A Field and Numerical Study of Heat and Solute Transport in a Discrete Fracture
Two tracer experiments were conducted in a highly-transmissive discrete rock fracture to explore the transport properties of solute and heat, and the influence of fracture heterogeneities. The experiments were conducted between two boreholes located approximately 10 m apart in a granitic gneiss. The discrete fracture used for the experiments was identified from prior pulse interference testing and geophysical logging. Test sections were minimized in both boreholes to isolate the discrete feature. Heat and solute tracers were applied simultaneously in the same divergent flow field developed by injecting water (6-10 Lpm). Both solute and thermal breakthrough were observed with peak arrival within ½ hour for solute and within two hours for heat. Based on the analytical analysis of pulse interference tests that were conducted prior to the experiments (in the same test section), hydraulic conditions measured during the experiments, and the results of the solute breakthroughs, a complicated fracture geometry with channelized flow was suggested. Numerical analysis conducted with uniform aperture and a radial flow field did not fit either of the tracers, so variable aperture configurations were investigated. Channelized flow fields with variable aperture provided reasonable fits for both solute and heat breakthrough curves. The tracers, however, were fit using different fracture geometry. The best fit for the heat tracer was achieved using a 0.2 m-wide channel with a 4.0 mm aperture and an unrealistic matrix thermal conductivity of 28 W/mK. The best fit for the solute tracer was achieved using a 5 m-wide channel, 1.4 mm aperture and a longitudinal dispersivity of 1 m. Both fits were achieved with similar hydraulic head rises to those measured during the field experiments. The substantial difference in flow geometry, and the need to use an unrealistic thermal conductivity is not easily attributed to any known feature of the flow system.