Résumé
Hydrogeochemical analysis and geological mapping, together with water Isotopes analysis, were performed to identify the recharge area of Umbulan spring, a high discharge spring located in the Bromo-Tengger volcano. The volcanic edifice, situated in a tropical climatic context, is the origin of exceptionally high discharge springs in such a volcanic context. This is the case of Umbulan spring whose discharge is about 3500 l/s that supply drinking water to the city of Surabaya, the second biggest city of Indonesia. Groundwater flows through fractured/weathered andesitic lava flow and pyroclastic deposits. The main groundwater outlet corresponds to gravity springs on the flanks of the volcano and to artesian springs in the plain. To improve the hydrogeological knowledge of the study area, the geological mapping of the North volcano flank has been performed to identify the aquiferous formations and refine the geological limits defined in the literature. Based on this geological survey, a new geological map was proposed. Water samples of gravity springs, artesian springs and deep wells were collected with elevations ranging from 40 to 2700 m above sea level, for water major ions elements and stable isotope (delta (super 18) O, delta D). The meteoric local gradient of delta (super 18) O is assessed from the water isotope contents of springs, which are considered as "local pluviometer" representative of the precipitation isotope content at a given elevation corresponding to the mean elevation of their recharge catchment. Based on the analysis of the meteoric local gradient of delta (super 18) O, the mean elevation of Umbulan spring recharge catchment ranges between 700 to 1300 m a.s.l, which in agreement with geological observations. Many interrogations subsist but these first hydrogeological data collected in the field allowed to propose a first conceptual model of the Bromo-Tengger volcano, which will help improving the hydrogeological knowledge of the study area and thus preserve and manage the groundwater resource of Bromo-Tengger volcano.