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United States Department of Agriculture

Agricultural Research Service

Title: Reactive Transport of Bromide in Two Mobile Regions with Linear Diffusive Exchange

item Schwartz, Robert

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: December 9, 1998
Publication Date: N/A

Technical Abstract: The exchange of solutes between macropores and soil matrix regions is an important consideration in describing solute transport in structured soils when the time scales of interest are short in comparison to diffusive time scales of the matrix region. In this study, the dual-porosity approach is used to describe the steady-state reactive transport of a bromide tracer through undisturbed soil columns over a range of pore water velocities and corresponding levels of soil water saturation. Theory and methodology are presented to estimate diffusive exchange and transport coefficients for soil columns at a given water flux. Nonlinear least-squares regression was used in conjunction with physical information derived from displacement experiments to obtain estimates of transport parameters. The estimation of pore water velocity and water content of each mobile region, using a pair of displacement experiments conducted at different pressure heads, was found to be a suitable procedure for ascribing solute flux to each region. The fit of the dual-porosity model to observed effluent concentrations resulted in lower residual standard deviations than the fit of the advective-dispersive equation. A principle difficulty of the application of the dual-porosity model to solute transport, for the soils used in this study, was related to the nonlinear behavior of the diffusive exchange term at early times after a step change in inlet concentration. Another problem was that fitted solutions predicted nearly all adsorption sites to be in equilibrium with solute in the macropore region rather than with solute in the matrix region. Despite these difficulties, the dual-porosity model led to differentiation of transport processes that corresponded to observed structural differences in soil horizons.

Last Modified: 4/24/2014
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