Quantifying heterogeneous transport of a tracer and a degradable contaminant in the field, with snowmelt and irrigation

Abstract

To examine the persistence of preferential flow paths in a field soil, and to compare the leaching of a degradable contaminant with the leaching of a tracer, two field experiments were performed using a multi-compartment sampler (MCS). The first experiment was carried out during the snowmelt period in early spring, characterized by high infiltration fluxes from snowmelt. The second experiment was carried out in early summer with irrigation to mimic homogeneous rainfall. During the second experiment, the soil was warmer and degradation of the degradable contaminant was observed. For both experiments, the highest tracer concentrations were found in the same area of the sampler, but the leached tracer masses of the individual locations were not highly correlated. Thus, the preferential flow paths were stable between the two experiments. With a lower infiltration rate, in the second experiment, more isolated peaks in the drainage and the leached masses were found than in the first experiment. Therefore, it is concluded that the soil heterogeneity is mainly caused by local differences in the soil hydraulic properties, and not by macropores. With higher infiltration rates, the high and low leaching cells were more clustered. The leached masses of the degradable contaminant were lower than the leached masses of the tracer, but the masses were highly correlated. The first-order degradation rate and the dispersivity were fitted with CXTFIT; the first-order degradation rate was 0.02 d<sup>−1</sup>, and the dispersivity varied between 1.9 and 7.1 cm. The persistence of the flow paths during the experiments suggests soil heterogeneity as the driver for heterogeneous flow and solute transport in this soil. At the MCS scale, heterogeneous snowmelt did not seem to have much influence on the flow and solute paths

Understanding Preferential Flow in the Vadose Zone: Recent Advances and Future Prospects

“…Multi‐compartment samplers provide additional data on the spatial distributions of water and solute fluxes for a two‐dimensional surface orthogonal to the main flow direction as well as BTCs for each individual compartment (Bloem et al, 2010). Thus, spatiotemporal “leaching surfaces” and statistical indices calculated from these data can be used to both quantify the strength of preferential flow as well as give some indication of the prevailing preferential flow process (e.g., Schotanus et al, 2012). In contrast, dye staining experiments only give two‐dimensional “snapshots” of spatial variations in water flow patterns but at the larger pedon scale (Flury et al, 1994).…”

Characterization of water content dynamics and tracer breakthrough by 3-D electrical resistivity tomography (ERT) under transient unsaturated conditions

“…They showed that preferential flow and transport would be considerably underestimated with a single compartment suction plate. A multicompartment suction sampler has also been recently applied to characterize preferential transport of degradable compounds [ Schotanus et al ., ]. However, multicompartment samplers require a large effort for sample handling and analysis.…”

Links between soil properties and steady‐state solute transport through cultivated topsoil at the field scale

“…It mostly takes place along preferential flow paths instead of homogeneously distributed through the whole soil pore space. Ever‐increasing evidence has accumulated in recent decades that this is true at all spatial scales from the pore scale [e.g., Carminati et al ., ], over the Darcy or column scale [e.g., Bloem et al ., ; Kasteel et al ., ; Schotanus et al ., ], the plot scale [e.g., Bouma and Dekker , ; Flury et al ., ], and the field scale [e.g., Bronswijk et al ., ; Butters et al ., ; Wild and Babiker , ] up to the regional scale [e.g., Kurunc et al ., ; Shaffer et al ., ; Iversen et al ., ] and larger. It is crucial to take the spatial heterogeneity at all scales into account for obtaining quantitative predictions of water flow and solute transport through soils, which are predominantly needed at the field scale or larger, namely, for managing environmental and agricultural resources.…”