Demonstrating a natural origin of chloroform in groundwater using stable isotope analysis
| Responsable du projet | Daniel Hunkeler |
| Collaborateur |
Cédric Weber
Bibiane Schlunegger Florian Breider |
| Résumé |
In recent years, an increasing number of naturally produced
organohalogens has been discovered. Environmental studies on
organohalogens have frequently focused on chloromethane, a
naturally produced ozone-killer, and its release to the atmosphere.
However, there is increasing evidence that organohalogens produced
in soils, especially chloroform, can also be transported to
groundwater. The relatively frequent detection of chloroform in
ambient groundwater suggests that natural chloroform production may
be widespread. However, it is difficult to demonstrate a natural
origin of chloroform in groundwater given its widespread use as an
industrial solvent. For long-term evaluation of groundwater quality
and because anthropogenic chloroform may frequently be associated
with other contaminants, it is important to know to what extent
chloroform in groundwater originates from natural sources. This
project is a continuation of the project n°200020-117860, which
corresponds to the first two years of a PhD thesis. In the previous
project, soil gas measurements were carried out at several sites
with natural chloroform production. Carbon isotope ratios between
-25 and -30‰ were observed which are significantly more enriched in
13C compared to anthropogenic chloroform (-50 to -70‰). This clearly
demonstrates the potential of using carbon isotope data to
distinguish between natural and anthropogenic chloroform. At the
study sites, a strong correlation between the chloroform
concentration in soil gas and the quantity of base-hydrolysable
trichloroacetyl-groups in soil samples was observed suggesting that
the latter could be a precursor of naturally formed chloroform.
Using base-hydrolysis, the site-specific isotope ratio of the
trichloromethyl position was determined and values of -10 to -15‰
were obtained. If this position is the chloroform precursor, a
substantial carbon isotope fractionation has to occur during their
release. This is indeed the case as demonstrated in experiments
with model compound and humic acids, and as was also shown in a
study recently published by another research group. A carbon
isotope enrichment factor of about -15‰ was found which explains
the isotope ratio of soil gas chloroform very well. The fairly
stable isotope ratio of soil gas chloroform implies a fairly stable
isotope signature of the trichloromethyl-position despite
fractionation during their release. A stable isotope ratio can be
achieved if the precursor pool is large compared to the rate of
chloroform production or if a steady state between formation and
release of this position is established. Based on these
considerations a simplified mathematical process model was
developed incorporating different carbon pools leading to
chloroform that reproduces the observed isotope values and temporal
trends well. In the next project phase (year 3 and 4 of the PhD), we
plan to explore these hypotheses of chloroform formation in more
detail and to “validate” the mathematical process model with
laboratory experiments. In addition, we plan to include chlorine
isotopes as well for two reasons. On the one hand, by measuring
chlorine isotope ratios, it will be possible to further confirm the
conceptual model of chloroform production in soils. On the other
hand, chlorine isotope ratios could also be used to distinguish
between natural and anthropogenic sources of chloroform. Indeed, an
adaptation of the simplified process model to chlorine suggests that
chlorine isotope ratios in chloroform should substantially deviate
from those of inorganic chlorine in soil. Since part of the
soil-produced chloroform escapes to the atmosphere, its isotope
ratio might be influenced by diffusion in addition to reactive
processes. This possible effect will also be explored using column
experiments and, in case of chlorine, diffusion isotope effects
will be evaluated on an isotopologue basis. In the further project,
the well-established collaboration with Geological Survey of Denmark
and Greenland will be continued and some of the studies will be
carried out at their well-characterized research field sites.
Moreover, field sampling and measurement will be carried out in
different Swiss sites selected on the basis of the data of the
national groundwater monitoring network (NAQUA) to study the
dynamics and isotope evolution of the trichloroactyl-pool and
chloroform under field conditions. |
| Mots-clés |
groundwater, organic contaminants, isotopes, stable isotopes, chloroform, Chlorinated hydrocarbons |
| Type de projet | Recherche fondamentale |
| Domaine de recherche | Géochimie |
| Source de financement | FNS - Encouragement de projets (Div. I-III) |
| Etat | Terminé |
| Début de projet | 1-10-2010 |
| Fin du projet | 30-9-2012 |
| Budget alloué | 201'540.00 |
| Autre information |
http://p3.snf.ch/project-132740# |
| Contact | Daniel Hunkeler |