Masahiro Munakata

Validation studies of tracer tests in a fracture zone at the Finnsjön research area

Abstract Tracer experiments were performed in a fracture zone, extending several hundred metres, in crystalline rock in Sweden. This paper describes modellings of tracer experiments (radially converging and dipole test) and their numerical results. We have applied a variable aperture channeling model to both tracer tests and evaluated steady-state channel flows in the fracture zone. Solute transport in the channel flows was simulated by a particle-tracking technique considering matrix diffusion. Calculated breakthrough curves and pressures were compared with experimental ones. The calculated breakthrough curve obtained by an equivalen porous medium model was also compared with data from the dipole experiment. Our models seem to explain the experimental results well, but some important assumptions are necessary for calibration of the breakthrough curves. Further experimental data related to the assumptions and geostatistics would be needed for the full validation of the flow and transport model. Study shows that the mean apertures of fractures calibrated with the tracer tests increase with increasing flow rates.

Mathematical Modeling of Groundwater Flow and Radionuclide Transport in Heterogeneous Aquifer

Atomic Energy of Canada Limited carried out a series of tracer tests in the Twin Lake site, to study geologic heterogeneity on the aquifer dispersion properties. Moltyaner et al. [6] analyzed the heterogeneity in statistical terms as variability in the hydraulic conductivity. Their statistical results were used here to develop a geostatistical approach based on the correlation between observed hydraulic conductivity values. The geostatistical model assumes that the correlation strength of the values of hydraulic conductivities between any two locations depends on the distances between these locations, and is expressed as an exponential function. Numerically, a matrix decomposition method is used. The groundwater flow field is calculated by three-dimensional finite element method using the realized spatial distribution of hydraulic conductivity that is calculated by the geostatistical model. The flow simulation results are used as input into the transport computer code, which in turn is used to simulate the tracer breakthrough curves at different observation wells. A random walk method is used for the radionuclide transport simulation. The simulated tracer plumes of tracer tests explain favorably the experimental tracer plumes. The simulations showed that the correlation length of the geostatistical model is a key parameter that characterizes the heterogeneous flow field, and the value of 5m in the flow direction and 0.5m in the direction perpendicular to the flow is obtained through the geostatistical analysis. The heterogeneous flow field in this aquifer is adequately characterized by the statistical spatial distribution of hydraulic conductivity based on the geostatistical model.