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

Bayartungalag Batsaikhan; Seong-Taek Yun; Kyoung-Ho Kim; Soonyoung Yu; Kyung-Jin Lee; Young-Joon Lee; Jadambaa Namjil

doi:10.1016/j.scitotenv.2020.142790

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

Sci Total Environ. 2021 Apr 15:765:142790. doi: 10.1016/j.scitotenv.2020.142790. Epub 2020 Oct 6.

Authors

Bayartungalag Batsaikhan¹, Seong-Taek Yun², Kyoung-Ho Kim³, Soonyoung Yu⁴, Kyung-Jin Lee⁵, Young-Joon Lee⁶, Jadambaa Namjil⁷

Affiliations

¹ Department of Earth and Environmental Sciences, Korea University, Seoul 02841, Republic of Korea; Institute of Geography and Geoecology, Mongolian Academy of Sciences, Ulaanbaatar 15170, Mongolia.
² Department of Earth and Environmental Sciences, Korea University, Seoul 02841, Republic of Korea. Electronic address: styun@korea.ac.kr.
³ Department of Earth and Environmental Sciences, Korea University, Seoul 02841, Republic of Korea; Korea Environment Institute, Sejong 30147, Republic of Korea.
⁴ Korea-CO(2) Storage Environmental Research (K-COSEM) Research Center, Korea University, Seoul 02841, Republic of Korea.
⁵ Department of Earth and Environmental Sciences, Korea University, Seoul 02841, Republic of Korea.
⁶ Korea Environment Institute, Sejong 30147, Republic of Korea.
⁷ Institute of Geography and Geoecology, Mongolian Academy of Sciences, Ulaanbaatar 15170, Mongolia.

PMID: 33069480
DOI: 10.1016/j.scitotenv.2020.142790

Abstract

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) 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.

Keywords: Groundwater Quality Index (GQI); Nitrate contamination; Peri-urban, Mongolia; Sewage and pit latrine; Urbanization; Water chemistry and environmental isotope.