Flow and tracer movement in some natural fractures in the Stripa granite. Results from the Stripa project Phase I

Tracer experiments have been performed in two natural fractures in the Stripa granite 360 m below the ground. The experiments have utilized the natural inflow to the drifts which are located in water saturated rock. The water flow was found to vary considerably along the fractures. Typically one collection section, out of 15, carried more than half the total flow. Non-sorbing tracers were injected at approximately 5 and 10 m distance from the drift. From the tracer breakthrough curves and from pressure drop data, the fracture openings, or rather the openings of the channels in the fractures, were calculated. A mixture of sorbing tracers was injected during more than 7 months in one of the fractures. None of these tracers arrived in measurable quantities at the collection points. About 0.5 m2 of the fracture was excavated and the surface was analysed for the sorbing tracers. In several points it was found that these tracers has migrated into the rock matrix up to several millimeters, confirming the matrix diffusion mechanisms which have been postulated and found in the laboratory