Most cyanide in surface water will form hydrogen cyanide and evaporate.
As long as it has a surface to evaporate, it will degenerate.
… Hydrogen cyanide is literally what has been used to execute people in gas chambers and genocide during the Holocaust. The LC(Lo), the lowest recorded lethal concentration is 107ppm, resulting in death in 10 minutes. That’s, objectively, far more dangerous than the respective material that firefighters were exposed to at Chernobyl. You don’t want that in any appreciable quantity in the air around people that you want to continue living.
Oh yeah, you could totally just leave it in a giant pool and ignore it. It’ll react, evaporate and eventually break down into cyanide again, rain down, subtly poison the area, react again, evaporate again, etc.
And that’s great for the owner of the big pool of cyanide, and very bad for everyone else. Stuff that evaporates doesn’t disappear, the cyanide doesn’t magically change into cookiedough. You’re just spreading it around more.
Hydrogen cyanide will turn into “cookie dough” in 1-5 years. Which is way shorter than “forever”.
The way you said it in your first comment made it seem longer lasting than radioactive waste. Which it isn’t according to the linked PDF. That is the only point I was trying to make.
