under similar pressure gradients. The preferentially rapid transport
of water in large pores is called channelling, and results in solutes
travelling further and more rapidly than simple piston-displacement
concepts would allow.
The parameter f represents the fraction of solutes present in the
mobile region under equilibrium conditions. The mobile-water fraction,
0, may be calculated with Eq. 4-21:
0 = O/J = OR f(R-1) [4-21]
where 0 is the mobile-water fraction, Om is the mobile-water content, 8
is the total-water content, R is the chemical-retardation factor, and f
is the fraction of total adsorption sites in the mobile-water region.
The parameter f is typically approximated. Nkedi-Kizza et al.(1982)
argued that, since the surface area associated with a unit volume of
water in the small pores of the immobile region is probably much
greater than the surface area associated with a unit volume of water in
the mobile region, f may be approximated to be zero. However, NKedi-
Kizza et al. (1983) have also proposed equal distribution of the sites
between the two regions such that f = p and, therefore, 0 = #.
Seyfried and Rao (1987) proposed an intermediate approximation of f =
0/2. In all approximations, the severity of any error in the eventual
estimation of the mobile-water content is influenced by the value of R.
If there is almost no chemical adsorption or repulsion (R approaches
1.0), then the location of the sites becomes less important because the
value OR f(R-1) approaches both f and the mobile-water fraction, 0.
The dimensionless parameter, w, relates the mass-transfer
coefficient [Eq. 4-16] to column length and solution flux (volume per