Quantifying heterogeneous transport of a tracer and a degradable contaminant in the field, under two infiltration rates

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 non-degradable tracer, we did two field experiments, using a multicompartment sampler. The first experiment was done during the snowmelt period in early spring, characterized by high infiltration fluxes from snowmelt. The second experiment was done in early summer with irrigation to mimic homogeneous rainfall. In 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 seasons. 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 clustering of high and low leaching cells was higher. The leached masses of the degradable contaminant were lower than the leached masses of the non-degradable tracer, but the masses were highly correlated. The first-order degradation rate was 0.02 d−1. The dispersivity varied between 1.9 and 7.1 cm. Soil heterogeneity is the main reason for the heterogeneous water flow and solute transport in this soil. Heterogeneous melting of snow does not influence the heterogeneous flow in the soil much at this scale.