Prediction of Dispersivity for Undisturbed Soil Columns from Water Retention Parameters

ity D/v (L), that can be related solely to characteristics of the porous medium (Fried and Combarnous, Dispersivity ( ) is a required input parameter in solute-transport 1971). Dispersivity is a required input parameter in conmodels based on the advection-dispersion equation (ADE). Normally is obtained from miscible-displacement experiments. This depentaminant transport models based on the ADE (Zheng dency on inverse procedures imposes a severe limitation on our preand Bennett, 1995). Except for a few simple systems such dictive capability. If solute breakthrough curves and soil hydraulic as packed beds of uniformly sized particles, cannot be properties were measured simultaneously, pedotransfer functions obtained from independent measurements (e.g., Koch could be developed to predict from independent measurements. In and Flühler, 1993). Since most natural porous media are this study, short (6 cm long) undisturbed columns were employed to heterogeneous, soil physicists are currently unable to preinvestigate the relationship between and the water-retention curve dict solute dispersion in undisturbed soil without first as parameterized by the air-entry value ( a ) and Campbell exponent conducting a miscible-displacement experiment to mea(b ). We worked with 69 columns from six soil types ranging in texture from loamy sand to silty clay, conventional-till and no-till management sure it. This dependency on inverse procedures imposes practices, steady-state saturated flow conditions, and a step decrease a severe limitation on our predictive capability. in CaCl2 concentration from 0.009 to 0.001 M. Breakthrough curves If solute breakthrough curves and static physical were measured by monitoring changes in effluent electrical conductivproperties were determined on the same sample, empiriity using a computerized data acquisition system. Estimates of (calcal relations could be developed to predict from indeculated using the method of moments) ranged from 1 to 192 mm pendent measurements. However, such studies are refor the six soil types. Stepwise multiple-regression analysis explained markably rare, and most of them have used packed beds 50% of the total variation in , and indicated that dispersion inof disturbed media (Passioura and Rose, 1971; Han et creased as a and b increased. Since both a and b increase with increasing clay content, also increases moving from coarseto fineal., 1985; Xu and Eckstein, 1997). Under saturated contextured soils. Our regression equation can be used as a pedotransfer ditions, the magnitude of solute dispersion at any given function to predict from existing databases of soil hydraulic properflow rate is controlled by the pore-space geometry (Perties. Further research is needed to independently validate its predicfect and Sukop, 2001). Since it is the geometrical chartive capability, and to develop strategies for upscaling the model preacteristics of solids or aggregates rather than the pore dictions. space that are measured in the packed bed approach, the resulting relationships are not directly applicable N mathematical models are available for to undisturbed soil. While studies involving artificial macropores (Kanchanasut et al., 1978; Li and Ghodrati, describing solute transport in soil. Of these, the 1997) may provide more information on the relationship ADE is the most widely used. For steady state, onebetween solute spreading and pore-space geometry, dimensional water flow, the ADE for a nonreactive they are subject to similar criticisms regarding their apsolute is given by (Fried and Combarnous, 1971): plicability to natural systems. In terms of heterogeneous systems, Anderson and C