Unravelling Recharge Mechanisms Supporting Urban Water Supply: Isotopic Evidence From A Mid‐Latitude Coastal Aquifer (Mar Del Plata, Argentina)

ABSTRACT Groundwater recharge in coastal aquifers is often characterised by the coexistence of multiple recharge pathways and complex hydrodynamic conditions, which may complicate the interpretation of isotopic tracers. In this study, we evaluate the strengths and limitations of stable water isotopes (δ 18 O, δ 2 H) for identifying groundwater recharge sources in an urban coastal aquifer, using the Mar del Plata aquifer system (Argentina) as a case study. An integrated approach combining stable isotopes, major ion chemistry, spatial analysis and Bayesian mixing models was applied to groundwater, surface water and rainfall samples. Isotopic compositions reveal overlapping signatures between continental waters affected by evaporation and potential marine influence, highlighting intrinsic ambiguities in isotope‐only interpretations in coastal settings. Hydrochemical indicators and mixing model results demonstrate the contribution of multiple recharge sources, including rainfall, surface‐water bodies and minor seawater inputs and help constrain isotopic interpretations within a robust conceptual framework. The results show that stable isotopes provide valuable insights into recharge processes but require complementary tracers and hydrological context to avoid misinterpretation in coastal aquifers. These findings are transferable to other urbanised coastal systems and emphasise the need for integrated methodologies to improve understanding of groundwater recharge mechanisms.