It's likely at least a brownfield, if not a future superfund site. It probably hasn't been submitted for superfund status because that would put it under very strict "do not touch" guidelines. I bet the current thought is to get someone to buy it and treat it as a brownfield, clean it up, and redevelop it instead. That's prolly why it isn't a superfund, but the sheer cost of cleaning up 400 acres would preclude a lot of development, including the Bears stadium