Departamento de Ingeniería Estructural y Geotécnica, Pontificia Universidad Católica de Chile, Santiago, Chile; Centro de Excelencia en Geotermia de los Andes (CEGA), Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile.
Centro de Excelencia en Geotermia de los Andes (CEGA), Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile; Instituto de Ciencias de la Ingeniería, Universidad de O'Higgins, Rancagua, Chile.
Sci Total Environ. 2021 Jun 1;771:145456. doi: 10.1016/j.scitotenv.2021.145456. Epub 2021 Jan 29.
The Mountain-Block Recharge (MBR), also referred to as the hidden recharge, consists of groundwater inflows from 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 Mountain Front Zones (MFZs), the hydrogeological limit between the mountain-block and adjacent alluvial basins is complex and, consequently, the groundwater flow-paths reflect that setting. To cope with the typical low density of boreholes in MFZs hindering a proper assessment of MBR, a combined geoelectrical-gravity approach was proposed to decipher groundwater flow-paths in fault-controlled MFZs. The study took place in the semiarid Western Andean Front separating the Central Depression from the Principal Cordillera at the Aconcagua Basin (Central Chile). Our results, corroborated by field observations and compared with worldwide literature, indicate that: (i) The limit between the two domains consists of N-S-oriented faults with clay-rich core (several tens of meters 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 basement faults (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 aquifers, which are recharged by indirect infiltration along ephemeral streams and focused inflows from oblique basement faults. This study also revealed seepage from irrigation canals highlighting their key role in the recharge of alluvial aquifers in the Central Depression. The proposed combined geophysical approach successfully incorporated (hydro)geological features and geophysical forward/inverse modelling into a robust hydrogeological conceptual model to decipher groundwater flow-paths in fault-controlled MFZs, even in the absence of direct observation points.
山区块补给（MBR），也称为隐式补给，由山区块向相邻冲积含水层的地下水流入组成。这是干旱环境中的一个重要补给过程，但由于从地面上无法察觉，因此经常被忽略。在断层控制的山前带（MFZ）中，山区块与相邻冲积盆地之间的水文地质边界复杂，因此地下水流动路径反映了这种情况。为了解决 MFZ 中典型的钻孔密度低，难以对 MBR 进行适当评估的问题，提出了一种结合地球物理和重力的方法来解析断层控制的 MFZ 中的地下水流动路径。该研究位于西安第斯山前带，该山前带将中央洼地与安第斯山脉主脉分隔开来，位于阿空加瓜盆地（智利中部）。我们的结果得到了实地观测的证实，并与全球文献进行了比较，表明：（i）两个区域之间的边界由 N-S 走向的断层组成，这些断层富含粘土（几十米宽的低电阻率亚垂直带），阻碍了弥散的 MBR。沿西安第斯山前带的“隐式补给”是通过（ii）集中的 MBR 过程实现的，这些过程包括：（ii.a）与 MFZ 斜交的开放和离散的基底断层（质量缺陷和泉水），这些断层横切 N-S 走向的断层；（ii.b）峡谷中高水力传导性的冲积走廊。冲积走廊中容纳狭窄的无约束山区含水层，这些含水层通过沿短暂溪流的间接渗透和来自斜交基底断层的集中补给来补给。本研究还揭示了灌溉渠的渗漏，强调了它们在中央洼地冲积含水层补给中的关键作用。所提出的综合地球物理方法成功地将（水文）地质特征和地球物理正/反演模型纳入到一个稳健的水文地质概念模型中，以解析断层控制的 MFZ 中的地下水流动路径，即使在没有直接观测点的情况下也是如此。
