Assessment of Groundwater Quality of the Khan-Uul District of Ulaanbaatar, Mongolia

Khan-Uul is one of the nine districts with the largest number of wells unconnected to the centralized drinking water supply line of Ulaanbaatar, the capital of Mongolia. In this study, a total of 134 groundwater samples were collected from 9 sub-districts of Khan-uul, and their physico-chemical parameters including pH, EC, TDS, Na+, Ca2+, Mg2+, Cl-, SO42-, HCO3-, NO3-, As, Cd, Zn, Cu, Pb, Cr, U contents were determined. For quality assessment, the measured values of the groundwater samples were compared against the standards set by the Mongolian standard for Drinking Water Quality (MNS 0900:2018) and the guidelines established by the World Health Organization (WHO) for Drinking Water Quality. Additionally, the water quality index, Gibbs diagram, and Piper diagram were employed to evaluate the suitability of the groundwater for drinking purposes. The electrical conductivity (4.48% of the samples) and the contents of total calcium (9%), magnesium (6%) and nitrate (20%) exceeded the permissible limit set by the guidelines of the MNS and WHO, both. According to the Gibbs diagram, all of the groundwater samples seemed to be ion-exchanged mainly from rock weathering dominance and the dominant hydro-chemical facies were the Ca·Mg-HCO3 type. In order to calculate the water quality index (WQI), we selected sixteen parameters encompassing physical and chemical properties alongside the content of microelements. Although a majority of the groundwater samples, comprising 70.1% were classified as excellent for drinking purposes, 25.4% were deemed good and 4.5% were classified as poor, rendering them unsuitable for consumption.