Some new insights offered by Discrete Fracture Network simulation into the characterization of the Santa Cruz basal aquifer, Galapagos- Ecuador
Résumé
This work forms part of a more general research program GIIWS (ANR) lead by UPMC-Sorbonne Universités, dedicated to the evaluation of the hydrogeological potential of volcanic islands, and in particular of the occurrence of groundwater and/or freshwater aquifers in the Galapagos Archipelago, Ecuador. Indeed water resource problems there are rising very fast, in conjunction with the local economic development driven by the tourism industry. Drinkable water shortage and wastewater management problems have to be urgently faced. We focus here on the south-eastern part of Santa Cruz Island close to Puerto Ayora city, which benefited from a very exhaustive evaluation program . The island is formed by sequences of fractured basaltic lava flows, with some occurrence in the southern part of faulted zones of normal type, with opening and/or escarpments, which may reflect extensive periods. These zones, kilometric in size are known as grietas, and were favoured until now for water exploitation of the basal aquifer body. It was observed that these structures do not assemble at large into a connected network, but some of them may have a direct connection to the sea. At the hectometre scale, seismic refraction and P wave seismic velocity analysis has been used to characterize an effective medium away from grietas, with a mean crack density and an equivalent porosity . Both types of structures, cracks and faults, are embedded in a Discrete Fracture Network approach , and a number of numerical hydraulic experiments are performed to discuss various hydraulic observations (tidal attenuation at various locations) and to provide an estimation of their hydraulic properties. Piezometric levels that correspond to depth of the fresh/salted interface as delineated by low resistivity zones identified by heli-borne geophysical survey are also addressed. It comes out from our analysis that rock mass permeability might be in the range of 10-4 to 10-3 m/s, that the aquifer is not isotropic and might behave as a compartmentalized object being easily drained toward south east or south west by the large faulted zones of high diffusivity.
It is therefore likely that recharge areas identified by rainfall gradients contribute to the basal aquifer recharge, but further inland than the location of Pozo Profundo deep well, and that these fresh water most probably migrates along fracture corridors delineated by faults toward the south-eastern shore line. Finally, we consider that these findings should help to define further drilling exploration phases.