Coupling 3D groundwater modeling with CFC-based age dating to evaluate Residence Time Distribution

International audienceAgricultural activity has increased nitrate concentration in groundwater worldwide, incurring substantial costs to society and representing one of the major environmental challenges of our generation. However, due to the high cost and difficulty of direct measurement of groundwater processes, the dynamics that govern nitrate transport and removal at the catchment scale, such as Residence Time Distribution (RTD) and flowpath, are still poorly understood. We used a three-dimensional groundwater model in combination with atmospheric tracers as chlorofluorocarbons (CFCs) to map water and solute flow in a 76 km2 agricultural catchment in western France.CFCs were produced and released to the atmosphere in the 1940s, since then they are taking part in the hydrological cycle and can be used as an environmental tracer. CFC-based dating is usually used to get globally information of the aquifer and to calculate the average time since water was in contact with the atmosphere, but it does not provide information of how the water was transported through the aquifer. In combination with the finite element subsurface flow model FEFLOW, flowpaths could be analyzed to understand the relation between aquifer properties, wells position in the catchment and RTD. The computed transit time distributions convoluted with the CFC chronicles result in a modeled groundwater age, which was compared with the CFC-based groundwater age and accounted for 40-50 % of variation. The spare input data (topography, aquifer depth, homogeneous porosity and homogeneous hydraulic conductivity) of the steady-state model already yield consistent results with the CFC-based groundwater ages. If the model results prove robust across different catchments and seasonal variability, this method may allow low-cost mapping of water flow and nitrate pollutio