Linking Multi-Scale Observations to Determine Hyporheic Nitrate Removal in a Stream

Abstract:

Surplus nitrate (NO₃^-) in streams is a persistent problem for many aquatic ecosystems and denitrification represents the primary removal process for NO₃^- in streams. Hyporheic zones can have high denitrification potentials, but the role of the hyporheic denitrification on reach and network scale NO₃^- removal is unknown because it is difficult to estimate using current methods. Here, we develop a new approach that links existing independent and complementary multi-scale measurements of denitrification and total NO₃^- uptake. This approach is then used to quantify the role of hyporheic NO₃^- removal in a 303m reach of a third-order agricultural stream in western Oregon, USA. The reach scale NO₃^- dynamics were characterized with steady-state ¹⁵N-NO₃^- tracer addition experiments and solute transport modeling, while the hyporheic conditions were measured via in situ biogeochemical and groundwater modeling. Our linking of multi-scale approaches revealed that the hyporheic NO₃^- removal (rate coefficient λ_HZ = 0.007 h^-1) accounted for 17% of the observed total reach NO₃^- uptake, and 32% of the reach denitrification estimated from the ¹⁵N experiments. The primary limitations of hyporheic denitrification at the reach scale were labile dissolved organic carbon availability (low hyporheic SUVA₂₅₄) and the restricted size of the hyporheic zone due to anthropogenic channelization (sediment thickness ≤ 1.5 m). Linking multi-scale methods enabled us to make one of the first ever reach-scale estimates of hyporheic influence on stream NO₃^- and denitrification dynamics. Further, this study also demonstrates 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.