Remobilization of LNAPL in unsaturated porous media subject to freeze-thaw cycles using modified light transmission visualization technique

The frequent use of light non-aqueous phase liquids (LNAPLs) in cold regions creates serious risks of soil and groundwater contamination. However, the subsurface remobilization behaviors of LNAPL in freezing and thawing unsaturated porous media are not well understood. The light transmission visualization (LTV) technique, which be previously used in two-phase system (water–air or water-NAPL), is modified to quantify LNAPL in three-(water–air-LNAPL) or four-phase (water–ice-air-LNAPL) systems during freezing and thawing. The pattern of LNAPL remobilization was observed in a two-dimensional flow chamber filled with unsaturated porous media subject to freeze–thaw cycles (FTCs) using the modified LTV technique. It is shown that LNAPL remobilization mainly occurs in the direction of the freezing front and horizontal with the number of FTCs. Meanwhile, LNAPL in the area below the freezing front is promoted to remigrate downward during freezing and the opposite direction during thawing. The analysis suggests that LNAPL remobilization is caused by a combination of freezing-induced pressure and changes in capillary pressure controlled by water migration, rather than through microcracks in the frozen porous media. The proposed modified LTV technique shows great promise for investigating LNAPL transport and distribution in freezing (unfreezing) unsaturated porous media that is non-destructive. The study offers a scientific basis for the prediction of the subsurface behavior of LNAPL at contaminated sites in widely distributed cold regions.