The Grimsel Migration Experiment: field injection–withdrawal experiments in fractured rock with sorbing tracers

In the framework of the Migration Experiment at the Grimsel Test Site, Switzerland, breakthrough experiments with a conservative tracer, uranine, and sorbing cationic γ-emitting radionuclides (, , , and ) were carried out to test models of radionuclide migration in the field. An array of bore holes drilled from an experimental drift penetrated a transmissive, water-saturated shear zone in a granodiorite. The shear zone consists of a set of fractures filled with a fine-grained porous fault gouge. A two bore hole injection-withdrawal (`dipole') flow field was superimposed on the natural hydraulic gradient and two different linear flow distances, 1.7 and 4.9 m, were studied in detail. The concentrations of the radionuclides were, in general, much smaller than the natural background levels of the corresponding stable isotopes of Na, Sr, Rb, and Cs, in the groundwater of the shear zone. In-line tracer detection techniques included down-hole point fluorescence measurement and NaI scintillation counting of γ-emitters in the withdrawal flow line. Pulse-stimulus tracer injections resulted in monomodal breakthrough curves and recovery curves. The reduction of the maximum (peak) radionuclide activity (relative to the injected activity), peak retardation, and retardation of the time of 50% recovery were compared to those of uranine. A selectivity sequence of relative affinity of the studied cations for an exchange site was found to be Na (radionuclides exhibited little peak-height reduction relative to uranine and little or no retardation. In the longer flow field, Sr and Cs showed a considerable peak-height reduction and a significant retardation relative to uranine and Na. Increasing the flow distance affected the retardation of the radionuclides. The results were compared with both pre- and post-experiment model simulations and earlier model predictions.