De⊥ = 4, φ = 0.15 (HDO) and De|| = 8.3 × 10−12 m2/s, De||/De⊥ = 4, φ = 0.08 (I−). Differences in the results may be due to the two geometries (3D / 2D), to various aspects of the calculations or to comparison methods between model and experimental data. A previous sensitivity analysis performed with the 3D model had shown that the effect of considering only diffusion along the bedding planes (De||/De⊥ = 100) was relatively minor, causing only a significant effect on tracer distribution in the rock near the top and bottom edges of the injection interval. The differences between the two sets of results are small and confirm the smaller effective diffusion coefficient and accessible porosity of I− with respect to HDO. The results corresponding to 6Li+ and 87Rb+ have also been modeled using both 2D and 3D approaches. For 6Li+ (C0 = 3 × 10−5 mol/l), the best fit for both models was achieved with De|| ≈ 7 × 10−11 m2/s, Kd ≈ 0.2 l/kg and φ = 0.15–0.16. The De and Kd values are very similar to those obtained previously for Na+ at Mont Terri. This Kd value is certainly larger than what was initially expected. Regarding 87Rb+ (C0 = 3 × 10−7 mol/l), only three profiles were finally measured, and they showed significant variability. This variability has also been observed for other strongly sorbing tracers (Cs+, Co2+) in the DI-A and DI-A2 in situ experiments at Mont Terri. It has not been possible to obtain unique values for De|| and Kd for 87Rb+, although strong sorption has been confirmed.