Spatial and Temporal Isotopic and Hydrochemical Characteristics of Groundwater and Surface Water in the Tuul River Basin, Mongolia

Hydrochemistry and stable isotope tracing approaches were applied to groundwater and surface waters (rivers, springs, stormwater drainage canals, and treated wastewater discharges) to study the spatiotemporal geochemical and stable isotope characteristics and their interactions. The analysis revealed spatiotemporal variations and interactions between groundwater and surface water. Ca-HCO₃-type water dominated the study area; representing 70.7%, (summer) and 81% (winter) of the samples. The chemical and isotopic values presented some spatial differences but minimal temporal changes, except in Tuul River water downstream of Ulaanbaatar. The Tuul River was mostly characterized as Ca-HCO₃-type water, but varied temporally and spatially, shifting to Ca-Na-HCO₃-type water in summer and Na-Ca-HCO₃-type in winter in the downstream region after wastewater discharge. Also, electrical conductivity (EC) values of Tuul River are increasing gradually with distance, and more than double in concentration after mixing with treated wastewater discharges and stormwater drainage canals, providing clear evidence of anthropogenic impacts. The similarity in the stable isotopes and chemical characteristics of floodplain groundwater and river water suggest that local recharge of the alluvial groundwater with Tuul River water, whereas distant groundwater quality exhibited higher levels of minerals and stable isotopes. Cluster analysis clearly indicated a connection between floodplain groundwater and river water, and also the effect of anthropogenic discharges associated with stormwater and treated wastewater discharges. The analysis results also show that cluster analysis is a useful approach for optimizing future spatial sampling strategies, and offers a reliable classification of sampling stations in the region, especially along Tuul River.