The interplay between transport and reaction rates as controls on nitrate attenuation in permeable, streambed sediments

Anthropogenic nitrogen fixation and subsequent use of this nitrogen as fertilizer has greatly disturbed the global nitrogen cycle. Rivers are recognized hotspots of nitrogen removal in the landscape as interaction between surface water and sediments creates heterogeneous redox environments conducive for nitrogen transformations. Our understanding of riverbed nitrogen dynamics to date comes mainly from shallow sediments or hyporheic exchange flow pathways with comparatively little attention paid to groundwater-fed, gaining reaches. We have used 15N techniques to quantify in situ rates of nitrate removal to 1m depth within a groundwater-fed riverbed where subsurface hydrology ranged from strong upwelling to predominantly horizontal water fluxes. We combine these rates with detailed hydrologic measurements to investigate the interplay between biogeochemical activity and water transport in controlling nitrogen attenuation along upwelling flow pathways. Nitrate attenuation occurred via denitrification rather than dissimilatory nitrate reduction to ammonium or anammox (range = 12 to >17000 nmol 15N L-1 h-1). Overall, nitrate removal within the upwelling groundwater was controlled by water flux rather than reaction rate (i.e. Damköhler numbers 80% of nitrate removal occurs within sediments not exposed to hyporheic exchange flows under baseflow conditions, illustrating the importance of deep sediments as nitrate sinks in upwelling systems

Fine-Scale in Situ Measurement of Riverbed Nitrate Production and Consumption in an Armored Permeable Riverbed

Alteration of the global nitrogen cycle by man has increased nitrogen loading in waterways considerably, often with harmful consequences for aquatic ecosystems. Dynamic redox conditions within riverbeds support a variety of nitrogen transformations, some of which can attenuate this burden. In reality, however, assessing the importance of processes besides perhaps denitrification is difficult, due to a sparseness of data, especially in situ, where sediment structure and hydrologic pathways are intact. Here we show in situ within a permeable riverbed, through injections of 15N-labeled substrates, that nitrate can be either consumed through denitrification or produced through nitrification, at a previously unresolved fine (centimeter) scale. Nitrification and denitrification occupy different niches in the riverbed, with denitrification occurring across a broad chemical gradient while nitrification is restricted to more oxic sediments. The narrow niche width for nitrification is in effect a break point, with the switch from activity “on” to activity “off” regulated by interactions between subsurface chemistry and hydrology. Although maxima for denitrification and nitrification occur at opposing ends of a chemical gradient, high potentials for both nitrate production and consumption can overlap when groundwater upwelling is strong

Recommendations for effective documentation in regional anesthesia: an expert panel Delphi consensus project

Background and objectives: Documentation is important for quality improvement, education, and research. There is currently a lack of recommendations regarding key aspects of documentation in regional anesthesia. The aim of this study was to establish recommendations for documentation in regional anesthesia. Methods: Following the formation of the executive committee and a directed literature review, a long list of potential documentation components was created. A modified Delphi process was then employed to achieve consensus amongst a group of international experts in regional anesthesia. This consisted of 2 rounds of anonymous electronic voting and a final virtual round table discussion with live polling on items not yet excluded or accepted from previous rounds. Progression or exclusion of potential components through the rounds was based on the achievement of strong consensus. Strong consensus was defined as ≥75% agreement and weak consensus as 50%-74% agreement. Results: Seventy-seven collaborators participated in both rounds 1 and 2, while 50 collaborators took part in round 3. In total, experts voted on 83 items and achieved a strong consensus on 51 items, weak consensus on 3 and rejected 29. Conclusion: By means of a modified Delphi process, we have established expert consensus on documentation in regional anesthesia

Impact of microforms on nitrate transport at the groundwater –surface water interface

Small streambed structures (or microforms, 0.01-1 m in length) exist ubiquitously in riverbed systems. Small-scale topography is potentially important in controlling hyporheic exchange flow and transport of conservative and reactive solutes at the groundwater-surface water interface. The role of microforms on NO3- transfer in a riffle-scale (macroforms of > 1 m length) hyporheic zone within a gaining river setting is investigated using a 2-D flow and transport model which accounts for both nitrification and denitrification. Results show that the short pathlines caused by microforms lead to more NO3- discharge to the river compared with a macroform-only condition due to shortened residence times of both surface water and groundwater in mixing zones. Short hyporheic exchange flow pathways caused by microforms could remain oxic along their entire length or switch from nitrate producing to nitrate consuming as oxygen concentrations decline. Microforms affect net NO3- flux by the combined effect of introducing more stream mass flux and reducing their residence time in mixing zones under different hydrological and biogeochemical conditions. Our findings underscore that ignoring microforms in river beds may underestimate NO3- load to the river and have practical implications for porewater sampling strategies in groundwater-surface water studies

In situ measurement of redox sensitive solutes at high spatial resolution in a riverbed using Diffusive Equilibrium in Thin Films (DET)

Here we evaluate a new field methodology for the deployment of Diffusive Equilibrium in Thin Films (DET, protected in stainless steel holders) in coarse riverbed sediments based on that originally developed for fine-grained sediments and soils. Concentration gradients of NO 3 -, NH 4 + and dissolved Mn were measured at cm resolution. We observed fine scale changes in NO 3 -, NH 4 +, and Mn concentrations in the river bed (0-30cm) that were only evident at high resolution and compared them to profiles of NO 3 - and NH 4 + obtained with low resolution multilevel piezometers. The range in concentrations of NO 3 - and NH 4 + measured through DET was larger than those measured at coarse resolution through pore water sampling from multilevel piezometer in the riverbed over the 30cm depth. According to the results, high resolution profiles of redox sensitive chemical species in riverbeds could help in identifying and resolving hotspots of biogeochemical activity. Measurements of NH 4 + using DET were higher than measurements in pore water collected from the multilevel samplers. Further studies are needed to establish whether there is a systematic bias associated with either procedure for the measurement of NH 4 +.</p

In situ measurement of redox sensitive solutes at high spatial resolution in a riverbed using Diffusive Equilibrium in Thin Films (DET)

Here we evaluate a new field methodology for the deployment of Diffusive Equilibrium in Thin Films (DET, protected in stainless steel holders) in coarse riverbed sediments based on that originally developed for fine-grained sediments and soils. Concentration gradients of NO 3 -, NH 4 + and dissolved Mn were measured at cm resolution. We observed fine scale changes in NO 3 -, NH 4 +, and Mn concentrations in the river bed (0-30cm) that were only evident at high resolution and compared them to profiles of NO 3 - and NH 4 + obtained with low resolution multilevel piezometers. The range in concentrations of NO 3 - and NH 4 + measured through DET was larger than those measured at coarse resolution through pore water sampling from multilevel piezometer in the riverbed over the 30cm depth. According to the results, high resolution profiles of redox sensitive chemical species in riverbeds could help in identifying and resolving hotspots of biogeochemical activity. Measurements of NH 4 + using DET were higher than measurements in pore water collected from the multilevel samplers. Further studies are needed to establish whether there is a systematic bias associated with either procedure for the measurement of NH 4 +.</p

Influence of emergent vegetation on nitrate cycling in sediments of a groundwater-fed river

Emergent vegetation in river beds can play a significant role in nutrient cycling in riverine sediments. We analysed and compared pore water NO3 (-) concentration gradients in the sediments of the River Leith, Cumbria, UK, in the presence and absence of emergent vegetation (dominated by Sparganium spp.). High resolution (1 cm interval), in situ, vertical profiles of NO3 (-) to 30 cm depth were measured using deployment of diffusive equilibrium in thin films probes on four occasions from July to September 2010. We found significantly (p <0.05) lower NO3 (-) concentration under vegetated sediments (VS) compared to those under adjacent un-vegetated sediments (UVS). Concentrations of dissolved oxygen, measured in pore water collected from multi-level piezometers at 10, 20, 25, 30 and 35 cm depths at the VS and UVS sties, were generally lower under VS (median concentration = 28 mu M) than under UVS (median = 132 mu M) and correlated significantly with NO3 (-) concentration (Spearman's r = 0.74, p <0.05). Similarly, pore water dissolved organic carbon (DOC) concentration was 2.8 times higher under VS compared to UVS, and correlated negatively with NO3 (-) concentration (Spearman's r = -0.39, p <0.05). Specific ultra-violet absorption at 254 nm (SUVA) and per cent aromaticity values of DOC were significantly higher under VS (p <0.05), suggesting that the DOC contained more complex (aromatic) compounds than DOC recorded under UVS. We suggest that, the higher quantity of DOC and its distinct SUVA and percentage aromaticity under VS may have supported faster dissolved oxygen consumption, with the creation of anoxic zones conducive for NO3 (-) reduction mainly through denitrification. Metabolic uptake and immobilization of NO3 (-) by plants and microbes may have further contributed to lower NO3 (-) concentrations under VS. As Sparganium spp. is a common plant growing in river beds in the UK, its role in NO3 (-) cycling should be considered in attempts to accurately budget N cycling in river beds

River bed carbon and nitrogen cycling:state of play and some new directions

The significance of freshwaters as key players in the global budget of both carbon dioxide and methane has recently been highlighted. In particular, rivers clearly do not act simply as inert conduits merely piping carbon from catchment to coast, but, on the whole, their metabolic activity transforms a considerable fraction of the carbon that they convey. In addition, nitrogen is cycled, sometimes in tight unison with carbon, with appreciable amounts being ‘denitrified’ between catchment and coast. However, shortfalls in our knowledge about the significance of exchange and interaction between rivers and their catchments, particularly the significance of interactions mediated through hyporheic sediments, are still apparent. From humble beginnings of quantifying the consumption of oxygen by small samples of gravel, to an integrated measurement of reach scale transformations of carbon and nitrogen, our understanding of the cycling of these two macro elements in rivers has improved markedly in the past few decades. However, recent discoveries of novel metabolic pathways in both the nitrogen and carbon cycle across a spectrum of aquatic ecosystems, highlights the need for new directions and a truly multidisciplinary approach to quantifying the flux of carbon and nitrogen through rivers

Passive pore water sampling provides evidence of suppression of hyporheic exchange and ntrate transformation in a groundwater-fed river

In recent years, the potential of riverbed hyporheic sediments to attenuate nitrate from polluted groundwater has received much attention. However, transformation of reactive nitrate along upwelling flow paths is dependent on redox conditions and the availability of electron donors; these biogeochemical conditions are in turn strongly dependent on hydro-morphologic drivers including riverbed structure and water flux. We worked on a 50m stretch of the River Leith, Cumbria, UK, which is dominated by upwelling nitrate-rich groundwater. Previous investigations of hyporheic connectivity and pore water chemistry in our sub-reach suggest strong groundwater upwelling might suppress the hyporheic zone, possibly restraining its ability to attenuate nitrate in the upwelling groundwater. However, this hypothesis could not be verified previously due to the difficulty of measuring pore water chemistry at depths in the riverbed <10cm. The goal of this paper is to test whether nitrate attenuation is occurring in riverbed sediments characterised by strong vertical water fluxes. We utilised diffusive equilibrium in thin-films (DET) samplers to capture high resolution (cm-scale) vertical concentration profiles of nitrate in the upper 15cm of the riverbed sediments. The vertical concentration profiles obtained from DET probes indicate considerable cm-scale variability in concentrations. However, the overall trend suggests the absence of substantial net chemical transformations and surface-subsurface water mixing in the shallow sediments of our sub-reach