Sediment deposition in riparian zones exacerbates saltwater intrusion

Coastal farmland is becoming increasingly exposed to flooding due to climate change. Inundation can lead to groundwater and soil degradation through saltwater intrusion. Much of the research investigating saltwater intrusion is focused along the marine coast; however, as storm intensity and sea levels rise, transitional coastal areas not previously susceptible to salinization may be at risk. Flood-derived sediment deposits may provide an overlooked salinity source in estuarine and upriver areas, even where floodwater salinity is relatively low. This study was conducted to evaluate the impact of subaerial flood deposits on underlying soil and porewater. A parcel of agricultural land in an estuarine floodplain in Nova Scotia, Canada, was selected to assess the subsurface response to repeated, low-salinity flooding. The site experienced inundation by fortnightly tidal floodwater following a managed dike realignment, resulting in dynamic surficial alteration. A three-year field campaign, including soil and water monitoring, geophysical surveying, and drone-based LiDAR surveying, was conducted to monitor changes to the site geomorphology and water and sediment chemistry. A one-dimensional numerical solute transport and vertical water flow model informed by field data was applied to investigate the hypothesis that saline sediment deposits can drive downward saltwater intrusion in areas experiencing brackish or low-salinity flooding. Results revealed that the soil concentrations exceeded that of the brackish floodwater by up to 50 times, with the highest salinization occurring preferentially in areas experiencing persistent deposition. Model results showed that soil salinization may persist for decades longer than the duration of flooding; however, removing these deposits through erosion resulted in soil and groundwater recovery. This study highlights the potential importance of flood-derived sediments for exacerbating saltwater intrusion in riparian areas along estuaries, which were not previously thought to be at risk of saline flooding.