Dynamic characteristics and mechanism of representative elementary volume of LNAPL saturation under freeze–thaw cycles

The exploitation and transportation of petroleum can result in serious subsurface contamination by light non-aqueous phase liquids (LNAPLs) in seasonally frozen regions. Freeze-thaw cycles (FTCs) can alter soil structure and redistribute LNAPLs. In these regions, the representative elementary volume (REV) of LNAPL saturation (Sn) undergoes dynamic changes. To investigate the impact of FTCs on the REV of Sn (Sn-REV), an experiment on LNAPL migration was conducted in two-dimensional homogeneous sand under unidirectional FTCs. This study used the modified light transmission visualization (LTV) technology to monitor the two-dimensional distribution of LNAPL. The results indicated that the freezing decreased the mean Sn, and the thawing increased it to a lesser extent. A negative correlation was observed between Sn and Sn-REV (R2 = 0.868). The Sn-REV increased during the freezing and decreased to a lesser extent during the thawing, resulting in a rising trend of Sn-REV with FTCs until reaching the stability. This trend established the empirical interval of Sn-REV in a homogeneous sandy vadose zone under freeze–thaw conditions, ranging from 2.18 mm to 5.92 mm (mean value: 4.08 mm). This study offered an accurate analysis of scale effects for LNAPL contamination remediation under freeze–thaw conditions.