Tracing Urban Hydrological Connectivity in the Highly Altered Patiño Aquifer, Asunción, Paraguay

The Patiño Aquifer is a key groundwater source for the metropolitan region of Asunción, Paraguay, supporting domestic, industrial and agricultural demands. Increasing pressure on this resource highlights the need to better understand recharge processes and groundwater flow dynamics. This study (2019–2026) applied stable water isotopes (δ 2 H, δ 18 O and d ‐excess) and dissolved radon ( 222 Rn) to evaluate urban hydrological connectivity. Precipitation isotopes showed a clear seasonal pattern controlled by atmospheric systems, particularly the South American Low‐Level Jet and the South Atlantic Subtropical High. Surface waters and urban wells, especially in the northern metropolitan sector, exhibited higher δ 18 O and lower d ‐excess values, indicating potential secondary evaporation effects and influence of urban return flows. In contrast, groundwater in the middle and southeastern more rural sectors displayed isotopic compositions closer to precipitation, suggesting recharge with limited evaporation. Radon activities showed a consistent trend, with low values in surface waters and increasing concentrations in groundwater towards the southeast. This spatial pattern may reflect greater interaction between groundwater and aquifer materials and is consistent with progressive evolution along inferred flow paths towards the Paraguay River. These findings improve understanding of aquifer dynamics and highlight the influence of urbanisation and extreme rainfall events on groundwater recharge and quality under changing climatic conditions.