| Combined sewer overflows (CSOs), urban flooding and pollution of the receiving environment have become increasingly chronic problems.;The trend is likely to intensify due to urban growth and significant changes in precipitation. The goals of this thesis work were to analyse the impact of rainfall variables on event mean concentrations (EMCs) and event fluxes (EFs), to evaluate the ecotoxicological risks of discharges from a combined sewer overflow on receiving aquatic ecosystems, and to evaluate the quantitative and qualitative impact of climate change (CC) on urban wet weather discharges (UWWD). The study was carried out simultaneously on two catchment areas in the Longueuil urban agglomeration (Quebec, Canada) for common rainfall events to characterize the discharges from a combined sewer system and from a separated storm sewer system. A large panel of pollution parameters (n=125) and four bioassays were considered. A two-pronged " substance and bioassay " approach was used to analyse the ecotoxicological risks of the discharges under both current climate conditions and future climate conditions.;The findings revealed that, for the pollutants considered, there was wide intra-site, inter-site and inter-event variability for EMCs and EFs, linked primarily to the variability of the rainfall events monitored. The results also highlighted the significant contribution of the erosion of accumulated deposits in the combined sewer system, suggesting a scouring of this system. Analysis of the correlations indicate that the length of dry antecedent time is the best hydrological variable (61%) in predicting EMC values for organic and nutrient pollutants, while the best variables to estimate EF values are total rainfall (69%) and volume discharged (VD, 57%). Total suspended solids (TSS) act as tracers for numerous pollutants (organics and nutrients), accounting for 97% of EF variability in this study and suggesting that the choice of a technological solution to reduce TSS would also help limit these pollutants.;The two-pronged " substance and bioassay " approach is a promising management tool for monitoring untreated combined effluents. The evaluation of the impact of CC highlighted an increase in VD from 15% to 500% and in peak flow (Qcso) from 13% to 148% by 2050, suggesting that (i) the relationships between rainfall and the variables characterizing sewer overflows (VD, Qcso) are not linear, and (ii) changes in rainfall require adjustments in the design criteria of water infrastructures. Ecotoxicological risk indices will double under future climate conditions (2050) compared to current climate conditions (2013). However, the data from the present study did not show risk caused by discharges from Longueuil's combined sewer overflow alone, mainly because of the strong diluting effect of the receiving St. Lawrence River.;Lastly, this study also highlighted potential ecotoxicological risks associated with untreated municipal effluents and provides a framework for investigating sustainable management strategies to reduce this pollution. It suggests the need to rethink urban land development through densification, best management practices (BMP) such as urban greening, storm water infiltration and the filtration of runoff by introducing drainage trenches, ditches, green roofs, etc. Although there are many uncertainties regarding CC scenarios, adjustments are required in response to the hydrological and environment effects of significant changes in precipitation. In light of the considerable volumes projected under future climate conditions, future studies should focus more on evaluating strategies that will limit the impact of CC and of rapidly expanding urbanization. |
