Phase-Dependent Spectral Induced Polarization Signatures During DNAPL Transport in Porous Media

Dense non-aqueous phase liquids (DNAPLs) pose persistent challenges for subsurface characterization because their migration, retention, and phase partitioning are difficult to resolve using conventional monitoring approaches. Spectral induced polarization (SIP) provides a non-invasive approach sensitive to pore-fluid properties and interfacial processes; however, the interpretation of SIP responses during DNAPL phase transitions remains challenging because electrical signals may vary with contaminant phase state and pore-scale distribution. In this study, we investigate the SIP responses associated with the multiphase transport of tetrachloroethylene (PCE) in heterogeneous porous media using coupled two-dimensional sandbox imaging and one-dimensional column experiments. The sandbox experiments captured the spatial evolution of PCE migration and imaging-derived apparent PCE saturation (SPCE) distributions, while column experiments separately characterized SIP responses under dissolved-phase and non-aqueous-phase PCE conditions. Results show that SIP responses are influenced not only by DNAPL mass, but also by changes in pore-scale phase distribution, aqueous-phase connectivity, and interfacial conditions associated with DNAPL phase transitions. The real conductivity (σ′) decreases monotonically with increasing SPCE, showing a strong relationship with changes in effective aqueous-phase connectivity. The imaginary conductivity (σ″) exhibits a non-monotonic response, with the largest response variation occurring within an intermediate range of imaging-derived apparent PCE saturation. This behavior indicates an experiment-specific transition in SIP response rather than a uniquely defined critical saturation threshold. The combined SIP and imaging observations indicate that SIP responses during DNAPL migration are associated with the coupled effects of phase distribution, aqueous-phase connectivity, and interfacial conditions. A conceptual framework is proposed to interpret the evolution of SIP signatures across dissolved, mixed, and non-aqueous phase states. These findings improve the understanding of phase-dependent SIP responses and provide guidance for interpreting DNAPL-related SIP signals under controlled laboratory conditions, while field-scale applicability requires further validation.