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United States Environmental Protection Agency
Applications of Stable Isotope Analyses to Environmental Forensics (Part 3), and to Understand the Degradation of Chlorinated Organic Contaminants (Part 4)
Sponsored by: U.S. EPA Technology Innovation and Field Services Division
Original Time/Date of Presentation:

October 27, 2010, 2:00 PM - 3:30 PM, EDT (18:00-19:30 GMT)

You may also be interested in the following archive of this seminar:

Presentation Overview:

This webinar will consider advanced topics in the applications of compound specific stable isotope analyses (CSIA). It will assume some understanding of the fundamental principles of CSIA as presented in Parts 1 and 2. Please consult the archived presentations if a review of the fundamental principles would be useful before considering the advanced topics.

Part 3 of the webinar will discuss applications CSIA to various problems in environmental forensics. Many ground water contaminants, such as fuel spills, are complex mixtures of many compounds. CSIA data can be combined with data on concentrations of individual compounds (as determined by conventional GC or GCMS analysis) to associate contamination in ground water plumes with specific sources or releases. This is particularly important where there might be multiple sources potentially responsible for a particular spill. In cases where the contaminant has multiple components, and the GC and GCMS data are ambiguous, relationships between source and plume can only be determined through the CSIA of individual components of the mixture. The application of CSIA is also extremely valuable for single component contaminants, such as PCE or TCE, where data on concentrations as provided by GC and GCMS are of little or no use for correlation. Most early applications of CSIA to environmental problems were limited to carbon and hydrogen isotopes. Efforts are being made to introduce the use of chlorine isotopes as an additional tool for monitoring chlorinated compounds. The approach is not as mature as with carbon and hydrogen isotopes but the methodology involved will be discussed along with problems associated with the use of chlorine isotopes.

Part 4 of the webinar will consider the degradation of chlorinated solvents and their transformation products and will focus particularly on the evolution of CSIA as a novel method for investigation of biodegradation at contaminated sites. For chlorinated solvents, petroleum hydrocarbons and fuel additives, degradation can involve large and reproducible kinetic isotope effects, producing systematic changes in the delta 13C or delta 2H values of the residual contaminant as the light (12C; or 1H) versus heavy isotope (13C; or 2H) bonds are preferentially degraded, resulting in isotopic enrichment of the residual contaminant in 13C, or 2H. In many cases, stable isotope fractionation during degradation can be modeled by a Rayleigh distillation model that relates the change in observed stable isotope compositions to the extent of degradation in the system. Stable isotope analysis can provide a direct indication of the effects of degradation on specific contaminants, and in some cases an independent means to quantify the extent of degradation and estimate degradation rates. At some sites, the concentrations of TCE and cis-DCE decline without the production of expected daughter products such as vinyl chloride and ethylene. Has degradation of TCE really stalled at the level of cis-DCE, or is the cis-DCE being further degraded? Can the effects of abiotic degradation of chlorinated solvents be distinguished from biologically mediated remediation? The same questions apply to natural products. At some fuel spill sites or manufactured gas plant sites, the other BTEX compounds are removed by anaerobic biodegradation, but concentrations of benzene persist. Is the benzene also degrading, although more slowly than the other compounds, or is it entirely recalcitrant?

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