Modeling Coastal Salinity in Quasi 2D and 3D Using a DUALEM-421 and Inversion Software

Area of Science:

• Geophysics
• Hydrogeology
• Environmental Science

Background:

• Rising sea levels due to climate change pose a significant threat to coastal freshwater aquifers.
• Saltwater intrusion into these aquifers is exacerbated by sea level rise, wave climate, tidal cycles, and beach morphology changes.
• Traditional methods like borehole and galvanic contact resistivity (GCR) are time-consuming for assessing these dynamics.

Purpose of the Study:

• To investigate the dynamics of saltwater intrusion in coastal aquifers using a non-invasive geophysical method.
• To develop quasi-two-dimensional (2D) and quasi-three-dimensional (3D) electromagnetic conductivity images (EMCI) of Long Reef Beach.
• To determine the primary drivers of saline intrusion at the study site.

Main Methods:

• Employed frequency-domain electromagnetic (FEM) induction using a DUALEM-421 instrument.
• Utilized EM4Soil inversion software to process data and generate EMCI.
• Conducted surveys across Long Reef Beach, New South Wales, Australia, analyzing data for low and high tides.

Main Results:

• Quasi-2D models successfully differentiated zones of dry sand, freshwater-saturated sand, mixed water, and saline sand based on conductivity.
• Quasi-3D EMCIs indicated that daily tidal cycles have a minimal impact on local groundwater salinity.
• Saline intrusion appears to be predominantly controlled by larger-scale factors such as wave climate and beach morphology.

Conclusions:

• Frequency-domain electromagnetic (FEM) induction is a viable, time-efficient alternative to traditional methods for mapping coastal aquifer salinity.
• Wave climate and beach morphology are more significant drivers of saline intrusion at Long Reef Beach than daily tidal fluctuations.
• Further research is needed to understand the influence of spring-neap tides, varying beach states, and long-term sea level rise.