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.

As a major and popular groundwater extraction structure, seepage wells are often used to transfer river water into aquifers for harvesting water resources. It can help ameliorate the imbalance between supply and demand, especially in areas of water shortage. Large drawdowns due to pumping may cause the river to disconnect from the groundwater and to form an unsaturated zone, which seriously affects the efficiency of seepage wells. However, most of the current models of extraction structures of non-tube wells only account for saturated flow and do not consider unsaturated conditions. To address this limitation, a saturated-unsaturated coupling model has been developed using the exchange flow rate between the well pipe and the aquifer as the coupling point. Moreover, the model was evaluated with physical simulation test data. The statistical results indicated that the model can estimate the drawdown and pumping rate well with root-mean-square deviations (RMSD) of 0.0114 m and 0.0079 L/s, respectively, for a river with strong leakage capacity, and 0.0129 m and 0.0099 L/s, respectively, for a river with weak leakage capacity. The critical drawdown, where the river disconnects from the aquifer, as well as variations of the unsaturated zone, is also discussed. The present study provides important information for the design of seepage wells with reasonable drawdown while being able to predict the potential water yield, and at the same time help protect the groundwater environment.

As a major and popular groundwater extraction structure, seepage wells are often used to transfer river water into aquifers for harvesting water resources. It can help ameliorate the imbalance between supply and demand, especially in areas of water shortage. Large drawdowns due to pumping may cause the river to disconnect from the groundwater and to form an unsaturated zone, which seriously affects the efficiency of seepage wells. However, most of the current models of extraction structures of non-tube wells only account for saturated flow and do not consider unsaturated conditions. To address this limitation, a saturated-unsaturated coupling model has been developed using the exchange flow rate between the well pipe and the aquifer as the coupling point. Moreover, the model was evaluated with physical simulation test data. The statistical results indicated that the model can estimate the drawdown and pumping rate well with root-mean-square deviations (RMSD) of 0.0114 m and 0.0079 L/s, respectively, for a river with strong leakage capacity, and 0.0129 m and 0.0099 L/s, respectively, for a river with weak leakage capacity. The critical drawdown, where the river disconnects from the aquifer, as well as variations of the unsaturated zone, is also discussed. The present study provides important information for the design of seepage wells with reasonable drawdown while being able to predict the potential water yield, and at the same time help protect the groundwater environment.

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