HR: 0800h
AN: H11C-0685    [Abstracts]
TI: Uranium Geochemistry in Hypersaline Soda Lakes in Eastern Mongolia
AU: * Linhoff, B S
EM: benlinhoff@gmail.com
AF: University of Texas at Austin, Geol Science Dept 1 University Station C1100, Austin, TX 78712-0254, United States
AU: Bennett, P
EM: pbennett@mail.utexas.edu
AF: University of Texas at Austin, Geol Science Dept 1 University Station C1100, Austin, TX 78712-0254, United States
AU: Puntsag, T
EM: p_tamir2001@yahoo.com
AF: Mongolian University of Science and Technology, School of Geology Mongolian University of Science and Technology P.O. 46 Box 520 Ulaanbaatar 210646 Mongolia, Ulaanbaatar, 210646, Mongolia
AB: Extremely high concentrations of uranium were discovered in water samples from hypersaline soda lakes in eastern Mongolia. The origin and fate of uranium in these lakes was examined using geochemical analyses and modeling, using samples collected from five lakes, six wells and one stream. Samples were analyzed for strontium and uranium isotopes, cations and trace metals, anions, alkalinity, and unstable field parameters. The lakes are small, shallow (<1Km2, <1m) and terminal; their size fluctuates seasonally and they periodically completely desiccate. The region is characterized by rolling semi arid grassland steppe covered by a thick loess deposit of unknown thickness that is underlain by Neogene rhyolite. A typical groundwater in the field area is alkaline (pH = 7.9, 10.7 meq alk/L), 4.4 ° C, with an average T.D.S. of 1500 and low calcium concentration (20 ppm). A strong linear correlation was found between groundwater and lake water chlorine to bromine ratios implying groundwater discharges to lake water and is subsequently evaporated. Evaporation is intense with lake waters having average chlorine concentrations 300 times that of well waters. Uranium in well samples is higher than typical for shallow groundwaters (7-101ppb) suggesting discharging groundwater as a probable source of uranium in lake water. Concentrations of uranium in lake water ranges from 57-14,900ppb making these lakes possibly the highest naturally occurring uranium concentration reported. Lake water alkalinity is strongly correlated to uranium abundance suggesting uranium is complexed with carbonate as the aqueous species UO₂CO₃. Consequently, the extremely high alkalinity of the most alkaline lake (pH = 9.8, 1288.8 meq alk/L) also has the highest uranium concentrations. Stable strontium isotopes were used to assess the degree of water rock interactions and the presence of ⁹⁰Sr was checked for to test the possibility of input of nuclear fallout. ⁹⁰Sr was not detected in lake water samples suggesting the high uranium is of natural origins. A large difference in the ⁸⁷Sr/⁸⁶Sr ratio was found between groundwater and lake water samples. Groundwater samples displayed large variation in the ⁸⁷Sr/⁸⁶Sr ratio (0.70612-0.709776) whereas lake water samples averaged a high radiogenic ratio (0.709432). The large variation in the strontium isotopes in groundwater samples suggests varying degrees of water rock interactions, however the least radiogenic samples likely are derived from interaction with the young rhyolite. The strontium isotopes of the lake water samples are likely most affected by windblown sediment of continental sources.
DE: 1000 GEOCHEMISTRY
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1065 Major and trace element geochemistry
DE: 1800 HYDROLOGY
DE: 1830 Groundwater/surface water interaction
SC: Hydrology [H]
MN: 2007 Fall Meeting