Water flow and solute transport through a frozen clay soil

Abstract: Freezing and thawing of soil affect water flow in the vadose zone for instance by altering soil structure or ice blocking parts of the pore system. Changes in soil hydraulic properties impact solute transport and therefore the risk that pesticides and other agrochemicals are transported towards groundwater. In models currently used for pesticide registration freezing of soil is not accounted for, even though some substances –especially those applied in autumn– may persist throughout the winter. The objective of this study was to quantify the effect of soil freezing on pesticide leaching for a Swedish clayey topsoil. The herbicides clomazone, propyzamide and diflufenican and the non-reactive tracer bromide were applied to undisturbed soil columns (20 cm high, 12.5 cm diameter), which were frozen to -2 °C and exposed to simulated rain. Three repetitions of the freezing-irrigation cycle were performed. Pesticide and bromide concentrations were measured in the effluent and compared to the results of non-frozen columns. X-ray tomography was used to visualise and quantify the macropore structure. Most concentrations of the least mobile pesticide (diflufenican) were below the limit of quantification. Relative leaching of pesticides was between 0.15% and 2.67% of the applied amount for clomazone and between 0.10% and 1.67% for propyzamide. Considering all three irrigation events no significant difference was found between frozen and non-frozen soil columns regarding pesticide transport. However, relative leaching in percent of the applied amount as well as concentrations of clomazone and propyzamide were significantly higher in the non-frozen columns during the second irrigation event. The non-reactive tracer showed the same trend, with the difference that the major part of the transport appeared already during the first irrigation event, showing significantly higher amounts of bromide transported in the non-frozen columns. A possible explanation is that soil freezing created fine voids and therefore increased diffusion into soil aggregates and reduced preferential transport.