Forensic Applications for 1,4-Dioxane and Solvent Stabilizers - Environmental Investigation and Remediation - page 440

vary over time. A further limitation to using CSIA for solvent source identii cation is that once
delivered to the site of use, the solvent may undergo additional isotopic depletion during vapor
degreasing, dry cleaning, on-site recycling including distillation and carbon i ltering, and evapora-
tion during cold-cleaning operations. Carbon isotope ratios for solvents from different manufactur-
ers in different years were analyzed by Shouakar-Stash et al. (2003), Van Wamerdam et al. (1995),
and Beneteau et al. (1999) and compared in Morrison et al. (2006). Source identii cation applica-
tions of CSIA are best suited to the rare circumstance in which a sample of the original solvent
waste is available. A source-zone sample may be a reasonable surrogate, particularly for studying
biodegradation of solvents with CSIA.
One challenge to using CSIA has been the mass of the contaminant required on the gas chroma-
tography column in GC-IRMS, which previously limited or even prevented CSIA from contaminant
fate studies and the differentiation between sources of contaminants based on their isotopic signa-
ture. Because of the large mass required for CSIA, studies of in situ biotransformation of contami-
nants were previously restricted to source-zone samples at sites with high aqueous concentrations of
contaminants to avoid collecting and processing very large volumes of samples in which the subject
contaminant is present at low concentrations (Blessing et al., 2008). New methods such as SPME
have been developed to improve the isotopic resolution of GC-IRMS on low-concentration samples
(Zwank et al., 2003; Jochmann et al., 2006; Blessing et al., 2008). Currently, most CSIA laboratories
use purge and trap as the GC-IRMS concentration method to achieve detection limits of

13 C of
about 1 ppb for chlorinated solvents and MTBE (Robert J. Pirkle, personal communication,
Microseeps, Pittsburgh, Pennsylvania, 2008).
The limitations and uncertainty of CSIA for i ngerprinting sources of chlorinated hydrocarbons
may discourage its use in forensic investigations; however, it may nevertheless prove useful if
approached in a stepwise fashion, beginning with a screening-level evaluation. To ascertain whether
CSIA has the potential to be a useful tool for a particular investigation, the expected isotopic “end-
members” of the system should be identii ed and sampled as a screening-level evaluation of the
potential viability of CSIA. For example, where groundwater analyses show a sequence of geo-
chemical conditions suggestive of a reducing environment and reductive dechlorination of chlori-
nated solvents, samples of the original solvent and its primary breakdown product(s) can be analyzed
by using CSIA to coni rm whether there is a sufi cient shift in carbon, hydrogen, or chlorine isotope
ratios to justify additional investigation. Similarly, if CSIA is to be useful for solvent-source identi-
i cation for allocating cost in a commingled plume of the same solvent from two or more sources,
sampling the different source areas and a few monitoring wells in the commingled plume should be
the i rst step. If the source-area isotope ratios show a wide enough differentiation from the isotope
ratios in the commingled plume, it may be possible to apply this method to establish mixing
patterns. The range in values of carbon isotope ratios (

δ

13 C) should be larger than the margin of
analytical error, that is, typically 1.0-0.5‰ (Morrison et al., 2006).
One effort to coni rm the biodegradation of TCE in a plume from a landi ll in Switzerland pro-
duced the surprising result that the presence of cis -1,2-DCE was not the result of reductive dechlo-
rination of TCE. Isotopic signatures showed the absence of in situ degradation of TCE, despite the
presence of cis -1,2-DCE, a known metabolite of TCE (Zwank et al., 2003).
To constrain the uncertainty or ambiguity that a CSIA investigation may produce, investigators
may include additional lines of evidence, such as the presence of proprietary solvent-stabilizer for-
mulations, which together with the basic geochemical and hydrogeological framework may result in
an improved conceptual model.

δ

9.2.5 C OMPOUND -S PECIFIC I SOTOPE A NALYSIS OF 1,4-D IOXANE B IODEGRADATION
CSIA has not yet been applied to assess the isotopic fractionation that may occur during the biodeg-
radation of 1,4-dioxane. A grant application was i led with the National Science Foundation by
Jennifer A. Field of Oregon State University to study CSIA of 1,4-dioxane biodegradation jointly