Extensive perched aquifer and structural implications revealed by 3D resistivity mapping in a Galapagos volcano

Due to the complexity of geological formations and limited subsurface data in volcanic islands, hydrogeological conceptual models can differ from one island to another and for the same island. The Galapagos Islands, like most inhabited volcanic islands, face important water resource problems which might have a major impact on their unique and pristine ecosystems, Endanger World Heritage list (June 2007). The scarcity of geological and hydrological data combined with the difficulty of access for field measurements lead to a poor understanding of the island hydrogeology and unconvincing interpretation of traditional geophysical data. Here we present three dimensional (3-D) resistivity maps for the supposedly "waterless" Santa Cruz Island, obtained by using the SkyTEM device, a helicopter-borne transient electromagnetic method. The latter is non-invasive and measurements over inaccessible terrain were of vital importance. We show that even in high-relief terrain with extreme subsurface resistivity contrasts [1-6 000 ohm-m], the method is sensitive to low-resistivity layers of hydrogeological interest [50-200 ohm-m] to a depth of approximately 300 m. The unique spatial resolution and 3-D view of the subsurface resistivity structures allow identification of two zones of hydrogeological importance: a previously unknown extensive perched aquifer (50 km²) on the southern mountain side and the geometry of the salt-water wedge in the basal aquifer located with an accuracy of a few meters. This finding supports the existence of hidden perched aquifers on basaltic islands, until now only inferred from hydrogeological studies. It is seen to be affected by faulting. Beyond the vital implications for water resource and ecosystem management in the unique yet severely threatened Galapagos Islands, some conceptual flow models of volcanic islands have to be reconsidered. The presence of such structures in a volcanic edifice recognized as low strength layer may help to explain the origin of potentially catastrophic landslides.