Insights on Chlorate Occurrence, Intra‐system Variability, and Source Water Concentrations

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.