Ann Mulligan, Rob Evans and Dan Lizarralde, The role of paleochannels in groundwater/seawater exchange, pre-print
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. 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,
North Carolina, 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.
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