Tidal controls on riverbed denitrification along a tidal freshwater zone

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.

Drought Conditions Maximize the Impact of High‐Frequency Flow Variations on Thermal Regimes and Biogeochemical Function in the Hyporheic Zone

“…Our system characteristics are representative of most large gravel river with large bank storage, high HEF rates, oxygenated, and low DOC conditions in both river water and groundwater. While the conceptual model and findings could vary in other river corridor systems, such as those with finer sediments, low HEF rates, different river/subsurface geochemical conditions, and reaction pathways (Knights et al, ; Shuai et al, ; Trauth & Fleckenstein, ), the numerical modeling approaches and analyses used to identify the effects of high‐frequency flow variations are general and readily transferable.…”

Spectral analysis of continuous redox data reveals geochemical dynamics near the stream–aquifer interface

“…All streambed depths tended to be anaerobic (E h < 800 mV) for the majority (>99%) of the study period. Despite significant tidal exchange between surface water and the bed, oxic conditions typically extended less than 15 cm into the sediment where the shallowest electrode was located, consistent with reactive transport models (Knights et al, ). Nitrate (NO 3 − ) concentrations in the streambed ranged from 0.19 to 0.51 mg N/L over 75‐cm depth and were low relative to NO 3 − concentrations in either surface water (2.69 mg NO 3 − N/L) or deeper groundwater, consistent with possible denitrification.…”

“…Redox amplitude spectral densities (ASDs) at the 0.5‐day period from streambed FFTs were larger than those in the bank (e.g., 2.66 mV at 50 cm in the bed at Location C, compared with 1.83 mV at 146 cm in the bank at Location E; Figure b–c). In the bed, the ASD was greatest at 50 cm near a change in lithology (Figure b) but was reduced at 15 cm near the sediment–water interface (not shown), where significant interaction with surface water maintained relatively oxic conditions (Knights et al, ). Cross wavelet analysis shows that the streambed E h response lagged stage by 1.5 hr at 15 cm and by 3 hr at 50 cm (Figure S1).…”

“…Beneath the sediment–water interface, oscillatory surface water–groundwater exchange (or tidal pumping) in sandy sediments effectively transports oxygen, NO 3 − , DOC, and other reactants (Bianchin et al, ; Rocha et al, ). Advective transport distances over individual tidal cycles were likely on the order of centimetres or less (Knights et al, ), but the long‐term effect of oscillatory flow would enhance solute dispersion producing a shallow vertical redox gradient that fluctuates slightly over each tidal cycle. Tidal oscillations in transport were particularly evident near the lithologic boundary at 50 cm (Figure a).…”

“…The riparian aquifers of tidal rivers have the potential to reduce nutrient export to the coast through denitrification and other transformations. Though their capacity to remove redox-sensitive pollutants has been recognized (Ensign et al, 2008;Ensign, Noe, & Hupp, 2014;Knights, Sawyer, Barnes, Musial, & Bray, 2017;Musial, Sawyer, Barnes, Bray, & Knights, 2016), few studies have quantified the link between tidal hydrodynamics and redox conditions. Furthermore, to our knowledge, no study has used spectral analysis of a continuous geochemical signal to evaluate hydrologic forcings across multiple timescales.…”