Solute generation and export to rivers are a result of different processes that occur in the Critical Zone. Chemical weathering, climatic forcing, and biotic effects control solute concentration in streams. This paper presents a comprehensive dataset of major ions, 87Sr/86Sr ratios and Ge/Si ratios from the Maipo River watershed in the Andes of Central Chile, a water-stressed region that has undergone a decade-long megadrought and overexploitation. To assess the spatial and temporal variability of solute sources and fluxes, we conducted sampling in the period August 2021-August 2022 at the selected Mapocho Alto and Maipo Alto sub-watersheds. Concentration-discharge relationships revealed mostly diluting behavior for most solutes in the streams at the Maipo Alto sub-watershed, except for the Melosas stream with enriching pattern, and the Yeso stream that showed chemostasis. Solutes in the Mapocho Alto sub-watershed were mostly chemostatic to weakly diluting. Nitrate exhibited predominantly enriching behavior. Regarding Sr isotopes, the Maipo Alto sub-watershed streams yielded 87Sr/86Sr ratios between 0.70589–0.70732, indicating mixing of Jurassic marine carbonatic-evaporitic rocks and magmatic arc rocks. Conversely, the Mapocho Alto streams presented 87Sr/86Sr ratios between 0.70371–0.70417, indicating weathering of the magmatic arc rocks. Last, low Ge/Si ratios (< 1.5 µmol/mol) suggested incongruent weathering and equilibrium between water and clays at the Mapocho Alto streams, while at the Maipo Alto streams, higher Ge/Si ratios (up to 45 µmol/mol) imply action of hydrothermal fluids or sulfide dissolution. This study provides a geochemical baseline that can be used for water quality management, especially in semiarid regions undergoing water stress. Graphical Abstract The Maipo River watershed in the Central Chilean Andes is the main water supply for the most populated region in Chile where the capital city Santiago is located with over 8 million people (INE, Censo 2024. National Institute of Statistics, Chile. <a href="https://censo2024.ine.gob.cl/">https://censo2024.ine.gob.cl/</a>, 2024). It also provides solutes and sediments from the Main Cordillera to the Pacific Ocean. By means of concentration-discharge (C-Q) relationships of major ions and trace elements, Sr isotopes (87Sr/86Sr ratios), and Ge/Si ratios, we studied two sub-watersheds with contrasting lithology: the Maipo Alto and the Mapocho Alto sub-watersheds, which are shown in the map. C-Q relationships of the Maipo Alto sub-watershed (red shade) indicate dilution patterns, reflecting the more soluble and reactive nature of the bedrock. By contrast, the Mapocho alto sub-watershed (blue shade) shows mostly a chemostatic behaviour, meaning that solute concentrations have reached thermodynamic equilibrium. Moreover, 87Sr/86Sr ratios suggest that mostly carbonatic-evaporitic rocks provide solutes in the Maipo Alto sub-watershed (red field), whereas volcanic (silicic) rocks are the main solute sources in the Mapocho Alto sub-watershed (blue field). Last, low Ge/Si ratios indicate that incongruent weathering dominates in the Mapocho Alto sub-watershed (blue field). In the Maipo alto sub-watershed, however, Ge is supplied by hydrothermal fluids and/or sulphide dissolution, yielding high Ge/Si ratios (red field). Therefore, lithology and climate/hydrology control solute contents. Understanding solute behaviour is crucial to improve water quality assessments and geochemical baselines, especially in such a densely populated area, which currently undergoes water scarcity due to overexploitation and a decadal drought scenario. This dataset can be used by policy makers, regional governments and other public affairs decision makers of Central Chile, who deal with water resources management.

Solute Generation and Transport in Semiarid Mountain Catchments of the Central Chilean Andes

期刊论文
发表日期：

2025年5月12日

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Marcos Macchioli-Grande,
Agustín Soto-Maass,
Marco Pfeiffer,
José Ignacio Covarrubias,
Amanda Peña-Echeverría,
Alida Perez-Fodich

摘要

Solute generation and export to rivers are a result of different processes that occur in the Critical Zone. Chemical weathering, climatic forcing, and biotic effects control solute concentration in streams. This paper presents a comprehensive dataset of major ions, ⁸⁷Sr/⁸⁶Sr ratios and Ge/Si ratios from the Maipo River watershed in the Andes of Central Chile, a water-stressed region that has undergone a decade-long megadrought and overexploitation. To assess the spatial and temporal variability of solute sources and fluxes, we conducted sampling in the period August 2021-August 2022 at the selected Mapocho Alto and Maipo Alto sub-watersheds. Concentration-discharge relationships revealed mostly diluting behavior for most solutes in the streams at the Maipo Alto sub-watershed, except for the Melosas stream with enriching pattern, and the Yeso stream that showed chemostasis. Solutes in the Mapocho Alto sub-watershed were mostly chemostatic to weakly diluting. Nitrate exhibited predominantly enriching behavior. Regarding Sr isotopes, the Maipo Alto sub-watershed streams yielded ⁸⁷Sr/⁸⁶Sr ratios between 0.70589–0.70732, indicating mixing of Jurassic marine carbonatic-evaporitic rocks and magmatic arc rocks. Conversely, the Mapocho Alto streams presented ⁸⁷Sr/⁸⁶Sr ratios between 0.70371–0.70417, indicating weathering of the magmatic arc rocks. Last, low Ge/Si ratios (< 1.5 µmol/mol) suggested incongruent weathering and equilibrium between water and clays at the Mapocho Alto streams, while at the Maipo Alto streams, higher Ge/Si ratios (up to 45 µmol/mol) imply action of hydrothermal fluids or sulfide dissolution. This study provides a geochemical baseline that can be used for water quality management, especially in semiarid regions undergoing water stress.

Graphical Abstract

The Maipo River watershed in the Central Chilean Andes is the main water supply for the most populated region in Chile where the capital city Santiago is located with over 8 million people (INE, Censo 2024. National Institute of Statistics, Chile. https://censo2024.ine.gob.cl/, 2024). It also provides solutes and sediments from the Main Cordillera to the Pacific Ocean. By means of concentration-discharge (C-Q) relationships of major ions and trace elements, Sr isotopes (⁸⁷Sr/⁸⁶Sr ratios), and Ge/Si ratios, we studied two sub-watersheds with contrasting lithology: the Maipo Alto and the Mapocho Alto sub-watersheds, which are shown in the map. C-Q relationships of the Maipo Alto sub-watershed (red shade) indicate dilution patterns, reflecting the more soluble and reactive nature of the bedrock. By contrast, the Mapocho alto sub-watershed (blue shade) shows mostly a chemostatic behaviour, meaning that solute concentrations have reached thermodynamic equilibrium. Moreover, ⁸⁷Sr/⁸⁶Sr ratios suggest that mostly carbonatic-evaporitic rocks provide solutes in the Maipo Alto sub-watershed (red field), whereas volcanic (silicic) rocks are the main solute sources in the Mapocho Alto sub-watershed (blue field). Last, low Ge/Si ratios indicate that incongruent weathering dominates in the Mapocho Alto sub-watershed (blue field). In the Maipo alto sub-watershed, however, Ge is supplied by hydrothermal fluids and/or sulphide dissolution, yielding high Ge/Si ratios (red field). Therefore, lithology and climate/hydrology control solute contents. Understanding solute behaviour is crucial to improve water quality assessments and geochemical baselines, especially in such a densely populated area, which currently undergoes water scarcity due to overexploitation and a decadal drought scenario. This dataset can be used by policy makers, regional governments and other public affairs decision makers of Central Chile, who deal with water resources management.