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assumption of first-order exchange kinetics. However, conceptually the
assumption is only valid for dead-end pores with a neck of negligible
volume (Coats and Smith, 1964). van Genuchten and Dalton (1986) have
shown that first-order kinetics represent a very close approximation in
the case of radial diffusive exchange between the soil matrix and
hollow, cylindrical macropores but, for other pore geometries, first-
order exchange is only a crude approximation. Because a is a lumped
parameter, it depends not only on the pore-space geometry, solute
diffusivity and the magnitude of the immobile region, but also on the
changing solute concentration within the two regions. The increase in
the mass-transfer coefficient with the increase in solution flux is due
to the rapidity at which the concentration gradient reaches its extreme
(more diffusive force) as a solute front approaches and leaves a given
point in a soil column.
Estimation of Column II Parameters
An additional enhancement to the validity of the estimated soil-
column parameters lies in their transferability to a different sample
of the same soil. Model-simulated BTC parameters derived from
experiments using Column I were applied to the experimental data from
Column II (Figs. 4-11 and 4-12). The values of R and # were set at
1.05 and 0.053, respectively, and the values for P and c were derived
from the line:- and curvilinear relationships presented in Figs. 4-9
and 4-10. The derivations included consideration of slight differences
in column lengths and bulk densities (Table 4-1). The simulated BTCs
based on the parameters derived from Column I exhibit later break