where R, the chemical retardation factor, is defined by
R = 1 + pKd/. [4-4]
where p is the soil bulk density (g/cm3) and 0 is the volumetric water
content (mL/mL). Eq. 4-3 can be rearranged to include the
dimensionless parameters:
T = vo t/L [4-5]
x = z/L [4-6]
P = vo L/D and [4-7]
C = C/C, [4-8]
where vo, t, z, and D have been previously defined and L is the column
length (cm), P is the Peclet number, T is the pore volumes of solution,
x is dimensionless distance, and C the ratio of effluent concentration
(Cb) to influent concentration (Co) to give the convective-dispersive
(CD) model,
R(aC/aT) = (1/P)(a2c/ax2) ac/ax [4-9]
The CD water-flow model has been used satisfactorily to simulate
nonadsorbed solute transport under laboratory and field conditions
(Nielsen and Biggar, 1961; Warrick et al., 1971). However, the model
has been relatively poor at simulating solute transport through well-
aggregated and structured soils (Green et al., 1972; Rao et al., 1974;
van Genuchten and Wierenga, 1976 and 1977).
Solutions of Eq. [4-1] predict nearly sigmoidal or symmetrical
concentration distributions (Coats and Smith, 1964; Gershon and Nir,
1969; van Genuchten and Wierenga, 1976). However, numerous
experimental studies have shown distinctly asymmetrical effluent curves
(Nielsen and Biggar, 1961; Biggar and Nielsen, 1962; Green et al.,