Name of toxic liquid product that can be cleared with flocking agent

I'm writing a paper rpg scenario, and even if we don't need to be 100% accurate about chemistry and physics in such games, one of my players has an advanced level in chemistry and I don't want him to cringe at the situation (and I can't ask him, obviously, or he'll be spoiled).

So, I need a river to be polluted with an invisible and odorless product, that slowly destroys the fauna. As it happens in Siberia, I thought about tritium first, but I don't think it's easy to clean. So I thought about a product that could be cleaned with a flocking agent. Problem is, when I was searching for "products reacting to flocking agent" on Google I didn't find any name of product that I could use.

So, which invisible, toxic and odorless product react to flocking agent to the point the river is cleaned in one or two months maximum (and eventually produced in the 1980's) ?

Thanks a lot, and sorry if it's not the SE to ask this question.

If your impurity is tritium, it can not be purified, whatever the method used. If your impurity is another substance, it can be purified by adding aluminum sulfate. Aluminum sulfate is slowly hydrolyzed in dilute solutions. And this phenomena produces a light and insoluble substance called aluminum hydroxide $$\ce{Al(OH)3}$$. The particles of this substance look a bit like snow flakes and have the property of fixing impurities on its surface, between its branches. So after a couple of hours, this aluminum hydroxide will be deposited on the ground of the river or the lake, with the impurities fixed inside its particles.