p = density of bulk dry soil
n = porosity
S = adsorbed phase concentration of compound (M/M)
The terms on the right hand side of Equation (1-1) are
referred to as dispersive transport, convective transport,
and adsorption, respectively. In a linear adsorption the
distribution coefficient Kd= S/C, since aS/ at = Kd*(aC/at),
Equation (1-1) can be written as
aC 2C ac
R ---- = D -- --- (1-2)
St L 3x2 ax
P Kd
R = 1 + ------- (1-3)
n
where R is the retardation factor. If degradation (decay)
is incorporated then the equation becomes
aC a2C aC
R -- = D V* -- K C (1-4)
a t L ax ax D
where KBD is the degradation rate (1/T). Many numerical
solutions have been presented by various researchers
(Amoozegar-Fard et al., 1983; Fuller and Warrick, 1985; Van
Genuchten, 1981). As an example, if given the boundary
conditions C=C0 at x=0 and C=0 at x=infinity, and with an
initial condition C=0 at t<0, Equation (1-4) can be solved
by a numerical solution proposed by Sauty (1980):
C = C0/2 { exp[(v-u)x/2D] erfc[(Rx-ut)/(4DRt)1/2
+ exp[(v+u)x/2D] erfc[(Rx+ut)/(4DRt)1/2] (1-5)
where u = (v + 4 kD R D)/2. With this solution and the
BD