Single- and dual-porosity modelling of multiple tracer transport through soil columns: effects of initial moisture and mode of application

We investigated the effect of initial moisture contents and mode of applica tion on the displacement of multiple conservative tracers through undisturb ed columns of a Humic Gleysol. Bromide was applied at the soil surface and chloride was injected at 5 cm depth. The columns were irrigated with deuter ium-enriched water. A dual-porosity model and two single-porosity models we re calibrated separately to Br and Cl- elution curves in the two columns. Elution curves were almost identical for Br- and Cl- under initially wet co nditions, whereas the displacement of Br- was faster than that of Cl- in th e initially dry column, indicating rapid transport with preferential how. O nly the dual-porosity model described the long-tailing breakthrough of Cl- in the initially dry column adequately. The parameter values giving accepta ble fits for 'Br dry' were not compatible with the description of the three other elution curves, which could be adequately modelled with a single set of parameter values. The estimated set of common parameters was validated by comparing with the elution curves of deuterium water, nitrate and sulphate, as well as with re sident tracer concentrations at four depths. The results showed that solute s can be displaced much faster when applied at the surface of initially dry soil than when applied to wet soil or when resident in the soil matrix. Th e simulation results suggest that solute transport under initially dry cond itions was governed by preferential flow of infiltration water through macr opores by-passing the matrix due to shrinkage cracks and water repellence o f matrix surfaces.