COMPARISON OF SINGLE- AND DUAL-DOMAIN SUBSURFACE TRANSPORT MODELS IN AN ATLANTIC COASTAL PLAIN SETTING

Sub-grid modeling of some type is typically used to account for heterogeneity at scales below the grid scale. The single-domain model (SDM), employing field-scale dispersion, and the dual-domain model (DDM), employing local hydrodynamic dispersion and exchange between domains having large hydraulic conductivity contrasts, are well-known examples. In this work, the two modeling approaches are applied to tritium migration from the H-area seepage basins to a nearby stream (Fourmile Branch) at the Savannah River Site. This location has been monitored since 1955, so an extensive data set exists for formulating realistic simulations and comparing the results to field-scale data. Both models were calibrated to produce the best fit to recorded tritium data. When all attributes of the data set were considered, including cumulative tritium activity discharged to Fourmile Branch, plume arrival time, and plume attenuation due to closure of the seepage basins in 1988, the DDM produced break-through results superior to the SDM, while causing no unrealistic up-gradient dispersion. A sensitivity analysis showed that only the DDM was able to accurately produce both the instantaneous activity discharge and cumulative activity with a single parameter set. This is thought to be due to the advection-dominated nature of transport in natural porous media and the more realistic treatment of this type of transport in the DDM relative to the SDM. It is concluded that the main parameters of both models are scale-dependent, and methods are discussed for making initial estimates of the DDM parameters, which include mobile/immobile porosities and the mass exchange coefficient.