Saline sediment deposition in estuarine floodplains exacerbates vertical saltwater intrusion

• An agricultural field was contaminated following exposure to brackish flooding. • Marine-derived novel sediment deposited during flooding concentrated salinity. • Modeling and field results suggest the sediment deposits drove saltwater intrusion. • Removal of sediment by erosion enabled flushing and recovery of the underlying soil. Low-elevation coastal zones are increasingly exposed to flooding due to the effects of climate change. Inundation can lead to groundwater and soil degradation through saltwater intrusion. In transitional coastal areas, such as the upper reaches of estuaries, where floodwater is relatively fresh, flood-derived sediment deposits may provide an overlooked salinity source. A parcel of land in an estuarine floodplain was selected to assess the subsurface salinity response to episodic flooding. The site experienced intermittent inundation by low-salinity floodwater following a managed dike realignment, resulting in sediment deposition and alterations to the land surface topography. A three-year field campaign involving soil and water monitoring and geophysical surveying was conducted to map the subsurface response to the flooding, while aerial LiDAR was used to monitor geomorphologic changes. The development of a one-dimensional numerical model of coupled vertical water flow and solute transport informed by field data was applied to investigate the hypothesis that saline sediment deposits can drive saltwater intrusion in areas experiencing low-salinity flooding. Soil salt concentrations exceeded that of the floodwater by up to 50 times, while the highest salinization occurred preferentially in areas experiencing persistent deposition. Model simulations showed that novel sediments contributed to soil salinization for decades longer than the initial flood; however, the removal of these deposits through erosion could, in turn, drive soil recovery. This study suggests that marine-derived sediments may exacerbate saltwater intrusion in settings upriver from the marine coast and may help guide management decisions in coastal areas expected to undergo future flooding.