The Channel Network Model and Field Applications
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ABSTRACT The Channel Network model describes the fluid flow and solute transport in fractured media. The model is based on field observations, which indicate that flow and transport take place in a threedimensional network of connected channels. The channels are generated in the model from observed stochastic distributions and solute transport is modelled taking into account advection and rock interactions, such as matrix diffusion and sorption within the rock. The most important site-specific data for the Channel Network model are the conductance distribution of the channels and the flow-wetted surface. The latter is the surface area of the rock in contact with the flowing water. These parameters may be estimated from hydraulic measurements. For the Aspd site, several borehole data sets are available, where a packer distance of 3 metres was used. Numerical experiments were performed in order to study the uncertainties in the determination of the flowwetted surface and conductance distribution. Synthetic data were generated along a borehole and hydraulic tests with different packer distances were simulated. The model has previously been used to study the Long-term Pumping and Tracer Test (LPT2) carried out in the Aspb Hard Rock Laboratory (HRL) in Sweden, where the distance travelled by the tracers was of the order hundreds of metres. Recently, the model has been used to simulate the tracer tests performed in the TRUE experiment at HRL, with travel distance of the order of tens of metres. Several tracer tests with non-sorbing and sorbing species have been performed.
INTRODUCTION Deep repositories in crystalline rocks are a possible solution to the disposal of hazardous waste, including nuclear waste. The performance assessment of these repositories requires use of models that calculate the transport of contaminants through the geosphere including the interaction with the rock matrix, i.e., advection, diffusion and sorption. Field observations show that there are strong channelling effects and that when attempts are made to evaluate the dispersion coefficient, it appears to increase with distance. Fluid flow and solute transport in fractured rock occur mainly through fractures and fracture zones. Due to the heterogeneities on the fracture surfaces the fluid flow occurs mainly through preferential paths, channelling. From these channels the solutes may migrate into the stagnant water in the rock matrix surrounding the channels. Sorbing solutes may also be sorbed on the micropore surfaces. These two mechanisms may significantly retard the transport of solutes in fractured media. On the other hand, channelling may cause solutes in such paths to travel considerably faster than the average velocity. 721 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
The Channel Network model [I] has been used to carried out a performance assessment of a repository for final disposal of radioactive waste [2]. The model has previously been applied to the study of water flow in the SCV Stripa experiment [3] and to a
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