Groundwater contamination assessment in Ulaanbaatar City, Mongolia with combined use of hydrochemical, environmental isotopic, and statistical approaches

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 delta N-15-NO3- along the first principal component of the principal component analysis (PCA). Based on the concentrations and delta N-15 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% delta N-15-NO3-), while pristine hydrochemistry is observed restrictedly in grassland and forest areas. Other ions (e.g., Cl- and SO42-) are also higher in Group II water. The delta N-15-NO3- values in Group II water in combination with the spatial distribution on the land use map indicate that nitrate originates fromuntreated sewage effluents including pit-latrine leakage in peri-urban areas, while nitrate in Group I water originates fromsoil organic matter. The relationship between nitrate concentrations and delta H-2 (and delta O-18) 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) that consists of three variables (concentrations of dissolved silica, nitrate and chloride) and can be used to delineate zones vulnerable to nitrate contamination as a crucial step for the efficient monitoring and management of groundwater quality. The study results suggest an urgent need for the management of unsealed pit latrines that are common in peri-urban areas with high population density. (C) 2020 Elsevier B.V. All rights reserved.