A physical explanation for the development of redox microzones in hyporheic flow

Abstract

Recent observations reveal a paradox of anaerobic respiration occurring in seemingly oxic‐saturated sediments. Here we demonstrate a residence time‐based explanation for this paradox. Specifically, we show how microzones favorable to anaerobic respiration processes (e.g., denitrification, metal reduction, and methanogenesis) can develop in the embedded less mobile porosity of bulk‐oxic hyporheic zones. Anoxic microzones develop when transport time from the streambed to the pore center exceeds a characteristic uptake time of oxygen. A two‐dimensional pore‐network model was used to quantify how anoxic microzones develop across a range of hyporheic flow and oxygen uptake conditions. Two types of microzones develop: flow invariant and flow dependent. The former is stable across variable hydrologic conditions, whereas the formation and extent of the latter are sensitive to flow rate and orientation. Therefore, pore‐scale residence time heterogeneity, which can now be evaluated in situ, offers a simple explanation for anaerobic signals occurring in oxic pore waters.

Bedforms as Biocatalytic Filters: A Pumping and Streamline Segregation Model for Nitrate Removal in Permeable Sediments

“…53,54 This heterogeneity can lead to mixing across streamlines 55,56 and facilitate the formation of redox microzones (e.g., localized pockets of denitrification embedded within well-oxygenated downwelling regions) that enhance coupled nitrification-denitrification and overall nitrate removal rates in natural sediments. 36,55 Third, organic carbon and microbial respiration rates are spatially variable, not homogeneous as assumed in the present modeling effort. For example, …”

Recent advances in the study of mercury methylation in aquatic systems

“…In a boreal peatland study, oxidation during droughts encouraged the mobilization of MeHg to peat porewaters when the land was re-flooded, allowing for increased MeHg production, particularly in the areas of peatland that had increased atmospheric sulfate deposition (Coleman Wasik et al 2015). In addition, inundation and stagnation may reduce oxygen availability in the sediment as pores fill with water, thus encouraging the activity of anaerobic bacteria (Briggs et al 2015;Singer et al 2016). In this case, the frequency of inundation should be considered when determining the risks of mercury contamination in particular aquatic environments, as the frequency and duration of inundation have been found to be correlated with estimated MeHg production potential (Singer et al 2016).…”

Indicator and Pathogen Removal by Low Impact Development Best Management Practices

“…Ongoing research should elucidate how pollutant removal in biofilters is affected by heterogeneity in the permeability field and reaction field. Judging based on analogies to pollutant treatment in river sediments, such heterogeneity (sometimes referred to as "microzone heterogeneity") could play a key role in pollutant destruction and sequestration [111]. …”