Material Safety Data Sheets are your friend.
They are stored right next to the HAZMAT level A suits locker.
LOL... I guess. I reviewed a couple after seeing your post. They seem rather ill-equipped to deal with a spill of this magnitude.
I have a circa 1985 booklet published by DuPont titled: "Sodium Cyanide: Properties, Uses, Storage and Safe Handling". In it, it says:
Mild oxidants, such as alkaline hypochlorite solutions, hydrogen peroxide solutions, and permanganate solutions can oxidize cyanide to sodium cyanate. The reactions find wide use in the control of cyanides in effluent. These oxidation reactions MUST BE in dilute solutions to avoid formation of highly toxic gases (Wee "Waste Disposal).
In the Waste Disposal sections, it states the following:
CAUTION: CONCENTRATED HYPOCHLORITE SHOULD NOT BE MIXED WITH CONCENTRATED CYANIDE SOLUTIONS SOLID CYANIDE BECAUSE HIGHLY TOXIC CYANOGEN CHLORIDE GAS WILL BE RELEASED
A similar warning is listed later regarding use of peroxides. I think either is acceptable for use in a controlled environment. Over a wide, varying contamination? I'm not so sure.
The best solution for them, I expect it... clean up as much dry stuff as possible before the rain... then, get people OUT of the immediate area, and downwind.... let mother nature do to work. Hopefully, they can contain much of the water. But, mother nature can win that battle if she wants.