NITRATE REMOVAL WITHIN HETEROGENEOUS FLUVIAL SEDIMENTS UNDER TIDAL INFLUENCE

Denitrification serves as an important sink for nitrate loads to the coast, which otherwise degrade coastal waters. Along tidal rivers, stage fluctuations enhance surface water-groundwater exchange (hyporheic exchange). Nitrification and denitrification are strongly coupled near the fluctuating water table and across heterogeneous high- and low-permeability floodplain deposits. Preferential flow along permeable pathways causes relatively high velocities and mobilizes oxygen and other nutrients, while low-permeability facies high in organic matter supply dissolved organic carbon but limit groundwater movement. Nitrogen processes reflect a balance of transport and reactant availability; nitrification dominates within oxic, high-permeability facies at the fluctuating water table, while denitrification dominates within anoxic, low-permeability layers where water movement is more limited. The net nitrate removal rate from surface water increases with permeability but accounts for a smaller fraction of the nitrate exchanged through sediments, amounting to a decline in removal efficiency. Knowledge of the effects of tidal flow conditions and sediment lithology is necessary for a more comprehensive understanding of the processing, retention, and export of nitrogen within these heterogeneous systems.