How can inverse reactive transport modelling of radiocarbon improve the estimation of sustainable yield in confined aquifer systems?

The sustainable yield of aquifer systems in sedimentary basins can theoretically be derived by long-term model simulations considering the adverse effects of pumping, such as streamflow depletion, subsidence, or contamination induced by flow reversals. The implementation of a model-based approach for sustainable yield estimation is often challenged by the lack of knowledge on system properties and (paleo)-recharge rates. Specifically, leakage flows through aquitards generally drive the hydrodynamic response of confined aquifers to pumping, but their properties are poorly constrained by typical observational datasets. Dating methods such as radiocarbon have been widely used to infer residence time in confined aquifer systems. However, their use to constrain flow hydrodynamics in multi-layer systems with a state-of-the-art inverse modeling approach is scarce.

In this study, we investigated the flow dynamics of the Aquitaine Basin located in Southwest France, with an extensive repository of hydrologic and geochemical data spanning several decades. A 2D cross-sectional numerical flow model was developed and extended to simulate reactive transport of radiogenic carbon. An inverse modeling approach was then implemented to estimate model parameters using observed hydraulic heads and ¹⁴C activity. This paves the way to a more rational quantification of the sustainable yield of critical resources for the resilience of water supply in a changing world.