Evaluation of the hydrogeological response to neotectonics of the Quequén Grande River Watershed using geoelectrical methods

Abstract

The Quequén Grande River Watershed (Southeast of Buenos Aires Province, Argentina) is a strong asymmetrical watershed in the Pampean Plain. It is marked by the stream capture of the Quequén Grande River eastward. The topographic slopes and hydraulic gradients are typically low in this area, with streams and rivers primarily effluent along their courses. This behavior is attributable to the influence of the unconfined and shallow Pampean aquifer, which serves as the region's main source of water supply. In the southwestern limit of this drainage basin, several shallow lakes, disconnected from the surface drainage network and related to an old deflation/accumulation relief, occur over hillock formations. In this area, Paleozoic rocks crop out and constitute the hydrological basement of the aquifer. The bedrock architecture of this formation could explain the stream piracy of the Quequén Grande River eastward and the local geomorphology, where structural lineaments related to neotectonic movements have been reported. However, there is a lack of information related to the hydrological basement in depth, which is needed to understand the surface water dynamics of this drainage basin, linked to the topographic control of the landscape. To fill this gap, a geoelectric model of the hydrological basement is proposed from 37 Vertical Electrical Soundings and 6 Electric Resistivity Tomographies, aiming to contribute to understanding this portion of the Earth's crust. Geoelectrical surveys were performed over 3000 km2 of the southwestern limit of the Quequén Grande River Watershed. Results obtained and field observations suggest the deviation of the Quequén Grande River catchment eastward is caused by the hydrogeological basement structure in depth, in response to neotectonic movements inferred by structural lineaments.