Intraval-project test case 5: tracer tests at Finnsjoen

A concept of varying aperture channels in a fracture plane has been worked through and applied to the Intraval project case 5 to predict tracer transport in a radially converging flow geometry. The aim of the tracer tests is to investigate the transport properties in a highly conductive rock over distances up to about 400 m. In the radially converging experiment water is pumped from one borehole and tracers are injected into three different isolated sections of a fracture zone, into the so called subzones. The subzones appear as highest local transmissivities in the three injection boreholes existing at distances of 123 m, 168 m and 179 m from the pumping hole. Mean flow times have been calculated using the reported hydrological data and the conceptual model described in this report. The tracer transport has been modelled applying a concept of linear flow velocity field across the channel width. Molecular diffusion combined to the advection in the velocity field has been taken into account in the transport calculations. The break-through as mean flux out of the whole fracture cross-section has been obtained as a function of time. A dilution factor has been evaluated from the geometrical and hydrological data and absolute break-through curves have been predicted. The real time dependence and concentrations for all of the injected tracers were not known at the time of writing of this report. The planned injection period of about nine weeks, 1500 hours, was used for most of the predictions and the injected concentrations were assumed to be 1000 ppm. For two injection sections the actual injection concentrations were known and taken into account. Calculations have been done also for short injection periods. An injection period of 1.5 h has been used in the calculations. The dilution factor has not been taken into account for these short duration injections. No predetermined injection concentrations have been assumed and the results correspond thus the break-through just of the considered channel for an injected concentration of unity