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.