Hyporheic zone denitrification: Controls on effective reaction depth and contribution to whole-stream mass balance

Abstract

[1] Stream denitrification is thought to be enhanced by hyporheic transport but there is little direct evidence from the field. To investigate at a field site, we injected 15 NO 3 À , Br (conservative tracer), and SF 6 (gas exchange tracer) and compared measured whole-stream denitrification with in situ hyporheic denitrification in shallow and deeper flow paths of contrasting geomorphic units. Hyporheic denitrification accounted for between 1 and 200% of whole-stream denitrification. The reaction rate constant was positively related to hyporheic exchange rate (greater substrate delivery), concentrations of substrates DOC and nitrate, microbial denitrifier abundance (nirS), and measures of granular surface area and presence of anoxic microzones. The dimensionless product of the reaction rate constant and hyporheic residence time, hz hz define a Damköhler number, Da den-hz that was optimal in the subset of hyporheic flow paths where Da den-hz % 1. Optimal conditions exclude inefficient deep pathways where substrates are used up and also exclude inefficient shallow pathways that require repeated hyporheic entries and exits to complete the reaction. The whole-stream reaction significance, R s (dimensionless), was quantified by multiplying Da den-hz by the proportion of stream discharge passing through the hyporheic zone. Together these two dimensionless metrics, one flow-path scale and the other reach-scale, quantify the whole-stream significance of hyporheic denitrification. One consequence is that the effective zone of significant denitrification often differs from the full depth of the hyporheic zone, which is one reason why whole-stream denitrification rates have not previously been explained based on total hyporheic-zone metrics such as hyporheic-zone size or residence time.

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.…”

Active heat pulse sensing of 3-D-flow fields in streambeds

“…Using this method, spatial and temporal flow dynamics within the hyporheic zone, particularly hyporheic transport and exchange (e.g. longer attenuation), have been shown to enhance stream denitrification (Harvey et al, 2013;Gomez-Velez et al, 2015;Zarnetske et al, 2011), degradation of mine-pollutants (Gandy et al, 2007) and the degradation of wastewater micro-pollutants (Engelhardt et al, 2013). It is also widely used by other disciplines, e.g.…”