Transport and Accumulation of Uranium in a Fluvial Wetland: Importance of Hydrology and Colloids
Daniel Kaplan1,2* (email@example.com), Ronald J. Smith3, Connor Parker4, Kimberly A. Roberts2, Pieter Hazenberg5, Juan Morales,2,5, Edward O’Loughlin6, Maxim Boyanov6,7, Pamela Weisenhorn6, Brian Powell2,4, Kent Orlandini6, Maggie Bowman8, Rosey Chu8, Alice Dohnalkova8, Libor Kovarik8, Ravi Kukkadapu8, Odeta Qafoku8, Jason Toyoda8, Kenneth Kemner6
1Savannah River Ecology Laboratory, University of Georgia, Aiken, SC; 2Savannah River National Laboratory, Aiken, SC; 3Savannah River Nuclear Solutions, Aiken, SC; 4Department of Environmental Engineering and Earth Sciences, Clemson University, Clemson, SC; 5Applied Research Center, Florida International University, Miami, FL; 6Argonne National Laboratory, Lemont, IL; 7Bulgarian Academy of Sciences, Sofia, Bulgaria; 8Pacific Northwest National Laboratory, Richland, WA
A nuclear fuel fabrication facility released 43,500 kg of uranium (U) into a riparian wetland located on the Savannah River Site between 1955 to 1988. Studies were undertaken to evaluate hydrology, geochemical processes, and suspended colloids on U accumulation in the wetland. Gamma radiation mapping surveys were conducted by systematically walking over the contaminated wetland with backpacks equipped with global positioning systems and sodium iodide gamma detectors. Based on maps compiled from >700,000 gamma spectra and eight sediment U depth profiles, it was determined that 94% of the released U remained in the wetland. The U in the wetland is concentrated in five multi-hectare areas along the stream, accounting for ~11% of the land area adjacent to the stream.
While land type (upland or wetland) and topography provided a reasonable first approximation of where much of the U was deposited, hydrological watershed modeling revealed that the stream velocity was especially slow through the accumulation areas. Using autoradiography combined with scanning electron microscopy and energy dispersive X-ray analysis measurements of contaminated sediments, surprisingly few hot particles were detected. Instead, U was evenly distributed throughout the sampled sediment, suggesting that soon after released into the wetland dissolved U had bound to sediment particles that became suspended and later deposited in low energy (low flow velocity) portions of the stream and floodplain.
Fe extended X-ray absorption fine structure (EXAFS) measurements of the colloidal particles (three size fractions between 3kDa and 0.1 μm) indicated that they consisted predominantly of ferrihydrite and a smaller fraction of OM-bound Fe(III), without significant differences between groundwater and surface water colloids. Uranium LIII-edge EXAFS indicated that the U atoms in the larger colloids were predominantly associated with the mineral fraction as innersphere complexes (U–Fe distance of 3.44 Å). These studies show that wetlands can be extraordinarily effective at binding and retaining U, thereby providing a natural barrier to the transport of U out of a watershed. However, significant anthropogenic or climatic changes to wetlands, such as those associated with flooding, forest fires, or land use, may disrupt the complex hydrological and biogeochemical balance necessary to maintain long-term immobilization of U.