Colorado today adopted the nation's toughest rule requiring oil and gas drillers to disclose all the chemicals used in the fracking fluids they pump down wells. "The level of detail required in this rule is much greater than other states require," said Mike Freeman, an attorney for the environmental law group Earthjustice. The Colorado Oil and Gas Commission unanimously adopted the rule after last-minute negotiations among environmental groups, industry and state regulators. Fracking fluid is pumped down a well under high pressure to fracture rock and release oil and gas. About 90 percent of the wells in the U.S. have been fracked, but the fluids and the process have become controversial over concerns that they threaten groundwater. Some states, such as New Mexico, only require that chemicals identified by the U.S. Occupational Safety and Health Administration as hazardous in the workplace, be listed. "We know that is about half the chemicals in fracking fluid, so we are doubling the number of chemicals" to be disclosed, said David Neslin, the commission's executive director. Other states, such as Wyoming and Arkansas, require all chemicals be listed, but do not require revealing the concentrations of the ingredients, Freeman said. The oil and gas industry had opposed the requirement in the Colorado rule calling for the disclosure of chemical concentrations.