Groundwater flow and salinity dynamics in swash Zones: Combined effects of Evaporation, Waves, and geologic heterogeneity

Abstract

This study employs groundwater simulations to examine the combined effects of waves, evaporation, and geologic heterogeneity on the flow and salinity dynamics in a shallow beach environment. The modeling results reveal that wave motion generates a saline plume beneath the swash zone, with hypersalinity near the surface triggered by evaporation. Geologic heterogeneity critically controls the temporal and spatial patterns of evaporation, moisture content, and salinity in the swash zone. Heterogeneous capillarity creates localized moisture hotspots within the unsaturated zone, which support enhanced evaporation and therefore facilitate salt accumulation at the surface, even when the overall moisture conditions along the swash zone are not conducive to high rates of evaporation. The formation of capillary barriers allows these moisture hotspots to persist over tidal cycles, leading to the retention of saltwater pockets within heterogeneous unsaturated finer sediments. As the swash zone recedes and evaporation intensifies, salt begins to accumulate near the beach surface. The moisture hotspots create preferential pathways that facilitate the penetration of hypersaline water into deeper, saturated sediments. In contrast, within the saturated zone, groundwater flow and salt transport are predominantly driven by preferential flow within high-permeability coarse sediments where capillarity is relatively low. Such transport and capillary mechanisms are crucial for a better understanding of coastal groundwater flow, interstitial habitats, biogeochemical conditions, and consequent nutrient cycling and contaminant transport in coastal zones. This highlights the necessity of considering integrated coastal physical drivers when investigating flow and transport processes in coastal swash zones.