Effects of boundary conditions and aquifer parameters on salinity distribution and mixing controlled reactions in high-energy beach aquifers

Abstract. In high-energy beach aquifers fresh groundwater mixes with recirculating saltwater and biogeochemical reactions modify the composition of groundwater discharging to the sea. Changing beach morphology, hydrodynamic forces as well as hydrogeological properties control density-driven groundwater flow and transport processes that affect the distribution of chemical reactants. In the present study, density-driven flow and transport modelling of a generic 2-D cross-shore transect was conducted. Boundary conditions and aquifer parameters were varied in a systematic manner in a suite of twenty-four cases. The objective was to investigate their individual effects on flow regime, salt distribution, and potential for mixing controlled chemical reactions in a system with a temporally-variable beach morphology. Our results show that a changing beach morphology causes the migration of infiltration and exfiltration locations along the beach transect that lead to transient flow and salt transport patterns in the subsurface, thereby enhancing mixing controlled reactions. The shape and extent of the zone where mixing controlled reactions potentially take place as well as the spatio-temporal variability of the freshwater-saltwater interfaces are most sensitive to variable beach morphology, storm floods, hydraulic conductivity and dispersivity.