[Colonel Hamish de Bretton Gordon, former commanding officer of the UK Chemical, Biological, Radiological and Nuclear Regiment (CBRN)] told the Telegraph that Peshmerga commanders have intelligence that Isil has rigged a chemical plant at Misraq with explosives. The plant lies just 25 miles south of Mosul and six miles north of where the US troops are at Qayyarah.
An explosion at Misraq, which holds thousands of tonnes of sulphur dioxide and hydrogen sulphide, could be catastrophic.
The good news (for friendly forces if not local residents, of course) is we could avoid such a threat by going around the plant until the cloud dissipates without wrecking the offensive to liberate Mosul.
I don't know how we'd continue the offensive if ISIL manages to set off a "water bomb."