Abstract
Hyporheic zones act as critical ecological links between terrestrial and aquatic systems where redox-sensitive metals of iron (Fe) and manganese (Mn) significantly impact nutrient cycling and water quality. However, the geochemical controls on the release and speciation of Fe(II) and Mn(II) in these biogeochemical hotspots are still poorly understood. Here we conducted batch incubation experiments and analyzed Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy data using sediment samples from a hyporheic zone of the East River floodplain in Colorado to understand the production, release and speciation of Fe(II) and Mn(II) in groundwater. Our results indicate that the production and release of Fe(II) and Mn(II) vary with sediment reducing conditions and subsurface positions, and the rates were determined either by a zero- or first-order rate equation. The sediments with higher Fe(II) production did not necessarily result in higher release of dissolved Fe(II), and ≥97% Fe(II) is accumulated in solid phase. We found that the majority of Fe(II) exists as siderite (FeCO3), Fe(II)-natural organic matter (NOM) complexes and ferrosmectite, and the equilibrium concentrations of dissolved Fe(II) are controlled primarily by siderite solubility, and enhanced greatly by formation of strong Fe(II)-NOM complexes as dominant aqueous Fe(II) species. By contract, dissolved Mn(II) increases slowly and linearly, and an equilibrium concentration was not reached during the incubation period, and the roles of rhodochrosite (MnCO3) and Mn(II)-NOM complexes are insignificant. Furthermore, we reviewed and calibrated the literature reported binding constants (log K) of Fe(II)-NOM complexes which successfully predicted our experimental data. This work reveals that siderite and dissolved NOM are the controlling phases in release and speciation of dissolved Fe(II), and the finding is expected to be applicable in many hyporheic zones and subsurface environments with similar geochemical conditions.
Highlights
River hyporheic zones are defined as regions of sediment beneath and alongside a streambed where mixing and bidirectional exchange of shallow groundwater and river water occurs
Our observation shows that the release extent [i.e., equilibrium concentration (Ceq)] of dissolved Fe(II) varies with Fe-reducing conditions and positions (Table 4B)
Under the aqueous pH (7.5–8.2) and dissolved organic carbon (DOC) concentrations (0.5–1.1 mM) in these experiments, we hypothesize the release of dissolved Fe(II) is primarily controlled by siderite solubility, which is enhanced by Fe(II) complexation with dissolved natural organic matter (NOM)
Summary
River hyporheic zones are defined as regions of sediment beneath and alongside a streambed where mixing and bidirectional exchange of shallow groundwater and river water occurs. These zones perform important ecological functions by linking terrestrial and aquatic systems within watersheds and have been recognized as hotspots for biological activity and cycling of metals and nutrients (Boulton et al, 1998; Gomez et al, 2012; Boano et al, 2014; Dwivedi et al, 2018; Saup et al, 2019). Soluble Fe(II) and Mn(II) species in groundwater gets re-oxidized and precipitated as insoluble Fe(III) and Mn(III/IV) oxides during transport, thereby changing the porosity and permeability of sediment, clogging water pathway as well as staining water supply systems
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