Influence of hydrogeological and numerical parameters on the simulation of an upper saline plume and fresh submarine groundwater discharge

At the land-sea transition, the distribution of salt and fresh groundwater is the result of a complex balance between land- and sea-related processes and the subsurface properties. Fresh rainwater, recharging on land, passes through the so-called subterranean estuary (STE) which constitutes an important entry gate for nutrients and/or pollutants to the sea. In some conditions, tidal-induced seawater recirculation forms an upper saline plume (USP), which overlies the discharging freshwater. From below, the freshwater discharge tube is confined by the classic density-driven saltwater wedge. The size, geometry and location of the USP and wedge, as well as the water fluxes and residence times, are dependent on the tidal cycle, the recharge on land, and the aquifer and beach properties. Groundwater modelling has been found useful to analyse the sensitivity of aquifer properties and forcing factors on the salinity distribution within the STE. However, the dynamics of the shore makes it difficult to accurately represent the seaside boundary condition of STE groundwater models. This contribution shows that the boundary condition type and the accompanying heads should be well-chosen and that phase-averaged boundary conditions applied in steady-state can provide a realistic salinity distribution, similar to transient simulations. Next to this, the sensitivity of the recharge rate, hydraulic conductivity, and the heterogeneity by means of a LPL in the intertidal zone on the fresh submarine groundwater discharge (FSGD) is systematically assessed. The findings are applied to successfully simulate the FSGD patterns observed along the high energy beach of the western Belgian coast.