Abstract

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, Sr-87/Sr-86 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 Sr-87/Sr-86 ratios between 0.70589-0.70732, indicating mixing of Jurassic marine carbonatic-evaporitic rocks and magmatic arc rocks. Conversely, the Mapocho Alto streams presented Sr-87/Sr-86 ratios between 0.70371-0.70417, indicating weathering of the magmatic arc rocks. Last, low Ge/Si ratios (< 1.5 mol/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 mu mol/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.