The variability of the apparent diffusion coefficient in undisturbed soil columns

In column studies in the laboratory the apparent diffusion coefficients(D)of chloride and TOH for undisturbed and unsaturated soils were determined as a function of the pore water velocity. Parallel the effective diffusion coefficients (D+) of Cl− were measured as a function of water content. It was found that the values of D varied significantly between soil layers and soil types. This findings also held for the low flow velocities studied (0.3 and 1.0 cm/d). The impedance factors to calculate the effective diffusion coefficients (D+) from D values in the bulk water, decreased with decreasing bulk densities. In model calculations it was demonstrated that in spite of the differences of D values measured, for many purposes in the field good estimates of the solute distribution in the soils can be obtained by using simple relations between D and vo.