A Physical Explanation for the Development of Redox Microzones at Stream-Groundwater Interfaces

Abstract:

A growing number of stream-groundwater interface observations reveal the paradox of anaerobic respiration occurring in seemingly oxic-saturated sediments. Here we demonstrate a physical 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 less mobile porosity of bulk-oxic hyporheic zones. These anoxic microzones develop when the transport time from the streambed to the pore centers, referred to as the pore scale residence time, exceeds a characteristic uptake time of oxygen. We used a two-dimensional pore-network model to quantify how anoxic microzones develop across a range of hyporheic flow and oxygen uptake conditions. Ultimately, two types of anaerobic microzones develop: flow invariant and flow dependent. The flow invariant microzones are stable across variable hydrologic conditions, whereas the formation and extent of the flow dependent microzones are sensitive to flow rates and orientation. Therefore, pore-scale residence time heterogeneity offers a simple physical explanation for the paradox of anaerobic signals occurring in oxic pore waters without the prerequisite of heterogeneity in reaction substrate availability. Fortunately, electrical geophysical methods are sensitive to ionic tracers within less mobile pore spaces, and thus can help quantify pore scale residence time heterogeneity in situ. These emerging methods present an opportunity to estimate the role of microzones at larger scales.