Impacts of Co-formulants on pesticide sorption and leaching through soil

A large variety of co-formulants are added to commercial pesticide formulations to enhance their biological effectiveness, but their impacts on the behaviour of active substances are largely unknown due to lack of regulatory measures concerning their use. The overall aim of this research was to investigate the impacts of co-formulants on pesticide sorption and leaching through soil. Leaching and sorption experiments were carried out using technical grade and commercial formulations of four different pesticides at various intervals from application in two soils from the Blackwood and Bishampton associations. The effects of formulation, residence time and soil type on the leaching behaviour of pesticides were all found to be highly statistically significant (p<0.001). Both solubility of the active substance and the type of formulation also influenced the effect of formulation on leaching behaviour of pesticides. The relative difference in mass leached between formulated and technical material of low solubility pesticides was less than that for pesticides with greater water solubility. Greater leaching losses of pesticide were observed from an emulsifiable concentrate (EC) formulation compared to suspension concentrate (SC) formulation for the fungicide azoxystrobin. Results from sorption and desorption experiments on propyzamide in the two soils showed that the batch-equilibrium method is not suitable to study the effect of formulation on pesticide sorption. Rather it is important to use a centrifugation technique under natural moisture conditions to characterise these interactions. The effect of formulation on sorption of propyzamide was highly significant (p<0.001) in sandy loam soil, but there was no effect of formulation in the sandy silt loam soil when characterised by a centrifugation technique. Results suggested that any effect of formulation on pesticide sorption was not sufficient to explain fully the effect of formulation on leaching behaviour. Two-site and three-site sorption models were applied and fitted the sorption phase of the experiment well. However, the models failed to describe the observed desorption behaviour of propyzamide. These results highlight gaps in the existing knowledge about the formulation effects on pesticide sorption and leaching through soil and suggest that these effects should be considered during the risk assessment of environmental fate and behaviour of pesticides.