Study on the release of LNAPLs in the capillary zone via ERT and GPR in an outdoor experiment

The capillary fringe serves as a zone of persistent release for Light Non-Aqueous Phase Liquids (LNAPLs). Investigating the migration and diffusion of LNAPLs within this zone is beneficial for subsequent remediation processes. In this study, the release process of LNAPLs in the capillary fringe was simulated in outdoor experimental pits. By combining cross-hole electrical resistivity tomography (CHERT), ground-penetrating radar (GPR) technology, and chemical analysis of samples, the study comprehensively explored the migration process of LNAPLs in both the capillary fringe and the aquifer, as well as the detection efficacy. The results indicate that, under certain liquid phase pressures, LNAPLs persistently release into the capillary zone. Their downward migration is influenced by buoyancy from the aquifer and capillary zone water, hindering the downward migration of some LNAPLs, which instead migrate horizontally, displacing water to form circular high-saturation LNAPL source zones resembling concave lenses. After the cessation of LNAPL release, the source zones evolve under the combined effects of LNAPL liquid phase pressure and diffusion, with plumes continuously spreading. CHERT reflects the diffusion process of LNAPLs in the capillary fringe and aquifer; however, the calculation of saturation values presents a certain degree of error due to the uncertainty of the oil-water state. GPR can delineate the distribution boundaries of high-saturation source zones but has limited accuracy in monitoring the diffusion of plumes with lower oil saturation. This study provides important references for the application of CHERT and GPR in detecting the distribution of LNAPLs in the capillary fringe and aquifer.