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Publications > Briggs and others, 2015

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

Martin A. Briggs1, Frederick D. Day-Lewis1, Jay P. Zarnetske2, and Judson W. Harvey3

1 Office of Groundwater, Branch of Geophysics, U.S. Geological Survey, Storrs, Connecticut, USA
2 Department of Geological Sciences, Michigan State University, East Lansing, Michigan, USA,
3 National Research Program, U.S. Geological Survey, Reston, Virginia, USA

Student Opportunity

The authors of this work, and Kamini Singha of the Colorado School of Mines, are currently recruiting 2 PhD students to work on a 3 yr NSF-funded project to investigate streambed less-mobile porosity and associated redox microzones. One student will be based at Michigan State University with Jay Zarnetske, the other at the University of Connecticut with Martin Briggs and Fred Day-Lewis. The MSU student will focus on reactive nitrogen experimentation, while the UConn student will focus on geophysical evaluations, but there will inherently be much overlap and combined lab/field efforts. Interested potential students should contact Jay Zarnetske (jpz@cns.msu.edu ) or Martin Briggs (mbriggs@usgs.gov).

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.

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 [Figure from journal article]

Figure 1. Figure 1. (a) The overarching conceptual model which shows dissolved reactants in oxic stream water being carried into the hyporheic pore network. As O2 is consumed, a bulk transition to anaerobic respiration occurs if mean flow path residence times exceed tlim. (b) A hypothetical flow path is simulated with a 2-D pore-network model that incorporates pore-scale heterogeneity in pore throat size, creating microzones of anoxic conditions. (c) Anoxic microzones (red) are classified based on center-of-pore arrival times relative to the threshold time for anoxia, tA. (d) Microzone designation can be further refined into flow-dependent (yellow) and flow-invariant (red) microzones.



Final copy as submitted to Geophysical Research Letters for publication as: Briggs, M.A., Day-Lewis, F.D., Zarnetske, J.P., and Harvey, J.W., 2015, A physical explanation for the development of redox microzones in hyporheic flow: Geophysical Research Letters, vol. 42, doi: 10.1002/2015GL064200.

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