Groundwater Contributions to Baseflow in a Mountain Stream of the Central Andes: Insights From Hydrochemistry, Stable Isotopes and Radon

Climate change is altering the hydrological dynamics of mountain systems by modifying precipitation regimes, increasing temperatures and accelerating snow and ice melt. In the Central Andes, these processes directly affect water availability in semiarid basins that strongly depend on cryospheric inputs. In this context, groundwater (GW) is increasingly recognised as a potential buffer against hydrological variability, although its interaction with surface water (SW) in high‐mountain environments remains poorly constrained. This study investigates SW–GW interactions in the San Alberto stream, Uspallata basin (Central Andes, Argentina), using a multi‐tracer approach that combines hydrochemistry, stable water isotopes and 222 Rn under high‐ and low‐flow conditions across different hydrological years. Surface water samples collected along a 15 km stream reach were predominantly characterised as Ca–SO 4 waters, shifting towards a Ca–SO 4 –HCO 3 facies during wetter hydrological years. Stable isotope data indicate that snowmelt is the primary recharge source for both SW and GW, as reflected by the similarity among snow, stream water and groundwater isotopic signatures. Spatial variations in stream‐water 222 Rn activity enabled the identification of groundwater discharge zones and their differentiation from lateral subsurface and parafluvial contributions. Radon mass balance calculations estimated cumulative groundwater inflows of 6307 m 3 day −1 during high‐flow conditions (95% uncertainty interval: 4250–8331 m 3 day −1 ), decreasing to 2880 and 1375 m 3 day −1 during low‐flow conditions in October 2018 and September 2021, respectively (95% uncertainty intervals: 2330–3879 and 899–1734 m 3 day −1 ). Despite these lower volumetric inflows, the highest stream‐water 222 Rn activities occurred during low‐flow periods due to reduced dilution, indicating that groundwater remained the main component sustaining baseflow. Differences between radon‐ and water‐mass balance estimates further suggest additional contributions from parafluvial and lateral subsurface flows lacking measurable 222 Rn activity. These results improve the conceptual understanding of SW–GW interactions in fractured Andean headwaters and provide a basis for water resources management under increasing hydroclimatic stress.