Influence of tides and waves on the fate of nutrients in a nearshore aquifer: Numerical simulations

A numerical investigation is presented that demonstrates the influence of tides and waves on the transport and transformation of nutrients (NO3-; NH4+; PO43-) in a homogeneous unconfined nearshore aquifer and subsequent fluxes to the sea. Simulations of an aquifer subject to semi-diurnal tides and constant waves acting on a sloping beach face were conducted using SEAWAT-2005 combined with PHT3D v2.10. Tidal amplitude (A) and wave height (H-rms) varying from 0.25 to 0.75 m and 1 to 2 m, respectively, were examined. Results show that tides and waves modify the subsurface discharge pathway of land-derived nutrients by changing the nearshore groundwater flow dynamics. More importantly, the oceanic forcing impacts nutrient cycling as it causes significant seawater exchange (along with dissolved O-2 and organic matter) across the aquifer-ocean interface. Although steady wave forcing caused higher seawater influx, 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 dissolved organic matter (DOM) as its degradation consumed O-2, released inorganic N and P, and altered redox conditions in the salt-freshwater mixing zones. For the conditions and reaction network simulated, nutrient regeneration by marine DOM degradation was independent of the seawater-freshwater mixing intensity, and therefore was greatest for the wave case due to the high seawater influx. For 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 PO43- removal was high due to its adsorption to Fe oxide minerals. The model enables evaluation of the complex coupled physical-biogeochemical processes controlling nutrient loading to the sea via submarine groundwater discharge in dynamic coastal environments. (C) 2014 Elsevier Ltd. All rights reserved.