Unraveling Non-aqueous Phase Liquid (NAPL) Migration Regimes during Water–NAPL Co-boiling under Heating-Induced Unsaturated Conditions: Insights from Two-Dimensional Sandbox Visualization

-octane, trichloroethylene, and perchloroethylene) were examined. The results show that, under saturated conditions, co-boiling remains spatially localized and temperature plateaus correspond to local removal behavior. Under heating-induced desaturation, reduced capillary constraints and evolving phase connectivity allow NAPL migration, leading to shortened, shifted, or absent temperature plateaus. LNAPLs primarily migrate with declining water saturation, while DNAPLs tend to redistribute toward deeper and cooler regions. These observations provide a mechanistic basis for understanding how the temperature response is influenced by saturation-dependent migration behavior. The results indicate that temperature plateaus remained representative of local NAPL removal when co-boiling occurred in place but became less representative of local residual distribution when redistribution occurred under heating-induced desaturation.