Assessing the Groundwater Heritage of Veterinary Antibiotics: Insights from Field Measurements and Modeling in an intensive agricultural watershed

Veterinary antibiotics, widely used in agriculture, are emerging contaminants with significant implications for water quality and ecosystem health. This study investigates the presence and heritage of antibiotics and other pollutants in groundwater within the Naizin watershed (~5 km²) in Brittany (western France), a region marked by intensive agricultural activity. Field measurements revealed the occurrence of veterinary antibiotics in surface water and groundwater, alongside other pollutants. Additionally, CFCs and SF6 gas tracers provided groundwater age estimates ranging from 20 to 30 years, suggesting potential long-term heritage effects. A particle-tracking approach using HydroModPy, a calibrated MODFLOW-coupled groundwater flow model, was implemented to complement this study. Hydraulic conductivity and porosity, determined from hydrological characterization and calibration of the watershed, were incorporated to estimate residence times. The simulated groundwater residence times closely matched those inferred from tracer data, strengthening the linkage between field measurements and model outputs. These results highlight the persistence of pollutants such as antibiotics and underscore the need for integrated field and modelling approaches to assess contaminant load and transport  in agricultural catchments. This study offers critical insights into the interactions between anthropogenic activities, pollutant dynamics, and groundwater quality, providing a foundation for improved water resource management and pollution mitigation strategies.