Groundwater resources of the Western Andean Front: insights from the Aconcagua Basin, Central Chile

Abstract

The misunderstanding of hydrogeological processes together with the oversimplification of aquifer conceptual models result in numerous inaccuracies in the management of groundwater resources. Despite that Chile is a mountainous country, hydrogeological studies have exclusively focused to alluvial deposits in valleys (~15 % of total area of Chile) and mountain front zones are considered arbitrarily impermeable. Given that in arid zones (and semiarid) a significant component of the alluvial aquifers recharge occurs along the mountain front zone, this PhD thesis aims to develop a reliable conceptual mode that explains the Western Andean Front hydrogeological functioning. The Aconcagua Basin was selected because there several perennial springs show evidence of groundwater flows in the Western Andean Front. The major springs outflow along the NS-oriented Pocuro Fault Zone (PFZ), which separates the volcanic rocks of the Principal Cordillera from the alluvial deposits of the Central Depression. Thus, the study was addressed by means of: i) hydrogeochemistry and water stable isotope; ii) structural geology; and iii) a topological approach and fracture analysis. The groundwater circulation and recharge processes occurring at the Western Andean Front were addressed using hydrogeochemical and water stable isotope analyses of 23 perennial springs, 10 boreholes, 5 rain-collectors and 5 leaching-rocks samples. Most groundwater in the Western Andean Front is HCO3-Ca and results from the interaction with Ca-silicate. The Hierarchical Cluster Analysis groups the samples according to its elevation along the Western Andean Front and supports a clear correlation between the increasing groundwater mineralization (31 1188 µS/cm) and residence time. Whereas, the Factorial Analysis point that Cl, NO3, Sr and Ba concentrations seems to be related to agriculture practices in the Central Depression. After defining the regional meteoric water line at 33°S in Chile, water isotopes demonstrate the role of rain and snowmelt above ~2000 m asl in the recharge of groundwater. Also, irrigation canals contribute to the high-altitude isotopic signature in the alluvial aquifer of Central Depression. The multi-scale mapping of fractures (from regional to the outcrop scale) was conducted in the PFZ. Three non-coetaneous major tectonic features were recognized within PFZ: i) NS oriented normal-sinistral faults with sealed fractures presence (laumontite, quartz and calcite), ii) NS oriented reverse faults consisting in shear bands (gouge) of 30 60 cm thick, and iii) NW oriented reverse faults consisting in open fractures plane. Then, topology allows for quantification of the density of connected fractures within the PFZ and its relationship with groundwater circulation. The study results identify two areas of high density of connected fractures that are related to the main springs of the PFZ: Termas de Jahuel (discharge ~14.0 m3/h at 22 °C) and Termas El Corazón (discharge ~7.2 m3/h at 20 °C). Outcrop-scale mapping reveals that groundwater outflows from NW reverse faults, which is consistent with the preferential orientation of the fracture network (N30 60W) within the PFZ. Thus, while NS oriented faults act as a hydraulic barrier, the NW oriented faults are high-permeability axes for groundwater circulation. Finally, an original conceptual model applicable to the entire Central Chile is proposed. The water releases from high-elevation areas infiltrate in mid-mountain gullies feeding groundwater circulation in the fractured rocks of Western Andean Front. To the downstream, mountain-block recharge occurs through NW-reverse faults taking advantage of the high density of connected fractures of the PFZ. Likewise, mountain-front processes recharge the alluvial aquifers in the piedmont zones through the focused infiltration of perennial and ephemeral streams.