Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Dr. Clare Robinson

Abstract

A reactive groundwater transport model has been developed to investigate the fate of nutrients (ammonium, nitrate, and phosphate) in a near-shore coastal aquifer subject to oceanic forcing (tides and waves) and their subsequent discharge to coastal waters. The model is developed by combining the variable-density groundwater flow model SEAWAT-2005 with the reactive multi-component transport model PHT3D v2.10. The influence of tides and waves are typically neglected in prior studies that have examined the transport and transformation of nutrients in coastal aquifers. Oceanic forcing however can induce a highly dynamic surficial salt-freshwater mixing and reaction zone in a near-shore aquifer and this may modify the transport pathways and concentrations of discharging nutrients. The reactions considered in the model include denitrification, nitrification, aerobic degradation of dissolved organic matter, iron oxidation, and phosphate adsorption. The reaction network implemented, including the kinetic rate expressions, has been verified previously by numerical simulations conducted for a near-shore aquifer not exposed to oceanic forcing. The simulations conducted reveal that oceanic forcing significantly modifies the discharge pathway of the groundwater-derived nutrients and the reactions occurring along this pathway. This alters the net production and consumption of nutrients in the near-shore aquifer and their subsequent loading rates to coastal waters. It is further shown that the fate of the nutrients is strongly controlled by the availability of chemical species including dissolved organic matter in seawater recirculating through the near-shore sediments. Moreover, for the conditions simulated, tides led to more intense salt-freshwater mixing in the near-shore aquifer and thus greater transformations of nutrients in the near-shore aquifer compared to regular wave forcing. This study significantly enhances conceptual understanding of the processes controlling the fate of nutrients in a near-shore aquifer and hence provides a valuable tool for improving prediction of nutrient loading rates to coastal waters.