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Uranium Redistribution Due to Water Table Fluctuations in Sandy Wetland Mesocosms
Citation:
Gilson, E., S. Huang, P. Koster van Groos, K. Scheckel, O. Qafoku, A. Peacock, D. Kaplan, AND P. Jaffé. Uranium Redistribution Due to Water Table Fluctuations in Sandy Wetland Mesocosms. David L. Sedlak (ed.), ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 49(20):12214-12222, (2015).
Impact/Purpose:
Waste from uranium (U) mining and milling for energy and weapons production can contribute to U contamination of soils and groundwater. While dissolved in groundwater, U can travel from its source to contaminate surface waters used for drinking and animal habitat. For example, migration of U contamination has led to excessive U levels in wells, mines, and rivers near abandoned U mines in the southwestern United States, and in wetlands near a Department of Energy nuclear facility at the Savannah River Site in Aiken, SC. Consumption of U-contaminated waters can have toxic effects on the kidneys, bones, and livers of humans and animals. The objective of this research was to determine how sediment drying and rewetting affects the fate of U immobilized in wetland sediments. To study U immobilization, greenhouse mesocosms were built and dosed with uranyl acetate for several months. Effluent U concentrations from mesocosms were measured and mesocosms were dissected after being dried and rewetted. Chemical extractions, speciation measurements, and imaging analyses of U on sediments and roots were used to gain new insights into U immobilization in wetlands and the effects of drying and rewetting on the stability of immobilized U.
Description:
To understand better the fate and stability of immobilized uranium (U) in wetland sediments, and how intermittent dry periods affect U stability, we dosed saturated wetland mesocosms planted with Scirpus acutus with low levels of uranyl acetate for 4 months before imposing a short drying and rewetting period. Concentrations of U in mesocosm effluent increased after drying and rewetting, but the cumulative amount of U released following the dry period constituted less than 1% of the total U immobilized in the soil during the 4 months prior. This low level of remobilization suggests, and XANES analyses confirm, that microbial reduction was not the primary means of U immobilization, as the U immobilized in mesocosms was primarily U(VI) rather than U(IV). Drying followed by rewetting caused a redistribution of U downward in the soil profile and on to root surfaces. Although the U on roots before drying was primarily associated with minerals, the U that relocated to the roots during drying and rewetting was bound diffusely. Results show that short periods of drought conditions in a sandy wetland, which expose reduced sediments to air, may impact U distribution without causing large releases of soil-bound U to surface waters.
