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