1-s2.0-S0048969720363191-main.pdf

Ulaanbaatar City, Mongolia is rapidly becoming urbanized and attracts great attention because of environmental issues. This study was performed to assess the status of groundwater quality in Ulaanbaatar at an early but growing stage of urbanization, focusing on nitrate contamination in relation to land use. Along with high total dissolved solids and NO3 − concentrations, significant contamination of groundwater is indicated by positive loadings of NO3 −, Cl− and δ15N-NO3 − along the first principal component of the principal component analysis (PCA). Based on the concentrations and δ15N values of nitrate, groundwater is classified into two groups: Group I (baseline quality) and II (contaminated). Nitrate in Group II water in urbanized (esp. peri-urban) areas is higher in concentration (> 10 mg/l NO3 −) and N-isotopic values (> 10‰ δ15N-NO3 −), while pristine hydrochemistry is observed restrictedly in grassland and forest areas. Other ions (e.g., Cl− and SO4 2−) are also higher in Group II water. The δ15N-NO3 − values in Group II water in combination with the spatial distribution on the land use map indicate that nitrate originates from untreated sewage effluents including pit-latrine leakage in peri-urban areas, while nitrate in Group I water originates from soil organic matter. The relationship between nitrate concentrations and δ2H (and δ18O) values of water suggests that nitrate enrichment is also influenced by evaporation during groundwater recharge. With the help of PCA for compositional data, we suggest a hydrochemical index for groundwater contamination assessment; i.e., the Groundwater Quality Index (GQI)