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

In situ thermal remediation of non-aqueous phase liquid (NAPL)-contaminated soils involves coupled heat transfer, phase change, and multiphase migration processes, which complicate the interpretation of temperature signals. Under fully saturated conditions, NAPL pools remain capillarity confined and co-boiling proceeds in place, often producing temperature plateaus that reflect local phase change and removal. However, heating-induced desaturation can alter capillary entry resistance and phase connectivity, enabling NAPL redistribution and modifying the temperature response. In this study, a controlled two-dimensional sandbox system with spatially resolved temperature measurements and visualization was used to investigate NAPL migration and temperature behavior under saturated and heating-induced unsaturated conditions. Four representative NAPLs (n-hexane, n-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.