Simulation of Hydraulic Heterogeneity and Upscaling Permeability and Dispersivity in Sandy-Clay Formations

Since 1963, radioactive waste has been injected in deep artesian aquifers of Cretaceous terrigenous deposits in western Siberia. It is well known that geologic heterogeneity strongly affects contaminant transport in unconsolidated formations. To predict the long-term migration of this radioactive waste the effective hydraulic and macrodispersion parameters are estimated by using a three-dimensional high-resolution hydraulic heterogeneity model. The heterogeneous model of the injection area is developed by applying transition probability geostatistics. This model is used to simulate local steady-state groundwater flow and advective transport, leading to numerical estimates of effective hydraulic conductivity and macrodispersion parameters. Mean seepage velocities and effective longitudinal macrodispersion are calculated from observed breakthrough curves for a conservative tracer. Results show that mean horizontal lengths exceed vertical lengths by a factor of more than 30. As a result, vertical effective hydraulic conductivity is two orders of magnitude less than the horizontal effective conductivity. Observed breakthrough curves exhibit long tails and appear to be non-Fickian. Estimated effective longitudinal macrodispersivity in the vertical direction is one order of magnitude less than that in the horizontal direction. Under a Fickian framework, this implies that dispersion modeling for regional transport simulations requires an anisotropic-media dispersion model.