Técnicas geofísicas aplicadas a la evaluación de acuíferos profundos en la provincia de Entre Ríos, Argentina

DAPEÑA, CRISTINA; FAVETTO, ALICIA; POMPOSIELLO, MARIA CRISTINA

ASUNCIÓN, PARAGUAY

Congreso; VIII Congreso Latinoamericano de Hidrología Subterránea.; 2006

ASOCIACIÓN LATINOAMERICANA DE HIDROLOGÍA SUBTERRANEA PARA EL DESARROLO (ALHSUD)

Geophysical exploration methods are commonly used in geothermal research, since these non-invasive tools are very useful in providing detailed information about sedimentary basins.  The most used methods in hydrogeology and environmental research are the Schlumberger array Vertical Electrical Sounding (VES) and the multielectrode Dipole-Dipole (DD) or Wenner array profiling. These methods provide electrical resistivity models (1D), in the first case, and tomographies (2D), in the second case. VESs are generally interpreted through horizontal-layered models that allow the correlation of geological data with electrical resistivity. In Argentina, deep thermal aquifers are manly exploited for recreation and balneotheraphy purposes. This geothermal resource is usually related to a large transboundary groundwater reservoir shared by Brazil, Paraguay, Uruguay and Argentina, and known as the Guaraní Aquifer System (GAS). This aquifer is part of the Paraná and Chacoparanense sedimentary basins. The geological units related to this system are the Triassic-Jurassic eolian and fluvial sandstones of the Piramboia and Botucatu Formations, and the Serra Geral Basalts which present clastic intercalations. This effusive Cretaceous complex covers the sandstones and provides a high-confinement degree. The GAS is mainly represented by the confined sandstones. Nevertheless, some authors also include both the clastic intercalations found within the volcanic rocks, and the fractured basalts. The geological, hydrogeological, hydraulic, hydrochemical, and thermal characteristics of the GAS vary throughout the basin, and are controlled by the different conditions of the deposition environments, the structural evolution, and the water residence time. Geoelectrical methods were used to describe the geometry of the geological units and to determine the depth of the crystalline basement, the presence of intercalated clastic deposits, and the existence of thermal waters in the southeast of the Entre Rios province, Argentina. Geoelectrical models were obtained from a set of fourteen Vertical Electrical Soundings carried out in the vicinity of Gualeguaychú, Aldea San Antonio, Concepción del Uruguay, and San José towns. These models indicate that the basement is at a depth of around 900 m, except in those places where the sedimentary sequence does not appear, in which case it is found between 470 and 800 meters, and no low-resistivity value layer associated to the thermal-aquifer bearing sediments was found. This structural setting is similar to that found in Colón and differs from the one found in Villa Elisa where there is a 1030m deep well that did not reach the basement. In the remaining localities, a conductive layer that could hold the thermal aquifer was identified beneath the more resistive basalts. The depth of the basement was also studied at a magnetotelluric East-West profile measured near Villaguay town and was later compared with the drilling profile of the thermal well. Analyzing all the features and the results obtained, we conclude that the main factor determining the presence of the sediments which contain the Guarani Aquifer System in this area is the structural setting which is directly related to the depth of the crystalline basement.