Dual‐domain model of advective‐diffusion transport with variable mass transfer rate for long-term contaminant transport simulation at radwaste injection site

Abstract

Since 1963, liquid radioactive waste has been injected into deep artesian aquifer waste disposal sites at the Siberian Chemical Combine. The injection and overlaying zones are sandy clay formations with complex internal architectures. Therefore, obtaining relevant forecasts of long-term waste migration largely depends on accounting for the heterogeneity of water-bearing sediments. At present, it is impossible to create regional models that consider detailed hydraulic heterogeneity (due to the insufficient provision parameters and limitations of computing capacities). Therefore, for long-term forecasts of the migration of contaminants in groundwater, the solution is to use a dual-domain model.To use a dual-domain model, one must set the mass transfer parameter. To estimate this parameter, the article proposes an approach using an auxiliary three-dimensional (3D) high-resolution model of hydraulic heterogeneity. This model was constructed using the transition probability/Markov chain geostatistical approach (TP/MC method) and detailed lithological analysis data of the sections of 295 boreholes in the Siberian Chemical Combine region, which were obtained by core studies and using geophysical methods for studying boreholes. An important feature of this model was that the thicknesses of the low-permeability hydrofacies have an exponential-like distribution with an average thickness of about 6 m. This thickness heterogeneity, as it appeared from the simulation results, essentially affects the overall subsurface contaminant spreading during long time. The effective mass transfer rate coefficient was obtained by processing the breakthrough curves of the flux concentrations by modeling horizontal migration on the auxiliary model, providing setting the highest weight coefficients in the “tail” part of the breakthrough curve. As a result of the selection, the effective mass transfer rate coefficient for the assessment of long-term subsurface waste migration for the conditions of the Siberian Chemical Combine was 6.4 × 10−8 day−1.