Hydrogeology of the Galapagos Islands
Abstract
Due to the heterogeneity of geological formations, volcanic islands present complex and contrasting hydrogeological settings. A young discipline in the Galapagos, hydrogeology requires an understanding of geology, geomorphology, climate and hydrology. Throughout history, navigators, scientists and inhabitants noted the lack of surface freshwater; and water availability limited settlement of the islands. Today, this limitation is overcome through groundwater exploitation and expensive desalination, fed by economic growth. This shift has freed the field of hydrogeology from the existing premise of water being the principal drive for human development. Within this context, our approach is to lead a pluri-disciplinary research to characterize Galapagos hydrogeology. It involves a long-term commitment with international, national and local partners. Field investigations conducted on the inhabited islands of Santa Cruz, San Cristobal, Isabela and Floreana reveal three types of aquifers. A low-lying basal aquifer outcrops on Santa Cruz and Isabela. Due to the high permeability of fractured shield series forming coastal aprons, intruding sea water mixes with discharging freshwater, and confers a high salt content to groundwater. In order to characterize the hydraulic properties of this aquifer, the propagation of the tidal signal into the basal aquifer has been investigated through piezometric monitoring in three open coastal fractures and the deep well on Santa Cruz. Springs are scarce in the Archipelago, but have been identified historically on Santa Cruz and Floreana, located on the flanks of volcanic cones, and fed by small perched aquifers. On San Cristobal, high-level aquifers feed springs on the southern mountainside that contribute to a network of permanent rivers that reach the sea, a unique feature in the whole archipelago. They are independent from El Junco, a unique summital freshwater, and semi-endoreic lake. Internal resistivity structure of Santa Cruz and San Cristobal was revealed through a helicopter borne TEM geophysical survey using the SkyTEM system. An area of over 350 km2 was covered, with depth of investigation of up to 200 m. It allows visualization of 3-dimensional internal low-resistivity layers on the southern windward mountainsides and salt-water intrusion beneath both islands, which relate to some known hydrological features. However, validation of the resistivity model requires drilling of exploration boreholes. Evaporation, runoff and infiltration estimations were obtained from monitoring rainfall and surface runoff in the highlands of Santa Cruz. As an on-going study, new topics of interest are explored: soil properties and contribution of fog water in the hydrological budget; geomorphological context of groundwater emergence; and stable isotopes and noble gases characterization of water masses. The hydrogeology of the central islands provides a window to understand hydro-geomorphological evolution of the Galapagos Islands; and a link between their biological content and the physical environment.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V51D..06D
- Keywords:
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- 0900 EXPLORATION GEOPHYSICS;
- 1829 HYDROLOGY / Groundwater hydrology;
- 1876 HYDROLOGY / Water budgets;
- 8400 VOLCANOLOGY