Deciphering groundwaterflow-paths in fault-controlled semiaridmountain front zones (Central Chile)

Abstract

The Mountain-Block Recharge (MBR), also referred to as the hidden recharge, consists of groundwater inflowsfrom the mountain block into adjacent alluvial aquifers. This is a significant recharge process in arid environ-ments, but frequently discarded since it is imperceptible from the ground surface. In fault-controlled MountainFront Zones (MFZs), the hydrogeological limit between the mountain-block and adjacent alluvial basins is com-plex and, consequently, the groundwaterflow-paths reflect that setting. To cope with the typical low density ofboreholes in MFZs hindering a proper assessment of MBR, a combined geoelectrical-gravity approach was pro-posed to decipher groundwaterflow-paths in fault-controlled MFZs. The study took place in the semiaridWestern Andean Front separating the Central Depression from the Principal Cordillera at the Aconcagua Basin(Central Chile). Our results, corroborated byfield observations and compared with worldwide literature, indicatethat: (i) The limit between the two domains consists of N-S-oriented faults with clay-rich core (several tens ofmeters width low electrical-resistivity subvertical bands) that impede the diffuse MBR. The“hidden recharge”along the Western Andean Front occurs through (ii) focused MBR processes by (ii.a) open and discrete basementfaults (mass defect and springs) oblique to the MFZ that cross-cut the N-S-oriented faults, and (ii.b) high-hydraulic transmissivity alluvial corridors in canyons. Alluvial corridors host narrow unconfined mountain aqui-fers, which are recharged by indirect infiltration along ephemeral streams and focused inflows from obliquebasement faults. This study also revealed seepage from irrigation canals highlighting their key role in the re-charge of alluvial aquifers in the Central Depression. The proposed combined geophysical approach successfully incorporated (hydro)geological features and geophysical forward/inverse modelling into a robusthydrogeological conceptual model to decipher groundwaterflow-paths in fault-controlled MFZs, even in the ab-sence of direct observation points.