Combined Fe and S speciation tracks redox conditions in shallow alluvial sediments

Noël V, Boye K, Kukkadapu RK, Bone SE, Lezama Pacheco JS, Cardarelli E, Janot N, Fendorf S, Williams KH, Bargar JR. (2017) Understanding controls on redox processes in floodplain sediments of the Upper Colorado River Basin. Science of The Total Environment.

Detailed characterization of the molecular structures of Fe and S, on depth-resolved field samples from floodplain sediments, were performed using x-ray absorption spectroscopy (XAS) and X-ray microspectroscopy at SSRL combined with mössbauer spectroscopy at EMSL. The results revealed that organic carbon content, moisture, and particle size control the distribution and reactivity of redox constraints in the floodplain subsurface. 

Floodplains are important repositories of organic carbon, nutrients, and metal contaminants. The accumulation and release of these species is mediated by redox processes. In the upper Colorado River Basin, floodplains impacted by former uranium ore processing, host large inventories of U that has accumulated in sulfidic organic carbon-enriched bodies. These naturally reduced zones (NRZs) may be regionally important to the maintenance of contaminant groundwater plumes, acting as secondary sources during diffusion of oxidants, releasing U back to the aquifer. Indeed, the susceptibility of U to redox processes creates the need to better predict the biogeochemical controls on redox conditions in NRZs. The chemical forms of Fe and S provide precise tracers for defining the distribution and resilience of redox regimes. Combined the Fe and S speciation in natural sediments can help to develop conceptual and numerical models of redox-active contaminant behavior, such as U, within alluvial aquifers.

Using a combination of SSRL based XAS and x-ray microspectroscopy with mössbauer spectroscopy at EMSL the vertical distributions of Fe and S redox states and sulfide mineralogy were assessed in sediment cores from three floodplain sites spanning a 250 km transect of the central UCRB. We found that the presence of organic carbon, together with water saturation, are key requirements for maintaining reducing conditions, which were dominated by sulfate-reduction products. Sediment texture was found to be of secondary importance,having a moderating effect on the response of the system to external forcing, such as oxidant diffusion. Consequently, fine-grain sediments are relatively resistant to oxidation in comparison to coarser-grained sediments. Exposure to oxidants consumes precipitated sulfides, with a disproportionate loss of mackinawite (FeS) as compared to the more stable pyrite.  

Comparison of the distributions of Fe and S reactive species with that of U species suggests that the relative abundance of reactive iron sulfides is a functional proxi for estimating the susceptibility of U stocks in NRZs to oxidative remobilization.