High-frequency sampling reveals increased pesticide leaching to groundwater linked to autumn spraying

Pesticide contamination of groundwater is a persistent environmental concern, particularly in structured clay soils where preferential flow can rapidly transfer compounds from agricultural fields to shallow aquifers. Using propyzamide as a case study, we evaluated how sampling frequency affects the detection and interpretation of pesticide leaching at the Silstrup field of the Danish Pesticide Leaching Assessment Programme. Propyzamide was applied to winter rapeseed in autumn of 2005, 2018, and 2023, enabling comparison between historical monitoring based mainly on monthly or less frequent groundwater sampling and an intensified weekly sampling strategy implemented immediately after the 2023 application. Weekly sampling revealed rapid leaching to shallow groundwater, with first detections within two days of application and a maximum concentration of 9.6 μg/L detected within ten days in a shallow groundwater monitoring well. In contrast, the lower-frequency strategies used in 2005 and 2018 recorded markedly fewer detections and substantially lower peak concentrations, generally near or slightly above the 0.1 μg/L groundwater limit. Flow-proportional drainage monitoring showed concentration peaks within the first 20 days after all three applications, supporting rapid preferential transport as a key control on early pesticide movement. Trend analyses of precipitation, drainage, groundwater depth, and soil water content showed no systematic hydrological changes sufficient to explain the much higher groundwater concentrations observed in 2023. Our results demonstrate that temporally sparse monitoring can substantially underestimate transient pesticide concentration peaks in groundwater and, consequently, pesticide leaching risk. High-frequency post-application sampling should therefore be considered when assessing groundwater contamination in agricultural settings with preferential flow.