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

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. Key Points Hydrologic fluctuations drive diel nitrogen dynamics within the tidal freshwater zone Organic matter content and hydrologic connectivity determine whether nitrate production or nitrate removal dominates within bank storage zones Tidal freshwater zones are a sink of groundwater borne nitrate