Solutes and Water Sources, and Concentration-Discharge Relations in Semiarid Mountain Catchments in the Andes

Abstract

The chemical composition of stream water is a result of water-rock interactions, climatic forcing, and water transit times in the catchment. Here we examine Sr isotopic ratios, water stable isotopes, and concentration-discharge relations at the Maipo (Melosas, Toyo, Yeso, and Volcán streams) and Mapocho (Molina, Covarrubias, and Mapocho streams) headwater catchments in the Andean Cordillera in Central Chile. The ⁸⁷Sr/⁸⁶Sr ratios of the stream water samples from the Maipo watershed present an average of 0.70653 (n = 29), suggest provenance from the Mesozoic sedimentary rocks, of both marine and continental origin (i.e., Lo Valdés and Río Colina Formations), with mean ratios of 0.70728 and 0.70595, respectively. In the Mapocho watershed, the stream water samples show 87Sr/86Sr ratios with an average of 0.70397, pointing to a less radiogenic source, such as the local Miocene volcanic rocks of the Farellones and Abanico Formations, with relations of 0.70497 and 0.70368, respectively. Water stable isotopes indicate different degrees of diverse source mixing in the studied streams. All the Maipo streams, plus the Molina and Mapocho streams at the Mapocho watershed exhibit a more depleted signature in heavy isotopes, indicating a higher contribution from snowmelt. However, the Covarrubias stream in the Mapocho area shows a more enriched signature in heavy isotopes, suggesting a higher precipitation input. The concentration-discharge relations of major ions and trace elements exhibit different behavior at the different areas. At all the streams in the Maipo watershed, Ca, Mg, sulfate, and Sr show a chemostatic behavior; Na, chloride, and Si show a high dispersion, being Si close to chemostasis, similar to alkalinity, and Na and chloride to enrichment. At the Mapocho area, Ca, Mg, sulfate, chloride and Sr exhibit an enriching pattern. Na, alkalinity and Si exhibit a chemostatic pattern except for the Mapocho stream which shows dilution. These results indicate that solute sources are controlled by the lithological units at each region (silicates vs silicates plus marine formations), with mixed, snowmelt-dominated water sources.