Groundwater flow and salt transport in a subterranean estuary 2 driven by intensified wave conditions

25 A numerical study, based on a density-dependent variably saturated groundwater flow model, 26 was conducted to investigate flow and salt transport in a near-shore aquifer under intensified 27 wave conditions caused by offshore storms. Temporally varying onshore hydraulic gradients due 28 to wave set-up were determined as the seaward boundary condition for the simulated aquifer. 29 The results showed a rapid increase in influxes across the aquifer-ocean interface in response to 30 the wave event followed by a more gradual increase in effluxes. The upper saline plume first 31 widened horizontally as the wave set-up point moved landward. It then expanded vertically with 32 recirculating seawater pushed downwards by the wave-induced hydraulic gradient. The time for 33 the salt distribution to return to the pre-storm condition was up to a hundred days, and correlated 34 strongly with the time for seawater to recirculate through the aquifer. The pathways of 35 recirculating seawater and fresh groundwater were largely modified by the wave event. These 36 pathways crossed through the same spatial locations at similar times, indicating significant salt37 freshwater mixing. The flow and salt transport dynamics were more responsive to wave events of 38 longer duration and higher intensity, especially in more permeable aquifers with lower fresh 39 groundwater discharge. Despite their larger response, aquifers with higher permeability and 40 beach slope recovered more rapidly post-event. The rapid recovery of the flows compared with 41 the salinity distribution should be considered in field data interpretation. Due to their long-lasting 42 impact, wave events may significantly influence the geochemical conditions and the fate of 43 chemicals in a subterranean estuary. 44