Geochemical Controls on Release and Speciation of Fe(II) and Mn(II) from Hyporheic Sediments, East River, Colorado

Hyporheic zones act as an important ecological link between terrestrial and aquatic systems where the coupling of redox-sensitive metals and dissolved organic carbon (DOC) can impact nutrient cycling and water quality significantly. In order to understand the release rates and speciation of Fe(II) and Mn(II) in groundwater, we conducted laboratory batch incubation experiments using sediment samples from a transect within an intra-meander hyporheic zone at East River floodplain in Colorado. Our results indicate that the release rates of Fe(II) and Mn(II) vary with redox conditions and subsurface positions. Total Fe(II) and Mn(II) production increased steadily over the 57-day incubation period, with higher Fe(II) and Mn(II) produced from more reducing sediments. Dissolved Fe(II) reaches equilibrium within 3 weeks, while the release of dissolved Mn(II) is a slow process, and equilibrium is not reached within the 57-day period. Fe(II) and Mn(II) production and the release of dissolved Fe(II) and DOC can be described by zero- and first-order rate equations respectively. Interestingly, a higher production of total Fe(II) did not necessarily result in a higher release of dissolved Fe(II). Our geochemical modeling indicates that the equilibrium concentrations of dissolved Fe(II) are controlled by the solubility of siderite, and strongly affected by the geochemical conditions of pH, partial pressure of CO₂, and DOC concentrations. Dissolved organic matter (DOM) can significantly enhance the dissolution of siderite by forming strong Fe(II)-DOM complex as dominant aqueous Fe(II) species and therefore impact the mobility and release of Fe. These results are supported by Fe K-edge X-ray adsorption fine structure (EXAFS) spectra where the bulk Fe(II) in sediments is determined existing as siderite and the Fe(II)-organic matter complexes. Our observations highlight that the coupled cycling of Fe/Mn with DOC is critical for predicting the release and speciation of redox-sensitive elements in similar hyporheic zones and subsurface environments.