A TRACER MIGRATION EXPERIMENT IN A SMALL FRACTURE-ZONE IN GRANITE

The water flow mte and tracer distribution has been studied in a fracture zone and averagely fractured rock in the Stripa underground research laboratory. The experimental site is located in granitic rock at a depth of 385 m below the ground surface. A 50-m-long drift with a diameter of 3 m was excavated. The drift was intersected by a 6-m-wide fracture zone. The upper part of the drift was covered by 150 plastic sheets in which water was collected. The water in the lower part of the drift was collected in sump holes. Different tracers were injected in seven locations at distances between 9.5 and 25 m from the drift. More than 50% of the water was found in one sampling area in the drift, and more than 90% in only eight sampling areas. The recovery of the tracers was also concentrated in a few sheets. Most of the flow preferentially takes place in a few paths. Two sets of tracers were used in tandem; one set of dyes which had previously been used in Stripa and one set of lanthanide-DTPA complexes. Although shown to be nonsorbing in long-time high-sensitivity laboratory tests, the dyes were found to have a somewhat smaller recovery than the metal tracers. This observation was used in an attempt to estimate the flow-wetted surface into which the dyes can diffuse and sorb. The flow porosity was estimated from the tracer residence times for all injection points. The flow in the intersected fracture zone is not like that in a porous medium; it is highly channeled.