Surface water–groundwater exchange dynamics in a tidal freshwater zone

Musial, Cole T.; Sawyer, Audrey H.; Barnes, Rebecca T.; Bray, Samuel; Knights, Deon

doi:10.1002/hyp.10623

Cited by 37 publications

(60 citation statements)

References 49 publications

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“…The vertical redox gradient, as determined from median values, was significantly greater in the banks than the bed and increased with distance from the stream. Vertical gradients were 6.4 and 8.5 mV/cm at Locations E and G, respectively; smaller water table fluctuations farther from the stream may explain the steeper vertical gradients (Musial et al, ). Anaerobic conditions persisted for more than 95% of the study period at Location E, where the upper two electrodes (depths 146 and 166 cm) were above the water table occasionally but likely within the capillary fringe for about 99% of the study period.…”

Section: Results and Interpretationmentioning

confidence: 99%

“…Though storm events can raise river stage by more than a meter, the floodplain is elevated ~2 m above the river, and therefore, overbank flow occurs rarely and did not occur during the monitoring period. Water table fluctuations extend from the channel ~30 m into the riparian aquifer (Musial et al, ). The streambed and bank are composed primarily of alluvial, interbedded sands, and silts (Figure ).…”

Section: Methodsmentioning

confidence: 99%

“…(c) Water table elevation time series. Redox probes are located in the streambed (Location C of Musial et al, ) and bank (~65 cm east of Location E and ~550 cm west of Location G of Musial et al, ). The piezometer contained the reference electrode.…”

Section: Methodsmentioning

confidence: 99%

“…Beginning in May of 2016, water level and temperature were monitored every 15 min using nonvented shallow water‐level data loggers (In‐Situ TROLL 100) in the stream and one bank piezometer located approximately 5 m from the channel. Redox probes were installed in the streambed near Locations C (on sand bar), E (~6 m from stream), and G (~14 m from stream), as described in Musial et al (), to capture a range of water table fluctuations (Figure ). Each probe consisted of a fibreglass‐epoxy tube embedded with an array of 4–6 platinum electrodes spaced 15–25 cm apart (Paleo Terra, Amsterdam, Netherlands).…”

Section: Methodsmentioning

confidence: 99%

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

Wallace

Sawyer

Barnes

2018

Hydrological Processes

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Changes in streamflow and water table elevation influence oxidation–reduction (redox) conditions near river–aquifer interfaces, with potentially important consequences for solute fluxes and biogeochemical reaction rates. Although continuous measurements of groundwater chemistry can be arduous, in situ sensors reveal chemistry dynamics across a wide range of timescales. We monitored redox potential in an aquifer adjacent to a tidal river and used spectral and wavelet analyses to link redox responses to hydrologic perturbations within the bed and banks. Storms perturb redox potential within both the bed and banks over timescales of days to weeks. Tides drive semidiurnal oscillations in redox potential within the streambed that are absent in the banks. Wavelet analysis shows that tidal redox oscillations in the bed are greatest during late summer (wavelet magnitude of 5.62 mV) when river stage fluctuations are on the order of 70 cm and microbial activity is relatively high. Tidal redox oscillations diminish during the winter (wavelet magnitude of 2.73 mV) when river stage fluctuations are smaller (on the order of 50 cm) and microbial activity is presumably low. Although traditional geochemical observations are often limited to summer baseflow conditions, in situ redox sensing provides continuous, high‐resolution chemical characterization of the subsurface, revealing transport and reaction processes across spatial and temporal scales in aquifers.

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Section: Results and Interpretationmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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

Wallace

Sawyer

Barnes

2018

Hydrological Processes

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“…Previous hydrostratigraphic work illustrates that the stream sediment and aquifer materials consist of a lower sandy unit with alternating silt and sand layers above (Figure ; Musial et al, ). The various lithological units have similar porosities (0.42 to 0.49) with hydraulic conductivities varying by two orders of magnitude (1.5 × 10 −3 m/s in sand to 1.46 × 10 −15 m/s in silt units; Musial et al, ).…”

Section: Methodsmentioning

confidence: 96%

Hydrogeologic Controls of Surface Water‐Groundwater Nitrogen Dynamics Within a Tidal Freshwater Zone

et al. 2019

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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 toward 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) 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. In most of the riparian aquifer and streambed, nitrate concentrations are <0.5 mg N/L, 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.Plain Language Summary Human activities related to energy and food production add large amounts of reactive nitrogen to the landscape. Rain and snow wash some of that nitrogen into rivers and eventually to the coast. The addition of excess nitrogen to coastal ecosystems can cause excessive algal growth and low-oxygen conditions, which can lead to fish kills. As nitrogen travels to the coast, microbes in the sediment beneath and near the river process and remove large portions of this nitrogen. It is unclear how daily tidal fluctuations within the freshwater tidal zone alter these processes. Geochemical measurements of pore water beneath the stream and within the stream bank reveal that there are different zones of nitrogen processing, where differences in sediment type and water exchange control the supply of reactants. Zones of nitrate production exist within the stream bank aquifer, but conditions favoring nitrate removal dominate the aquifer. Therefore, depending on how water moves through the subsurface, it is possible that tidal fresh water zones could act as a source of nitrate to the stream channel, exacerbating coastal management challenges.

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Tidal controls on riverbed denitrification along a tidal freshwater zone

Knights

Sawyer

Barnes³

et al. 2017

Water Resources Research

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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.

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Surface water–groundwater exchange dynamics in a tidal freshwater zone

Cited by 37 publications

References 49 publications

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

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

Hydrogeologic Controls of Surface Water‐Groundwater Nitrogen Dynamics Within a Tidal Freshwater Zone

Tidal controls on riverbed denitrification along a tidal freshwater zone

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