Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/27/1999
Publication Date: N/A
Citation: Interpretive Summary: In humid regions, leaching can account for significant losses of applied agricultural chemicals. This not only leads to economic loss to the farmer but also can cause the contamination of surface and ground water resources. Accurate prediction of chemical transport within the crop root zone is necessary to develop and evaluate soil management strategies that lead to a amore efficient use of agricultural chemicals. Attempts to predict the effects of management practices upon the movement of chemicals have met with limited success due, in part, to our poor understanding of transport processes in structured soils. Laboratory investigations were carried out to characterize and quantify the important mechanisms governing the transport of water and mobile chemicals through a soil in southern Costa Rica. Experimental and simulated results were presented to evaluate the suitability of a dual porosity model to describe reactive solute transport in soil columns. This model divides the soil into slow and fast moving pore regions. Methodology was developed to estimate the parameters needed to describe transport using the dual-porosity model. Results of fitting the dual-porosity model to measured concentrations led to a distinction of transport processes that corresponded to observed structural differences in soil horizons. The results of this study show that the important transport mechanisms in the subsoil differed greatly from those in the surface horizon. Adsorption and slow exchange of solutes between regions were found to be important factors that influenced the transport of bromide in the subsoil. These transport mechanisms were not important in the surface horizon under steady state flow. This research has improved our understanding of the mechanisms responsible for solute movement in soils.
Technical Abstract: The exchange of solutes between macropores and soil matrix regions is an important consideration in describing solute transport in structured soils when the time scales of interest are short in comparison to diffusive time scales of the matrix region. In this study, a dual-porosity approach is used to describe the steady-state reactive transport of a bromide tracer through undisturbed soil in columns over a range of pore water velocities and levels of soil water saturation. This model partitions the soil into two mobile regions that represent the soil matrix and macropores. Theory and methodology are presented to estimate diffusive exchange between regions and transport coefficients in each region for soil columns under steady-state water flow. Nonlinear least-squares regression was used in conjunction with data derived from displacement experiments to obtain estimates of transport parameters. The estimation of pore water velocity and water content of each mobile region using a pair of displacement experiments conducted at different pressure heads was found to be a suitable procedure for ascribing solute flux to each region. The fit of the dual-porosity model to observed effluent concentrations resulted in lower residual standard deviations than the fit of the advective-dispersive equation. A major difficulty of the application of the dual-porosity model was the nonlinear behavior of the diffusive exchange term at early times. Another problem was that fitted solutions predicted nearly all adsorption sites to be in equilibrium with solute in the macropore region rather than with solute in the matrix region. Despite these difficulties, the dual-porosity model led to differentiation of transport processes that corresponded to observed structural differences in soil horizons.