Multidisciplinary study with quantitative analysis of isotopic data for the assessment of recharge and functioning of volcanic aquifers: Case of Bromo-Tengger volcano, Indonesia

•Architecture and hydrogeological implications of volcanic deposits of the Bromo-Tengger volcano.•Hydrogeological water balance at the scale of the Northern volcano flank.•A simple method for groundwater recharge assessment in volcanic context, coupling isotope analysis with hydrological mass balance.•The first hydrogeological conceptual model of the Bromo-Tengger aquifer system. The Bromo-Tengger volcanic aquifer system, (East Java, Indonesia). Andesitic volcanic aquifers in Indonesia are an important source of groundwater supply for the population. The artesian flow from high discharge springs facilitates the access for irrigation and drinking water but continuously flowing artesian wells exert an increasing pressure on the groundwater resource. Given the complexity of the volcanic edifice, a multidisciplinary approach including geological, hydrometeorological, hydrochemical and isotopic measurements was performed to characterise the hydrogeological functioning of the volcano’s northern flank. Two main hydrogeological systems are identified: a system with perched aquifers in the lava flows and pyroclastic complex, supplying low discharge springs or infiltrating to a deep flow system. The latter system is unconfined and provides groundwater to the downstream plain that hosts a confined multi-layer volcano-sedimentary aquifer. The joint implementation of a quantitative isotopic approach and a groundwater budget allows characterising aquifer recharge along the whole volcano flank. It is mainly controlled by the rainfall distribution that peaks around 1200 m elevation. The caldera basin on the top of the volcano has a very limited contribution to recharge. A conceptual hydrogeological model is proposed and compared to the main concepts of volcanic islands. This study is a first step for further scientific and management discussions to implement protection policies on the Bromo-Tengger aquifer system.