Identification of Hydrogeochemical Processes and Controlling Factors in Groundwater and Surface Water Using Integrated Approaches, Tuul River Basin (Ulaanbaatar, Mongolia)

Abstract

Insights into hydrogeochemical evolution and its controlling factors are crucial for sustainable water resources management in many countries worldwide, especially in arid and semi-arid regions. Here we present the hydrogeochemical characteristics of surface and subsurface water resources in a semi-arid region of Mongolia using stable isotopes, hydrochemistry, and multivariate statistics. A total of 42 surface water and 87 groundwater samples were collected during the 2016 summer and winter seasons, and major ions and stable isotopes (δ2H, δ18O) were analyzed. Hierarchical Cluster Analysis (HCA) classified water samples into 4 clusters (C1–C4) according to hydrochemical facies. Four principal components in summer and three principal components (PC1 through PC4) in winter were identified which explained 86.9 and 83.8% of geochemistry variance. In summer, PC1 represents the pollution related to domestic and industrial waste, and mineral weathering processes and rock-water interactions, PC2 indicates the regional flow system with strong loading of stable isotopes and mineral weathering, PC3 represents silicate weathering and pollution, and PC4 indicates physical processes in surface water samples, showing high positive loading for pH and temperature. In the winter, the number of factors decreased, indicating the minor variation in the chemical composition of water samples due to the freezing of the river and solid-state precipitation, and natural processes more influenced the behavior than the summer season. In winter, PC1 and PC2 represent mineral dissolution and anthropogenic inputs, whereas PC3 has positive loading of pH, negative loading of temperature indicates physical processes. The scatter diagrams show that carbonate and silicate weathering, and ion exchange are the dominant processes controlling the mineral water quality For this study area, the correlations indicate that water mineralization increases from the groundwater recharge area to the discharge area; and mineral weathering processes resulting in dissolution dominated, followed by human impacts associated with municipal waste discharges that also elevated the mineral concentrations of the groundwater.