Abstract
Saline lakes represent 23% of the area of all lakes on Earth and located mostly in arid and semi-arid climates. Olgoi and Orog lakes are located in semi-arid climates of the Valley of the Gobi lakes in Mongolia. It is the desert area that one of the sources of Asian dust (kousa) comes to Japan. Moreover, lake shrinkage has occurred in recent decades if the lake dried up it is possible to be a source of trace elements contamination. It can affect the environment and possible to move to a greater distance beyond the continent by the westerly winds. Then we need to clarify the geochemical behavior (solubility and mobility) of trace elements especially As and U in saline lake water and sediment in this area. Olgoi lake has no inflow and outflow river. Orog lake has one inflow river but no outflow. The depth of both lakes is around 1m. The water samples were collected from both lakes from 2015 to 2020. The major cations and anions of the water sample were analyzed using inductively coupled plasma optical emission spectroscopy and high-performance liquid chromatography. The trace elements were analyzed by using an inductively coupled plasma mass spectrometer. Sediment cores were taken by a gravity corer in August 2018 from Olgoi and Orog lake. Sediment samples were analyzed by using X-ray diffraction (XRD), and synchrotron-based X-ray absorption spectroscopy (XAFS), and 2 step chemical extraction for chemical speciation of As and U using 1M HCl for extractable and using a mixture of 48%HF, 60%HNO3, and 30%HCl, for a residual fraction.Olgoi and Orog lakes exhibited pH >9 and salinity above 1300 mg/kg. The major components of the lake's water are Na+, Cl- and SO42-. The high Na and Cl concentrations are the results of the evaporation of the lake waters. Also, the 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 a positive correlation between As and U with Na and Cl. The total As and U concentrations of Olgoi lake sediment was 14.5ppm and 0.9ppm, respectively. The total As and U in Orog lake sediment ranged from 2.4 to 3ppm and 2.2 to 2.7ppm, respectively. The partial extraction results showed that As and U in the sediments are distributed among extractable fraction and residual fraction almost equally. XRD analysis showed the presence of authigenic minerals including calcite and monohydrocalcite. The results of XAFS analysis revealed As in Orog lake sediment was As (V) and primarily coprecipitated with calcite and secondarily adsorbed on ferrihydrite. As in Olgoi lake sediment was mainly As (V) and 20-30% was As (III) and coprecipitated with calcite. As and U possibly are removed by the authigenic carbonates. An extractable fraction of partial extraction is including ferrihydrite and calcium carbonates. This study showed that high concentrations of As and U in saline lake water and low in sediment. Therefore, little amount of As and U were adsorbed into sediments but mostly accumulated in water due to high evaporation. This study suggests that the As and U may be precipitated as salts after drying out of the lake water, which must be highly soluble.
