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1
CHARACTERISATION OF THE HYDROCARBON POLLUTION IN
THE SEWER BED SEDIMENT IN THE WHOLE COMBINED
SEWER OF PARIS Rocher V. , Garnaud S. ,
Moilleron R. , Chebbo G.
CEREVE, Université Paris XII-Val de Marne,
Faculté de Sciences et Technologie, 61 Avenue du
Général de Gaulle, 94010 Créteil Cedex Cellule
Contrôle des Eaux, Section de lAssainissement de
Paris, 17 rue Delesseux, 75019 Paris Faculté
de Génie, Université Libanaise, Route de
laéroport, Beyrouth, Liban
Introduction During dry weather flow, part of
the suspended solids going through the sewer
network settles and leads to bed sediment
accumulation. To avoid excessive accumulation of
bed sediment in the sewer trunk, which may hamper
the waste water flow, 99 deeper trunk portions of
several meter lengths are spread along the sewer
system of Paris. These trunk portions can be
called Silt Traps (STs). These STs which volumes
varied from 21 to 325 m3 are cleaned out as soon
as their filling capacities are reached. The aim
of this research program was to investigate the
hydrocarbon pollution of the bed deposit in the
whole combined sewer of Paris. From this
perspective, we studied the hydrocarbon pollution
in the sediments accumulated in the STs. During
two years (2000/2001), sediment samples were
extracted from 40 STs spread out all over the
Paris network. We measured the Total Hydrocarbons
(THs) and the 16 Polycyclic Aromatic Hydrocarbons
(PAHs) from the priority list of the US-EPA
following the T90-115 and the NFX 31-410 AFNOR
methods, respectively.

• Objectives
• 1. A quantitative approach. Assessment of the
  hydrocarbon contamination levels (THs and Total
  PAHs) in the sewer deposit.
• 2. A qualitative approach. Estimation of the PAH
  fingerprint variations in the whole Paris network
  and identification of the PAH origins.

Sites and sampling procedures During two years,
sediment samples were extracted from STs before
each clean out procedure. 6 sediment samples were
extracted from each ST 3 were extracted from the
sediment surface (5-10 cm depth) and 3 were
extracted from a deeper sediment layer
(approximately at 1 m depth). The location of
these sampling points allowed us to take into
account the spatial variability of pollutant
loads within each ST.
Results
1. Quantitative approach

• Total hydrocarbon contamination levels in the
  sewer deposit

Except for two very strong values measured in the
sediments coming from two different STs (2600 and
7400 µg.g-1), TH contents varied from 200 to 1300
µg.g-1 and the mean value was 625 (? 300) µg.g-1
(Figure 1a). Our results were in a good agreement
with previous study carried out in the "Le
Marais" network located in central Paris. The
level of THs determined in this catchment was 500
µg.g-1( Gonzalez, 2001).

• Aromatic hydrocarbon contamination levels in the
  sewer deposit

The mean total PAH contents measured in the
sediment extracted from each ST varied from 2 to
90 µg.g-1 and the mean value was 26 (? 21) µg.g-1
(Figure 1b). As well as the TH contents, this
mean content was in the same order of magnitude
as the PAH contents measured in the sewer
sediment sampled in the "Le Marais" network.
Indeed, recent study showed that PAH contents
in the sewer sediment were between 22 and 30
µg.g-1 (Rocher, 2002).
2. Qualitative approach

• Spatial fluctuation of the PAH distributions in
  the whole Paris network

• Everywhere in the Paris network, the global PAH
  distribution pattern was similar, i.e.
  characterised by an abundance of heavy compounds.
  Indeed, 95 of the sites showed a Light
  Molecular Weight / Hight Molecular Weight ratio
  below unit and the mean value was 0.5 (figure
  2a). Moreover, in every ST, the major compounds
  were Phenanthrene, BenzokbFluoranthene,
  Fluoranthene, Pyrene, BenzoaPyrene,
  BenzoaAnthracene and Chrysene. The relative
  abundance of this PAH group was quite constant
  (mean value was 70 ? 10 ) (Figure 2b).
• These results suggest that there is no
  significant spatial fluctuation of PAH
  distributions in the 40 STs. There is a quite
  homogeneous PAH pollution in the combined sewer
  network of Paris.

• Determination of the PAH origins

Pyrolysis appears as the main mechanism of
formation for PAHs found in the Paris network.
Indeed, the predominance of the pyrolytic origin
for PAH was underlined by

• the abundance of HMW PAHs (Wang, 1999)

• the predominance of 4 compounds mainly emitted by
  combustion processes (car exhausts, residential
  heating, industrial fumes, etc.) Fluoranthene
  (11 ), Pyrene (10 ), BenzoaAnthracene (8 )
  and Chrysene (7 ) (Figure 3) (Colombo, 1989
  Fromme, 1998 Wang, 1999)

• the combined ratios between Fluo/Pyr vs P/A
  (Figure 4) (Budzinski, 1997)

Conclusions

• Hydrocarbon contamination levels in the sewer
  sediment

Investigation of the hydrocarbon pollution sorbed
onto the sediment accumulated in the 40 STs
spread out all over the sewer system has shown
that the mean TH and PAH contents were 625 (?
300) and 26 (? 21) µg.g-1, respectively.

• Predominance of the pyrolytic origin for PAH
  sorbed into the sewer sediment

Investigation of the PAH fingerprints suggested
that there was, at the Paris network scale, a
quite homogeneous PAH distribution. Moreover, the
predominance of the pyrolytic origin for PAHs
found in the sewer sediment was demonstrated by
the important relative abundance of high
molecular weight PAHs, the identification of
specific compounds and the values of the origin
indexes. Pyrolytic contamination of the Paris
sewer system can be attributed to the very
important road trafic, the residential heating
and the great number of industrial plants (power
plants, waste incinerators, etc.) spread out all
over the Paris conurbation.

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