For almost 20 years, US public-health advocates have worried that toxic chemicals are getting into ground water and harming human health because of an exemption to the federal Safe Drinking Water Act that allows operators of oil and gas fracking operations to use chemicals that would be regulated if used for any other purpose.

The so-called Halliburton Loophole, named after the oil and gas services company once headed by former Vice President Dick Cheney, means that the industry can use fracking fluid containing chemicals linked to negative health effects including kidney and liver disease, fertility impairment, and reduced sperm counts without being subject to regulation under the act.

While environmentalists and public-health campaigners have long called for closing the loophole, they haven’t known how many of the regulated chemicals are used by the industry, how often the industry reports their use in its fracking disclosures, what quantities of the chemicals are used, and how often the industry chooses not to identify its chemicals on the grounds that they are proprietary.