Abstract
AbstractVolcanoes are environments combining hazards and natural resources, the latter justifying the high population density living on many active volcanoes in tropical contexts. More than a source of construction materials, some volcanoes are considered as "water castles", providing important possibilities for drinking groundwater supply through complex and sometimes artesian aquifer systems. The Bromo-Tengger volcano aquifer system in East Java, Indonesia, is a strategic resource with its main artesian spring Umbulan, which partly supplies the second biggest city of Indonesia (Surabaya). For more than a decade, this resource has been under increasing pressure with an increase of water abstraction for agricultural practices, industrial development, population growth and is amplified by increasingly accessible drilling techniques. Known as one of the most visited stratovolcanoes in worldwide, the aquifer potential of the Bromo-Tengger was barely studied until now.As part of this thesis, a multidisciplinary approach was used to characterize the hydrogeology of the northern flank of the volcano and its downstream artesian plain. Geological and geophysical results have highlighted the lithology and the geometry of the volcano's aquifer formations. The outflow context of natural major artesian springs of volcanic piedmonts area such as Umbulan has been identified.The climatic conditions were characterized with the installation of new meteorological stations along the northern flank and completed by the monitoring of a representative watershed. Compared with hydrochemical and isotopic data, they allowed a quantitative analysis of the distribution of the recharge and the elaboration of a first hydrogeological conceptual model. The recharge covers the whole northern flank of the volcano (≈66% of the rain) with a preferential recharge zone identified at about 1000 m elevation mainly due to orographic effects. Conceptually, infiltrated waters supply a binary system composed of: a network of high elevation perched aquifers feeding all or part of a low-lying basal aquifer. This latter becomes confined towards the plain through a multi-layer artesian aquifer, explaining the high aquifer productivity of the area.A new measurement device at free-flowing artesian well has been developed, combining hydraulic tests and piezometric measurement. Their interpretation allows both estimating the hydrodynamic parameters and drawing a piezometric map of the artesian aquifer of Pasuruan. These last results, made it possible to define the main flow directions and systems as well as an estimate of groundwater transit time.A first steady-state numerical flow model shows good consistency with the conceptual model previously developed. It allows a better understanding of the hydrogeological functioning in the present and pre-exploitation conditions. Compared with the water age dating analyses, it allows to discuss the most realistic flow model to consider at the scale of the volcano.This research provides new insights in the understanding of hydrogeology of andesitic stratovolcanoes in volcanic arc position under tropical climate including a caldera, high-elevation perched aquifers with a multi-layered and artesian volcano-sedimentary plain.Keywords: Hydrogeology, Bromo-Tengger volcano, conceptual hydrogeolgoical model of volcanic aquifer, volcano-sedimentary plain, artesianism, recharge, multidisciplinary