Effect of Hydrological Variations on Biogeochemical Processes Within Contaminated Stream Sediments and the Use of Uranium as a Geochemical Tracer for Recent Changes to the Sediment Redox State

Authors

Anthony Boever¹* (tonyboever@gatech.edu), Evan Magette¹, Daniel I. Kaplan², Maxim I. Boyanov³, Edward J. O’Loughlin³, Kenneth M. Kemner³, Martial Taillefert¹

Institutions

¹Georgia Institute of Technology, Atlanta, GA; ²Savannah River National Laboratory, Aiken, SC; ³Argonne National Laboratory, Lemont, IL

Abstract

Riparian and streambed sediments are highly dynamic environments sensitive to hydrological changes in which small variations in flow regimes may result in extensive changes to the bulk redox state of the sediments. For example, intermittent stream sediments may be subjected to net influent (losing) or effluent (gaining) conditions with respect to subsurface flow, although the degree of surface and subsurface mixing varies both temporally and spatially and results in unique and shifting redox regimes. Such redox toggling makes short-term fluxes of greenhouse gases difficult to predict and long-term carbon budget estimates less accurate. In this field-based study of uranium-contaminated stream sediments, the spatiotemporal variations in biogeochemical redox processes were revealed through a combination of in situ semicontinuous voltametric measurements at four different depths in the sediment and in situ pore water depth profiling at discrete timepoints. Dissolved oxygen (DO) was consistently detected at centimeters depth at the losing reach, whereas DO was depleted within millimeters of the sediment–water interface at the gaining reach. Dissolved ferrous iron was consistently abundant in the gaining reach and often accompanied by aqueous clusters of electrochemically active iron sulfide, whose formation indicates recent or concomitant sulfate reduction. Signs of active sulfate reduction were also captured in the form of pulsing electrochemical signals of dissolved sulfides that migrated upward during the summer months. Electrochemically active soluble organic-iron(III) complexes were absent in the losing reach and ubiquitous in the gaining reach. These signals often peaked immediately below the oxic–anoxic interface and persisted at depth, possibly indicative of the dynamic recycling of iron(III). Dissolved uranium covaried with dissolved iron and orthophosphates in both locations, indicating that uranium distribution may trace dynamic redox interfaces. Storm events disrupted redox zonation by entraining oxygenated waters deeper into the sediment at both locations and temporarily washing out reduced metabolites at the gaining reach. Contrary to the opposing electrochemical signatures and despite a tenfold difference in total dissolved iron, similar bulk aqueous ferric-to-ferrous iron ratios were observed in both systems. These findings imply common, broad-scale processes involved in iron cycling that may result from rapid, hydrologically driven redox oscillations. In a changing climate and as demand for natural resources continues, the pressure exerted on hydrologically sensitive ecosystems increases. Improved understanding of these hydro-biogeochemical dynamics may unravel the complexities of carbon remineralization and better inform carbon budgets as well as elucidate the timing and extent of greenhouse gas emissions.