Estimation of residual coal tar saturation in soils

Coal tars are viscous polycyclic aromatic hydrocarbon (PAH)-rich nonaqueous phase liquids (NAPLs) that persist as subsurface contaminants at former manufactured gas plant (MGP) sites. Accurate determination of residual coal tar saturation (S_Tr) in soils is essential for estimating tar mass, predicting long-term source longevity, and designing cost-effective remediation strategies. However, limited S_Tr data exist for natural soils, and no predictive framework currently relates S_Tr to measurable tar and soil properties. In this study, coal tars and soils from nine former MGP sites were comprehensively characterized to identify key factors governing tar entrapment. Two-phase (water-tar) column experiments using paired and unpaired tar-soil combinations were conducted to determine S_Tr, employing a nonreactive tracer approach to overcome challenges associated with viscous tars and small tar-water density differences. Measured S_Tr values ranged from 7.70% to 22.60%, and were strongly influenced by soil texture, porosity, carbon content, and tar viscosity. Using the combined dataset, linear and nonlinear regression models (e.g., r² = 0.84 for a linear model incorporating soil carbon content, median grain size, tar viscosity and tar-water interfacial tension) were developed to provide a practical, cost-effective tool for estimating S_Tr at MGP sites without labor-intensive column testing.