25th International Meeting on Organic Geochemistry IMOG 2011

P-140
Effects of natural and artificial oxidation on dissolved organic
matter: example of Boom Clay
Pascale Blanchart 1 , Pierre Faure 1 , Raymond Michels 1 , Christophe Bruggeman 2 , Mieke
De Craen 2
1 UMR7566 G2R, CNRS, Nancy Université, Vandoeuvres-lès-Nancy, France, 2 SCK-CEN, Mol, Belgium
(corresponding author:Pascale.blanchart@g2r.uhp-nancy.fr)
The Boom Clay (Belgium) is studied as a
reference host rock for methodological studies on
the geological disposal of high-level and longlived
radioactive waste. The drilling of galleries in
the Boom Clay at Mol lead to perturbations of the
initial physical and chemical conditions. Since
organic matter is present in this argillaceous
formation, it is important to know its response to
these new conditions.
The Boom Clay pore water (20% in mass of
rock) contains significant quantities of Dissolved
Organic Carbon (DOC) with a mean concentration
of 115+/-15 mg/L, determined on the basis of
piezometer water as well as rock squeezing and
leaching experiments. Yet, in piezometers, the
DOC may show considerable and irregular
variations through time, with values ranging
between 80 and 425 mg/L [1]. The origin and biophysico-chemical
controls of such variations are
yet unknown. In this presentation the influence of
oxidation on the aforementioned observations is
studied.
Three categories of samples were studied:
1) Fresh as well as air-oxidized Boom Clay
samples were collected in the
Underground Research Facility (URF) of
SCK•CEN (Mol, Belgium); they represent
a natural series of oxidation
2) A fresh Boom Clay sample was
submitted to air oxidation (artificial
series). In these experiments, powdered
clay was heated at 80°C under air flow
during 1, 3, 6, 9, 12, 18 months,
3) Four water samples were collected from
different horizons in the Boom Clay by
means of piezometers located in the
Underground Research Facility during
January 2010.
The Dissolved Organic Matter (DOM) of natural
and artificial oxidation series was extracted by
soxhlets and leaching experiments using pure
water.
Different analyses were conducted and their
results will be presented in detail: DOC
measurements give a quantitative evolution of
DOM with the oxidation process, while PyGC, 3D-
Fluorescence, FTIR, GPC and APPI-QTOF were
used to characterize the DOM.
The evolution of DOM as a function of oxidation
could be evidenced in the natural and the artificial
oxidation series. The composition of the DOM
samples collected from the piezometer waters
was homogeneous.
All the results allow to describe the different
stages of organic matter oxidation in relationship
to the formation of dissolved organic matter:
kerogen decomposition, formation of free organic
matter, clay catalysis, neoformation of water
solubles. Geochemical tracer parameters of the
alteration process could be identified. These can
be used to assess the alteration state of samples
in the Boom Clay and to infer if air oxidation has
an influence on the composition of the formation
water.
This study thus brings a sound background to
the understanding of the reactivity and mass
balances implied in the oxidation of fossil organic
matter from clay. This may be of broader interest
to issues like continental erosion in relationship to
fossil carbon input into the surface carbon cycle.
[1] De Craen M., Wang L., Van Geet, M. & Moors H. (2004)
The geochemistry of Boom Clay pore water at the
Mol site, status 2004. SCK•CEN Scientific Report.
BLG 990.
281