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In coastal rivers, tides facilitate surface water-groundwater exchange and strongly coupled nitrification-denitrification near the fluctuating water table. We used numerical fluid flow and reactive transport models to explore hydrogeologic and biogeochemical controls on nitrogen transport along an idealized tidal freshwater zone based on field observations from White Clay Creek, Delaware, USA. The capacity of the riparian aquifer to remove nitrate depends largely on nitrate transport rates, which initially increase with increasing tidal range but then decline as sediments become muddier and permeability decreases. Over the entire model reach, local nitrification provides a similar amount of nitrate as surface and groundwater contributions combined. More than half (similar to 66%) of nitrate removed via denitrification is produced in situ, while the vast majority of remaining nitrate removed comes from groundwater sources. In contrast, average nitrate removal from surface water due to tidal pumping amounts to only similar to 1% of the average daily in-channel riverine nitrate load or 1.77 kg of nitrate along the reach each day. As a result, tidal bank storage zones may not be major sinks for nitrate in coastal rivers but can act as effective sinks for groundwater nitrate. By extension, tidal bank storage zones provide a critical ecosystem service, reducing contributions of groundwater nitrate, which is often derived from septic tanks and fertilizers, to coastal rivers.

Plain Language Summary Nitrate is one of the most common pollutants and can degrade coastal water quality. In the beds and banks of coastal rivers, tides increase the mixing of river and groundwater, which creates opportunities for nitrate production by nitrification and removal by denitrification. It is unknown which process prevails along tidal rivers and how these processes vary in strength as sediments become muddier and tides become larger near the coast. In this study, we used computer models to estimate rates of nitrate production and removal and where they are greatest along tidal rivers. We found that nitrate is ultimately removed from the aquifer, but that the majority of nitrate removed is from nitrification near the water table and groundwater sources. Although aquifers along tidal rivers are only marginally effective at removing nitrate from river water, their ability to remove groundwater nitrate is vital for maintaining coastal water quality.