Response of the salt–freshwater interface in a coastal aquifer to a wave-induced groundwater pulse: field observations and modelling

Field observations on an unconfined coastal aquifer showed that a groundwater pulse, generated by a moderate (significant wave height, Hsig∼4.5 m) wave/storm event, induced significant oscillations in the salt–freshwater “interface” of the order of several metres in the horizontal direction. A dynamic sharp-interface model is developed to quantify the mechanism of these interface oscillations. The model uses the 50% seawater salinity contour as the location of the equivalent sharp-interface. The model was calibrated against the observed groundwater table fluctuations. It predicted reasonably well the interface oscillations with a slight over-prediction of the oscillation magnitude and a steepening of the interface. The neglect of mixing in the salt–freshwater mixing zone by the sharp-interface model is suggested as a possible contributor to the discrepancies between the model predictions and observations. In contrast with the significant wave effects, there was no observable response of the interface to diurnal or semi-diurnal tides.