Groundwater in the Qinghai–Tibet Plateau (QTP) is mainly distributed in the active layer. Rising temperatures and human activities have influenced the hydrogeochemical characteristics of the QTP in recent years. This study investigates the hydrochemistry and stable isotope (δ18O and δD) variations of the groundwater in the Beiluhe Basin which is located in the QTP of China during a freezing–thawing period of the active layer. Results show the chemical types of the groundwater are mainly HCO3 • Cl–Na • Ca • Mg, which are converted to Cl • HCO3–Na • Ca • Mg during the freezing period of the active layer. At different stages of the freezing–thawing period of the active layer, evaporation, concentration, and rock weathering control the chemical composition of the groundwater in this region. The main ion ratio coefficients of groundwater and the saturation indices of related minerals in groundwater indicate that halite, dolomite, and calcite control the relevant chemical components of water in the study area. The stable isotopic results show that δ18O and δD fluctuations in the lake water and the groundwater are mainly affected by groundwater runoff, temperature, and evaporation. This study provides the scientific basis for groundwater evolution and utilization in high-altitude areas.

With the gradual increase of global temperature, thermokarst lakes are widely developed and become major environmental disasters in the Tundra Plateau which have impacted the stability of the project such as the Qinghai–Tibetan highway. In this study, some typical thermokarst lakes in the Qinghai–Tibet Plateau (QTP) were selected as the research object. And four samples were taken from different freezing–thawing processes of the lakes in 2019 to analyze the hydrogeochemical process of the thermokarst lake in the context of climate change. Results show that the main hydrogeochemical types of the lake water in the northern part of the study area were HCO3·Cl − Na·Ca·Mg or Cl·HCO3 − Na·Mg, whereas in the central and southern parts were mainly Cl − Na·Mg. The variations of hydrogeochemical concentration in thermokarst lake water are mainly affected by evaporation concentration, rock differentiation, freezing desalination in the active layer, and plant photosynthesis, which are mainly due to temperature changes. Furthermore, the results of the saturation index (SI) show that dolomite and calcite leaching control the hydrogeochemical composition in thermokarst lakes. In addition, the evaporation-to-inflow (E/I) ratios of the lake reach the maximum in the middle and later periods of the active layer thawing. On the contrary, the E/I values of the lakes decrease during the initial thawing or freezing periods of the active layer.

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Thermokarst lakes and permafrost degradation in the Qinghai-Tibet Plateau (QTP) resulting from global warming have been considerably affected the local hydrological and ecological process in recent decades. Simulation with coupled moisture-heat models that follows talik formation in the Beiluhe Basin (BLB) in the hinterland of permafrost regions on the QTP provides insight into the interaction between groundwater flow and freezing-thawing process. A total of 30 modified SUTRA schemes have been established to examine the effect of hydrodynamic forces, permeability and climate. The simulated results show that the hydrodynamic conditions impact the permafrost degradation surrounding the lake, thereby further affecting groundwater flow and late-stage freezing-thawing process. The thickness of the active layer varies with time and location under different permeability conditions, which significantly influences the occurrence of a breakthrough of the lake bottom. Warmer climate accelerates thawing and decreases the required time of formation of the breakthrough zone. Overall, these results indicate that explicit consideration of hydrologic process is critical to improve the understanding of environmental and ecological changes in cold regions.

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