Abstract
Abstract. Transport of a tracer and a degradable solute in a heterogeneous soil was measured in the field, and simulated with several transient and steady state infiltration rates. Leaching surfaces were used to investigate the solute leaching in space and time simultaneously. In the simulations, a random field for the scaling factor in the retention curve was used for the heterogeneous soil, which was based on the spatial distribution of drainage in an experiment with a multi-compartment sampler. As a criterion to compare the results from simulations and observations, the sorted and cumulative total drainage in a cell was used. The effect of the ratio of the infiltration rate over the degradation rate on leaching of degradable solutes was investigated. Furthermore, the spatial distribution of the leaching of degradable and non-degradable solutes was compared. The infiltration rate determines the amount of leaching of the degradable solute. This can be partly explained by a decreasing travel time with an increasing infiltration rate. The spatial distribution of the leaching also depends on the infiltration rate. When the infiltration rate is high compared to the degradation rate, the leaching of the degradable solute is similar as for the tracer. The fraction of the pore space of the soil that contributes to solute leaching increases with an increasing infiltration rate. This fraction is similar for a tracer and a degradable solute. With increasing depth, the leaching becomes more homogeneous, as a result of dispersion. The spatial distribution of the solute leaching is different under different transient infiltration rates, therefore, also the amount of leaching is different. With independent stream tube approaches, this effect would be ignored.
Highlights
Groundwater contamination by nutrients or chemicals will be enhanced by preferential flow in the unsaturated zone
Preferential flow can be caused by macropore flow (Jarvis, 2007), by small scale differences in hydraulic properties (Roth, 1995), or by water repellency (Van Dam et al, 1990), amongst others
To account for preferential flow in modelling heterogeneous soils, several approaches exist, which were reviewed by Feyen et al (1998) and Simunek et al (2003)
Summary
Groundwater contamination by nutrients or chemicals will be enhanced by preferential flow in the unsaturated zone. Roth (1995) included soil heterogeneity in a numerical model, by the use of a random scaling factor for the retention curve and the saturated hydraulic conductivity, where heterogeneous water flow was studied. Roth (1995) showed that the infiltration rate determines which parts of the soil will transport most water and solutes Another modelling approach is the use of independent stream tubes, which each have a different velocity and dispersion coefficient (Vanderborght et al, 2006; Russo and Fiori, 2009). Solute transport can evolve from a stochastic-convective (independent stream tubes) to a convective-dispersive regime with increasing depth (Seuntjens et al, 2001)
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