EFFECT OF AN OFFSHORE STORM EVENT ON GROUNDWATER FLOW AND SALT TRANSPORT IN A NEAR-SHORE AQUIFER

Most coastal aquifers are exposed to varying wave conditions at the seaward boundary. A numerical study was conducted to investigate nearshore groundwater flow and salt-freshwater mixing affected by intensified wave conditions due to offshore storms. Synthetic time series of wave height (H_rms) were generated to simulate single storm events of differing duration (1.5 – 6 d) and amplitude (H_rms increases by 3 – 5 m). Simulation of time-varying wave-induced onshore pressure gradients (wave-set up) during these events determined the seaward boundary condition of a variable-density variably saturated groundwater flow model for a coastal aquifer. Results show a rapid increase in the water and salt influx across the aquifer-ocean interface in response to the wave event followed by a slower increase in the water and salt efflux. The upper saline plume expands horizontally in direct response to onshore movement of the wave set-up point and then expands vertically as the recirculating seawater is pushed downwards by the wave-induced pressure gradient. The time for the salt distribution to return to pre-storm conditions is long and this recovery period correlates strongly with the transit time of seawater infiltrating the beach near the maximum wave-set-up point to discharge through the aquifer. The pathways of recirculating seawater and fresh groundwater, revealed by particle tracking, are largely modified by the storm event. These pathways cross each other, indicating significant salt-freshwater mixing in the aquifer which may alter the geochemical conditions and fate of chemical species discharging and recirculating through the near-shore aquifer. Finally, simulations also show that the flow and salt transport dynamics are more responsive to longer duration and higher amplitude storm events, especially in aquifers with lower fresh groundwater discharge.