title: Contrasting distributions of groundwater arsenic and uranium in the western Hetao basin, Inner Mongolia: Implication for origins and fate controls.
abstract: Although As concentrations have been investigated in shallow groundwater from the Hetao basin, China, less is known about U and As distributions in deep groundwater, which would help to better understand their origins and fate controls. Two hundred and ninety-nine groundwater samples, 122 sediment samples, and 14 rock samples were taken from the northwest portion of the Hetao basin, and analyzed for geochemical parameters. Results showed contrasting distributions of groundwater U and As, with high U and low As concentrations in the alluvial fans along the basin margins, and low U and high As concentrations downgradient in the flat plain. The probable sources of both As and U in groundwater were ultimately traced to the bedrocks in the local mountains (the Langshan Mountains). Chemical weathering of U-bearing rocks (schist, phyllite, and carbonate veins) released and mobilized U as UO2(CO3)2(2-) and UO2(CO3)3(4-) species in the alluvial fans under oxic conditions and suboxic conditions where reductions of Mn and NO3(-) were favorable (OSO), resulting in high groundwater U concentrations. Conversely, the recent weathering of As-bearing rocks (schist, phyllite, and sulfides) led to the formation of As-bearing Fe(III) (hydr)oxides in sediments, resulting in low groundwater As concentrations. Arsenic mobilization and U immobilization occurred in suboxic conditions where reduction of Fe(III) oxides was favorable and reducing conditions (SOR). Reduction of As-bearing Fe(III) (hydr)oxides, which were formed during palaeo-weathering and transported and deposited as Quaternary aquifer sediments, was believed to release As into groundwater. Reduction of U(VI) to U(IV) would lead to the formation of uraninite, and therefore remove U from groundwater. We conclude that the contrasting distributions of groundwater As and U present a challenge to ensuring safe drinking water in analogous areas, especially with high background values of U and As.
authors.affiliation: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, PR China; School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China. Electronic address: hmguo@cugb.edu.cn.
authors.forename: Huaming
authors.initials: H
authors.lastname: Guo
authors.affiliation: School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China.
authors.forename: Yongfeng
authors.initials: Y
authors.lastname: Jia
authors.affiliation: U.S. Geological Survey, MS 964d Denver Federal Center, Denver, CO 80225, USA.
authors.forename: Richard B
authors.initials: RB
authors.lastname: Wanty
authors.affiliation: School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China.
authors.forename: Yuxiao
authors.initials: Y
authors.lastname: Jiang
authors.affiliation: School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China.
authors.forename: Weiguang
authors.initials: W
authors.lastname: Zhao
authors.affiliation: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, PR China; School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China.
authors.forename: Wei
authors.initials: W
authors.lastname: Xiu
authors.affiliation: School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China.
authors.forename: Jiaxing
authors.initials: J
authors.lastname: Shen
authors.affiliation: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, PR China; School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China.
authors.forename: Yuan
authors.initials: Y
authors.lastname: Li
authors.affiliation: School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China.
authors.forename: Yongsheng
authors.initials: Y
authors.lastname: Cao
authors.affiliation: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, PR China.
authors.forename: Yang
authors.initials: Y
authors.lastname: Wu
authors.affiliation: School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China.
authors.forename: Di
authors.initials: D
authors.lastname: Zhang
authors.affiliation: School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, PR China; The National Institute of Metrology, Beijing 100013, PR China.
authors.forename: Chao
authors.initials: C
authors.lastname: Wei
authors.affiliation: Institute of Hydrogeology and Environmental Geology, China Academy of Geological Sciences, Shijiazhuang, Hebei, 050061 PR China.
authors.forename: Yilong
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Institute of Hydrogeology and Environmental Geology, China Academy of Geological Sciences, Shijiazhuang, Hebei, 050061 PR China.
authors.forename: Wengeng
authors.initials: W
authors.lastname: Cao
authors.affiliation: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA.
authors.forename: Andrea
authors.initials: A
authors.lastname: Foster
keywords: As species; Coprecipitation; Oxidative dissolution; Redox-sensitive element; Reductive dissolution; U species