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.