Poisons dumped into the River or sunk in barreels in the River are the danger, not barrels floating down the River, for they may be retrieved and disposed of. The traces are more likely due to leakage from munitions dumped to the bottom of the River, and that will be a major problem to clean up once peace is established.
It's going to be a mess, no doubt. But time and nature, along with a concerted effort to retrieve the sunken barrels, will someday work.
There's a famous river near where I grew up that once "caught fire". When I was a kid the water looked like a swirl of day-glo paints and smelled like nail polish remover. Today, at least according to the EPA, you can eat the pike, bass, and blue gill that live in it.