Imaging LNAPL distribution at a former chemical plant with time-domain induced polarization

Abstract

Light non-aqueous phase liquids (LNAPLs) that infiltrate into the subsurface are commonly described in two distinct zones: the source zone and the plume zone. A precise differentiation between these zones is essential for constraining further migration and selecting an effective remediation method. In this study, we employ the induced polarization (IP) method to characterize the contaminants. Six time domain IP survey lines were conducted at a former chemical plant contaminated with LNAPLs. Even though the contaminated areas corresponding to BTEX concentration above 180 mg/kg are less than 5 mS/m, the source and plume zones cannot be distinguished by conductivity alone. However, a noticeable difference in phase (φ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varphi$$\\end{document}) between the two zones is observed, and the threshold phase value corresponding to a critical concentration of 450 mg/kg is 20 mrad. Moreover, the normalized chargeability (Mn\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_{n}$$\\end{document}) threshold for the source zone is 80 mS/m, and the corresponding Mn\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_{n}$$\\end{document} differences between the source and plume zones are more significant than those in φ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varphi$$\\end{document}. These results illustrate that changes in polarization characteristics associated with BTEX concentrations can aid in further distinguishing between the source and plume zones. Ultimately, it is concluded that IP imaging is a well-suited method for LNAPL investigations that permits an improved characterization of different contaminated zones, which can facilitate the optimization of drillings for further site assessment and remediation.