Groundwater controls on legacy antibiotics and pesticides in an intensive agricultural headwater catchment

Pesticides and veterinary antibiotics, widely used in agricultural systems for crop production and livestock farming, have significant impacts on groundwater quality and ecosystem health. This study investigates their occurrence and long-term persistence in a small temperate agricultural watershed (Kervidy-Naizin, Brittany, France). Field measurements revealed that antibiotics appeared sporadically in surface water only, in contrast to pesticides (including atrazine and metolachlor), which were detected broadly in both surface water and groundwater. Additionally, chlorofluorocarbons (CFCs) were employed as age tracers to constrain a 3D physically-based hydrogeological model of the crystalline bedrock aquifer, with particle tracking used to simulate groundwater transit time distributions (TTDs). In steady state, the groundwater flow model accurately represents both the local water table observed in piezometers and the spatial extent of the average hydrographic network. The TTDs were directly linked to the pesticide measurements, providing a consistent explanation for the long-term persistence of these contaminants. The results confirmed that atrazine and its metabolites were detected in groundwater with a mean age of 10 years, decades after its ban, while metolachlor residues were found in even older groundwater (>35 years), as inferred from CFC-constrained TTDs. This demonstrates the critical role of groundwater, governed by aquifer hydraulic properties, in storing and releasing legacy pollutants long after their application ceases. Our findings demonstrate the value of integrated field-modeling approaches for elucidating legacy pollutant pathways. They emphasize the need for expanded groundwater monitoring programs that account for hydrological time lags to effectively assess long-term pollution risks and inform mitigation strategies in agro-intensive regions.