With the increasing demand for water resources, the utilization of marginal water resources of poor-quality has become a focus of attention. The brackish water developed in the Loess Plateau is not only salty but also famous for its "bitterness". In the present work, multi-isotope analysis (Sr, B) was combined with geochemical analysis to gain insight into the hydrogeochemical evolution and formation mechanisms of brackish water. These results demonstrate that groundwater in the headwater is influenced by carbonate weathering. After the confluence of several tributaries in the headwater, the total dissolved solids (TDS) of water is significantly increased. The dissolution of evaporates is shown to be the main source of salinity in brackish water, which also greatly affects the Sr isotopic composition of water. This includes the dissolution of Mg-rich minerals, which is the main cause of the bitterness. Furthermore, the release of calcium from the dissolution of gypsum may induce calcite precipitation and incongruent dissolution of dolomite, which also contributes to the enrichment of magnesium. The highly fractionated boron isotopic values observed in the upstream groundwater were explained by boron interacting with clays, illustrating the important role played by the cationic exchange reaction. The inflow of brackish groundwater is the source of the observed quality of the river water. River water with relatively enriched 11B contents reflects the occurrence of evaporation along the flow path of the river. This process further aggravates the salinization of river water, with water quality evolving to saline conditions in the lower reach. When the river reaches the valley plain, the 87Sr/86Sr ratios decreases significantly, which is primarily related to erosion of the riverbanks during runoff. These results indicate that water resource sustainability could be enhanced by directing focus to mitigating salinization in the source area of the catchment.

Abstract. Climate warming accelerates the global water cycle. However, the relationships between climate warming and hydrological processes in the alpine arid regions remain unclear. Herein, high spatiotemporal resolution sampling of surface water and groundwater was performed at the Qaidam Basin, an extremely arid area in the northeastern Tibetan Plateau. Stable H-O isotopes and radioactive 3H isotopes were combined with atmospheric simulations to examine climate change and hydrogeological characteristics. The surface water heavy isotopes enrich during the wet season and deplete during the dry season. The contribution of precipitation to river discharge was considerably higher in the eastern region of the basin (approximately 45 %) than in the central and western regions (10 %–15 %). The H-O isotopic compositions showed a gradually negative spatial pattern from the west to the east in the Eastern Kunlun Mountains water system; a reverse pattern occurred in the Qilian Mountains water system. This distribution pattern was jointly regulated by the westerly water vapor transport intensity and local hydrothermal conditions. Increased precipitation and cryosphere shrinkage caused by climate warming mainly accelerated basin groundwater cycle. In the eastern and southwestern Qaidam Basin, precipitation and ice/snow meltwater infiltrate structural channels that favor water flow, such as fractures and fissures, facilitating rapid seasonal groundwater recharge and increased terrestrial water storage. However, under future increases in precipitation in the southwestern Qaidam Basin, compensating for water loss from long-term melting of ice and snow will be challenging, and the total water resources may show an initially increasing and then decreasing trend.