We tested the association of common events in drinking water distribution systems with contamination of household tap water with human enteric viruses. Viruses were enumerated by qPCR in the tap water of 14 municipal systems that use non-disinfected groundwater. Ultraviolet disinfection was installed at all active wellheads to reduce virus contributions from groundwater to the distribution systems. As no residual disinfectant was added to the water, any increase in virus levels measured downstream at household taps would be indicative of distribution system intrusions. Utility operators reported events through written questionnaires. Virus outcome measures were related to distribution system events using binomial and gamma regression. Virus concentrations were elevated in the wells, reduced or eliminated by ultraviolet disinfection, and elevated again in distribution systems, showing that viruses were, indeed, directly entering the systems. Pipe installation was significantly associated with higher virus levels, whereas hydrant flushing was significantly associated with lower virus levels. Weak positive associations were observed for water tower maintenance, valve exercising, and cutting open a water main. Coliform bacteria detections from routine monitoring were not associated with viruses. Understanding when distribution systems are most vulnerable to virus contamination, and taking precautionary measures, will ensure delivery of safe drinking water.

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