I think it is reasonable to assert that the health and survival of both human populations and the natural ecosystem is of critical importance to human society.

I think it is reasonable to assert that the health and survival of both human populations and the natural ecosystem is of critical importance to human society. Therefore, it seems easy to argue that making a water system uninhabitable and causing the collapse of communities or ecosystems should be prevented. Whether the pollution in question is in the form of toxic heavy metals, forever chemicals, etc., or just excessive farm runoff that causes algal blooms and kills marine life (or anything else that consumes the water), that seems like the line to draw. If you're making a water system incapable of sustaining life, then... don't.