CONICET | Buscador de Institutos y Recursos Humanos

Streams in semiarid mountains are the main water supply in Northwestern Argentina (Llanes et al., 2022). Understanding the variability in their isotopic and geochemical composition is important for assessment on water quality and management. This is crucial in this kind of environments, where water scarcity is rising due to climate change. We report isotopic and hydrochemical data of the Huaco stream (Sierra de Velasco, at La Rioja province), which provides water to the capital city of the province. The catchment is 775 km2 of size, and has a longitudinal shape with a NNW-oriented long axis. The geology is mainly composed of slightly peraluminous to peraluminous A-type granites of early carboniferous age (Grosse et al., 2009; Macchioli Grande et al., 2020 ). The climate is semiarid, with a wet season during the summer and a dry season during the winter. We collected samples on a seasonal basis between April 2021 (end of wet season) to October 2022 (end of dry season), from four sites along an altitudinal gradient, from 1500 to 1100 m asl. They are at the localities of Las Peñas (1500 m asl), Huaco (1240 m asl), Finca Vista Larga (1175 m asl), and Sanagasta (1110 m asl). Streamflow was also measured at each sampling site. In general, stream Electrical Conductivity values range from 89 to 438 mg/L, and Cl varies between 1.7 and 143.0 ppm. δ18O values range between -5.44 and -0.09 , with a mean value of -4.29 ± 1.63 (n = 26), and δ2H varies between -25.18 and -9.99 , with a mean value of -21.34 ± 4.37 (n = 26). The d-excess ranges between -9.27 and 18.92 with a mean value of 13.01 ± 8.83 . Interestingly, the averages from each sampling site reveal an increase in electrical conductivity and chloride values with decreasing altitude. Streamflow decreased with increasing catchment size. The dual δ18O vs δ2H plot reveals that water from the wet and dry periods of the same hydrological year plot closely together, and above the Global Meteoric Water Line except for samples from June 2021 (dry season). This suggests a uniform isotopic composition of stream waters at intra and inter-annual scale. Moreover, the Huaco stream samples show similar isotopic signature than dry season samples from other Sierras Pampeanas streams (Poca et al., 2020).Furthermore, the averages between the wet and dry seasons of electrical conductivity, chloride, δ18O and δ2H show no significant differences. To better understand the seasonal dynamics of this stream, we measured major ions from samples from the end of wet season of 2022 and end of dry season of the same year. Surface waters are diluted, and mixed bicarbonate in composition. The results revealed no significant differences between both seasons, and an increment on total dissolved solids, electrical conductivity and fluoride with increasing catchment size (downstream). These findings are in agreement with other semi-arid mountain catchments, which exhibit similar values throughout the year, despite inter-annual variability in isotopic composition of rainwater (Jin et al., 2012). The same pattern is observed for major ions stream chemistry, in consistence with other rivers located in the same range, north from the study area (Uran et al., 2022). The uniform isotopic and chemical composition indicates that groundwater is the dominant source in these streams, as reported for other semiarid mountain catchments (e.g. Hrachowitz et al., 2011; Mokua et al., 2020). In addition, increasing total dissolved solids and electrical conductivity, may indicate that stream waters undergo evaporative concentration downstream.

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