3.3 cm2"n dn'1 with n = 1.3, which is of a magnitude similar to values

from the two previously mentioned studies on stony soils. Several

factors may be contributing to the high dispersivity of this soil.

Dispersion increases as the range of small to large pore sizes in a

soil increases, thereby providing a wide range of water velocities

within a single soil sample. Edwards et al. (1984) have shown that the

presence of non-porous gravel increases the total macropore volume at

the expense of the micropore volume. The large reduction in the water

content of the soil under slight tensions (0 to 50 mbar) indicates that

the soil possesses a considerable volume of large pores (Fig. 3-1).

Similarly, the retention of nearly 33% of the total soil water at 15

bars of tension indicates that the soil also contains a large volume of

very small pores.

 The values of the dimensionless parameter w, estimated for each

experiment on Column 1, is presented in Table 4-5. It relates the

mass-transfer coefficient, a, to the column length and solution flux

(Eq. [4-16]). The mass-transfer coefficient is a lumped diffusion

parameter that relates the solute diffusion transfer to the molecular

diffusion coefficient, the mobile/immobile-water fraction, the

tortuosity, the radius of soil aggregates, and the solution flux. The

relationship between the mass-transfer coefficient and the solution

flux is shown in Fig. 4-10. The mass-transfer coefficient is not a

constant and has been shown to increase with solution flux using both

theoretical postulations and experimental methods (Rao, 1980a and

1980b; van Genuchten, 1985). The mathematical description of the

diffusion process employed by the MIM model has an underlying