The effect of freezing and thawing on water flow and pesticide leaching in partially frozen soil: Soil column experiments and model development

Increased pesticide concentrations have often been detected in soil leachate, drain discharge and surface runoff during freeze/thaw periods in late winter and early spring, both in Norway and Sweden. Limited knowledge and experimental data exist however on pesticide leaching through partially frozen soil, partly because of the complexity of the processes involved. Climate changes can add to this complexity and influence the use and the fate and behaviour of pesticides. Models that can capture this complexity may help us improve our understanding of the systems and enable more reliable predictions of the transport of pesticides in partially frozen soil and of the potential impacts climate changes may have on water quality. One of the objectives of this PhD project has been to generate new knowledge and hence be able to increase the understanding of the complex processes of freezing and thawing and the effects these processes have on water flow and on the transport of bromide and pesticides. A second objective has been to develop and evaluate a dual-permeability approach for water flow and heat transport in macroporous soils undergoing freezing and thawing. To achieve these objectives we have conducted a soil column irrigation experiment to quantify the transport of a non-reactive tracer (bromide) and five pesticides (MCPA, clomazone, boscalid, propiconazole, diflufenican) in partially frozen soil. Intact topsoil and subsoil columns (i.d. 9.2 cm, h 20 cm) from two agricultural soils (silt and loam) in South-East Norway were sampled and bromide and pesticides were applied on top of all columns. Half the columns were then frozen (-3 °C) while the other half were left unfrozen (+4 C). Columns were subjected to repeated irrigation events which were followed by periods of freezing or refrigeration. Leachate from the columns was collected and analysed for pesticides and bromide. Parallel to this, physically based equations for soil freezing and thawing was included in the MACRO model. We tested the new model for water flow in the micropore domain against available measured data on the redistribution of water during freezing and illustrative scenario simulations were performed to demonstrate the effects of soil macropores on water flow and heat transport in partially frozen soils. The column study generated a substantial dataset which showed that pesticide leaching was up to five orders of magnitude larger from frozen than unfrozen columns. Leaching patterns of bro