to differences in aggregate structure of the fine fraction than to
differences in gravel porosity.
The dimensionless parameters P and w are a function of
experimental conditions, and are used with their functional
relationships to determine soil-water properties. The Peclet number
relates the pore-water velocity and column length to the dispersion
coefficient (Eq. [4-14]). One theoretical exponential relationship of
the dispersion coefficient to the pore-water velocity is:
D = \ vmn [4-22]
where Dm is the hydrodynamic dispersion coefficient, X the
dispersivity, v, the mobile pore-water velocity, and n an empirical
constant. For most laboratory-displacement experiments involving
disturbed repackedd) soils, A is about 1.0 cm (van Genuchten and
Wierenga, 1986). For displacement experiments involving undisturbed
field soils, especially when aggregated, X can be one or two orders of
magnitude higher. The degree of dispersivity in a soil is increased as
the pore-size distribution in the mobile-water regions becomes broader.
The dispersivities of different soils are more easily compared if the
empirical constant, n, is assumed to be 1.0 and the equation is linear.
Schulin et al. (1987) and Russo (1983) determined dispersivities of
2.24 and 2.91 cm from soils containing 55 and 43% gravel by volume,
respectively, using a linear relationship. The Peclet numbers
estimated from the BTCs of Column I tended to increase with decreasing
flux, suggesting a nonlinear relation between dispersion coefficient
and pore-water velocity within the velocity range used in this study
(Fig. 4-9). The nonlinear plot provides a dispersivity of