Seasonality, Storms and Soil Properties Drive Groundwater and Salinity Dynamics in Retreating Coastal Forests

ABSTRACT

Sea level rise is causing the rapid conversion of upland habitats to salt marshes in low‐sloping areas due to increasing groundwater salinity and soil saturation. Despite this being a widespread phenomenon, questions remain about the factors shaping groundwater hydrology and salinity in these transitioning systems, particularly in the US Northeast. We examined water table elevations and specific conductivity (a proxy for salinity) in three northeastern forests with differing slopes where forest retreat has been occurring at rates of approximately 1 m yr ⁻¹ over the past 100 years. Our results indicate that water table levels were impacted by seasons, storms, precipitation and soil characteristics. Water tables were significantly higher during the dormant season than the growing season at all sites. Water table elevation relative to mean high water was approximately equal at all sites across the marsh–forest ecotone, with two key implications. First, forests on steeper terrain had a deeper water table. Second, because the overall hydraulic gradient was low, even sites with steeper slopes remain vulnerable to saltwater intrusion. Salinity dynamics were occasionally driven by storm surges and hydraulic gradients, but the magnitude and direction of these effects were not consistent across events. Finally, sites with greater soil hydraulic conductivity exhibited greater depths to the water table and enhanced tidal advection. This pattern suggests that well‐drained soils characteristic of the Pine Barrens Ecoregion may facilitate efficient drainage but simultaneously heighten susceptibility to saltwater intrusion. These findings underscore the need to consider local hydrologic and soil conditions when predicting the pace of marsh migration and the resilience of coastal forests under rising sea levels.