The potential regulation of chlorate, a by‐product of hypochlorite and chlorine dioxide disinfection, presents a significant challenge for the drinking water community. Data from the Third Unregulated Contaminant Monitoring Rule were analyzed to gain insights about chlorate occurrence. The number of utilities affected by a potential chlorate regulation would vary significantly depending on the concentration at which chlorate might be regulated and whether compliance calculations incorporate averaging. Seasonal trends in chlorate concentrations were observed, with the highest concentrations occurring during the summer. Chlorate concentrations also vary significantly between the entry point to the distribution system and the point of maximum residence time. Analysis of results for systems using gaseous chlorine, which is not known to form chlorate, suggests that source water chlorate concentrations may be significant for some source waters and that those source concentrations may increase during low‐flow or drought events.

Throughout its History, the Washington Aqueduct has Retained its Historic Character, Standing as a Testament to the Longevity of Great Engineering Works.

Water utilities must continuously ensure safe drinking water within the context of multiple competing objectives and considerable uncertainty. Systematic, transparent prioritization of contaminants can improve utility decision‐making. A methodology was developed to identify, screen, evaluate, and determine appropriate actions for a wide range of contaminants, including regulated parameters, contaminants of emerging concern, and aesthetic issues. Actions were assigned on the basis of “utility risk,” defined as the risk that the utility may produce undesirable water through potential health effects of unregulated contaminants, exceeding aesthetic levels, or regulatory noncompliance. Application of this methodology at Washington Aqueduct in Washington, D.C., narrowed a list of more than 700 candidate contaminants down to about a dozen priority contaminants, allowing the utility to focus on the most beneficial actions when considering future capital improvements and nontreatment approaches to enhance drinking water quality.

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Key Takeaways Cyanotoxin occurrence is difficult to predict and detect, yet cyanotoxins at high concentrations are acute health concerns, raising the stakes for utility response. There's no one‐size‐fits‐all method to respond to harmful algal blooms. Utilities should customize their response plans to their specific circumstances and characteristics. Two water utilities in the same US region developed different yet equally comprehensive, reliable plans.