Modélisation de scénarios de déversements dans un canal d'eau potable

摘要

Under the new Quebec regulations on source water protection, water quality and vulnerability of source waters must be evaluated. Events leading to toxic spills or the release of waste water containing pathogenic microorganisms may require real time changes to the operation of a drinking water treatment plant. If spill events are not adequately recognized, foreseen and reported, the health of the population served by the community water supply may be affected. This project aims to determine the types of contamination events that pose the greatest threats to a drinking water source, and the actions that can be taken to minimize the risk. The project is based on a real case study of a canal used to supply raw water to a drinking water plant located in an urban area.;Types of contamination events studied were based on preliminary studies that had identified the leakage from the sewer system, the re-suspension of contaminated sediments and chemical spills from the road and railroad transportation network as potential threats. In the context of the increased transportation of hydrocarbon products, we have studied in detail the case of a fuel spill following a road accident. Finally, the possibility of an intentional spill was considered.;The assessment of risks associated with different types of events required the definition of spill scenarios, establishment of contaminant concentration thresholds beyond which the operation of the drinking water treatment plant would be affected, and the development of a numerical model for contaminant transport. The model developed showed good performance when compared to one dimensional models and allowed the consideration of many processes, including advection, dispersion, adsorption, sedimentation, evaporation and dissolution of contaminants.;Simulation results have identified the characteristics of spills affecting their impacts on the quality of the raw water. Spill near the water intake in conditions of low longitudinal dispersion result in higher maximum concentrations and therefore more important consequences. In return, more distant spills with greater dispersion can prolong the duration of the contamination event and compromise the operation of the drinking water treatment plant for a longer period of time. Among the different contamination scenarios studied, accidental fuel spills appear to be of greatest concern with regards to their impact on plant operation. In fact, a fuel spill of as little as 30 litres up to 8km upstream could increase the concentration of benzene in raw water beyond the threshold of 0.5 mug / l. For a large spill of 500 kg undercritical conditions, the concentration of benzene could remain above the threshold for more than 48 h.;Among the proposed risk mitigation measures, several designed for oil spills in marine environments are not suitable for drinking water sources in a river. The fact that dissolved contaminants are of greatest concern means that the use of dispersants is not recommended. Modeling of short dissolution times of less than 60 minutes also indicates that in situ recovery of floating oil would be inefficient. Other measures were proposed and evaluated to reduce the vulnerability of the source water to contamination events. Simulations show that the continuous monitoring of raw water quality of by sensors could help avoid a situation where a spill goes unnoticed and could reduce the period of time that the operation of the drinking water treatment plant is affected by 60%. The installation of a second water intake 1 km upstream and the coordinated use of both water intakes could reduce by about 60% the probability of a spill resulting from road accidents reaching the intake and could reduce by almost half the duration of the impact on the plant.;The method and the numerical model developed in this paper as well as some of the findings may be applicable to other drinking water sources vulnerable to contamination events.