The role of paleochannels in groundwater/seawater exchange

Relict fluvial channels that are infilled with high permeability sediments act as preferred pathways for groundwater flow and solute transport. In coastal regions, such paleochannels can provide a hydraulic connection between freshwater aquifers and the sea, facilitating saltwater intrusion landward or freshwater discharge offshore. Although paleochannels have been implicated in several instances of saltwater intrusion, there has been little direct study of these channels to confirm their role. In this work, simulation modeling of a general multi-layered, coastal-plain-aquifer setting indicates that when a paleochannel breaches a confining unit offshore, submarine groundwater discharge of intermediate salinity occurs. This discharge is largely concentrated along the margins of the channel. Conversely, seawater inflow occurs along the channel axis, resulting in higher salinity in the middle of the channel relative to the flanks. Chirp seismic and electromagnetic data collected offshore Wrightsville Beach, NC, USA, confirm these simulation results and indicate fresher porewater along channel flanks and slightly higher porewater salinity along the channel axis. Hence, paleochannels contribute to the spatial variability in submarine groundwater discharge by serving as conduits of focused fluid exchange. Simulations also reveal that the freshwater/saltwater transition zone is closer to land below paleochannels than in locations with a continuous confining unit. This indicates that such channels are likely to be significant modes of saltwater intrusion into confined aquifers when excess freshwater extraction occurs on land.