Abstract
Introduction- The shrinkage of lake areas has been documented in many inland lakes. Inland lakes and their ecosystems are very sensitive to water chemistry. Therefore, it is crucial to understand the impact of the shrinkage of saline lakes on their water chemistry. Previous studies have shown that the elevated concentrations of As and U exhibited by inland lake waters are caused by evaporation. However, there is little understanding of the chemical reactions responsible for the enrichment of these trace elements. The purpose of this study was to quantitatively understand the dynamic chemical behaviors of As and U in alkaline saline lake during evaporative shrinkage. Methods- We used five-year monitoring data and geochemical modeling of the water chemistry of Orog Lake in the Valley of Gobi Lakes and characterized the chemical forms of As and U in lake sediments to understand the factors controlling the solubility of As and U.Results and Discussion- Five-year monitoring showed that intensive evaporation caused both As and U to accumulate in the lake water. A comparison of the observed concentrations with those predicted by a geochemical model suggested that As in the lake water could be removed during evaporation, whereas U accumulated conservatively in the lake water. Sequential extraction analyses and XAFS measurements of the sediments revealed that the primary host of As(V) in the sediments was ferrihydrite. The adsorption characteristics of As (V) on iron oxides indicate that As(V) can be removed by ferrihydrite during evaporation. In contrast, as neither carbonate minerals nor low- crystalline iron oxides act as sorbents of U, the concentration of U, like those of Na+ and Cl−, may increase dramatically in alkaline lakes during evaporation. This implies that the ubiquitous formation of extremely soluble U-bearing salts after the complete desiccation of inland saline lakes may become a serious contaminant source in arid areas. When the lake is partially dry or desiccated, U in the lake system can be a source of dust contamination. Westerlies can transport 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 has completely evaporated.
