Trace Elements Contamination in Water of Alkaline-Hyposaline Lake from Valley of the Gobi Lakes, Mongolia: Field Evidence of Elevated Level of As and U accumulated by evaporation

Unlike the freshwater lakes, the trace elements in saline lake water have rarely been studied. Some of the saline lakes in Mongolia and other countries were studied but mainly focusing on the physical and chemical concentrations of water, morphometric, biogeochemical features, macro, and microelements. In recent years, lake water shrinking has increased, it caused contaminated shallow water and sediments coming into the surface which might be enriched by trace elements and possibly transported into places by a dust storm. The purpose of the study is to define the contamination, distribution, and solubility of trace elements in a saline lake environment. Here, we investigated As and U in saline lake water and sediments of Olgoi and Orog lake, Mongolia. Water samples from 2015-2020 results showed that high concentrations of As (~3019ppb) and U (~590ppb) in lake water, which exceeded the WHO standards of As (10ppb) and U (30ppb). We found that lake level calculated from a satellite image data shown negatively correlated with concentrations of As and U. Specifically, the concentration of As and U were increasing when lake area and precipitation decreasing. Importantly, this is the first continuous field evidence of an ongoing increase of As and U contamination by evaporation in saline lake water. Therefore, total As and U concentrations in sediments of both lakes were ranging from 2.4 to 15ppm and from 0.9 to 2.7 ppm, respectively. The distribution coefficient reveals the potential mobilization of metals between the liquid and solid phases. The value of distribution coefficient ranging from 1.2 to 2.2 L/kg, shown As and U is available in the liquid phase in both lake water. The study shown the little amount of As and U were adsorbed into sediments but mostly accumulated in water. The As and U may precipitate as salts after drying out of the lake water, which must be highly soluble. This study reveals field evidence of trace elements contamination increased by evaporation based on annual measurements. Moreover, evaporation might be more important for trace elements accumulation in saline lake water since most of the saline lakes are in arid areas.