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

Abstract

Summary We investigated the effect of initial moisture contents and mode of application on the displacement of multiple conservative tracers through undisturbed 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 deuterium‐enriched water. A dual‐porosity model and two single‐porosity models were calibrated separately to Br– and Cl– elution curves in the two columns. Elution curves were almost identical for Br– and Cl– under initially wet conditions, whereas the displacement of Br– was faster than that of Cl– in the initially dry column, indicating rapid transport with preferential flow. Only the dual‐porosity model described the long‐tailing breakthrough of Cl– in the initially dry column adequately. The parameter values giving acceptable 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 resident tracer concentrations at four depths. The results showed that solutes 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. The simulation results suggest that solute transport under initially dry conditions was governed by preferential flow of infiltration water through macropores by‐passing the matrix due to shrinkage cracks and water repellence of matrix surfaces.

Dual-porosity and kinematic wave approaches to assess the degree of preferential flow in an unsaturated soil

“…The agreement between observed and measured water content was especially poor (negative E value) for Run 1 (low irrigation intensity) at 0.26 m depth, where the initial water content was low (about 34%). Kätterer et al (2001) noted a similar tendency for MACRO to overestimate water exchange between macropores and micropores when the soil was dry, which they attributed to water repellency in their organic-rich soil.…”

“…The MACRO model run under equilibrium conditions (d = 1 mm) described fairly well the soil water movement, and tracer transport in the sandy soil (St Peter's Island) even at high input rates. In general, some discrepancies between model predictions and measurements were noted at a depth of 0.26 m in the column in drier soil with initial water contents less than or equal to 34% by volume (see also Kätterer et al, 2001). …”

A review of model applications for structured soils: a) Water flow and tracer transport

“…MACRO-SUFI was applied to study the effects of the initial saturation and the mode of tracer application (in soil vs. in irrigation water). A single fitted parameter set could be applied to simulate almost all elution curves, irrespective of initial saturation and application mode, reasonably well (Kätterer et al, 2001).…”

Measurement and simulation of subsurface tracer migration to tile drains in low permeability, macroporous soil

“…One of the primary difficulties in evaluating the migration of water and solute is the spatially and temporally variable mass exchange that occurs between matrix and macropores. These processes have been studied using solute breakthrough measurements and associated one-dimensional dual continuum modeling of intact soil columns [Katterer et al, 2001;Alaoui et al, 2003;Jarvis et al, 2007]. Upscaling this understanding to the field is more challenging because of spatial variability in soil properties, the relative difficulty of conducting field experiments, and computational limitations arising from extension to two-dimensional and three-dimensional models.…”