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

Abstract

AbstractExcess 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 λHZ = 0.007/h) accounted for 17% of the observed total reach NO3– uptake and 32% of the reach denitrification estimated f...