Salinity, water table, and subsurface resistivity data from migrating marsh–forest ecotones

Upland habitats are converting to salt marsh at unprecedented rates due to
sea level rise. While increasing salinity is understood to be the main
driver of this conversion of upland to marsh, the factors that influence
salinity in the zone of change (i.e., the marsh-forest ecotone) are
unclear, especially in the northeastern USA, where marsh migration studies
are less common. This study examined spatial and temporal patterns in
salinity across the marsh-forest ecotone and potential drivers using a
combination of groundwater well measurements and geophysical surveys.
Across three study sites, groundwater salinity was influenced by multiple
factors, including topography, weather events, and tidal cycles.
Electrical resistivity tomography (ERT) showed differing patterns among
sites: at the Massachusetts site, there was a sharper transition from low
to high resistivity moving from the marsh to the forest. Whereas at the
New Jersey and New York sites, there was a low resistivity (high salinity)
layer overlaying high resistivity. The presence of this deeper fresh layer
contrasts with the ‘salt wedge’ configuration found in open water
estuarine systems and suggests that trees may be able to survive saltwater
inundation by accessing this deeper reserve of freshwater. 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 lower water tables 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.