Modeling of Nonequilibrium Bromide Transport through Alluvial Gravel Vadose Zones

Abstract

Understanding solute transport and flux through the vadose zone is important for predicting potential contaminant loading to groundwater systems. Two dual‐permeability models: mixing cell (MC) and HYDRUS 1D (H1DDP) were used to simulate long‐term nonequilibrium Br− leaching using data derived from suction cup samples at two field sites (sampled to 7‐ and 4‐m depths) on the Canterbury Plains, New Zealand. Effective model parameters were derived by inverse methods to represent average transport processes through the heterogeneous profiles. Suction cup samples indicated rapid initial movement of solute through the profiles followed by a long tail. The “management type” MC model results were comparable to the more complex HYDRUS model, providing similar overall fits to the observed data (similar RMSE ∼ 0.05–0.1). Modeling indicated that macropore flux was significant in transporting solute through the profile (MC range 5–59%; H1DDP range 15–54% of total fluxes) and simulations of cumulative fluxes from both models were similar. The MC model results indicated macropore transporting water contents of ∼0.003 (v/v) and matrix domain transporting water contents of around 0.1 to 0.15 (v/v) for the two field sites. These estimates suggest that only 3 mm of bulk drainage is required to transport solutes 1 m through the macropore domain, whereas approximately 100 to 150 mm of drainage is required to transport solutes the same distance through the matrix domain. More accurate representation of boundary conditions, texture spatial distributions, and hydraulic interactions is important for obtaining a better understanding of flux dynamics in future studies.