Sources of Chlorate Ion in US Drinking Water

Samples of untreated source water and finished drinking water were obtained from 42 water utilities which treated their water with oxidants–disinfectants that included chlorine dioxide (ClO2), gaseous chlorine, hypochlorite solutions, and chloramines. Chlorite ion was only detected in water from utilities that used ClO2. Finished water from utilities that used ClO2 or hypochlorite solutions contained comparable concentrations of chlorate ion (ClO3−) in the range 21–660 μg/L. Analysis of the hypochlorite solutions indicated ClO3− contamination was a greater problem in the solutions purchased in concentrations >10 percent available chlorine. Low levels of ClO3− (10–81 μg/L) were also detected in approximately one third of the untreated source samples.