From field to pond and beyond: bidirectional transport of pesticides and their transformation products in lentic small water bodies in Northern Germany

Lentic small water bodies (LSWBs) in agricultural landscapes are increasingly recognized for their ecological importance, but they remain vulnerable to agrochemicals, with pesticide contamination being of particular concern. This study investigated the spatio-temporal dynamics of 25 pesticides and their transformation products (TPs) in two LSWBs and their surrounding shallow groundwater in northern Germany. The aim was to identify dominant contamination pathways in LSWBs. Over two years, a high-resolution monitoring campaign collected pesticide concentration data on a daily to biweekly basis from surface water, shallow groundwater, and drainage inflows. Results revealed strong seasonal variations, with concentration peaks following pesticide applications and rainfall events. Metazachlor and flufenacet TPs dominated the compound spectrum due to their high mobility and persistence. Artificial drainage facilitated rapid pesticide transport and export, while shallow groundwater contributed to gradual, long-term inflows. Contamination patterns were shaped by the presence or absence of artificial drainage, groundwater inflow zones, and compound-specific behavior such as leaching potential and persistence. These findings emphasize the dual role of LSWBs as both sinks and sources of pesticide contamination and highlight the need to integrate subsurface and drainage pathways into monitoring and risk assessment frameworks of LSWBs.