Effects of variable beach morphology, storm surges and aquifer parameters on the salinity distribution in the deep subsurface of a high energy beach – a generic modelling approach

Beach aquifers are the contact zone between the terrestrial groundwater and the ocean. Where the fresh groundwater mixes with the recirculating saltwater biogeochemical reactions take place and change the composition of the water. These biogeochemical reactions and resulting element net fluxes are linked to residence times and dispersive mixing processes that depend on dynamic density-driven groundwater flow and transport processes. In the present study we investigate the effect of dynamic hydro- and morphological boundary conditions and aquifer parameters on the groundwater flow- and transport processes in the deep subsurface of a high energy beach along a 2D transect perpendicular to the shore by means of a density-driven flow and transport model (SEAWAT). The ‘classical’ concept of a beach aquifer in the intertidal zone describes the stable establishment of three separated, layered water bodies. The fresh water discharge tube (‘FDT’) separates the wave and tide induced upper saline plume (‘USP’) from the saltwater wedge (‘SW’). Recent research challenges this concept of stable position of water bodies under high energy beach conditions which are characterized by high wave and tidal amplitudes. Our modelling results show that dynamic beach morphology has an important effect on the spatio-temporal flow and transport patterns as well as on the salt distribution and residence times in the deep beach aquifer and results in the formation of several time variable FDTs and USPs. Moreover, our research shows the sensitivities of the freshwater-saltwater interface in the subsurface to hydraulic conductivity, anisotropy, dispersion and boundary conditions like storm surges and fresh water inflow. The result is a complex, temporally and spatially variable picture of the ‘classical’ stable USP with dynamic interfaces. Hence, our results elucidate the physical conditions in the deep subsurface that are relevant for the spatio-temporal distribution of chemical reactions that are linked to the mixing interfaces between the water bodies. As a consequence the biochemical reactions might be enhanced with effects on the element net fluxes to the sea.