Woods Hole Oceanographic Institution

Dr Rob. L. Evans

»Electrical Structure of the Central Cascadia Subduction Zone: The EMSLAB Lincoln Line Revisited
»Electrical Lithosphere Beneath the Kaapvaal Craton
»Gulf Of Mexico Gas Seep
»MELT MT Results
»Wrightsville Beach Geophysics and Hydrology
»MELT Area Off-Axis Structure
»Karst Formation off North Carolina
»Review of Shallow Offshore EM Work
»Towed EM System
»EPR MMR Experiment
»Offshore MT and Subduction Systems
»Shallow Porosity Structure on the Continental Shelf
»Oceanic and Continental Mantle Resistivity
»New Jersey EM Survey
»Eel River EM Survey
»Impact of groundwater on EM data
»Electrical structure of Slave Craton
»Report of Shoreline Change Workshop

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.

FILE » Mulligan_Hydrol_8123.pdf

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