SOLUTE TRANSPORT IN UNSATURATED SOIL - EXPERIMENTAL-DESIGN, PARAMETER-ESTIMATION, AND MODEL DISCRIMINATION

The objectives of this study were to: (i) examine the efficacy of two sampling techniques for characterizing solute transport under steady-s tate water how, (ii) study the variation in transport model parameters with increasing depth of solute leaching, and (iii) perform model dis crimination to examine the transport process operative within a field plot. Bromide, NO3-, and Cl- were applied sequentially to a plot instr umented with two sets of 12 solution samplers located at depths of 0.2 5 and 0.65 m. At the conclusion of the experiment we destructively sam pled the entire 2.0 by 2.0 m plot to a depth of 2.0 m. Mass recovery b y the solution samplers ranged from 63 to 83% for the three tracers, a nd recovery by soil excavation ranged from 96 to 105%. The mean solute velocity estimated with the solution sampler data was significantly l ess than that determined by soil excavation. Mean solute velocity dete rmined from soil excavation implied an effective transport volume equa l to 0.82 theta(v) (where theta(v) is volumetric water content) for th e three tracers. Solution samplers and soil excavation provided simila r measures of vertical dispersion. Both sampling methods revealed a sc ale-dependent dispersion process in which the dispersivity increased l inearly with mean residence time. The depth profiles for all three sol utes were accurately described with a stochastic convective lognormal transfer function model (CLT) using the applied mass and two constant parameters (estimated from simultaneous fitting to the depth profiles) .