An integrated study of the hydrogeology of volcanic islands using helicopter borne transient electromagnetic: Application in the Galلpagos Archipelago

The hydrology of volcanic islands is poorly characterized due to their complex internal structure and challenging access. The Galلpagos Islands, an isolated basaltic environment with unique ecosystems and growing anthropogenic pressure, suffer from scarcity of freshwater resources and from the lack of fundamental knowledge on their hydrology. To overtake these constraints and provide fresh water to the population, a geophysical survey and a hydroclimatologic network at watershed scale have been performed over Santa Cruz. An innovative helicopter borne transient electromagnetic method permitted a quasi-3D (an inversion scheme allowing for a 3D model description but with a local 1D forward algorithm) resistivity mapping of the massif and the identification of its hydrogeological potential. The latter is composed of a perched aquifer on the windward mountainside and a basal aquifer. Our hydroclimatologic network provided useful data records and allowed us to characterize the hydrodynamic properties of the basal aquifer. The geophysical data allow us to precise its extension. The conceptual hydrogeological model proposed for Santa Cruz confirms one of the models proposed for Le Piton de la Fournaise (Réunion Island). It has now to be confirmed with boreholes and a detailed geochemical study including noble gas analysis.