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.

Mining activities that shape geographical patterns of biodiversity in individual regions are increasingly understood, but the complex interactions on broad scales and in changing environments are still unclear. In this study, we developed a series of empirical models that simulate the changes in biodiversity and ecosystem functions in mine-affected regions along elevation gradients (1500-3600 m a.s.l) in the metal-rich Qilian Mountains (~800 km) on the northeastern Tibetan Plateau (China). Our results confirmed the crucial role of potentially toxic elements (PTEs) dispersal, topography, and climatic heterogeneity in the diversification of plant community composition. On average, 54% of the changes in ecosystem functions were explained by the interactions among topography, climate, and PTEs. However, merely 30% of the changes were correlated with a single driver. Plant species composition (explained variables = 94.8%) changed more in lowland than in highland areas. The changes in species composition (explained variables = 94.8%) in the PTE-polluted habitats located in the warm low-elevation deserts and grasslands were greater than those occurring in the alpine deserts and grasslands. The ecosystem functions (soil characteristics, nutrient migration, and plant biomass) experienced greater changes in the humid low-elevation grasslands and alpine deserts. The effect of topography (i.e., slope and aspect) was less important than that of climatic heterogeneity (MAT, win10, and ET0) in predicting ecosystem functions of PTE-polluted habitats. Our results suggest that the processes driven by climate or other factors can result in high-altitude PTE-affected habitat expansions or contractions.