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, and 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 24 cases. The objective was to investigate the individual effects of boundary conditions and hydrogeological parameters 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, leading 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 spatiotemporal variability of the freshwater–saltwater interfaces, are most sensitive to variable beach morphology, storm floods, hydraulic conductivity, and dispersivity. The present study advances the understanding of subsurface flow, transport, and mixing processes that are dynamic beneath high-energy beaches. These processes control biogeochemical reactions that regulate nutrient fluxes to coastal ecosystems.