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

Abstract

Excess NO 3 -in streams is a growing and persistent problem for both inland and coastal ecosystems, and denitrification is the primary removal process for NO 3 -. Hyporheic zones can have high denitrification potentials, but their role in reach-and network-scale NO 3 -removal is unknown because it is difficult to estimate.We used independent and complementary multiscale measurements of denitrification and total NO 3 -uptake to quantify the role of hyporheic NO 3 -removal in a 303-m reach of a 3 rd -order agricultural stream in western Oregon, USA. We characterized the reach-scale NO 3 -dynamics with steady-state 15 N-NO 3 -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 NO 3 -removal (rate coefficient λ HZ = 0.007/h) accounted for 17% of the observed total reach NO 3 -uptake and 32% of the reach denitrification estimated from the 15 N 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 NO 3 -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.

Effects of inset floodplains and hyporheic exchange induced by in-stream structures on nitrate removal in a headwater stream

“…Finally, removal rates calculated by Protocol 2 are zeroth order with respect to NO 3 − concentration, meaning the rate is flat regardless of inflowing concentration. By contrast, our approach was first order with respect to NO 3 − concentration, which is more in line with approaches typically taken in the scientific literature (Boyer et al, 2006;Stewart et al, 2011;Harvey et al, 2013;Zarnetske et al, 2015). This may mean that Protocol 2 may overestimate N removal at some concentrations and underestimate it at others.…”

“…MIKE SHE solves the advection-dispersion equation using the QUICKEST method (Leonard, 1979), an explicit scheme that applies upstream and central differencing for the advection and dispersion terms, respectively (DHI, 2011). Denitrification in hyporheic zone models is often simulated as Michaelis-Menten (Monod) kinetics (Gu et al, 2007, Zarnetske et al, 2012, however first-order kinetics are justified when the NO 3 − concentration is less than the reaction halfsaturation constant of 1.64 mg/L NO 3 − N (Boyer et al, 2006;Stewart et al, 2011;Zarnetske et al, 2012Zarnetske et al, , 2015Harvey et al, 2013). We used a base case NO 3 − concentration of 1.0 mg/L NO 3 − N, thus we were below this threshold during most of our model runs.…”

“…We simulated hyporheic zone NO 3 − removal via denitrification by assigning a first-order decay rate to the groundwater component of the model. While hyporheic zone denitrification can be simulated following Michaelis-Menten kinetics (Cardenas et al, 2008;Marzadri et al, 2011;Kessler et al, 2015), models using first-order kinetics have been successfully calibrated to field data (Boyer et al, 2006;Stewart et al, 2011;Harvey et al, 2013;Zarnetske et al, 2015), and it is considered reasonable to simulate it as a first order process when NO 3 − concentrations are low (Sheibley et al, 2003;Marzadri et al, 2011;Gu et al, 2012), which they are in this case. We varied this parameter in our sensitivity analysis using a range of literature values (Section 2.2.4, Table 1).…”

“…For example, we assumed first-order removal of NO 3 − from the hyporheic zone based on prior work (Boyer et al, 2006;Stewart et al, 2011;Harvey et al, 2013;Zarnetske et al, 2015). As discussed earlier, this was a reasonable approximation given our relatively low NO 3 − concentrations.…”

Groundwater–surface-water interactions: current research directions

“…Buried riparian litter fuels the microbial processes that can reduce stream N loads in areas with NO 3 − -rich ground water. Zarnetske et al (2015) investigated hyporheic denitrification and NO 3 − uptake at 2 spatial scales: the channelunit scale (a 6.1-m-long gravel bar) and the reach scale (a 303-m-long stream reach). Salt and 15 NO 3 − injections and solute transport and groundwater-flow models were used to quantify NO 3 − uptake, denitrification, advection, diffusion, transient storage, and hyporheic exchange flows.…”