Gas tube effect: A transport mode of deeply buried volatile DNAPLs to shallow strata

In recent years, there have been frequent incidents of DNAPL pollution in deep underground spaces caused by improper discharge of industrial wastewater and waste residue in China. In the DNAPL pollution event of a deeply buried mine in Jinan, Shandong Province, we found that gas enrichment occurred in the shallow strata of the deeply buried pollutant restoration area through geophysical methods, and the enriched gas mainly consisted of gaseous dichloromethane, gaseous dichloroethane and CO2. To achieve this type of gaseous pollutants tracing, we conducted a comprehensive analysis of the formation characteristics, hydrogeological conditions, pollutant properties, and blocking and remediation methods in the contaminated areas. Additionally, we developed corresponding hydrogeological models and utilized TOUGH2 software to simulate the diffusion process of dichloromethane within closed underground spaces under fluctuating groundwater table conditions. The results indicated that the deep buried volatile organic pollutants will be transported to the shallow strata in a gaseous state. The fault fracture zones developed at the site are the primary conduits for the upwards migration of gaseous pollutants, which is mainly driven by the rise in groundwater level. Additionally, the shallow limestone layer with high permeability is the secondary gas accumulation area. The installation of cut-off walls during the remediation and treatment promotes the accumulation and upwards migration of gaseous pollutants by blocking the transverse diffusion pathway of gaseous pollutants. The permeability coefficient of the fault directly impacts the distance to which gaseous pollutants can migrate upwards. The vertical diffusion process of volatile DNAPL caused by rising water level can be summarized by the gas tube effect. The study findings enrich the vertical migration theory of volatile organic compounds and provide a valuable contribution towards improving the accuracy of pollutant dispersion prediction and prevention strategies.