Application of Environmental Isotopes in Studies of Biodegradation of Organic Contaminants in Ground

1
Application of Environmental Isotopes in Studies
of Biodegradation of Organic Contaminants in
Groundwater Ramon Aravena, Department of Earth
Sciences, University of Waterloo, Waterloo,
Canada Daniel Hunkeler, Centre for Hydrogeology,
University of Neuchâtel, Switzerland

Results Isotope fractionation during
biodegradation organic contaminants
Case Study Reductive dechlorination of PCE in a
sandy aquifer in Toronto, Canada
Introduction Biodegradation can lead to
transformation of organic contaminants in
groundwater to non toxic products under natural
conditions (natural attenuation) or as part of a
engineered remediation strategy. However, it is
often difficult to assess biodegradation at field
sites because contaminant concentration vary also
due to dilution and sorption or as function of
varying water levels and groundwater flow
directions. Analysis of stable isotope ratios is
a possible way to trace biodegradation. A number
of studies have shown that no significant isotope
fractionation in organic contaminants occurs due
to physical processes such as sorption and
volatilization. This presentation summaries the
current knowledge of isotope fractionation during
biodegradation of organic compounds gained from
laboratory and field studies.
Part 1 Chlorinated hydrocarbons


Quantification of isotope fractionation in
laboratory experimentsExample Reductive
dechlorination of vinyl chloride (initial
concentration 40 ppm) to ethene
Cis-DCE and VC become strongly enriched in 13C
during biodegradation confirming that strong
carbon isotope fractionation also occurs under
field conditions.

Discussion The magnitude of carbon isotope
fractionation tends to be larger for chlorinated
hydrocarbons than for petroleum hydrocarbons. The
difference is likely due to differences in the
type of bonds that are broken and formed during
the initial transformation step and due to the
differences in size of molecules. The larger
number of carbon atoms in petroleum hydrocarbons
tends to  dilute  the isotope effect. For
petroleum hydrocarbons, the isotopic enrichment
factors are generally larger for hydrogen than
for carbon, which can be explained by the larger
relative mass difference between D and H than
between 13C and 12C. A particularly larger H
isotope effect occurs for pathways during which a
C-H bond is broken in the initial transformation
step (toluene degradation by methyl oxygenase and
under sulfate reducing conditions). For several
of the compounds, the carbon isotopic enrichment
factor varies relatively litlle between
microcosms from different sites or different
cultures suggesting that carbon isotope ratios
can be used to quantify biodegradation. Regarding
hydrogen isotopes, the enrichment factors are
larger but more variable suggesting that in some
cases hydrogen isotopes could be a very sensitive
qualitative parameter to confirm biodegradation.
Part 2 Petroleum hydrocarbons
Kinetic isotope effect k12 gt k13
1
Biodegradation of methyl tert-butyl ether (MTBE)
2
4
3

4
Quantification of carbon isotope fractionation
using modified Rayleigh equation

• References
• Hunkeler, D., Aravena, R. and Butler, B.J., 1999.
  Environmental Science and Technology, 33,
  2733-2738.
• Bloom, Y., Aravena, R., Hunkeler, D., Edwards,
  E., and Frape, S.K., 2000. Environmental Science
  and Technology, 34, 2768-2772.
• Hunkeler, D., Butler, B.J., Aravena, R. and
  Barker, J.F., 2001. Environmental Science and
  Technology, 35, 676-681.
• Hunkeler, D., Andersen, N., Aravena, R.,
  Bernasconi, S.M. and Butler, B.J., 2001.
  Environmental Science and Technology, 35,
  3462-3467.
• Hunkeler, D., Aravena, R., and Cox E., 2002.
  Environmental Science and Technology, 36,
  3378-3384.
• Morasch, B., Richnow, H.H., Schink, B., and
  Meckenstock, R.U., 2001. Applied and
  Environmental Microbiology, 67, 4842-4849.
• Morasch, B., Richnow, H.H., Schink, B., Vieth, A.
  and Meckenstock, R.U., 2002. Applied and
  Environmental Microbiology, 68, 5191-5194.
• Sherwood Lollar, B, Slater G.F., Ahad, J. et al.,
  1999. Organic Geochemistry, 30, 813-820.
• Slater, G.F., Sherwood Lollar, B., Sleep, B.E.,
  and Edwards, E.A., 2001. Environmental Science
  and Technology, 35, 901-907.
• Gray, J.R., Lacrampe-Couloume, G., Gandhi, D. et
  al., 2002. Environmental Science and Technology,
  36, 1931-1938.

d13C d13C0 e ? ln C/C0 d13C0 initial isotope
ratio Co initial concentration d13C0 initial
isotope ratio at time t C concentration at time
t e isotopic enrichment factor
Acknowledgements
This project was supported through grants from
the National Sciences and Engineering Research
Council of Canada, the Centre for Research in
Earth and Space Technology and the University
Consortium Solvents-in-Groundwater Research
Program.
2
(No Transcript)