Interactions of macropores with tides, evaporation and rainfall and their effects on pore-water salinity in salt marshes

Salt marshes, a type of coastal wetlands, are influenced by various hydrogeological processes. These processes affect water and salt transport and play a critical role in determining the ecosystem services of salt marshes. Responses of pore-water flow and soil moisture to individual factors (e.g., macropores, evaporation and semidiurnal tides) have been well examined. However, how macropores interact with spring-neap tides, evaporation and rainfall and affect pore-water salinity in salt marshes remains unclear. This study investigated this question by analyzing water exchange across the marsh surface and the distribution of pore-water salinity in marsh sediment, based on a three-dimensional marsh-creek model. The results reveal high spatial heterogeneity of porewater salinity in the presence of macropores. Due to the lateral recharge of macropore water, a negative salinity gradient is formed from macropore walls toward adjacent sediment. Additionally, macropores' impact on water exchange and salt transport varies with distance from creeks. This results in two mechanisms by which macropores affect pore-water salinity, flushing and recharging. In well-drained near-creek areas, macropores reduce salt accumulation (flushing) compared with salt marshes without macropores. Whereas in the poorly-drained marsh interior, the recharge from macropores inhibits rainfall infiltration. This, together with enhanced evaporation, increases pore-water salinity (recharging) in the shallow soil layer. Moreover, the macropores' impacts on pore-water salinity vary remarkably with macropore density. This study improves the understanding of porewater salinity distribution in coastal wetlands and has potential implications for related biogeochemical processes.