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Hydrogeologic controls of surface water-groundwater nitrogen dynamics within a tidal freshwater zone

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Created: Oct 07, 2019 at 2:20 p.m.
Last updated: Oct 07, 2019 at 3:26 p.m.
DOI: 10.4211/hs.39d9f1263d744c4c98325a80e8fca51a
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Sharing Status: Published
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Microbial processing of reactive nitrogen in stream sediments and connected aquifers can remove and transform nitrogen prior to its discharge into coastal waters, decreasing the likelihood of harmful algal blooms and low oxygen levels in estuaries. Canonical wisdom points to the decreased capacity of rivers to retain nitrogen as they flow towards the coast. However, how tidal freshwater zones, which often extend hundreds of kilometers inland, process and remove nitrogen remains unknown. Using geochemical measurements and numerical models, we show that tidal pumping results in the rapid cycling of nitrogen within distinct zones throughout the riparian aquifer. Near the fluctuating water table nitrification dominates, with high nitrate concentrations (>10 mg N L-1) and consistent isotopic composition. Beneath this zone, isotopes reveal that nitrate is both denitrified and added over the tidal cycle, maintaining nitrate concentrations >3-4 mg N L-1. In most of the riparian aquifer and streambed, nitrate concentrations are <0.5 mg N L-1, suggesting denitrification dominates. Model results reveal that oxygen delivery to groundwater from the overlying unsaturated soil fuels mineralization and nitrification, with subsequent denitrification in low oxygen, high organic matter regions. Depending on flow paths, tidal freshwater zones could be sources of nitrate in regions with permeable sediment and low organic matter content.

Subject Keywords



Coordinate System/Geographic Projection:
WGS 84 EPSG:4326
Coordinate Units:
Decimal degrees
Place/Area Name:
White Clay Creek


Start Date:
End Date:



Funding Agencies

This resource was created using funding from the following sources:
Agency Name Award Title Award Number
National Science Foundation EAR 1446763
National Science Foundation EAR 1446724

How to Cite

Barnes, R. T., A. H. Sawyer, D. M. Tight, C. D. Wallace, M. G. Hastings (2019). Hydrogeologic controls of surface water-groundwater nitrogen dynamics within a tidal freshwater zone, HydroShare,

This resource is shared under the Creative Commons Attribution CC BY.


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