25th International Meeting on Organic Geochemistry IMOG 2011

P-391
Environmental forensics-recent applications of C, H and Cl
isotopes in the determination of sources of groundwater
contaminants
Paul Philp, Tomasz Kuder
School of Geology and Geophyscis, University of Oklahoma, Norman, United States of America
(corresponding author:pphilp@ou.edu)
Since the commercial availability of gas
chromatograph-isotope ratio mass spectrometer
systems, the number of environmental forensic
applications has increased exponentially. There are
many well documented EPA methods used in
environmental studies but these methods are of little
use in environmental forensic studies, where the
major goal is to find responsible parties rather than
simply obtaining concentration data. In general
environmental forensic studies utilize a tiered
approach, with gas chromatography (GC) providing
an initial indication of the contaminant type; gas
chromatography-mass spectrometry (GCMS)
identifying specific compounds and undertaking
correlation or discrimination studies. However in many
cases the results at this point may still be ambiguous
and not clearly pointing to any specific source(s) for
the contaminant. This is particularly true in the case of
contaminated ground water where there might be a
single component such as MTBE or PCE etc., where
GC and GCMS, whilst telling us it is one compound,
will certainly not differentiate sources of the
contaminant. Differentiation of more complex mixtures
such as crude oils, refined products, and PAHs may
also benefit from using stable isotopes as an
additional line of evidence in distinguishing potential
sources.
There are two major applications of stable
isotopes in environmental forensics, the first being a
tool to differentiate sources and the second to monitor
the onset and extent of natural attenuation. This
paper will provide examples from both of these areas.
First though it is important to mention that early
applications of stable isotopes only considered
utilization of carbon isotopes, but in recent years
utilization of H and, even more recently, Cl isotopes
has become more widespread. A number of isotope
specific challenges have become apparent along the
way to developing these as routine tools for these
applications. For example whilst H isotopes can be
readily determined in the case of hydrocarbons, in a
compound such as TCE (trichloroethylene) with only
one H and 3 Cl, this requires higher contaminant
concentrations for the H but at the same time may
provide valuable information as to whether the TCE
was an original synthetic product or formed from
degradation of PCE. Determination of Cl isotopes of
individual compounds is now becoming a routine tool
and can be obtained through the use of GCMS and
does not require GCIRMS.
The outcome is that it is now possible to use
2D or 3D stable isotope fingerprinting to study both
sources of contaminants as well as the onset of
natural attenuation. In this paper a number of case
histories will be described where stable isotopes, C, H
and Cl, have been used to discriminate sources of
MTBE, chlorinated solvents and hydrocarbons in the
environment. Whilst this approach may not provide
the silver bullet every attorney is looking for, it will
certainly provide additional evidence not available
from the traditional techniques of GC and GCMS.
Ranges of isotope values for non-degraded samples
of many of the common groundwater samples are
now well established and samples with isotope values
heavier than these values are typically considered to
have undergone natural attenuation. At the same time
it is important to point out certain misconceptions
about utilization of stable isotopes in environmental
forensic studies. As a result of a small number of
papers published several years ago, it is believed in
some areas that one can use isotope compositions of
groundwater contaminants to actually identify the
company that made the contaminant of interest. This
is totally incorrect and any investigation into the
manufacturing process involved in making these
compounds will quickly show the potential for isotopic
variations as a result of changing feedstocks and
other variables.
In brief stable isotopes are playing an
invaluable role in environmental forensic
investigations but a huge potential for additional
applications remains.
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