25th International Meeting on Organic Geochemistry IMOG 2011

P-153
Differentiation of indoor and subsurface VOCs sources in
residential air by CSIA for vapor intrusion management
Tomasz Kuder 1 , Thomas McHugh 2 , Kyle Gorder 3 , Erik Dettenmaier 3 , Paul Philp 1
1 University of Oklahoma, Norman, United States of America, 2 GSI Environmental Inc., Houston, United
States of America, 3 Hill AFB Environmental Restoration Branch, Hill AFB, United States of America
(corresponding author:tkuder@ou.edu)
At corrective action sites with potential vapor intrusion
(VI) concerns, the presence of indoor sources of
volatile organic compounds (VOCs) complicates the
exposure pathway investigation. Indoor sources of
VOCs in residences are ubiquitous and the
background indoor air concentrations of VOCs can
increase or decrease over time based on changes in
the use of these VOCs in consumer products [1].
Because of this, the detection of a target VOCs in a
potentially affected building at a concentration above
the regulatory screening level does not necessarily
indicate a VI impact. Unfortunately, the current
methods for identification of indoor sources are
expensive and have limited effectiveness. A novel
alternative approach presented herein relies on
determination of stable isotope ratios of the target
VOCs present in the air and/or soil gas ( 13 C/ 12 C,
37 Cl/ 35 Cl for PCE and TCE; 13 C/ 12 C and 2 H/ 1 H in the
case of benzene) by compound-specific isotope
analysis (CSIA). The isotope ratios can be used to
differentiate between VOCs sourced from subsurface
(the true vapor intrusion) and those sourced from
miscellaneous household products. The premise for
such discrimination is that the isotope compositions of
a given chemical compound manufactured at different
facilities and/or at different times tend to vary,
reflecting the isotope ratios inherited from the
manufacturing precursors and processes. Moreover,
subsurface biodegradation of VOCs sources often
results with an enrichment of the heavier isotope
species in the remaining residue, while no such
enrichments are feasible for household product VOCs
[2]. The similarity/dissimilarity of the isotope ratios
between VOCs in indoor air, in soil gas and in
household products (if identified on site) permits
identification of those sources with best match to the
indoor air VOCs isotope composition.
Samples of TCE and/or PCE were collected and
analyzed by CSIA, to determine C and Cl isotope
ratios of the target compounds in residential indoor
air, soil gas and in the ground water in the proximity of
a chlorinated solvent plume at Hill AFB (Utah, USA).
At one of the residences, a household product
containing PCE was found and it was possible to
provide a specimen of that product for direct
comparison with the isotope ratios of indoor air PCE.
The residences were selected for the study based on
previous work – sites with and without likely VI impact
were chosen to allow independent validation of the
CSIA results.
Key challenge of applying CSIA to indoor VOCs is the
low concentration of the analytes. In the present
study, an application of thermal desorption sorbent
tubes was proven to be successful and isotope ratios
could be determined for TCE and PCE at low ug/m 3
concentrations. The results of the survey of the four
residences demonstrated that the isotope ratios of
VOCs present in indoor air may be similar to those of
the same VOCs in soil gas or in household products.
In the former case, VI scenario is suggested; in the
latter case, domestic product impact is suggested.
Combined carbon and chlorine data were more
informative than single isotope data, permitting 2Dcorrelation
of the sources and indoor air samples. In
one residence, the isotope ratios of TCE were clearly
indicative of extensive degradation of the compound.
In such case, even if soil gas samples are not
available, the TCE can be inferred to originate from
the subsurface, as degradation of household product
VOCs is not feasible. The conclusions from CSIA
matched the conclusions from traditional assessment
of VI impact, with a benefit of simplified
implementation.
References
[1] Hers, I., Zapf-Gilje, R. Li, L. Atwater J. (2001) J.
Air & Waste Management Association 51, 1318-1331.
[2] Meckenstock, R.U., Morasch, B., Griebler, C.,
Richnow, H.H. (2004) J. Contam. Hydrology 75, 215-255.
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