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Coupled transport and reaction kinetics control the nitrate source-sink function of hyporheic zones

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https://ir.library.oregonstate.edu/concern/articles/9306t0773

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  • The fate of biologically available nitrogen (N) and carbon (C) in stream ecosystems is controlled by the coupling of physical transport and biogeochemical reaction kinetics. However, determining the relative role of physical and biogeochemical controls at different temporal and spatial scales is difficult. The hyporheic zone (HZ), where groundwater–stream water mix, can be an important location controlling N and C transformations because it creates strong gradients in both the physical and biogeochemical conditions that control redox biogeochemistry. We evaluated the coupling of physical transport and biogeochemical redox reactions by linking an advection, dispersion, and residence time model with a multiple Monod kinetics model simulating the concentrations of oxygen (O₂), ammonium (NH₄), nitrate (NO₃), and dissolved organic carbon (DOC). We used global Monte Carlo sensitivity analyses with a nondimensional form of the model to examine coupled nitrification-denitrification dynamics across many scales of transport and reaction conditions. Results demonstrated that the residence time of water in the HZ and the uptake rate of O₂ from either respiration and/or nitrification determined whether the HZ was a source or a sink of NO₃ to the stream. We further show that whether the HZ is a net NO₃ source or net NO₃ sink is determined by the ratio of the characteristic transport time to the characteristic reaction time of O₂ (i.e., the Damköhler number, Da[subscript O2]), where HZs with Da[subscript O2] < 1 will be net nitrification environments and HZs with Da[subscript O2] ≪ 1 will be net denitrification environments. Our coupling of the hydrologic and biogeochemical limitations of N transformations across different temporal and spatial scales within the HZ allows us to explain the widely contrasting results of previous investigations of HZ N dynamics which variously identify the HZ as either a net source or sink of NO₃. Our model results suggest that only estimates of residence times and O₂ uptake rates are necessary to predict this nitrification-denitrification threshold and, ultimately, whether a HZ will be either a net source or sink of NO₃.
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  • Zarnetske, J. P., R. Haggerty, S. M. Wondzell, V. A. Bokil, and R. Gonza´lez-Pinzo´n (2012), Coupled transport and reaction kinetics control the nitrate source-sink function of hyporheic zones, Water Resources Research, 48, W11508, doi:10.1029/2012WR011894.
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  • 48
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  • W11508
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  • Support for this project was provided by a NSF Ecosystem Informatics IGERT fellowship (NSF grant DGE-0333257) to J.P.Z., research grants from the OSU Institute for Water and Watersheds and a Society for Freshwater Sciences Endowment Fund to J.P.Z., and the NSF grant EAR-0409534 to R.H. and S.M.W. V.A.B. would like to acknowledge the NSF Mathematical Biology grant DMS-1122699 for partial support. Further research support was provided by the Hollis M. Dole Environmental Geology Foundation at OSU and NSF Grant EAR-0838338.
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