10:50 〜 11:05
[AHW20-09] Distribution of Uranium and Arsenic in Surface Water around Alkaline-Hyposaline Lake from Valley of Gobi Lakes in Mongolia
キーワード：saline lake, water, uranium, arsenic
Saline lakes represent 23% of the area of all lakes on Earth and located mostly in arid and semiarid climates.
Saline lakes are mainly fed by precipitation and surface flow. But also, water lost by evaporation has significant role in saline lakes. Evaporation of the lake water results the formation of contaminated lake water and salt deposits containing high levels of dissolved species including trace elements such as arsenic and uranium. The chemical speciation of trace elements in sediments and distribution between the sediments and lake water are essential for the understanding of formation process and the mobility of trace elements.
This study aims to clarify distribution and formation process of As and U in saline lake water based on their sediment and water chemistry analysis of surface waters (springs, rivers, wells) around the study area. The sediment and water chemistry samples of alkaline-hyposaline lake catchment area (Orog and Olgoy lakes) in semi-arid climates of the Valley of Gobi lakes in Mongolia were studied. We measured lakes, river and spring water samples in different seasons from 2015 to 2020. Also, well water samples around the study area in 2017 and 2018 were measured. Sediment samples were analyzed by using sequential extraction procedure (SEP), X-ray diffraction (XRD) and synchrotron-based X-ray absorption spectroscopy (XAFS).
Olgoy and Orog lakes exhibited pH exceeded 9 and salinity above 1300 mg/kg. The lake waters major cation and anion is Na+, Cl- and SO42-. The high Na and Cl concentrations are result of evaporation of the lake waters. Also, our results showed that high concentrations of As (~3019ppb) and U (~590ppb) in lake water, respectively. Therefore, the concentration of arsenic and uranium differed in different months throughout the year which need more clarification to understand the process between lake-sediment interaction. The results of SEP of lake sediment showed that arsenic and uranium in sediments are distributed into calcium carbonates rather than amorphous iron oxide. XRD analysis results shows that sediment sample has authigenic minerals including calcite and Monohydrocalcite. The results of XAFS analysis shown 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. Carbonates are not “source” of these elements but “sink”. However, we need to clarify how much the carbonates play a role of “sink”. The springs, rivers, and well water have pH around 6.3-8.3 and low salinity. The major cation of surface water excluding lake water are Ca2+ and concentrations of Na+, Cl-, SO42- were low. But we found in some well and spring point has high concentration of U(~39ppb) and As (~61ppb). The high concentrations of As and U in lake water might be related to the surface water around lake and the evaporation of river water.
Saline lakes are mainly fed by precipitation and surface flow. But also, water lost by evaporation has significant role in saline lakes. Evaporation of the lake water results the formation of contaminated lake water and salt deposits containing high levels of dissolved species including trace elements such as arsenic and uranium. The chemical speciation of trace elements in sediments and distribution between the sediments and lake water are essential for the understanding of formation process and the mobility of trace elements.
This study aims to clarify distribution and formation process of As and U in saline lake water based on their sediment and water chemistry analysis of surface waters (springs, rivers, wells) around the study area. The sediment and water chemistry samples of alkaline-hyposaline lake catchment area (Orog and Olgoy lakes) in semi-arid climates of the Valley of Gobi lakes in Mongolia were studied. We measured lakes, river and spring water samples in different seasons from 2015 to 2020. Also, well water samples around the study area in 2017 and 2018 were measured. Sediment samples were analyzed by using sequential extraction procedure (SEP), X-ray diffraction (XRD) and synchrotron-based X-ray absorption spectroscopy (XAFS).
Olgoy and Orog lakes exhibited pH exceeded 9 and salinity above 1300 mg/kg. The lake waters major cation and anion is Na+, Cl- and SO42-. The high Na and Cl concentrations are result of evaporation of the lake waters. Also, our results showed that high concentrations of As (~3019ppb) and U (~590ppb) in lake water, respectively. Therefore, the concentration of arsenic and uranium differed in different months throughout the year which need more clarification to understand the process between lake-sediment interaction. The results of SEP of lake sediment showed that arsenic and uranium in sediments are distributed into calcium carbonates rather than amorphous iron oxide. XRD analysis results shows that sediment sample has authigenic minerals including calcite and Monohydrocalcite. The results of XAFS analysis shown 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. Carbonates are not “source” of these elements but “sink”. However, we need to clarify how much the carbonates play a role of “sink”. The springs, rivers, and well water have pH around 6.3-8.3 and low salinity. The major cation of surface water excluding lake water are Ca2+ and concentrations of Na+, Cl-, SO42- were low. But we found in some well and spring point has high concentration of U(~39ppb) and As (~61ppb). The high concentrations of As and U in lake water might be related to the surface water around lake and the evaporation of river water.