Particulate phase transport of pesticides is substantial for runoff and erosion in a small agricultural catchment

Agriculture on sloping lands is prone to processes of overland runoff and associated soil detachment, transportation, and deposition. The transport of pesticides to off-target areas related to runoff processes and soil erosion poses a threat of pollution to the downstream environment. This study aimed to quantify transport of pesticides both dissolved in water and in the particulate phase in transported sediments. Particulate phase transport of pesticides on short temporal time scales form agricultural fields is scarcely studied, and this study provides more insight into this process. During two growing seasons (2019 and 2020) rainfall – runoff events were monitored. We selected 32 different pesticides based on interviews with the farmers on the application pattern.  Concentrations for these 32 residues were analyzed in runoff water (dissolved phase - DP) and sediment (particulate phase - PP) and in soil samples taken in the agricultural fields. In all runoff events active substances (AS) were detected. There was a clear difference between DP and PP with a mean of 2 and 13 different AS per event respectively. The mean (± uncertainty) concentrations detected were 46 ±7 µg l^-1 in DP and 2900 ± 500 µg kg^-1 in PP. Although the transported mass of sediment is much lower than the total water discharge (Q_TSS : Q_w = 1 :73) the contribution of PP to total pesticide load discharged was 47%. We conclude that for agriculture on sloping lands overland transport of pesticide in the particulate phase is a substantial transport pathway, and that this process needs to be considered in future assessments for pesticide fate and environmental risk.