Modeling of a radionuclide transport experiment in granitic rock matrix at the Grimsel Test Site (Switzerland). The role of advection.

Within the framework of the GTS-LTD project (Grimsel Test Site - Long -Term Diffusion), a radionuclide transport experiment in unfractured granitic rock matrix was performed. Grimsel groundwater containing several radionuclide tracers (3H as HTO, 36Cl-, 22Na+, 134Cs+, 133Ba2+) was continuously circulated through a packed -off borehole interval. The decrease in tracer concentrations in the solution was monitored for a period of 1266 days (March 05, 2014-August 22, 2017). Additionally, tracer breakthrough was monitored in an observation borehole at a distance of 18.6 cm. Initial modeling of the experiment (1D radial), considering transport only by diffusion, showed that the evolution of tracer concentrations departed from the expected trend after some time, with concentrations in the injection borehole decreasing faster than expected. Additional 2D calculations (section normal to the boreholes) were performed to check the possible effect of advection through the rock matrix. Advection could explain the evolution of concentrations in the injection borehole, but concentrations in the observation borehole were overestimated. Core samples from new boreholes were collected immediately after the end of the experiment, allowing the measurement of tracer distributions in the rock. The observed patterns for the non -sorbing tracers (HTO, 36Cl-) showed clear preferential transport directions, consistent with advective flow towards the gallery from which the boreholes were drilled. Final 3D modeling of the experiment can explain the measured concentrations in the boreholes and in the rock. Tracer transport for the conservative tracers (HTO, 36Cl-) is affected by both diffusion and advection through the granitic rock matrix. Also, in situ accessible porosities deduced from the modeling (0.0014) are smaller than those measured in rock samples (about 0.009), pointing to unloading and destressing of the rock samples after drilling. At the spatial and temporal scales of the experiment, the effect of advection for the weakly sorbing 22Na+ is only minor, and it is practically negligible for the strongly sorbing tracers (134Cs+, 133Ba2+).