Dependency on high-altitude recharge and pollution legacy in a peri-urban, tropical volcanic aquifer

DD MMMM YYYY MMMM YYYY 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, and (iii) understand recharge governing processes, including mean potential recharge elevations (MREs). We hypothesize that the groundwater in the PVAS exhibits distinct geochemical and isotopic patterns (from high to low altitudes) that reflect a combination of altitude-dependent recharge and anthropogenic pollution influence. Despite the complex geological setting, the predominant facies across the PVAS were of the bicarbonate-calcic-magnesic type. Moderately high levels of nitrate and E. coli in more than 56% of the samples indicate anthropogenic pollution legacy. Isotope-inferred MREs ranged from 768 to 2,407 m a.s.l with a mean of 1,549±227 m a.s.l, overlapping a diverse land use mosaic. These findings show the high vulnerability of the PVAS to anthropogenic contamination and the strong dependency on high-elevation recharge. This information provides a valuable tool for prioritizing conservation efforts in key recharge areas and may be translated to other similar settings across the Central America Cordillera.