[A-HW19] Hydrology & Water Environment

[AHW19-04] Arsenic and uranium distribution in salt and surface soils around lakes in the Valley of the Gobi lakes, Mongolia

*Baasansuren Gankhurel¹, Keisuke Fukushi¹, Davaadorj Davaasuren², Uyangaa Udaanjargal², Tuvshin Gerelmaa², Yoshio Takahashi³, Noriko Hasebe¹ (1.Kanazawa University, 2.The National University of Mongolia, 3.The University of Tokyo)

Keywords:saline lakes, arsenic, uranium, dust storm, soluble salt

Many inland saline lakes have been shrinking throughout the world. When lakes shrink or desiccate it affects their water chemistry and ecosystems. Previous studies have shown that the enrichment of trace metals especially As and U increases in lake water due to intense evaporation (Gankhurel et al., 2022). If, the desiccation of the lake occurs high concentration of As and U in water can precipitate as a soluble salt in the dried lakebed. Therefore, the dried bed of saline lake will be the source of trace metals pollution. It also produces dust which is transported locally and regionally and influences the ecosystem and human health. The study aimed to understand the distribution and solubility of trace metals in salt and soils around saline lakes in the Valley of the Gobi Lakes, Mongolia which is one of the main sources of Asian dust storms.
We collected salt and soil samples from ten lakes (some of them already dried) in the Valley of the Gobi Lakes in June 2022. Salt and soil samples were analyzed by using a sequential extraction procedure to determine the chemical speciation of trace metals and X-ray diffraction to understand the mineralogy.
The concentration of As in salt samples ranges from 6-95 mg/L. U in salt samples ranges from 2-46 mg/L. The concentration of As in surface soil samples was 8-53 mg/L. U in surface soil samples ranges from 1-30 mg/L. The chemical speciation results show that arsenic and uranium in salt and surface soil samples are associated mainly with an easily soluble fraction such as exchangeable fraction and carbonates. The salt samples around the lake area have a relatively high concentration than the surface soil around the lakes. The XRD patterns of the salt samples indicate the common presence of thenardite (Na2SO4), halite (NaCl), gypsum (CaSO4・2H2O), and scarce carbonate minerals such as calcite and monohydrocalcite. The presence of thenardite and halite is consistent with the chemical composition of the lake water (Fukushi et al., 2020, Gankhurel et al., 2022). It provides evidence that supports the previous study result which is precipitated salts around the lake area have easily soluble As and U. Therefore, these salts can easily transport to the surrounding environment as a surface runoff with precipitation or transported by wind to become a source of contamination. Furthermore, we are planning to focus on the more detailed sources of dust storm areas nearby the lake area to collect dust samples and to compare the trace elements in salt and soil samples.