Interpretation of some in-situ tracer experiments in fractured crystalline rock at äspö hard rock laboratory

Abstract

Some recent, converging flow, tracer tests at the Äspö Hard rock laboratory in Sweden using non-sorbing as well as sorbing tracers in fractured crystalline rocks were analysed. The hydraulic and transport properties of the fracture networks were interpreted from hydraulic tests and borehole observations. A network model for flow was devised based on these observations. Laboratory data on diffusion and sorption properties were used to predict the Residence Time Distribution, RTD, of the sorbing tracers. Breakthrough curves for three strongly sorbing tracers were predicted using only laboratory data, the transmissivity data from borehole tests and the flow rate of the collected water. For the 5 m distance tests predictions were surprisingly accurate considering that no adjustments of any parameter values have been made. For the 14–33 m distance tests the pure predictions were not as good but the model is deemed to capture the main interaction mechanisms. The results show that what at first seemed to be prominent rather isolated fractures belong to a complex network of intersecting fractures that strongly influence the flow distribution in the rock.