[A-HW23] Hydrology & Water Environment

[AHW23-P01] Arsenic and uranium contamination in Orog lake in the Valley of the Gobi lakes, Mongolia: Field evidence of conservative accumulation of U in an alkaline, closed-basin lake during evaporation

*Baasansuren Gankhurel¹, Keisuke Fukushi¹, Davaadorj Davaasuren², Eigo Imai¹, TAKUMA KITAJIMA¹, Uyangaa Udaanjargal¹, Tuvshin Gerelmaa², Yasuhito Sekine³, Yoshio Takahashi⁴, Noriko Hasebe¹ (1.Kanazawa University, 2.National University of Mongolia, 3.Tokyo Institute of Technology, 4.The University of Tokyo)

Keywords:trace elements, saline alkaline lake, geochemical reaction modeling

Mongolia has many lakes, and almost half of them are alkaline, saline lakes. The Mongolian plateau is an arid region, and its lakes have experienced severe shrinkage during the past several decades. Elevated concentrations of As and U have recently been detected in the alkaline, saline lakes of western Mongolia. Previous studies have shown that the elevated concentrations of As and U exhibited by inland lake waters are due to evaporation. In contrast, there is currently little understanding of the chemical reactions responsible for the enrichments of these trace elements. The purpose of this study was to quantitatively understand the dynamic chemical behaviors of As and U in an alkaline, saline lake during evaporative shrinkage of the lake.
In this study, we used a five-year study of water chemistry, characterization of arsenic and uranium in sequentially extracted sediments, and X-ray absorption fine structure analyses (XANES) to examine the geochemical behavior of arsenic and uranium during evaporation of Lake Orog, a saline-alkaline lake in the arid region of Mongolia. Water chemistry analyses showed that the concentrations of arsenic and uranium increased as evaporation caused the lake to shrink. Comparisons of monitoring results with predictions of a geochemical model suggested that some of the arsenic was removed from the lake water under highly desiccated conditions. Sequential extraction and XANES analyses showed that As(V) could be removed by ferrihydrite, even at pHs greater than ~9. In contrast, the accumulation of uranium in the lake could be reproduced by consideration of only evaporation. The conservative behavior of uranium could be explained by the low affinity of U(VI) for carbonate and ferrihydrite at pHs > 9 and high concentrations of dissolved inorganic carbon. This study provided evidence of conservative accumulation of U in an alkaline lake.
It is likely that U(VI) eventually formed soluble salts after the lake was completely desiccated. Our study area has been considered to be a source of Asian yellow dust from the Gobi desert in southern Mongolia. When the lake is partially dry or desiccated, the U in the lake system could be a source of dust contamination. Westerlies could transport these dust particles to China, Korea, Japan, and the northwestern North Pacific. Furthermore, there is a need to consider the concentration and mobility of trace elements in salts after the lake water completely evaporates.