Hydrogeochemical characteristics and processes of thermokarst lake and groundwater during the melting of the active layer in a permafrost region of the Qinghai–Tibet Plateau, China

Permafrost degradation and the development of thermokarst lakes are important factors driving the variability of regional hydrologic processes. Hydrogeochemical and isotopic analyses are important methods for investigating the hydrologic processes of thermokarst lakes. This study focused on comparing the chemical and hydrogeochemical characteristics between lake water and groundwater during the melting of the active layer in a typical thermokarst lake region on the Qinghai–Tibet Plateau (QTP). Ninety-five samples were collected during different periods of active layer melting and analyzed using statistical, isotope, hydrogeochemical, and modeling methods. Statistical results showed that the average concentrations of almost all ions were lower in lake water than in groundwater, with wider spatial variability in groundwater. The lake water is of the ClNa and HCO₃-Ca type with low TDS (total dissolved solids), whereas groundwater is of the HCO₃-Ca and mixed type (or transition type) with high TDS. The chemical types of the lake water and groundwater are mainly driven by rock weathering. In terms of the saturation index (SI), halite and gypsum are unsaturated dissolved, whereas dolomite and calcite are generally saturated. Evaporation significantly affects the chemical composition of groundwater, while the hydrochemical compositions of lake water are relatively stable under the joint control of evaporation, precipitation, surface runoff, and groundwater. The isotopic analysis results showed that the contribution of permafrost meltwater and precipitation to groundwater and lake water varied during different stages of active layer melting. According to hydrogeochemical modeling, the main chemical reactions in groundwater are the precipitation of calcite and the dissolution of halite, dolomite, and gypsum. The intensity of groundwater flow determines the degree of chemical reactions along the flow path at different stages of active layer melting. The findings can provide deeper insight into hydrogeochemical processes in thermokarst lake regions under the background of permafrost degradation.