Modelling solute transport in structured soils: Performance evaluation of the ADR and TRM models

The movement of chemicals through the soil to the groundwater or discharged to surface waters represents a degradation of these resources. In many cas es, serious human and stock health implications are associated with this fo rm of pollution. The chemicals of interest include nutrients, pesticides, s alts, and industrial wastes. Recent studies have shown that current models and methods do not adequately describe the leaching of nutrients through so il, often underestimating the risk of groundwater contamination by surface- applied chemicals, and overestimating the concentration of resident solutes . This inaccuracy results primarily from ignoring soil structure and nonequ ilibrium between soil constituents, water, and solutes. A multiple sample p ercolation system (MSPS), consisting of 25 individual collection wells, was constructed to study the effects of localized soil heterogeneities on the transport of nutrients (NO3-, Cl-, PO43-) in the vadose zone of an agricult ural soil predominantly dominated by clay. Very significant variations in d rainage patterns across a small spatial scale were observed tone-way ANOVA, p < 0.001) indicating considerable heterogeneity in water flow patterns an d nutrient leaching. Using data collected from the multiple sample percolat ion experiments, this paper compares the performance of two mathematical mo dels for predicting solute transport, the advective-dispersion model with a reaction term (ADR), and a two-region preferential flow model (TRM) suitab le for modelling nonequilibrium transport. These results have implications for modelling solute transport and predicting nutrient loading on a larger scale. (C) 2001 Elsevier Science Ltd. All rights reserved.