Disclosure requirements vary considerably from state to state, as ProPublica recently charted. In many cases, the rules have been limited by a "trade secrets" provision under which companies can claim that a proprietary chemical doesn't have to be disclosed to regulators or the public.
One of the key controversies about fracking is the chemical makeup of the fluid that is pumped deep into the ground to break apart rock and release natural gas. Some companies have been reluctant to disclose what's in their fracking fluid. Scientists and environmental advocates argue that, without knowing its precise composition, they can't thoroughly investigate complaints of contamination.