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

The water flow mte and tracer distribution has been studied in a fract ure zone and averagely fractured rock in the Stripa underground resear ch 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 diam eter of 3 m was excavated. The drift was intersected by a 6-m-wide fra cture zone. The upper part of the drift was covered by 150 plastic she ets in which water was collected. The water in the lower part of the d rift was collected in sump holes. Different tracers were injected in s even 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 trace rs was also concentrated in a few sheets. Most of the flow preferentia lly takes place in a few paths. Two sets of tracers were used in tande m; one set of dyes which had previously been used in Stripa and one se t of lanthanide-DTPA complexes. Although shown to be nonsorbing in lon g-time high-sensitivity laboratory tests, the dyes were found to have a somewhat smaller recovery than the metal tracers. This observation w as used in an attempt to estimate the flow-wetted surface into which t he dyes can diffuse and sorb. The flow porosity was estimated from the tracer residence times for all injection points. The flow in the inte rsected fracture zone is not like that in a porous medium; it is highl y channeled.