Modeling of a field diffusion experiment in granite. The GTS-LTD project at the Grimsel Test Sit
A first in situ diffusion experiment in non-fractured granite (monopole 1) was already performed at the Grimsel Test Site. 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 789 days (June 2007 – August 2009). Subsequently, the borehole section was overcored and the tracer profiles in the rock analyzed. From the modeling of the experiment it was evident that a Borehole Disturbed Zone (BDZ) had to be taken into account. HTO seemed to display large rock capacity values in the BDZ. Also, modeling of out-leaching experiments (3H, I-) using overcored rock samples from monopole 1 gave apparent diffusion coefficients one order of magnitude larger than those obtained from the modeling of the in situ experiment. Given these results, it was decided to perform a second experiment (monopole 2), which includes a second observation borehole close to the new injection borehole.
Tracer circulation in the new monopole 2 experiment started in March 5th 2014 and ended in August 22nd 2017 (1266 days). The selected tracers were 3H, 22Na+, 134Cs+, 133Ba2+ and 36Cl-. A first predictive modeling exercise (1D-radial model) was performed early in the experiment using transport and sorption parameters from monopole 1, together with laboratory results for 133Ba2+ and 36Cl-. Those predictive calculations were compared with initial monitoring data from the in situ experiment (activities in the circulation system of the injection borehole). No apparent effect from a possible Borehole Disturbed Zone (BDZ) was observed from the experimental data. The very initial drop in activities for HTO, 22Na+ and 36Cl- (non- and weakly-sorbing tracers) during the first day of tracer circulation were clearly due to initial mixing in the circulation system. The initial drop in activities for 134Cs+ and 133Ba2+ showed clearly the effect of sorption. Bulk rock parameters for 134Cs+ from monopole 1 seemed to be applicable to monopole 2. However, 133Ba2+ seemed to sorb more strongly than expected (or to diffuse faster into the rock).
The final measurements are now compared with results from the existing calculations and also from new calculations using parameters from laboratory experiments for HTO and 36Cl-. Additionally, 2D calculations have been performed to check the possible effect of advection through the rock matrix.