GROUNDWATER TRANSPORT AND THE FRESHWATER-SALTWATER INTERFACE BELOW SANDY BEACHES

Current conceptual models for groundwater flow in beaches highlight an upper saline plume, which is separated from the lower salt wedge by a zone of brackish to fresh groundwater discharge. There is currently limited knowledge for what conditions an upper saline plume exists and what factors control its formation. We used variable-density, saturated-unsaturated, transient groundwater flow models to investigate the configuration of the freshwater-saltwater interface in beaches with slopes varying from 0.1 to 0.01. We also varied hydraulic conductivity, dispersivity, tidal amplitude and inflow of fresh groundwater. The simulated salinity configuration of the freshwater-saltwater interfaces varied significantly. No upper saline plumes formed in any beach with hydraulic conductivities less than 10 m/d. The slope of the beach was also a significant control. Steeper beach faces allowed stronger upper saline plumes to develop. We found the median sediment grain size of the beach to be strongly correlated to both beach slope and permeability, and therefore the development of an upper saline plume. Prior studies of groundwater flow and salinity in beaches have used a range of theoretical dispersivities. The appropriate values of dispersivity to be used to represent real beaches are still an ambiguous topic. We found the upper saline plume to weaken with the use of larger values of dispersivity. Our results suggest that upper saline plumes do not form in all beaches and may be less common than previously considered.