Abstract

Conceptual models in volcanic island aquifers remain an issue in many regions of the world, as usually, little hydrogeological information is available about these areas. This research evaluates the influence of the geological evolution of San Cristóbal (558 km2, Galápagos archipelago) in its aquifers and springs using classical field geological methods combined with a 3D resistivity model. The subaerial basaltic volcanism of the island (2.35±0.03 to 0.66±0.08 My) formed in the southwestern region, an older large shield volcano (710 masl), and a younger flat region to the northeast. The study area is located in the windward slope of the inhabited southwestern region, exposed to the tradewinds, where the only permanent springs of the archipelago are present. The field description of the outcrops includes exposure conditions, geological material and weathering degree as well as any other relevant geological or hydrogeological observations. These features allow us to classify the outcrops by their weathering degree and to identify a spatial differentiation of more or less permeable areas. In the eastern sector of the studied area, outcrops appear more altered/weathered whereas in the western sector lava flows still show their primary structure. The geophysical data obtained during the electromagnetic survey with the SkyTEM heli-born device reveals the existence of: (i) one or several shallow aquifers, where groundwater flows southwards and intersects the topography at several points that correspond to observed springs; and (ii) a deeper aquifer showing a southwestern flowpath. We consider that, the alternation of eruptive and calm periods in the island has developed a complex multi-layered aquifer in which non-weathered lava flows, weathered lava flows and paleo-soils are intercalated. Therefore, even though the geological evolution of San Cristóbal is relatively simple, the different hydrogeological features of these materials give as a result a heterogeneous and anisotropic aquifer with permeability values that should be studied in the future.