ESTIMATING GROUNDWATER RECHARGE USING THE CHLORIDE MASS-BALANCE METHOD IN THE ALTANTEVSH VALLEY (Ulaanbaatar, Mongolia)

Abstract:

Groundwater is an increasingly critical natural resource in the world. Ulaanbaatar city, the capital of Mongolia, is located in the Altantevsh Valley along the Tuul River with a semi-arid climate. 44% of the total population of Mongolia (3.2 million) live in Ulaanbaatar. The city’s population is expected to grow annually, resulting in increased industry, infrastructure requirements, and water demand. The city’s drinking water supply totally depends on groundwater from the alluvial aquifer along the Tuul River. Groundwater recharge must be estimated to manage the city’s water resources. The chloride mass-balance method is an economically efficient method that is widely used in arid and semi-arid regions. The method has never been used before to estimate groundwater recharge in the Altantevsh valley. A total of 71 water samples, 56 from groundwater, 2 from surface water and 13 from precipitation, were collected from October 2017 to December 2018. The water samples were analyzed for chemical ions and stable isotopes of hydrogen and oxygen to estimate groundwater recharge using the chloride mass balance and stable isotope methods. Hydrochemical characterization of the area indicated Ca-HCO₃ and Na-HCO₃ water types. Application of the chloride mass balance method showed that more than 10% of the annual 345 mm of precipitation went towards groundwater recharge. The stable isotope results show that 80% of groundwater recharge is from river water and nearly 10% is from precipitation.