A civil lawsuit. Litigation. They'd go to court if a plaintiff could prove damages and responsibility. If Monsanto dumps 400tons of eviljuice into the river and it kills off a farmer's entire crop that season, then Monsanto would be sued because it caused damage to somebody's property. Damage to life and property is obviously illegal.
Proving damages in most cases would be flat-out impossible. You're intentionally giving a case where it just happens to be clear-cut, but what about when you have hundreds of companies and facilities all contributing, in a general sense, to a worsening of environmental and health conditions? Who do you sue? How do you even pin down blame? What do you even blame them
for, exactly? You can't pick some guys out of a city and say "these plaintiffs all have lunger cancer, which
might be because of the undisclosed, unregulated chemicals the 427 defendant companies may or may not have been pumping into the soil and atmosphere, which may or may not heighten the statistical chance of such cancer". It just doesn't work that way. Litigation only works in cases where there is a
very clear-cut causal link between a problem and the actions of the perpetrating party. Usually, with issues like pollution, the link is there, but is indirect enough that you can't effectively point fingers and say "
this chemical by
this company definitely and directly caused
this problem". It's normally a hell of a lot more complicated than that.