Imaging Point Source Groundwater Discharges in a Confined Coastal Aquifer Using Electrical Resistivity.

Area of Science:

• Geophysics
• Hydrogeology
• Coastal Science

Background:

• Point-source submarine groundwater discharge (PSGD) is crucial for coastal solute and mass transport but its conduit geometry is poorly understood.
• Karstic coasts present unique challenges for imaging groundwater discharge due to complex geological structures.

Purpose of the Study:

• To present the first marine electrical resistivity tomography (ERT) characterization of PSGD.
• To evaluate marine ERT's effectiveness in detecting and characterizing PSGD conduits and confining aquitards in contrasting field settings.
• To establish a resistivity-based framework for quantifying PSGD fluxes.

Main Methods:

• Deployment of four dipole-dipole ERT profiles across land and offshore environments.
• Analysis of resistivity data to identify conductive anomalies indicative of PSGD conduits.
• Forward modeling to assess the influence of depth and salinity contrast on detection capabilities.

Main Results:

• ERT successfully imaged PSGD conduits as conductive features beneath a coastal lagoon and offshore vents.
• A cemented coastal aquitard was identified as a continuous resistive horizon.
• Conduit detectability decreases exponentially with increasing seafloor depth, with a characteristic depth of ~8m.

Conclusions:

• Marine ERT is a viable tool for characterizing PSGD in karstic coastal environments.
• The study provides a framework for integrating ERT with tracers and flow models to quantify PSGD.
• Performance estimates represent an upper bound, highlighting the limits of marine ERT for PSGD detection.