[1] There is widespread recognition that the groundwater-surface water interface can have significant influence on the pattern and form of the transfer of nutrient-rich groundwater to rivers. Characterizing and quantifying this influence is critical for successful management of water resources in many catchments, particularly those threatened by rising nitrate levels in groundwater. Building on previous experimental investigations in one such catchment: the River Leith, UK, we report on a multimeasurement, multiscale program aimed at developing a conceptualization of groundwater-surface water flow pathways along a 200 m reach. Key to this conceptualization is the quantification of vertical and horizontal water fluxes, which is achieved through a series of Darcian flow estimates coupled with in-stream piezometer tracer dilution tests. These data, enhanced by multilevel measurements of chloride concentration in riverbed pore water and water-borne geophysical surveying, reveal a contrast in the contribution of flow components along the reach. In the upper section of the reach, a localized connectivity to regional groundwater, that appears to suppress the hyporheic zone, is identified. Further downstream, horizontal (lateral and longitudinal) flows appear to contribute more to the total subsurface flow at the groundwater-surface water interface. Although variation in hydraulic conductivity of the riverbed is observed, localized variation that can account for the spatial variability in flow pathways is not evident. The study provides a hydrological conceptualization for the site, which is essential for future studies which address biogeochemical processes, in relation to nitrogen retention/release. Such a conceptualization would not have been possible without a multiexperimental program.Citation: Binley, A., S. Ullah, A. L. Heathwaite, C. Heppell, P. Byrne, K. Lansdown, M. Trimmer, and H. Zhang (2013), Revealing the spatial variability of water fluxes at the groundwater-surface water interface, Water Resour. Res., 49,[3978][3979][3980][3981][3982][3983][3984][3985][3986][3987][3988][3989][3990][3991][3992]
Rivers are an important global sink for excess bioavailable nitrogen: they convert approximately 4 40% of terrestrial N-runoff per year (~47 Tg) to biologically unavailable N 2 gas and return it to 5 the atmosphere. 1 Currently, riverine N 2 production is conceptualised and modelled as 6 denitrification. [2][3][4] The contribution of anaerobic ammonium oxidation (or anammox), an alternate 7 pathway of N 2 production important in marine environments, is not well understood. 5,6 Here we 8 use in situ and laboratory measurements of anammox activity using 15 N tracers and molecular 9 analyses of microbial communities to evaluate anammox in clay, sand, and chalk-dominated rates. In spite of requiring anoxic conditions, anammox, most likely coupled to partial 14 nitrification, contributed up to 58% of in situ N 2 production in oxic, permeable riverbeds.. In 15 contrast, denitrification dominated in low permeability clay-bed rivers, where anammox 16 contributes roughly 7% to the production of N 2 gas. We conclude that anammox can represent an 17 important nitrogen loss pathway in permeable river sediments. and increases a river's capacity to attenuate nitrogen. 49Much of what is known about anammox in the environment comes from estuaries and 50 coastal seas where anammox varies in response to sediment reactivity. The relative 51 contribution of anammox to marine N 2 production (ra) decreases with proximity to the shore 52 as supply of carbon stimulates denitrification over anammox. 12,13 Extrapolating this trend 53 further inshore suggested anammox activity would be insignificant in estuaries but anammox 54 potential actually increased. 14,15 In both estuaries and coastal seas, however, anammox is 55 important in low permeability sediments (ra <1 to 11 %) 9,16 , where oxygen penetration is 56 restricted 12,15 and it is these muddy sediments that the few studies of riverine anammox have 57 occurred. 5,6 In addition, anammox is widespread in marine sediments but the affiliated 64Using a combination of in situ and laboratory-based 15 N tracer techniques 12,18 and molecular 65 assays we characterised both the anammox community and its activity within rivers from 66 clay, sand and chalk-dominated sub-catchments under summer, base flow conditions (Table 67 S1). For rivers in which in situ measurements were performed we indexed catchment 68 permeability by calculating the base-flow index (BFI , Table S2), the proportion of river flow 69 4 derived from deep groundwater sources. In clay catchments, low soil permeability leads to 70 routing of rainfall overland or through shallow, more permeable soils into the river (low BFI). 71Whilst in chalk or sand catchments, the higher soil permeability allows infiltrated water to 72 percolate deeper into the aquifer and follow much longer flow paths to towards the river 73 (high BFI). 74We began by characterising the anammox hzo functional gene that encodes hydrazine 75 oxidoreductase which catalyses the oxidation of hydrazine to N 2 . The hzo gene was detected 76 in all sediment...
Laboratory incubations with river-bed sediment collected from riffles and pools were used to quantify potential pathways of dissimilatory nitrate reduction in the hyporheic zone of a groundwater-fed river. Sediments collected from between 5-cm and 86-cm depth in the bed of the River Leith, Cumbria, United Kingdom, were incubated with a suite of 15 N-labeled substrates ( 15 NO { 3 , 15 NH z 4 , and 14 NO { 3 ) to quantify nitrate reduction via denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox). Denitrification was the dominant pathway of dissimilatory nitrate reduction in the hyporheic sediments, although recovery of 15 N from the ammonium pool indicated that DNRA was also active. The potential for anammox was confirmed by the production of 29 N 2 during the 15 NH z 4 and 14 NO { 3 incubation, but it was much smaller than denitrification. Potential rates of denitrification were highest in shallow sediments and decayed exponentially with depth thereafter. There were clear differences in denitrification activity between riffle and pool sediments. After the production of 15 N-N 2 had stabilized, we added a spike of bacteriological peptone to determine the effect of complex organic substrates on denitrification potential. The potential rate of denitrification increased uniformly at all sediment depths but the total amount of denitrification fueled by the organic substrates decreased markedly with depth, from 90% in the shallow sediments to 30% in the deepest sediments. In addition, a considerable fraction of the 15 NO { 3 could not be accounted for, which suggested that up to 87% of it had been assimilated in the deepest sediments.
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