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Nitrogen-loads to streams across two different riparian zones (crop field and wetland): Importance of bypass flow and subsurface removal processes
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  • Peter Engesgaard,
  • Mads Steiness,
  • Soren Jessen,
  • Sofie GW van't Veen,
  • Tue Kofod,
  • Anker Lajer Hojberg
Peter Engesgaard
University of Copenhagen

Corresponding Author:pe@ign.ku.dk

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Mads Steiness
Univeristy of Copenhagen
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Soren Jessen
University of Copenhagen
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Sofie GW van't Veen
University of Aarhus
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Tue Kofod
Association of Danish Water Supply
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Anker Lajer Hojberg
Geological Survey of Denmark and Greenland
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N-loads from subsurface, drains, and groundwater-fed surface (bypass) flows via two riparian zones (crop field and wetland) to a second order stream were investigated by sampling of shallow and deep groundwater on both sides and monthly measurements of flows from springs, drains, and stream including water quality (nitrate). A push-pull test in the crop field gave estimates of first-order denitrification rate (0.23 day–1). Reactive transport modelling evaluated observations of water chemistry and denitrification processes in the groundwater below the crop field showing that nitrate was completely removed by denitrification with pyrite in the aquifer (model rates of 0.6–2.5 mmol NO3 L−1 yr−1). A drain in the crop field routed approximately 10% (bypass) of the regional groundwater inflow to the stream. Buffer efficiency was high at 90%. The wetland on the other side of the stream hosts several locations of focused nitrate-rich groundwater-fed spring discharge, predominantly through a non-maintained drainage system of drainpipes and ditches with bypass accounting for 59% of the regional flow input. Nitrate was completely removed in groundwater by denitrification with dissolved organic matter in shallow groundwater. The regional inflow and N load to the wetland is amongst the highest recorded and data shows that the N load to the stream is very high. The buffer efficiency ranged from 45–83% depending on if all springs contributed to the stream or only the two with visible outflow. A conceptual model for nitrate removal efficiency as a function of Damköhler number and percent bypass flow is proposed.