ReviewSpatial variability of the characteristics of combined wet weather pollutant loads in Paris
Introduction
A critical element to be investigated as part of a stormwater management program is an understanding of the local receiving water problems. Since the 1970s, numerous studies have emphasized the importance of pollutant loads conveyed by combined wet weather discharges, and their adverse impacts on receiving waters (US EPA, 1983; Chebbo, 1992; Saget, 1994; Seidl et al., 1998; Even et al., 2004). Therefore, an accurate knowledge of the production and transfer processes, along with the characteristics of pollutant loads, is necessary to develop management strategies that enable mitigating these impacts and thus meeting the legislative requirements of the Council Directive concerning Urban Waste Water Treatment (1991/271/EEC) as well as requirements associated with the European Water Directive Framework (2000/60/EEC).
Such was the context in 1994 when the CEREVE Laboratory (Education and Research Center on Water, the City and the Environment) decided to launch the “OPUR” research program (Observatory of Urban Pollutants) for the purpose of improving the knowledge on the generation and transfer of pollutants in combined sewers during both dry and wet weather periods. The first part of the OPUR research program (OPUR 1), between 1996 and 2001, addressed these mechanisms at the scale of a small combined urban catchment (41 ha), located in the historical center of the city of Paris, i.e. the Marais catchment (Gromaire, 1998; Garnaud, 1999; Gonzalez, 2001). During wet weather periods, results reflected an increase in suspended solids, organic matter and hydrocarbon concentrations and a decrease in heavy metal concentrations between the combined sewer inflows and outflows (Gromaire, 1998; Chebbo et al., 2001). These observations have confirmed that the sewer network not only acts as a transport system, but also constitutes a physicochemical reactor that influences the quality of urban water.
The conclusions drawn from the first part of the OPUR program could be site dependent because the Marais catchment has specific characteristics: small surface area and strong clogging of oversize collectors. For this reason, the objective focused on determining the extent to which these results could be extrapolated to larger catchment areas. In this pursuit, CEREVE, in partnership with the Municipality of Paris, the Interdepartmental Association for Sewage Disposal in the Paris Conurbation (SIAAP) and the Seine-Normandy Water Agency (AESN), extended in 2002 the onsite observatory to 6 bigger catchments (OPUR 2). This expanded facility then made it possible to study water quality trends upstream vs. downstream for a large catchment area.
For a wide range of pollutants including suspended solids (SS), volatile suspended solids (VSS), chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), heavy metals (Cd, Cu, Pb, Zn), aliphatic hydrocarbons (AHs) and polycyclic aromatic hydrocarbons (PAHs), this article seeks to evaluate the spatial variability of the pollutant loads during wet weather periods with respect to the characteristics of the catchment.
Section snippets
Site characterization and sampling procedure
The OPUR observatory is composed of six catchments all located on the right bank of the Seine River in Paris, along the Clichy interceptor (Fig. 1). The characteristics of these catchments (surface area, population, hydraulic length, etc.) are provided in Table 1. The catchments cover areas from 41 to 2581 ha, with a network length from 1 to 12.5 km and populations from 12 000 to 620 000 inhabitants. The sewerage system is combined and entirely accessible. The active or impervious surface was also
Combined WWF discharges
For each catchment, the range (d10, d50 and d90) of WWF discharges, expressed per unit of active surface area, is reported in Table 4. Depending on the site that was assessed, median flow rates varied between 4.2 and 6.3 l s−1 ha active−1. However, for a given rain event, flow rates were homogeneous for all the catchments except for Quais, which systematically displays lower discharges (Wilcoxon test, α=0.05). This latter catchment is characterized by a high proportion of railways lines (21% of the
Conclusion
The research work presented herein, as part of the OPUR 2 program (2001–2006), is aimed at evaluating for a wide range of pollutants (SS, VSS, COD, BOD5, TOC, TKN, Cd, Cu, Pb, Zn, AHs and PAHs) the potential scale influence on WWF quality within combined sewer systems. The characteristics of WWF pollutants were thus carefully examined at the scale of six OPUR catchments, covering land areas varying from 41 to 2581 ha and containing quite similar land uses.
The extensive data set presented in this
Acknowledgments
The authors are indeed grateful to the Seine-Normandy Water Agency, the Interdepartmental Association for Sewage Disposal in Paris Conurbation (SIAAP) and the Municipality of Paris for their combined financial support.
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