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amec— <br /> (approximately <br /> 99% of carbon is 12C; 14C is radioactive [unstable] and extremely low even in <br /> modern carbon). Although 13C/12C is approximately 1%, its relative abundance varies by a few <br /> to several tenths of a percent between different forms of carbon (i.e. marine carbonates have <br /> slightly more 13C content than atmospheric carbon dioxide, which has slightly more 13C content <br /> than plants and animals, which have slightly more 13C than biogenic methane). <br /> How is CSIA relevant to intrinsic remediation? <br /> The additional mass from the extra neutron(s) affects the rate at which the heavier isotopes <br /> participate in chemical reactions, phase changes and microbiological processes. Therefore, as <br /> a chemical or microbiological reaction proceeds, the stable isotope ratios of reactant and <br /> product can change. This change in isotope ratio is called "fractionation" and the extent of <br /> fractionation is often diagnostic of a specific reaction or process. Large fractionation occurs <br /> during the biodegradation of TCE, cDCE and vinyl chloride but not during the evaporation, <br /> dilution or sorption of these COCs. Therefore a change in the 13C/12C ratio of TCE is an <br /> indication that degradation is occurring. <br /> How are CSIA measurements performed by the laboratory? <br /> Groundwater samples for CSIA as applied to VOCs are collected in 40 milliliter glass vials with <br /> Teflon-lined septa, similar to routine VOC sample collection for EPA Method 8260. Specific <br /> compounds such as TCE are isolated with a gas chromatograph similar to that used for EPA <br /> Method 8260 except the mass spectrometer is different. For CSIA, stable isotope ratios are <br /> measured with an isotope-ratio mass-spectrometer (IRMS) instead of a quadrapole-type mass <br /> spectrometer used for Method 8260. There is an intermediate step where prior to IRMS, the <br /> compounds are broken down into carbon dioxide or hydrogen gas prior to IRMS, for stable <br /> carbon and hydrogen isotope ratios, respectively. Chlorine isotope ratios (37Cl/35C1) of <br /> chlorinated VOCs can be measured with IRMS by conversion of sample chloride to chlorine <br /> gas, or for chlorinated hydrocarbons such as TCE, they can also be carefully measured on a <br /> standard GC-MS instrument typically used for VOC analysis, by comparing the molecular ion <br /> signals of 35CI-TCE with 37CI-TCE and similarly for cDCE and vinyl chloride (EPA, 2008). <br /> How are the CSIA results reported? <br /> Every IRMS is calibrated to an isotope ratio of an internationally defined standard, so that <br /> isotope ratios can be referenced to a known standard value. Measurements of stable carbon <br /> isotope ratios (13C/12C) are reported as the deviation (6) from the 13C/12C of a Cretaceous <br /> marine carbonate called Pee Dee Belemnite (PDB). PDB was established as the stable carbon <br /> isotope standard by the International Atomic Energy Agency in Vienna (hence it is called <br /> VPDB for"Vienna" PDB). All stable carbon isotope ratios are reported as a difference in parts <br /> per thousand, or permil (%o), from VPDB as follows: <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 134-10 <br />