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Coupling multiscale observations to evaluate hyporheic nitrate removal at the reach scale

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https://ir.library.oregonstate.edu/concern/articles/zw12z688b

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  • Excess NO₃⁻ in streams is a growing and persistent problem for both inland and coastal ecosystems, and denitrification is the primary removal process for NO₃⁻. Hyporheic zones can have high denitrification potentials, but their role in reach- and network-scale NO₃⁻ removal is unknown because it is difficult to estimate. We used independent and complementary multiscale measurements of denitrification and total NO₃⁻ uptake to quantify the role of hyporheic NO₃⁻ removal in a 303-m reach of a 3rd-order agricultural stream in western Oregon, USA. We characterized the reach-scale NO₃⁻ dynamics with steady-state ¹⁵N-NO₃⁻ tracer-addition experiments and solute-transport modeling, and measured the hyporheic conditions via in-situ biogeochemical and groundwater modeling. We also developed a method to link these independent multiscale measurements. Hyporheic NO₃⁻ removal (rate coefficient λ[subscript HZ] = 0.007/h) accounted for 17% of the observed total reach NO₃⁻ uptake and 32% of the reach denitrification estimated from the ¹⁵N experiments. The primary limitations on hyporheic denitrification at the reach scale were availability of labile dissolved organic C and the restricted size of the hyporheic zone caused by anthropogenic channelization (sediment thickness ≤ 1.5 m). Linking multiscale methods made estimates possible for hyporheic influence on stream NO₃⁻ dynamics. However, it also demonstrated that the traditional reach-scale tracer experimental designs and subsequent transport modeling cannot be used alone to directly investigate the role of the hyporheic zone on reach-scale water and solute dynamics.
  • Keywords: residence time, denitrification, nitrogen, nutrient cycling, surface-water–groundwater interaction
  • Keywords: residence time, denitrification, nitrogen, nutrient cycling, surface-water–groundwater interaction
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  • Zarnetske, J. P., Haggerty, R., & Wondzell, S. M. (2015). Coupling multiscale observations to evaluate hyporheic nitrate removal at the reach scale. Freshwater Science, 34(1), 172-186. doi:10.1086/680011
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  • 34
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