Comparative modeling of an in situ diffusion experiment in granite at the Grimsel Test Site

Soler, Josep M.; Landa, Jiri; Havlova, Vaclava; Tachi, Yukio; Ebina, Takanori; Sardini, Paul; Siitari-Kauppi, Marja; Eikenberg, Jost; Martin, Andrew J.

doi:10.1016/j.jconhyd.2015.06.002

Comparative modeling of an in situ diffusion experiment in granite at the Grimsel Test Site

Soler JM, Landa J, Havlova V, Tachi Y, Ebina T, Sardini P, Siitari-Kauppi M, Eikenberg J & Martin AJ

Citation

Soler, J. M., Landa, J., Havlova, V., Tachi, Y., Ebina, T., Sardini, P., … Martin, A. J. (2015). Comparative modeling of an in situ diffusion experiment in granite at the Grimsel Test Site. Journal of Contaminant Hydrology, 179, 89-101. https://doi.org/10.1016/j.jconhyd.2015.06.002

Persistent URL https://www.dora.lib4ri.ch/psi/islandora/object/psi:50063

An in situ diffusion experiment was performed at the Grimsel Test Site (Switzerland). Several tracers (³H as HTO, ²²Na⁺, ¹³⁴Cs⁺, ¹³¹I⁻ with stable I⁻ as carrier) were continuously circulated through a packed-off borehole and the decrease in tracer concentrations in the liquid phase was monitored for a period of about 2 years. Subsequently, the borehole section was overcored and the tracer profiles in the rock analyzed (³H, ²²Na⁺, ¹³⁴Cs⁺). ³H and ²²Na⁺ showed a similar decrease in activity in the circulation system (slightly larger drop for ³H). The drop in activity for ¹³⁴Cs⁺ was much more pronounced. Transport distances in the rock were about 20 cm for ³H, 10 cm for ²²Na⁺, and 1 cm for ¹³⁴Cs⁺. The dataset (except for ¹³¹I⁻ because of complete decay at the end of the experiment) was analyzed with different diffusion-sorption models by different teams (IDAEA-CSIC, UJV-Rez, JAEA) using different codes, with the goal of obtaining effective diffusion coefficients (D_e) and porosity (ϕ) or rock capacity (α) values.
From the activity measurements in the rock, it was observed that it was not possible to recover the full tracer activity in the rock (no activity balance when adding the activities in the rock and in the fluid circulation system). A Borehole Disturbed Zone (BDZ) had to be taken into account to fit the experimental observations. The extension of the BDZ (1–2 mm) is about the same magnitude than the mean grain size of the quartz and feldspar grains. IDAEA-CSIC and UJV-Rez tried directly to match the results of the in situ experiment, without forcing any laboratory-based parameter values into the models. JAEA conducted a predictive modeling based on laboratory diffusion data and their scaling to in situ conditions. The results from the different codes have been compared, also with results from small-scale laboratory experiments. Outstanding issues to be resolved are the need for a very large capacity factor in the BDZ for ³H and the difference between apparent diffusion coefficients (D_a) from the in situ experiment and out-leaching laboratory tests.