[jeffreyg]

It completely depends on the type of chemical. Your scenario imagines something like radiation. Perhaps the chemical isn't incredibly toxic and can be flushed out. If someone spilled bleach you wouldn't be able to boil or filter it out but you could dilute it to where it's safe for consumption.

[maxerickson]

City of St Albans water, mentioned in the story as safe, appears do be drawn from the Kanawha, downstream of where the Elk River flows in.

So yeah, dilution appears to be the solution (really, the problem seems to be that one treatment plant drew in a large quantity of contaminated water).