Watershed Controls on Uranium Concentrations Tied into Natural Organic Matter and Iron Interactions in Streambeds and Wetlands

Authors

Peter H. Santschi¹* (santschi@tamug.edu), Chen Xu¹, Peng Lin², Daniel I. Kaplan², Chris M. Yeager³

Institutions

¹Texas A&M University–Galveston, TX; ²Savannah River Ecology Laboratory, University of Georgia, Aiken, SC; ³Los Alamos National Laboratory, Los Alamos, NM

Abstract

Wetlands and hyporheic zones are critical interfacial regions with complex and seasonally varied dynamics in hydraulic, chemical, physical and microbial properties. Different environmental conditions result in their functioning as a sink or source of organic matter (OM) and associated contaminants, such as uranium (U). In this project, researchers tested the central hypothesis that watershed interfacial zones, including wetlands and hyporheic zones, produce unique yet characterizable OM especially at gaining sites, where OM- and iron (Fe)-rich flocs form. The mixing models of Tims Branch combined with stable isotopes of ²H and ¹⁸O corroborate sources and sinks of the subsurface and surface waters along the flow path (Santschi et al. 2023). Sixteen sediment samples were collected from gaining and losing sections of Tims Branch to allow evaluation of the compositional differences of their sediment organic matter (SOM). The SOM samples were extracted from the sediment through the use of an optimized method (Xu et al. 2023a) with techniques including solvent extraction, ultrafiltration, and solid-phase extraction. Results showed the following SOM characteristics: (1) the average molecular weight of SOM in gaining site is lower than that in losing site; (2) sulfur to carbon ratio are higher in gaining site; (3) SOM is more oxygenated in gaining site; (4) nominal oxidation state of carbon (NOSC) is higher in gaining than losing; (5) the dominant compounds in losing site are protein, lignin, and lipid whereas in gaining site they are lipid, fatty acids, and condensed aromatics; (6) there are more aromatic or condensed aromatic in gaining than losing site; (7) there are more U-containing organic compounds in gaining site, especially in the low- molecular-weight fraction (< 3 kDa). Moreover, their moieties are spread over the van Krevelen diagram suggesting a possible unspecified compound class with a unique functional group responsible for U binding in these two regions. Microbial analysis (16s rRNA) concurrently supports that a unique microbial community evolves in a floc sample from a gaining site that participates in the modification of the SOM. The orange-colored NOM- and Fe-rich floc hotspots in the gaining stream showed a unique composition with abundant CONPS signatures in their ESI-FT-ICR-MS spectra, as well as in microbial community composition and genetic make-up, where Fe-driven redox and microbial processes can mutually control NOM functionalities, and vice versa, leading to flocs acting as terrestrial OM traps (Xu et al. 2023b).