Mobile, Immobile, and Multiflow Domain Approaches - Transport and Fate of Chemicals in Soils: Principles and Applications - page 292

contribution of large pores to preferential flow. Jarvis, Bergstrom, and Dik
(1991) applied the dual porosity model to a field study of water-unsaturated
and transient flow conditions. They found that a two-flow domain model
provided improved description of chloride movement compared with a
one-flow domain model. Gerke and van Genuchten (1993) also applied a
dual-porosity model to simulate the preferential movement of water in struc-
tured porous media. Othmer, Diekkruger, and Kutilek (1991) found that the
hydraulic conductivity [ K (ψ)] derived from a bimodal pore size distribution
was in agreement with the field-measured K (ψ). Here the term ψ refers to the
water suction (cm). Othmer, Diekkruger, and Kutilek (1991) concluded that
the bimodal porosity model represented the reality of soil system better than
a unimodal porosity model. Similar results were reported by Smettem and
Kirkby (1990) in an aggregate soil. Wilson, Jardine, and Gwo (1992) presented
a three-flow domain model to describe the K (ψ) - ψ or K (Θ) - Θ relationship
in different soil horizons.
Delineation of a flow domain in a multidomain approach has been at best
vague and somewhat arbitrary. Commonly used concepts rely on the defini-
tion of macropores and micropores, which are also arbitrary. The only known
experimental method for estimating two porosities is based on a measure of
the soil water content at an arbitrarily chosen water tension (ψ). From the
shape of the soil-moisture characteristic curve, Smettem and Kirkby (1990)
chose ψ of 14 cm as the matching point between the interaggregate (macro-)
and the intraaggregate (micro-) porosity. Jarvis, Bergstrom, and Dik (1991)
estimated macroporosity based on measured specific yield under a water
tension of 100 cm. Other water tensions used to differentiate macropores
from micropores include 10 cm by Wilson, Jardine, and Gwo (1992), 20 cm
by Selim, Schulin, and Flühler (1987), and 80 cm by Nkedi-Kizza et al. (1982).
Field and laboratory observations that support the two-flow domain con-
cept are the bimodal peaks for nonreactive solutes when steady soil water
flow was dominant. Hamlen and Kachanoski (1992) observed bimodal chlo-
ride breakthrough in the B horizon of a sandy soil. However, the bimodal
peaks were not observed in the A horizon. Skopp, Gardner, and Tyler (1981)
suspected that solute bimodal peaks were caused by the presence of dual
porosities in the soil.
The examples presented here to illustrate physical two-flow domains or
dual porosity are those of the work of Ma and Selim (1995), who investi-
gated transport of the nonreactive tracer tritium in packed columns. Ma
and Selim (1995) carried out a series of experiments to examine the effect
of water flux on the shape of the BTCs. Figure 8.21 shows BTCs for three
Mahan soil columns having flow velocities of 2.02, 3.82, and 5.28 cm/h,
respectively. These BTCs are for tritium pulse applications of approxi-
mately one pore volume. From Figure 8.21, tailing of the desorption
side was observed for all three BTCs. A hump was also observed on the