Compound-Specific Carbon Isotope Ratios and Extended PAH Profiles as Forensic Methods for Sediments near Former MGP Sites

摘要

Identifying the sources of polycyclic aromatic hydrocarbons (PAH) in sediments at former manufactured gas plant (MGP) sites can be difficult because of several factors. The long industrial histories of many of these sites often included multiple operations of various types utilizing several types of raw materials and producing many types of PAH-containing products and waste. In addition, many MGP sites are located in industrial areas with a variety of other sources. Finally, the molecular composition of many pyrogenic materials, including the tars which were produced in abundance at MGPs, are very similar and difficult to distinguish by molecular pattern analyses. Current approaches for identifying sources of PAHs in soil, groundwater, sediments, and surface water at former MGP sites have largely relied on the molecular fingerprint or the relative abundance of individual PAHs or groups of PAHs1. However, this approach suffers from two major problems: first, there are many sources of PAHs and their PAH patterns can be very similar, and second, environmental weathering (dissolution, evaporation, and chemical and biological transformations) can alter the PAH patterns and confound their interpretation. Recent studies have shown that certain ratios of alkylated PAHs and compound-specific carbon isotope ratios (13C/12C) are insensitive to environmental weathering processes and can be used to distinguish separate sources with similar PAH patterns even in weathered samples. This presentation reports some of the results of research by the Electric Power Research Institute, META Environmental, Inc., and the Gas Technology Institute to identify effective environmental forensic methods for application at former MGP sites.1 The work has partially included a review of literature on MGP chemistry, the collection and analysis of samples of MGP tar and other reference materials, and the testing of several emerging analytical methods for environmental forensics. The presentation discusses the application of extended PAH profiles and compound-specific carbon isotope ratios by gas chromatography with isotope ratio mass spectrometry (GC/IRMS) for the determination of PAH sources in non-aqueous phase liquids (NAPLs), soil, sediment, and water. Sample classification by GC/IRMS was compared to results obtained using hydrocarbon fingerprinting by GC/FID, extended PAH profiles by GC/MS, and bulk sample fingerprinting by FTIR. The isotopic ratio of C13/C12 for 16 parent PAHs was the most reliable technique for separating sources of PAHs in NAPL, soil, and sediment for weathered samples, and also was effective in determining the source(s) of groundwater plumes of MAHs and PAHs. Differences in carbon isotope ratios were noted for PAHs derived from coal tar, from oil tars, and from petroleum products. These differences correlated well with differences in certain PAH ratios and the amounts in the samples of other source indicators, such as saturated hydrocarbons and organic acids. Selected results from MGP sites will be used to illustrate the value and effectiveness of the methodology.