Field scale transport of bromide in an unsaturated soil: 2. Dispersion modeling

Abstract

The solute concentrations measured in the field experiment of G. L. Butters et al. (this issue) are used to compare two models of vadose zone solute transport: the deterministic one‐dimensional convection‐dispersion model, which represents solute transport far from the source of solute entry, and the stochastic‐convective lognormal transfer function model, which represents solute transport near the source. The stochastic‐convective model provided an excellent representation of the spreading of the solute pulse to a depth of 3 m after calibration at 0.3 m. Conversely, the deterministic model dramatically underpredicted solute spreading beyond 0.3 m after calibration. An analysis of the area‐averaged solute concentration revealed a nearly linear scale effect in the dispersivity to a depth of at least 14.8 m. A change in the growth pattern of dispersion observed in the breakthrough curve at 4.5 m was attributed to a soil texture change near 3 m, which caused the apparent dispersivity of the pulse to decrease between 3.0 and 4.5 m, after which it increased significantly between 4.5 m and the final profile sampling between 0 and 25 m.