T. Komatsu

Diffusion-limited mobilization and transport of natural colloids in macroporous soil

This study examines the dynamics of colloid mobilization and leaching from macroporous soil columns by means of laboratory experiments and numerical modeling. On the basis of a previous column study involving high and low water flow rates in structured soil, we designed a novel experiment emphasizing the time-dependence of the colloid release process. Intact macroporous soil columns were exposed to variable pauses in irrigation (flow interruption for 30 min, 1 d, or 7 d) followed by resumed infiltration. The experiments showed that (i) there was a seemingly unlimited source of in situ colloids even after prolonged leaching and (ii) the peak concentration of colloids in the effluent after the flow interruption increased with increasing length of the preceding pause. The results demonstrated that colloid mobilization is not controlled by hydrodynamic shear induced by the flowing water but is a time-dependent and possibly diffusion-limited process. We developed a simple, equivalent macropore model to investigate the hypothesis that colloid release to the flowing water is governed by two diffusion processes, one in a uniform water film lining the macropore and one in the crust of the macropore. The model was capable of reproducing and explaining the characteristic results of our soil column experiments and required no recalibration of exchange process parameters to simulate the particle mobilization after a flow interruption. However, model calibration yielded unexpected results with respect to the size of the diffusion coefficient of the crust and did not warrant accepting the dual diffusion model hypothesis. Using a simpler mass transfer concept to quantify the mobilization of colloids in 21 soil columns, we found mass transfer coefficients to be about 30 times higher in the water film than in the crust.

MODELING DIFFUSION AND REACTION IN SOILS: X. A UNIFYING MODEL FOR SOLUTE AND GAS DIFFUSIVITY IN UNSATURATED SOIL

Diffusion processes in the soil water and air phases often govern transport and fate of nutrients, pesticides, and toxic chemicals in the vadose zone. This final paper in a 10-part series on diffusion-reaction processes in soils concerns the development of a unifying model platform for predicting solute and gas diffusion coefficients as functions of fluid-phase (water or air) content and pore-size distribution in unsaturated soils. We find that the Buckingham (1904) expression predicts solute diffusivities in water-saturated porous media more accurately than other classical expressions and, extended with a pore-size distribution-based term, yields a new and accurate model for solute diffusivity in unsaturated soil. The same was shown for gas diffusivity in undisturbed soil in Part IX of this series. Thus, the predictive diffusivity models can be rewritten in a common form with two model parameters that vary between solute and gas diffusivity and, in the case of gas diffusivity, also between undisturbed and repacked soil. It is suggested that the two parameters in this unified diffusivity model (UDM) represent porous media (solids-induced) tortuosity (T) and water-induced fluid phase disconnectivity (W), respectively, with W increasing with clay content for solute diffusion but being constant (repacked soil) or decreasing (undisturbed soil) for gas diffusion. Tested against data for 77 soils, the UDM model was markedly more accurate than commonly used soil-type independent models, with 35–50% (gas diffusivity) and 75% (solute diffusivity) reduction in root mean square error of prediction. The use of the new UDM to predict effective diffusion of sorbing chemicals in the soil water and air phases is illustrated. The UDM concept enables a new definition of the relative diffusion coefficient in soil, i.e. relative to the diffusion coefficient in a fluid-saturated porous media instead of in free water or air. This provides new possibilities for analyzing tortuosity phenomena in the soil water and air phases and their effects on diffusive and convective transport parameters in unsaturated soil.