“The half-life determined in static tests for free chlorine, chlorine dioxide, and combined chlorine was 140, 93, and 1680 min.”
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1474301/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1474301/)
Large water districts that have the miles and miles of pipe you are referencing, typically use monochloramine (produced by the combination of free chlorine and ammonia) – far more persistent in water than free chlorine. To the point that it becomes a problem (and has to be removed before use) for particular end users like aquariums and brewers.
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