Unravelling the functioning of andesitic volcanic aquifers in the equatorial zone using isotopic, age-dating and geochemical tracers

The hydrogeological structure of volcanic aquifers is now rather well known, including for andesitic systems. However, despite the existence of well-established Hydrogeological Conceptual Models (HCM), the functioning of andesitic aquifers remains insufficiently understood. Environmental tracers provide valuable tools to address this issue. In this study, we applied a multi-tracer approach combining hydrochemistry, water stable isotopes ( δ 2 H – δ 18 O) and tritium ( 3 H) in the Kedunglarangan watershed (Pandaan, Indonesia). This combination of tracers enabled a refined delineation of groundwater recharge areas by introducing a bilinear model accounting for secondary evaporation effects. We also propose a continuous (from 1900 to present) 3 H input function for the equatorial Asian region. The generic HCM of andesitic volcanic aquifers and their hydrogeological functioning were significantly improved through the identification of seven groundwater groups based on tracer signatures and groundwater age modelling. At high elevations, volcanic edifices host unconfined aquifers recharged locally through permeable lava flows and characterized by short residence times (<25 years). The transition zone between permeable volcanic formations and less permeable laharic deposits contains aquifers and springs mainly recharged from the volcanic edifice, with increasing residence times (>25 years), although few systems show evidence of substantial local recharge from lower elevations. Further downstream, within the volcano-sedimentary plain, confined aquifers overlain by lahars naturally discharge through large springs fed predominantly by high-elevation recharge, with variable but higher mean residence times (>50 years). In addition, several shallow locally recharged aquifers display low recharge rates and relatively long residence times (>100 years), discharging through small springs. This zone is also characterized by the occurrence of very old hydrothermal waters outflowing near outcropping deep geological structures. These results demonstrate a clear hydrogeological continuity from the top of the volcanoes to their feet that could not be demonstrated based only on structural (geology) and hydrodynamic information. This research greatly improves the understanding of recharge dynamics and groundwater flow circulation in recent andesitic volcanic systems. Beyond the local case, the methodology provides tools that can be used for groundwater budget assessment and groundwater quality protection in similar volcanic contexts worldwide ( e.g. Pacific Ring of Fire, Caribbean, Mediterranean volcanoes etc.).