“what substance would they be most likely to use?”
It does not have to be terribly toxic or even effective.
People will go nuts if there is evidence that 2 kg of Diazinon pesticide was dumped in. Naturally it would be so diluted that there will be more Diazinon washed in from lawns, but people will go nuts. Especially if the MSM is in on it and creates a panic.
“Well Bob, the latest tests results show almost 14 attograms per liter measured at the trunk line. Diazinon is a dangerous pesticide that can cause convulsions, death, and autism, plus it is deadly to pets and fish. Authorities say there is no threat, but an empty 2 kilogram container of Diazinon was found only a quarter mile from one of the water systems most vulnerable spots.”
Quite true D. But public fear and actual danger are two different things. Although neither is desirable, and both should be taken seriously.