STOCHASTIC-ANALYSIS OF ADSORBING SOLUTE TRANSPORT IN 3-DIMENSIONAL, HETEROGENEOUS, UNSATURATED SOILS

We analyze adsorbing solute transport in heterogeneous three-dimension al unsaturated soils using a first-order perturbation method. Both the unsaturated hydraulic conductivity and water content are treated as s tatistically anisotropic and stationary random space functions. The wa ter flow is assumed to be steady state. Solute adsorption is described by a linear equilibrium isotherm with a spatially variable random ads orption coefficient. The macrodispersivity depends on the statistical properties of hydraulic and chemical parameters and on the distance tr aveled by the solute plume. The longitudinal macrodispersivity increas es with the solute travel distance and approaches an asymptotic value at long travel distances. The transverse macrodispersivity increases w ith travel distance for short distances and then drops to zero. The ef fect of water content variability on macrodispersivity can be neglecte d only for a soil near saturation. Compared with effects of hydraulic gradient and suction head variabilities, the effects of soil conductiv ity parameters on macrodispersivity are dominant at high water content or long travel distance. Heterogeneous adsorption has a larger impact on longitudinal macrodispersivity than on the transverse one. Macrodi spersivity increases as mean water content decreases and as mean retar dation factor increases. The correlation between the adsorption coeffi cient and saturated hydraulic conductivity plays an important role in the determination df macrodispersivity.