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Van Tuyl Lecture: Johanna Blake, University of New Mexico
January 17, 2019 @ 4:00 pm - 5:00 pm
Johanna Blake, University of New Mexico
241 Berthoud, 4-5PM
Topic: Uranium Mobility and Accumulation at the Jackpile Mine, Laguna Pueblo, NM
Abstract: The chemical interactions, mobility, and plant uptake of uranium (U) near abandoned mine wastes was investigated along the Rio Paguate, adjacent to the Jackpile Mine, located in Laguna Pueblo, New Mexico. Solid analyses using X-ray fluorescence determined that sediments located in the Jackpile Mine contain ranges of 320 to 9200 mg/kg U. The presence of coffinite, a U(IV)-bearing mineral, was identified by X-ray diffraction analyses in abandoned mine waste solids exposed to several decades of weathering and oxidation. Elevated U concentrations in surface water adjacent to mine waste range from 30 to 710 µg/L seasonally and decrease to 5.77 to 10.0 µg/L at a wetland 4.5 kilometers downstream of the mine. Although U concentrations in stream water are elevated, aqua regia acid digestions performed on co-located stream bed and stream bank sediments reveal that there is limited U accumulation on sediments along the reach between the mine and wetland, with most sediment concentrations being near the 3 mg/kg crustal average. However, U concentrations in sediments in the wetland are 4 times the background concentrations in the area. X-ray photoelectron spectroscopy analysis on sediment samples adjacent to the mine waste show a 75:25% ratio of Fe(III) to Fe(II), which can have an effect on adsorption properties. Individual results from salt cedar roots, stems, and leaves collected along the river transect show higher U concentrations in the roots adjacent to the mine waste (20 and 55 mg/kg) and lower in the stems and leaves. Translocation values calculated below 1 are evident in many of the plant samples, suggesting that U root to shoot translocation is minimal and U is accumulating in the roots. Concentrations of U in salt cedar roots from downstream of the mine waste decrease to 15 mg/kg. Synchrotron-based micro X-ray fluorescence analyses of plant roots collected from the field indicate that the accumulation of U occurs in the cortex of the root. However, Ternary Ca−U−CO3 complexes may affect a decrease in U bioavailability in plants. Our results suggest that dilution, uptake by plants, and U sorption to wetland sediments are the dominant factors that help to decrease the U concentrations downstream of the mine.