M. Saad

Microplastic contamination in an urban area: a case study in Greater Paris

This study investigates the microplastic contamination of both urban compartments (wastewater and total atmospheric fallout) and surface water in a continental environment (Greater Paris, France). These first investigations on urban environment confirm the presence of microplastics in sewage, freshwater and total atmospheric fallout and provide knowledge on the type and size distribution of microplastics in the [100 µm-5 000 µm] range. For the first time, the presence of microplastics, mostly fibers, is highlighted in total atmospheric fallout (29-280 particles/m2/day). High levels of fibers were found in wastewater (260-320 x103 particles/m3). In treated effluent, the contamination significantly decreases to 14-50 x103 particles/m3. In River Seine, two sampling devices are used to collect both large and small microplastic particles: i) a plankton net (80 µm mesh) and ii) a manta trawl (330 µm mesh). Sampling with the plankton net showed a predominance of fibers with concentrations ranging from 3 to 108 particles/m3. A greater diversity of both microplastic shapes and types was encountered during manta trawl sampling but at much lower concentrations (0.28-0.47 particles/m3). This combined approach could be relevant and implemented in future studies to provide an accurate overview of microplastic distribution in freshwater.

Partition of pollution between dissolved and particulate phases: what about emerging substances in urban stormwater catchments?

This paper presents results about the occurrence, the concentrations of urban priority substances on both the dissolved and the particulate phases in stormwater. Samples were collected at the outlet of a dense urban catchment in Paris suburb (2.30 km(2)). 13 chemical groups were investigated including 88 individual substances. Results showed that stormwater discharges contained 45 substances among them some metals, organotins, PAHs, PCBs, alkylphenols, pesticides, phthalates, cholorophenols and one volatile organic compound, i.e. methylene chloride. With respect to the European Water Framework Directive, these substances included 47% of the priority hazardous substances (n = 8), 38% of the priority substances (n = 10). The remaining substances (n = 27) belong to a list of others specific urban substances not included in the Water Framework Directive but monitored during this work. Finally, stormwater quality was evaluated by comparing the substance concentrations to environmental quality standards (EQS) and the particulate content to Canadian sediment quality guidelines. This showed that stormwater was highly contaminated and should be treated before being discharged to receiving waters in order to avoid any adverse impact on the river quality.

Micropollutants in urban stormwater: occurrence, concentrations, and atmospheric contributions for a wide range of contaminants in three French catchments

This study aimed at: (a) providing information on the occurrence and concentration ranges in urban stormwater for a wide array of pollutants (n = 77); (b) assessing whether despite the differences between various catchments (land use, climatic conditions, etc.), the trends in terms of contamination level are similar; and (c) analyzing the contribution of total atmospheric fallout (TAF) with respect to sources endogenous to this contamination. The studied contaminants include conventional stormwater contaminants (polycyclic aromatic hydrocarbons (PAHs), Zn, Cu, Pb, etc.), in addition to poorly or undocumented pollutants such as nonylphenol and octylphenol ethoxylates (NPnEO and OPnEO), bisphenol A (BPA), polybrominated diphenyl ethers (PBDEs), a wide variety of pesticides, and various metals of relevance (As, Ti, Sr, V). Sampling and analysis were performed using homogeneous methods on three urban catchments with different land use patterns located in three distinct French towns. For many of these pollutants, the results do not allow highlighting a significant difference in stormwater quality at the scale of the three urban catchments considered. Significant differences were, however, observed for several metals (As, Cr, Cu, Ni, Sr and Zn), PAHs, and PBDEs, though this assessment would need to be confirmed by further experiments. The pollutant distributions between dissolved and particulate phases were found to be similar across the three experimental sites, thus suggesting no site dependence. Lastly, the contributions of TAF to stormwater contamination for micropollutants were quite low. This finding held true not only for PAHs, as previously demonstrated in the literature, but also for a broader range of molecules such as BPA, NPnEO, OPnEO, and PBDEs, whose high local production is correlated with the leaching of urban surfaces, buildings, and vehicles.

Towards the determination of an optimal scale for stormwater quality management: micropollutants in a small residential catchment.

Stormwater and atmospheric deposits were collected on a small residential urban catchment (0.8 ha) near Paris in order to determine the levels of certain micropollutants (using a preliminary scan of 69 contaminants, followed by a more detailed quantification of PAHs, PCBs, alkylphenols and metals). Atmospheric inputs accounted for only 10%-38% of the stormwater contamination (except for PCBs), thus indicating substantial release within the catchment. On this small upstream catchment however, stormwater contamination is significantly lower than that observed downstream in storm sewers on larger adjacent urban catchments with similar land uses. These results likely stem from cross-contamination activity during transfers inside the sewer system and underscore the advantages of runoff management strategies at the source for controlling stormwater pollutant loads. Moreover, it has been shown that both contamination levels and contaminant speciation evolve with the scale of the catchment, in correlation with a large fraction of dissolved contaminants in upstream runoff, which differs from what has been traditionally assumed for stormwater. Consequently, the choice of treatment device/protocol must be adapted to the management scale as well as to the targeted type of contaminant.

Bed shear stress evaluation in combined sewers

Experiments have been undertaken on two sewer trunk lines in order to identify an accurate and practical technique for estimating bed shear stresses in combined sewers. Various methods were tested to determine both local bed shear stress values (one based on the logarithmic velocity profile and the other on the Reynolds shear stress distribution) and mean bed shear stress values (using a method based on the energy slope). Velocity measurements were performed using a micropropeller and an Acoustic Doppler Velocimeter (ADV) under dry weather flow conditions. In sewers without sediments, both the wall law and Reynolds shear stress distribution methods lead to the same bed shear stress estimation. A method based on the logarithmic velocity profile obtained by micropropeller is proposed herein to evaluate local shear stress in combined sewers. Calculations based on channel slope lead to an over-estimation of bed shear stress due to the inaccuracy of bed slope data. If the energy slope S c , as calculated from the Darcy-Weisbach or Manning formula, is used to calculate the mean shear stress in sewer sections without sediments, results are consistent with the local shear stress measurements.

Settling velocity of particulate pollutants from combined sewer wet weather discharges

Settling velocities of TSS and of particulate pollutants (COP, PDCO, PTKN, PCu, PPb, PZn, PPAH) measured on a wide range of wet weather flow (WWF) samples collected at different levels of the Parisian combined sewer system are reported. The recorded V30 (0.01 to 0.1 mm s(-1)) and V50 (0.09 to 0.6 mm s(-1)) values exceed by a factor 10 those of dry weather sewage and also exceed the values measured for pavement runoff. These values lie however often below the 0.28 mm s(-1) reference value considered in France for the design of WWF settling facilities. A decrease in settleability is observed between a small upstream catchment and larger scaled downstream catchments. The settling behaviour of particulate pollutants varies depending on the considered parameter and can differ significantly from the TSS behaviour, due to a non homogeneous distribution of micropolluants over the different classes of particles. PZn and PTKN appear far less settleable than TSS, whereas PPAH show higher settleability.