Advection in granitic rock matrix: Modeling a radionuclide tracer transport experiment at the Grimsel Test Site.

Within the GTS-LTD project (Grimsel Test Site – Long-Term Diffusion), a field radionuclide tracer transport experiment in unfractured granitic rock was performed. Grimsel groundwater containing several tracers (³H as HTO, ³⁶Cl^-, ²²Na⁺, ¹³⁴Cs⁺, ¹³³Ba²⁺) was continuously circulated through a packed-off borehole and the decrease in tracer concentrations in the liquid phase was monitored for a period of 1266 days (05/03/2014 – 22/08/2017). Additionally, tracer breakthrough was monitored in an observation borehole a few cm away.

Initial modeling of the experiment (1D radial) showed that the evolution of tracer concentrations seemed to depart from the expected trend after some time, with concentrations in the injection borehole decreasing faster than expected from pure diffusive transport. 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 could not fully explain the measurements in the observation borehole.

Rock samples around the experimental section were collected right 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 is affected by both diffusion and advection through the granitic rock matrix. Also, in situ accessible porosities (about 0.0014) are smaller than those measured in rock samples (about 0.009), pointing to the destressing of the rock samples after drilling.