Am I allowed to oppose dumping raw mercury into rivers & streams, if I support freedom to travel by airplane?
You are allowed to dislike anything you want. What you do about it, however, needs to be consistent. If you want government to fight pollution, for example, you should support governmental efforts to fight all of it. If, instead, you prefer the problem be solved by boycotts and lawsuits by the people actually suffering from the ill-effects, then that too view should, also apply to all kinds of pollution.
That said, could you not have come up with a less contrived example? Raw mercury is too valuable for anybody to just dump it into a river...