Dependency on High‐Altitude Recharge and Pollution Legacy in a Peri‐Urban, Tropical Volcanic Aquifer

ABSTRACT Groundwater supplies over 90% of Costa Rica's drinking water and supports vital ecosystem services, yet many aquifers, especially in high‐altitude headwater catchments, remain understudied. This study aimed to (i) characterize the hydrogeochemistry and isotopic composition of the Poás volcanic aquifer system (PVAS), (ii) identify the key drivers controlling the chemical and isotopic evolution within the groundwater flow, and (iii) understand recharge governing processes, including mean potential recharge elevations (MREs). Groundwater across the PVAS consistently showed a predominant bicarbonate‐calcic‐magnesic facies, with chemical evolution primarily driven by carbonate and silicate weathering. Moderately high levels of nitrate and Escherichia coli in more than 56% of the samples indicate anthropogenic pollution legacy. Stable isotope compositions revealed altitude‐dependent recharge patterns influenced by Pacific and Caribbean moisture sources. Isotope‐inferred MREs ranged from 768 to 2407 m a.s.l. with a mean of 1549 ± 227 m a.s.l., overlapping a diverse land use mosaic. These findings demonstrate the high vulnerability of the PVAS to anthropogenic contamination and its reliance on high‐elevation recharge. The integration of hydrochemical, isotopic, and multivariate statistical analyses establishes a transferable framework for the Central America Volcanic Arc region to identify groundwater chemical evolution, assess anthropogenic impacts, and better understand recharge dynamics. This framework supports local and regional water management agencies in prioritizing conservation efforts within recharge zones and in strengthening contingency plans for drought and contamination events across urban and peri‐urban settings.