A Numerical Study of Tidal Impacts on Denitrification and Redox Zonation in Coastal Sediments

Abstract:

In coastal settings, tidally induced surface water-groundwater exchange (tidal pumping) controls nitrate fluxes between sediments and the water column and denitrification rates in shallow sediments. A one-dimensional, coupled fluid flow and solute transport model was used to capture the influence of tidal pumping on redox zonation and nitrate removal in sediments within coastal settings such as tidal rivers, deltas, and marshes. Upwelling groundwater was assumed to be rich in nitrate but low in dissolved oxygen (DO) and dissolved organic carbon (DOC). Tidal pumping enhances exchange of DO-rich surface water across the sediment-water interface and creates a deep, oscillating zone of aerobic respiration. Relative to non-tidal conditions, denitrification rates are diminished. As redox conditions shift with spring-neap tides, redox gradients should also shift and drive a change in denitrification rates within aquatic sediments.