Tidal controls on riverbed denitrification along a tidal freshwater zone. Issue 1 (22nd January 2017)

• Record Type: Journal Article
• Authors: Knights, Deon; Sawyer, Audrey H.; Barnes, Rebecca T.; Musial, Cole T.; Bray, Samuel
• Is Part Of: Water resources research. Volume 53:Issue 1(2017)
• Page Start: 799
• Page End: 816
• Publication Date: 2017-01-22
• Subjects: tidal freshwater zone -- coastal rivers -- water quality -- hyporheic exchange -- tidal pumping; Hydrology -- Periodicals; 333.91
• DOI: 10.1002/2016WR019405
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: In coastal rivers, tidal pumping enhances the exchange of oxygen‐rich river water across the sediment‐water interface, controlling nitrogen cycling in riverbed sediment. We developed a one‐dimensional, fluid flow and solute transport model that quantifies the influence of tidal pumping on nitrate removal and applied it to the tidal freshwater zone (TFZ) of White Clay Creek (Delaware, USA). In field observations and models, both oxygenated river water and anoxic groundwater deliver nitrate to carbon‐rich riverbed sediment. A zone of nitrate removal forms beneath the aerobic interval, which expands and contracts over daily timescales due to tidal pumping. At high tide when oxygen‐rich river water infiltrates into the bed, denitrification rates decrease by 25% relative to low tide. In the absence of tidal pumping, our model predicts that the aerobic zone would be thinner, and denitrification rates would increase by 10%. As tidal amplitude increases toward the coast, nitrate removal rates should decrease due to enhanced oxygen exchange across the sediment‐water interface, based on sensitivity analysis. Denitrification hot spots in TFZs are more likely to occur in less permeable sediment under lower tidal ranges and higher rates of ambient groundwater discharge. Our models suggest that tidal pumping is not efficient at removing surface water nitrate but can remove up to 81% of nitrate from discharging groundwater in the TFZ of White Clay Creek. Given the high population densities of coastal watersheds, the reactive riverbeds of TFZs play a critical role in mitigating new nitrogen loads to coasts. Key Points: Denitrification rates decrease along tidal rivers as oxygen exchange increases Denitrification rates fluctuate with tides in coastal riverbeds Coastal riverbeds can remove a large portion of groundwater‐borne nitrate