Analysis of changes in water chemistry characteristics and influencing factors of three major lakes in Inner Mongolia in the last decade

The three major lakes in Inner Mongolia Autonomous Region, Dali Lake, Ulan Suhai Lake and Hulun Lake, are significantly affected by geographical location and climatic environment. In particular, Dali Lake, as a tail lake, has evolved from a freshwater lake to a medium-salt lake. In 2022, Dali Lake, Ulan Suhai Lake and Hulun Lake were investigated, sampled and compared with the collected historical data. Piper three-line diagram, Gibbs diagram, spatial difference analysis, endmember diagram, ion ratio, and correlation analysis were used to study the hydrochemical characteristics and formation mechanism of the three lakes. The primary ion concentrations of the three lakes over the years were used to analyze the changes of the primary ion concentrations of the lakes in the past 10 years, and the reasons for the salinization of Dali Lake were analyzed from the perspective of environment. The results showed that: (1) The hydrochemical types of Dali Lake, Ulan Suhai Lake and Hulun Lake were ${\mathrm{HCO}}_3^- $-Na⁺-Cl⁻ type, ${\mathrm{HCO}}_3^- $-Cl⁻-Na⁺ type and ${\mathrm{HCO}}_3^- $-Na⁺-Cl⁻ type, respectively. (2) The spatial distribution characteristics of ions with large coefficient of variation in each lake were studied. The spatial distribution characteristics of ions (${\mathrm{HCO}}_3^- $ and total dissolved solid (TDS) ) in Dali Lake were high in the west and low in the east. The ions (${\mathrm{HCO}}_3^- $ and ${\mathrm{SO}}_4^{2-} $) in Ulan Suhai Lake generally showed a decreasing trend from the periphery to the middle. The spatial distribution of ions (${\mathrm{HCO}}_3^- $ and ${\mathrm{SO}}_4^{2-} $) of Hulun Lake was affected by the ion concentration in its inflow rivers. (3) The hydrochemical characteristics of the three lakes were mainly affected by evaporation and rock weathering. Among them, Dali Lake was affected by the combination of evaporite minerals and silicate minerals, Ulan Suhai Lake and Hulun Lake were mainly affected by silicate minerals, and the groundwater of Dali Lake had cation adsorption. (4) In the past 10 years, the ion concentrations of Dali Lake had been increasing year by year. The evaporation of the lake surface of Dali Lake was much larger than the precipitation of the lake surface, and the river recharge was decreasing year by year, which led to the shrinkage of the lake surface and the gradual increase of the salinization of the lake water. Fundamentally speaking, environmental factors reduced the amount of water entering the lake and increased the evaporation, ultimately accelerating the salinization of the lake.