INFLUENCE OF OCEANIC FORCING ON NUTRIENT PROCESSING IN A NEARSHORE AQUIFER

The role of the nearshore subterranean estuary in regulating nutrient fluxes to coastal waters is well recognized. Complex interacting physical flow and biogeochemical processes determine whether a nearshore subterranean estuary will be a net source or sink of nitrogen (N) and phosphorous (P). A numerical model was applied to evaluate the influence of varying oceanic forcing (tides and waves) on the nutrient biogeochemistry in a permeable homogeneous nearshore aquifer. Simulations were performed using the variable-density groundwater flow model SEAWAT-2005 combined with the reactive multi-component transport model PHT3D v2.10. The model considered the transport and transformation of both marine- and land-derived chemical species (NH₄⁺, NO₃^-, PO₄^3-, Fe²⁺, DOM and O₂). Reactions considered include DOM oxic mineralization, nitrification, denitrification, Fe oxidation, and P adsorption. Varying DOM concentrations and mineralization rates were also examined for the different oceanic conditions simulated.

Although steady wave forcing caused higher seawater recirculation, tides led to greater seawater-freshwater mixing in the nearshore aquifer and subsequently greater transformation of land-derived nutrients. Nutrient processing was strongly controlled by the availability and reactivity of marine DOM as its degradation consumed O₂, released inorganic N and P, and altered redox conditions in the salt-freshwater mixing zones. For the simulations without marine DOM considered, NO₃^- discharge to the sea increased by 32% for the tidal case (A = 0.5 m) compared to only 13% and 8% for the wave (H_rms = 1 m) and no oceanic forcing cases. With labile marine DOM considered, the NO₃^- discharge decreased by 90% relative to the land-derived flux for the tidal case (A = 0.5 m). For all simulations PO₄^3- removal was high due to adsorption to Fe oxide minerals that precipitated along redox boundaries in the subterranean estuary.