Deciphering groundwater flow-paths in fault-controlled semiarid mountain front zones (Central Chile)

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 environments,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 complexand, consequently, the groundwater flow-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 proposedto decipher groundwater flow-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 by field observations and comparedwithworldwide 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 theWestern 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) highhydraulictransmissivity alluvial corridors in canyons. Alluvial corridors host narrow unconfined mountain aquifers,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 rechargeof alluvial aquifers in the Central Depression. The proposed combined geophysical approach successfully incorporated (hydro)geological features and geophysical forward/inverse modelling into a robusthydrogeological conceptualmodel to decipher groundwater flow-paths in fault-controlledMFZs, even in the absenceof direct observation points