A Forensic Approach for Discerning Hydrocarbon Sources during Vapor Intrusion Pathway Investigations

摘要

Determining the sources of hydrocarbons in soil gas and indoor and ambient air is an issue at many vapor intrusion sites. Vapor and air samples collected for chemical characterization at vapor intrusion sites are typically analyzed following EPA Method TO-15. The BTEX (benzene, toluene, ethylbenzene and xylenes) compounds, among the few petroleum- or tar-derived hydrocarbons included as target analytes in the standard TO-15 method, are commonly used by investigators to determine if hydrocarbons are present in subsurface soils, and to infer the source(s) of such hydrocarbons. Unfortunately, using only BTEX chemicals for identifying specific hydrocarbon sources in soil gas is problematic because BTEX is present in virtually every type of hydrocarbon product commonly encountered in the subsurface.rnOwners of former MGP sites, gasoline stations and refineries are particularly vulnerable to mistaken liability assignments, because the vapor phase of these petroleum and MGP residues are easily confused with background sources. Conventional analytical techniques (e.g. EPA TO-15) for soil gas and indoor air analysis include a limited number of hydrocarbons with little or no ability to differentiate petroleum and MGP residues from background influences. Although these methods are applicable for quantitative assessments to very low risk levels, these methods were primarily designed for chlorinated solvent analyses. They lack the analyte list and sensitivity for accurate hydrocarbon source identification.rnIn order to accurately characterize the nature and type or types of hydrocarbons sources in soil gas, indoor and ambient air, these environmental samples must be analyzed using a descriptive method of analysis that employs a more robust target analyte list. This paper presents a forensic air method (Forensic TO-15) specifically developed to differentiate tar, gasoline, and background sources during VI investigations. The robust target analyte list includes many Paraffin, Isoparaffin, Aromatic, Naphthene, and Olefin (PIANO) compounds. Forensic TO-15 can be a critical tool for site characterization by producing distinct fingerprints for source determination. The importance and advantages of being able to fingerprint hydrocarbons in the subsurface and indoor air will be discussed and case study data will be presented.