Andesitic volcanic aquifers are an important source for water supply in many countries of the world, particularly in subduction zones, such as Indonesia. Their sustainable management requires a thorough understanding of their hydrogeology. Given their complexity, multidisciplinary approaches are required and were implemented on the northern flank of the Bromo-Tengger volcano, Java Island, a 2700 m high strato-volcano, 40 km in diameter, comprising geological, hydrometeorological, hydrodynamical, hydrochemical, temperature and isotopic measurements.
Two main hydrogeological units are unveiled:
(I) An upstream volcanic unit mostly composed of a more than 1500 m thick lava flows series, Quaternary in age, topped by more recent pyroclastites up to several hundred meters thick. Despite the geological complexity of these formations in the detail, it hydrogeologically appears as homogeneous and permeable at the scale of the whole northern flank of the volcano. It hosts an unconfined aquifer, similar to a “basal aquifer” described in shield volcanoes. Some local perched aquifers (on ash layers and cooked paleo-soils) feed a few low discharge springs (few L/s).
Quantitative isotopic modeling, confirmed by thermal modeling to account for the “cold anomalies” observed downstream in the aquifer, demonstrates that aquifer recharge occurs on the whole Northern flank of the volcano. Recharge spatial distribution is mostly driven by the rainfall pattern, peaking around 4000 mm/y at about 1200 m amsl, and the area of the volcano’s flank that decreases with elevation.
(II) A more than 300 m thick downstream multilayer volcano-sedimentary unit, which roof extends up to about 50 m amsl, composed of coarse to fine volcanic sands, with a transmissivity ranging between 10-2 and 10-4 m2/s, interstratified with ashfalls, lahars and tuffs. It is confined by superficial decametric thick clayey layers and bounded to the North by distal clayey deposits. No geological limits were found West and East, as this aquifer gathers with the ones of neighbor volcanoes; then, for modelling purpose its hydrogeological limits were set-up based on groundwater flow lines. This geological unit hosts a confined aquifer, artesian in most places, exclusively fed by the upstream unit. Groundwater outflows correspond to (i) high discharge artesian springs (total discharge of about 4000 l/s and 3500 L/s for the Umbulan spring alone), (ii) more than 600 artesian wells (total discharge of about 2400 L/s), and (iii) vertical leakage estimated to about 600 l/s.
Hydrochemistry and water dating demonstrate a South-North groundwater aging, accentuated in the downstream distal part of the confined aquifer due to artesian wells development during the last few decades.This research unveiled the conceptual model of this aquifer and, more largely, of such type of andesitic volcanic and volcano-sedimentary aquifers. It also enabled its hydrogeological modeling, and the building-up of sustainable groundwater management scenarios.