Event-driven monitoring reveals multi-pesticide contamination of small standing waters beyond adjacent de facto farming practice risks

Abstract Agricultural practices are a major source of pesticide contamination in freshwater ecosystems. Lentic small water bodies (LSWBs), which make up the vast majority of freshwater systems, are particularly vulnerable to various pesticide entry routes, including surface runoff, spray drift and subsurface flow from adjacent agricultural land. This study aimed to assess whether event-driven monitoring could effectively detect pesticide contamination in LSWBs. Using 1 year of event-driven monitoring data from three LSWBs, we compared measured pesticide concentrations with predicted values from the SYNOPS-WEB risk assessment tool. The results revealed notable discrepancies. While SYNOPS-WEB predicted certain contamination events, on-site monitoring frequently showed the presence of pesticides that were not recently applied, suggesting long-term persistence or alternative transport pathways. A total of 28 active substances were detected, with certain herbicides and fungicides found in nearly all samples, irrespective of application time. Event-driven sampling shortly after rainfall or pesticide application events did not reliably reflect actual contamination from recent agricultural practices. Instead, the study found that legacy contamination, sediment desorption and subsurface flow likely played more significant roles. Overall, the study underscores the need for more tailored monitoring and modeling tools to understand pesticide dynamics in isolated small water bodies.