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, ura nine, and sorbing cationic gamma-emitting radionuclides (Na-22,24(+), Sr-85(2+), Rb-86(+), and Cs-137(+)) were carried out to test models of radionuclide migration in the field. An array of bore holes drilled f rom an experimental drift penetrated a transmissive, water-saturated s hear zone in a granodiorite. The shear zone consists of a set of fract ures filled with a fine-grained porous fault gouge. A two bore hole in jection-withdrawal ('dipole') now field was superimposed on the natura l 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 ground water of the shear zone. In-line tracer detection techniques included down-hole point fluorescence measurement and NaI scintillation countin g of gamma-emitters in the withdrawal flow line. Pulse-stimulus tracer injections resulted in monomodal breakthrough curves and recovery cur ves. The reduction of the maximum (peak) radionuclide activity (relati ve to the injected activity), peak retardation, and retardation of the time of 50% recovery were compared to those of uranine. A. selectivit y sequence of relative affinity of the studied cations for an exchange site was found to be Na (< Ca) < Sr < Rb < Cs. In the shorter flow fi eld, the 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 a ffected the retardation of the radionuclides. The results were compare d with both pre- and post-experiment model simulations and earlier mod el predictions. (C) 1998 Elsevier Science B.V. All rights reserved.