A mysterious byproduct of a chemical used to disinfect the tap water of about one-third of Americans has finally been identified, and the international research team behind the discovery is advocating rapid assessment of its potential toxicity. The research, reported Thursday in the journal Science, does not claim that tap water containing the byproduct is unsafe to drink or that the finding represents any kind of emergency. All water, including bottled water, contains contaminants. But the discovery of a new and previously unknown chemical, called chloronitramide anion, could have implications for municipal water systems that use a class of chlorine-based...