Mechanism for detecting NAPL using electrical resistivity imaging.

Area of Science:

• Environmental Geophysics
• Hydrogeology
• Contaminant Hydrogeology

Background:

• Electrical resistivity imaging (ERI) is used to detect non-aqueous phase liquid (NAPL) impacts in freshwater environments.
• The underlying mechanism generating the resistive signature of NAPLs during ERI remains poorly understood.

Purpose of the Study:

• To investigate the electrical barrier mechanism for detecting NAPLs using ERI.
• To develop a theoretical basis for the mechanism and validate it through laboratory experiments and modeling.

Main Methods:

• A two-dimensional sand tank experiment was conducted using ERI.
• Photographic techniques quantified petroleum saturation.
• Forward modeling was performed to simulate the laboratory experiment.

Main Results:

• The study confirmed that thin, saturated NAPL barriers can block pore throats, generating detectable electrical resistivity signals.
• NAPL thickness, not bulk volume, is critical; a threshold of 3.3cm NAPL thickness was identified for detectable resistivity changes (>3%).
• The maximum observed resistivity increase due to NAPL was 37%.

Conclusions:

• ERI can detect small quantities of NAPL if they form barriers of sufficient thickness.
• The primary mechanism for resistivity signal generation is a high-resistivity zone of separate-phase liquid blocking pore-scale current flow.
• Forward models validated the NAPL barrier mechanism theory for the experimental conditions.