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amec�9 <br /> 13 <br /> 13 � � <br /> 12 C)sample <br /> CS C= 13C —I X1000 (1) <br /> C)std <br /> where (13C/12C)",P/, is the isotope ratio of TCE and (13C/12C)Std is the isotope ratio of VPDB <br /> (equal to 0 %o), which has an absolute 13C/12C of 0.0112372. Similarly, for hydrogen, the <br /> standard is VSMOW, also set to 0%0, where SMOW is an abbreviation for standard mean <br /> ocean water (ocean water is the reference for both D/H and 180/160). The chlorine isotope <br /> ratio standard is VSMOC (Vienna standard mean ocean chloride). <br /> How are the CSIA results used to provide information on COC sources? <br /> Different chemical manufacturers have been shown to produce chemicals with different stable <br /> isotope ratios due to the isotopic composition of the chemical source material. If two sources <br /> exist, and the sources are from different manufacturers, it may be possible to use CSIA to <br /> identify separate sources. However, for TCE, published CSIA data is relatively sparse and the <br /> same manufacturer may have produced TCE with different isotope ratios over different times <br /> (EPA, 2008). Also, because only two chemical manufacturers dominated the TCE supply <br /> (Doherty, 2000), it can be expected that different releases could have the same isotope <br /> signature because there is a reasonably high probability that different industries were <br /> purchasing TCE from the same source. Perhaps the most powerful application of CSIA <br /> towards source identification lies in the ability to use the method to distinguish TCE derived <br /> from PCE biotransformation versus TCE that was produced as solvent at a chemical plant. <br /> TCE solvent has been shown to be greatly enriched in deuterium relative to TCE that is <br /> derived from the biotransformation of PCE in the subsurface (EPA, 2008). <br /> How can CSIA provide direct evidence of COC biodegradation? <br /> For degradation reactions involving the breaking of carbon bonds, such as for TCE <br /> degradation, the COC undergoing degradation becomes more enriched in 13C relative to its <br /> initial un-degraded condition (i.e. 6 <br /> 13Co is the starting isotope ratio and 613C values increase <br /> over time as degradation proceeds). This same situation applies for other isotope ratios such <br /> as hydrogen and chlorine, and other COCs such as cDCE and vinyl chloride, although the <br /> extent of enrichment (i.e. fractionation) with biodegradation varies. The isotope ratio of the <br /> source COC must be known or estimated within a reasonable degree of accuracy to compare <br /> with CSIA data from groundwater samples. The source carbon isotope ratio for TCE (613Co) <br /> can be estimated from site data or from published values on the range of product 613C values <br /> (Wilson, 2010 reports a range of-33.5%o to -27.8%o for TCE product 613C values). Therefore, a <br /> 613CTCE value >-27%o can be interpreted as direct evidence that TCE degradation has occurred. <br /> AMEC Geomatrix, Inc. <br /> \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTS Assessment_Apx B_012711\Attachment B.4\Attach B-4.docx 134-11 <br />