Laboratory and Field Studies of the Redox Dynamics of Iron and Uranium in Iron Flocs in Riparian Wetlands within the Tims Branch Watershed, Savannah River Site

Authors

Edward O’Loughlin¹* (oloughlin@anl.gov), Anthony Boever², Maxim Boyanov^1,3, Daniel Kaplan⁴, Connor Parker⁵, Brian Powell⁵, Martial Taillefert², Pamela Weisenhorn¹, and Kenneth Kemner¹

Institutions

¹Argonne National Laboratory, Lemont, IL; ²Georgia Institute of Technology, Atlanta, GA; ³Bulgarian Academy of Sciences, Sofia, Bulgaria; ⁴University of Georgia Research Institute, Athens, GA; ⁵Clemson University, Clemson, SC

URLs

https://www.anl.gov/bio/project/subsurface-biogeochemical-research

Abstract

The discharge of metallurgical process wastewater containing uranium (U) and co-contaminants (e.g., nickel, chromium, zinc, and lead) from a former nuclear material production facility (i.e., M Area at the Savannah River Site) led to extensive contamination of the downstream wetlands along Tims Branch. Abundant orange and reddish-brown flocs have been repeatedly observed along gaining sections of Tims Branch, where anoxic groundwater containing iron(II) discharges into oxygenated stream water. These flocs contain high levels of iron (Fe; 8–17 wt%, primarily in the form of ferrihydrite and lesser amounts of lepidocrocite and Fe-organic complexes as determined by Fe K-edge extended X-ray absorption fine structure spectroscopy), and are effective scavengers of phosphorus (P; 2–4 wt%), U (32–600 ppm), and trace metals. The flocs initially form under oxic conditions. However, accumulation of flocs can lead to burial of older floc material and the development of anoxic conditions.

Furthermore, during periods of enhanced stream flow (e.g., storm events), accumulated anoxic floc material can become resuspended in oxic stream water. Thus, biogeochemical processes and attendant changes in redox conditions within the flocs are likely to affect the speciation of redox active constituents (e.g., Fe and U) and, potentially the bioavailability of nutrients such as P. Laboratory microcosm studies of flocs show that the transition from oxic to anoxic conditions leads to the reduction of Fe(III) to Fe(II) and U(VI) to nonuraninite U(IV), as determined by X-ray absorption fine structure (XAFS) analysis; following a return to oxic conditions, Fe(II) and U(IV) oxidize back to Fe(III) and U(VI). XAFS analysis along the depth profile of accumulated floc material collected in Tims Branch shows a progressive reduction of Fe(III) to Fe(II) and U(VI) to U(IV) with depth, consistent with the observed decrease in redox potential and analogous to the behavior of Fe and U during redox manipulations in this microcosm study. Given that Fe flocs are frequently observed in a broad range of wetland environments, these studies of Fe floc biogeochemistry in Tims Branch and its potential impact on U speciation and transport expand the scientific community’s understanding of Fe floc’s role in the speciation and cycling of trace elements in wetlands, which in turn can lead to more robust modeling of trace element behavior in aquatic and terrestrial environments.