Review on migration and entrapment of light nonaqueous phase liquids in the subsurface environment

Groundwater contamination by Light Non-aqueous Phase liquids‎ (LNAPLs) has attracted ‎massive research attention over the past decades to ensure a safe environment for living beings. The knowledge of LNAPL distribution and entrapment in the subsurface environment is still scattered, preventing a clear view of the occurring processes. This review discusses ‎the effect of (1) groundwater table fluctuation, (2) sand arrangement and particle size, and (3) spilled LNAPL volume on LNAPL behavior and entrapment in ‎the subsurface. From the available literature, changes in the groundwater table increase LNAPL entrapment in the initial wet soil conditions by ‎over 130 % and reduce trapped LNAPL in saturated soil by 101.65 % promoting the ‎contaminant loss through enhanced dissolution and degradation in the unsaturated zone. ‎Researchers have investigated LNAPL movement in layered heterogeneous sand to ‎understand the effects of local low permeability on the contaminant ‎velocities. It showed that LNAPL displacement was 20.78 %, 20.5 %, and 10 % in the coarse, ‎medium, and fine sand, respectively. An increasing LNAPL volume with consistent ‎groundwater fluctuations has a limited impact on LNAPL behavior, although a higher LNAPL ‎spill volume can notably depress the capillary fringe. The influence of the three studied factors on ‎the migration and transformation of pollutants, however, remains uncertain, which necessitates further studies on the migration and entrapment of LNAPL in the capillary fringe ‎zone to mitigate LNAPL contamination in the saturated zone. ‎Additionally, future studies should investigate the LNAPL migration in double ‎porous soil and multiple LNAPL injections under groundwater fluctuation.‎ A deeper understanding of these factors is essential for developing predictive models and effective remediation techniques to mitigate groundwater contamination. This review serves as a foundation for future research, guiding efforts toward sustainable contamination management strategies. • LNAPLs migration and entrapment in subsurface layers were elucidated. • Influence of groundwater table fluctuation, sand particle size, and LNAPL volume on its spreading. • Changes in ‎groundwater table elevation increase the trapped LNAPL. ‎ • The roles of particle size, LNAPL volume, and dynamic conditions in capillary fringe zone are proposed as future research.