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| Titel |
Column experiment to study isotope fractionation of volatile organic contaminants in porous media under unsaturated conditions |
| VerfasserIn |
Simon Jeannottat, Daniel Hunkeler, Florian Breider |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250041792
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| Zusammenfassung |
| Pollution by organic contaminants such as petroleum hydrocarbons and chlorinated solvents
is common in industrialized countries. The use of stable isotope analysis is increasingly
recognized as a powerful technique for investigating the behaviour of organic or inorganic
contaminants. Recently, compound-specific isotope analysis (CSIA) has proven to be an
effective tool to confirm and quantify in-situ biodegradation by indigenous microbial
populations in groundwater.In contrast, only few studies have investigated the use of CSIA in
the unsaturated zone. In the unsaturated zone, the main potential applications of CSIA
include the assessment of biodegradation and the fingerprinting of different sources of
petroleum hydrocarbon or chlorinated solvents vapours. However, it has to be taken into
account that isotope ratios in the unsaturated zone can vary due to diffusion and volatilization
in addition to biodegradation. For application of isotope methods in the unsaturated zone, it is
crucial to quantify isotopic fractionation resulting from physico-chemical and transport
processes.
The study is focused on laboratory experiments that investigate the effect of vaporization
and diffusion on isotope ratios. The effect of diffusion is carried out using a column
experiment setup that can be considered to represent VOC transport from a floating NAPL
towards the atmosphere. Furthermore, additional column and batch experiments will be
conducted to better understand the effect of biodegradation. Volatilization is studied with an
other experimental setup. In addition, a mathematical framework was developed to simulate
the isotope evolution in the column study.
Since the initial experiments aimed at investigating the effect of vaporization and
diffusion only, the column is filled with dry quartz sand in order to avoid perturbations of
concentration profiles by humidity or adsorption on organic matter. An activated sand will
later be used for the biodegradation experiments. A mixture of nine contaminants (pentane,
MTBE, hexane, benzene, isooctane, methylcyclohexane, toluene, octane and xylene) that
represents a wide range of hydrocarbons is emplaced in the column. Periodical measurements
of concentrations and δ13C values were carried out in the source chamber and at
different distances along the column. A depletion of 13C with distance is observed,
which is due to faster diffusion of substances enriched in 12C. The shift of δ13C
values towards more negative values is more significant during the first hours of the
experiment. After some hours, the value stabilizes when a steady state is reached.
These results fit well the analytical models. These results demonstrate that stable
isotope profiles are reached under steady state conditions even though molecules with
light isotopes only diffuse faster than molecules with heavy isotopes. This is an
important finding for the application of isotope analysis to link VOC vapours to
contaminant sources and to demonstrate reactive processes based on shifts in isotope
ratios.
Further experiments will be conducted to study the isotopic response to diffusion,
vaporization and biodegradation of chlorinated solvents (PCE, TCE) using quite a similar
column setup. Stable hydrogen and chlorine isotopes will also be measured during the same
experiments in order to better constrain the different processes and fingerprinting sources of
contaminations. |
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