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, Créteil Cedex ** Cellule Contrôle des Eaux, Section de l’Assainissement de Paris, 17 rue Delesseux, Paris ***Faculté de Génie, Université Libanaise, Route de l’aéroport, Beyrouth, Liban Results 1. Quantitative approach 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). 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 m 3 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 T and the NFX 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.  Total hydrocarbon contamination levels in the sewer deposit  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 Figure 1 : (a) Total hydrocarbon and (b) Total PAH contents (mean values, µg.g -1 ) measured in the sediment extracted from the 40 silt traps (a)(b) (a) Figure 2 : (a) Ratio between Light Molecular Weight PAHs (ring number  3 cycles) and High Molecular Weigh PAHs (ring number > 3 cycles) and (b) Relative abundance (%) of the 7 major PAH group (Phenanthrene, Benzo[b,k]Fluoranthene, Fluoranthene, Pyrene, Benzo[a]Pyrene, Benzo[a]Anthracene and Chrysene) in the 40 silt traps 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, Benzo[k+b]Fluoranthene, Fluoranthene, Pyrene, Benzo[a]Pyrene, Benzo[a]Anthracene 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 %), Benzo[a]Anthracene (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) Figure 4 : Combined ratios between Fluoranthene / Pyrene vs Phenanthrene / Anthracene for sewer sediments Figure 3 : Relative abundance (%) of the 7 major PAHs in the sewer sediment Conclusions 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.  Hydrocarbon contamination levels in the sewer sediment  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. References: Budzinski, Jones, Bellocq, Piérard and Garrigues (1997). Evaluation of sediment contamination by PAHs in the Gironde estuary. Marine Chemistry. Vol.58, p° Colombo, Pelletier, Brochu and Khalil (1989). Determination of hydrocarbon sources using n-alkane and PAH distribution indexes. Case study: Rio de la Plata estuary, Argentina. Environmental Science and Technology. Vol.23, p° Fromme, Oddoy, Piloty, Krause and Lahrz (1998). 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