and maybe it wasn't methyl bromide itself in the water, but rather bromine (as ions, in other compounds) resulting from the breakdown of methyl bromide. (This gets interesting*.)
Bromine ions (from any source) could react with either chlorine dioxide (plus sunlight) or ozone to form bromate. (But generally you should be careful about using either to treat water if you know there's a lot of bromine in it.)
However, the methyl groups (if methyl bromide is involved: ie, if it hadn't broken down) might have combined with something, leaving some indicator (although perhaps an unstable/transient one).
It'd be nice to know that the problem isn't larger -- and that it'll be prevented in the future. Which brings us back to where did such concentrations of bromine come from.
*: It (apparently) turns out that while methyl bromide (CASRN 74-83-9) is a candidate for being considered (by the EPA) as a drinking water contaminant (
http://www.epa.gov/safewater/ccl/ccl2.html) and having limits set for it, currently it isn't considered a contaminant -- hence there are no federal limits. (Although the issue may be more leftover bromine from the breakdown of methyl bromide than methyl bromide itself.)
Moreover, methyl bromide is soluble in water at a level many times greater than that required to produce the EPA limit for bromate, which is 10 µg/L. (I've found a range of figures for its solubility on water: 13.4 grams/L and 16.1 grams/L at 25C for example.) However, supposedly it evaporates quickly from water (www.inchem.org/documents/sids/sids/methbrom.pdf -- if you can believe this).
But this doesn't mean that bromine (as ions, in other compounds) from broken-down (not evaporated; it apparently photodegrades for one path) methyl-bromide couldn't exist at levels (and stay dissolved long enough) in water sufficient to create 10µg/L (and above) of bromate.