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

• Published in: Journal of geophysical research. Biogeosciences 2015-06, Vol.120 (6), p.1093-1109
• Main Authors: Lansdown, K., Heppell, C. M., Trimmer, M., Binley, A., Heathwaite, A. L., Byrne, P., Zhang, H.
• Format: Article
• Language: English
• Subjects: Ammonium; Anthropogenic factors; Attenuation; Base flow; Biogeochemistry; Damköhler; Denitrification; Exchange; Fluxes; Freshwater; Groundwater; hot spots; Hydrology; hyporheic; nitrate consumption; Nitrate reduction; Nitrate removal; Nitrates; Nitrogen cycle; Nitrogen fixation; Nitrogen removal; Nutrient removal; Organic matter; residence time; River beds; Rivers; Sediments; Streambeds; Surface water; Upwelling; Water depth; Water transport
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1002/2014JG002874

Anthropogenic nitrogen fixation and subsequent use of this nitrogen as fertilizer have greatly disturbed the global nitrogen cycle. Rivers are recognized hot spots 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 1 m 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 >17,000 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 base flow conditions, illustrating the importance of deep sediments as nitrate sinks in upwelling systems. Key Points Deep sediments (>10 cm) are nitrate sinks in groundwater‐fed rivers Denitrification can be sustained without substantial buried organic matter Denitrification in a sand‐dominated reach can be transport‐controlled