Hmm... we need OT to explain what that means, I think!
It's the gubmint dumping Fluorine in the water again! :)
Series....halogens are fluorine, chlorine, bromine, iodine. They chlorinate the water some to kill the bad bugs but chlorine is toxic and you don't want too much of the stuff. From my environmental days, there's always some form of chloroform in the water: that's a carbon with 3 hydrogens and a chlorine stuck on it.
But then I've never worked at a water treatment plant. I expect they have ranges for these compounds that have been exceeded.
At least it ain't cryptosporidium. :)