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Tides in coastal rivers drive river-groundwater (hyporheic) exchange and provide opportunities for nitrate removal that may improve coastal water quality. Silt and sand layers in coastal floodplain sediments can alter the flow and transformation of nitrate. Our goal was to understand how sediment heterogeneity influences nitrogen dynamics near tidal rivers. Numerical simulations show that oxic, variably saturated sand layers and anoxic, organic-rich silt layers are sites of nitrification and denitrification, respectively. The exchange of river water and nitrate through heterogeneous sediments increases with sand fraction, as sand lenses become longer and more connected. The amount of nitrate removed from river water also increases but represents a smaller portion of total nitrate exchange through the hyporheic zone, causing removal efficiency to decline. Our results suggest that accurate characterization of aquifer heterogeneity leads to an improved understanding of sites of nutrient transformation within floodplain sediments.Plain Language Summary Excess nitrate can degrade coastal water quality, but microbial reactions reduce its concentration within the bed and banks of tidal rivers, where surface water and groundwater mix. Spatial arrangements of different sediments (sand and mud) affect the mixing of river water and groundwater and thus affect nitrate removal. Here, we use computer models to simulate nitrate transformation along a tidal river with different amounts of coarse and fine sediments. Coarse sediments promote groundwater flow and nitrate production, while fine sediments promote nitrate removal. The amount of nitrate removed from river water is greater in aquifers with coarser sediments, but the removal efficiency decreases. Removal also varies with the spatial distribution of sand and mud in sediments but to a lesser degree. Computer models of nitrate transport should consider the distribution of different sediment types.

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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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.