A Combined Field and Modelling Study of Interactions Between Evaporation and Oceanic Forcing in Coastal Aquifers

In coastal environments, evaporation is an important driver of subsurface salinity gradients in marsh systems. However, its interaction with oceanic forcing (e.g., tides and waves) has not been fully addressed in the coastal beaches. Here, we used field data on an estuarine beach foreshore with numerical simulations to show that evaporation caused upper intertidal zone pore-water salinity to be double that of seawater. We found the increase in pore-water salinity mainly depended on air temperature and relative humidity, and tide and wave actions diluted a fraction of the high salinity plume, resulting in a complex process. This is in contrast to previous studies that considered seawater as the most saline source to a coastal aquifer system, thereby concluding that seawater infiltration always increased pore-water salinity by seawater-groundwater mixing dynamics. It is also found that evaporation provided important forcing mechanisms, which not only facilitated pore water to flow upwards, but also intensification of the horizontal pore water flow due to the interaction with the tide. Our results demonstrated the combined effects of evaporation and tide and waves on subsurface salinity distribution on a beach face.