Risk evaluation of drinking water distribution system contamination due to operation and maintenance activities

The integrity of the infrastructure used to deliver drinking water from the treatment plant to the customers is of paramount importance to minimize the risk of water contamination. Recent work by a committee of experts (NRC, 2006) highlighted distribution system issues of greatest concern in terms of their potential health risk. The issues listed included new and repaired water mains (high priority) and low pressure transients and intrusion (medium priority). Low pressure may result from a wide variety of distribution system operational practices and maintenance activities such as pump starting or stopping, rapid opening or closing of valves, hydrant flushing, main breaks, and loss of power. The link between contamination of distributed water and occurrence of operation and maintenance activities in a distribution system is often based on specific cases when waterborne disease outbreaks are reported. Few data are available to evaluate the impact of operation and maintenance activities under normal operating conditions.; The main objective of this research project was to assess the conditions leading to intrusion of contaminants in a system during a set of standard operations and maintenance activities in order to identify solutions to minimize such events and protect public health. On a more detailed level, this project seeked to: (1) Demonstrate through data mining that a significant portion of non-compliance events may be linked to distribution system operation and maintenance activities; (2) Investigate, in full-scale distribution systems, the link between O&M activities, distribution system hydraulics and potential water quality changes (with an emphasis on the occurrence of negative transient pressures and microbial intrusion); (3) Achieve transient and intrusion modeling to assess the potential level of intrusion associated with negative pressure events in a full-scale distribution system.; The first part of the study consisted in the development and application of a data integration approach based on the combination and simultaneous consideration of water quality data, network structural data, system O&M data, information from a hydraulic model and a geographical information system (GIS) for data visualization. The Interactive Data Analyzer tool was used for the analysis. A total of 140 total coliform samples in five distribution systems were investigated. The results showed that the role of O&M activities on the occurrence of coliforms was variable from one system to another, explaining a minimum of 9% and up to 45% of the number of coliform cases investigated in each system.; In order to complement the information obtained from the data-based study, field work was conducted and consisted in monitoring some activities that could affect the physical/hydraulic integrity of a system and verify how these activities could lead to changes in water quality in full-scale distribution systems. The activities monitored included: repairs of water main breaks/leaks, water main flushings, hydrant flow tests, pump start-up and shutdowns, and normal distribution system operation. Two potential pathways of microbial intrusion were also investigated: soil and shallow groundwater surrounding buried water mains and water from flooded air-vacuum valve vaults. Monitoring took place in the Laval and Montreal distribution systems with most of the work conducted in the Laval system, in the same area where Payment and colleagues (1991, 1997) conducted their epidemiological studies and measured an increased rate of gastrointestinal illnesses associated with the consumption of tap water.; From the results obtained during this project, additional research is therefore needed to assess with confidence the risk of intrusion associated with the occurrence of transient negative pressures. The modeling work performed during this study was rather exploratory and allowed the prediction of locations and volumes of intrusion in the distribution system. Th...