Dynamics of nitrate production and removal as a function of residence time in the hyporheic zone

Abstract

[1] Biogeochemical reactions associated with stream nitrogen cycling, such as nitrification and denitrification, can be strongly controlled by water and solute residence times in the hyporheic zone (HZ , and hydraulic transport parameters (head, flow rates, flow paths, and residence time distributions) of the reach and along HZ flow paths of an instrumented gravel bar. HZ exchange was observed across the entire gravel bar (i.e., in all wells) with flow path lengths up to 4.2 m and corresponding median residence times greater than 28.5 h. The HZ transitioned from a net nitrification environment at its head (short residence times) to a net denitrification environment at its tail (long residence times). NO 3 − increased at short residence times from 0.32 to 0.54 mg-N L −1 until a threshold of 6.9 h and then consistently decreased from 0.54 to 0. ). The rates of the DO and DOC removal and net nitrification were greatest during short residence times, while the rate of denitrification was greatest at long residence times.15 NO 3 − tracing confirmed that a fraction of the NO 3 − removal was via denitrification as 15 N 2 was produced across the entire gravel bar HZ. Production of 15 N 2 across all observed flow paths and residence times indicated that denitrification microsites are present even where nitrification was the net outcome. These findings demonstrate that the HZ is an active nitrogen sink in this system and that the distinction between net nitrification and denitrification in the HZ is a function of residence time and exhibits threshold behavior. Consequently, incorporation of HZ exchange and water residence time characterizations will improve mechanistic predictions of nitrogen cycling in streams.Citation: Zarnetske, J. P., R. Haggerty, S. M. Wondzell, and M. A. Baker (2011), Dynamics of nitrate production and removal as a function of residence time in the hyporheic zone,

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

“…Our approach is consistent with prior studies (Gu et al, 2012;Hester et al, 2014), and is justified because the relatively low K values for most of the model scenarios (10 −4 -10 −8 m/s) result in long residence times, such that only a very small percentage of most hyporheic flowpaths would be considered nitrification zones (Zarnetske et al, 2011;Marzadri et al, 2012). In other words, oxygen would be used up by aerobic metabolism and nitrification relatively quickly at the beginning of each hyporheic flowpath, and then conditions would be conducive to denitrification thereafter.…”

“…In other words, oxygen would be used up by aerobic metabolism and nitrification relatively quickly at the beginning of each hyporheic flowpath, and then conditions would be conducive to denitrification thereafter. For example, at K = 10 −4 m/s, nitrification would be possible in approximately 3% of the hyporheic zone volume induced by the structures in our model based on the hyporheic zone residence times calculated for those hyporheic zones by Azinheira et al (2014) and the net nitrification/denitrification threshold of 7.5 h proposed by Zarnetske et al (2011). For all lower K's used in our sensitivity analysis (K = 10 −5 m/s, 10 −6 m/s, 10 −7 m/s, and 10 −8 m/s, Table 1), the percentage would be even smaller.…”

“…Given favorable microbial and redox conditions (i.e. an anoxic environment with sufficient labile organic carbon), NO 3 − may be removed by denitrification in the subsurface before upwelling downstream of the structure (Kasahara and Hill, 2006;Lautz and Fanelli, 2008;Zarnetske et al, 2011). Inset floodplains (i.e.…”

Upscaling Nitrogen Removal Capacity from Local Hotspots to Low Stream Orders’ Drainage Basins

“…Nitrate being transported by surface or subsurface flow into the riparian zone represents the second case, that is, the nitrate reactant being carried into a zone where the other conditions necessary for denitrification (availability of organic carbon and anoxia) are present. Current hyporheic zone modeling approaches focus on a hydrologically controlled spatial extent of the hyporheic zone, but with few exceptions (Zarnetske 2011;Bardini and others 2012;Trauth and others 2014), do not take into account that electron donors and acceptors might be delivered via different pathways. Hence, from a kinetic point of view, biogeochemical hotspots are zones where turnover rates are high and can be characterized by short characteristic reaction time scales s reaction .…”

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.…”