Using multiple environmental isotopic tracers to investigate groundwater flow and recharge from a mountain block to a basin-fill aquifer in semi-arid central Chile

The results of a multi-isotopic study are presented to understand the flow from a mountain block to an alluvial aquifer in a semi-arid climate, within the context of a prolonged megadrought in central Chile between 2010 and 2022. The research focused on the response of the Santiago de Chile alluvial aquifer to recharge from the Cordillera de los Andes, conducting a comparative analysis among the different sub-basins within the mountain block that reach the alluvial aquifer. These sub-basins vary in size, altitude, and the degree of incision into the mountain block. Tracers related to the specific lithological types recognized in the mountain block, which can provide specific chemical and isotopic signatures derived from water-rock interaction and hydrothermal processes, were selected to be traced downstream. Specifically, the molar Cl/Br ratio and isotopic tracers of δ¹⁸O and δ²H, ⁸⁷Sr/⁸⁶Sr, δ³⁴S and δ¹⁸O of dissolved sulfate, and δ⁷Li were used in surface and groundwater samples collected in the study area. The lighter isotopic values of δ¹⁸O and δ²H identified in the alluvial aquifer indicated that the primary recharge occurs through the infiltration of water from the Maipo and Mapocho rivers. The identification of Jurassic limestones and evaporites in the upper part of the Maipo River basin allows for the recognition of high Cl/Br ratios (7941) and more enriched δ³⁴S and δ¹⁸O values of dissolved sulfate (δ³⁴S = 10.9 ‰ and δ¹⁸O = 7.3 ‰) in the Maipo River waters. These values were also identified in the waters of the alluvial aquifer and in water samples obtained in the upper course of the Mapocho and Angostura rivers (basins that do not have the previously mentioned lithological types). The presence of these isotopic signals in the waters of the Mapocho and Angostura rivers indicates the possibility of inter-basin flow from the upper part of the Cordillera de los Andes to the Mapocho and Angostura river basins. This conclusion is also supported by the identification of lighter δ⁷Li isotopic values in the Mapocho River basin (8.2 ‰), which are found in the upper part of the Cordillera de los Andes, where thermal springs with similar δ⁷Li isotopic values exist, associated with active volcanism and magmatism. The ⁸⁷Sr/⁸⁶Sr isotopic ratio of surface and groundwater in the study area varies within a narrow range from 0.70375 to 0.70674. The higher ⁸⁷Sr/⁸⁶Sr isotopic ratio values identified in the waters of the alluvial aquifer are likely related to inputs of water from the Maipo River, which interacts with the limestone. The identification of high ⁸⁷Sr/⁸⁶Sr isotopic ratio values in the waters of the Mapocho River (0.704073) also suggests a possible inter-basin flow from the upper part of the Alto Maipo basin to the Mapocho River basin, although the mixing signal is not as clear as that of the other isotopes used in this study.