Drillers are not required to disclose the chemicals they use in their fracking fluids due to an exemption under the federal Safe Water Drinking Act.

In 2004, EPA reported that the process "poses little or no threat" to underground sources of drinking and "does not justify additional study at this time," a finding supported by the oil and gas industry.

But with a shale gas boom underway nationwide and water safety in doubt, federal regulation may be needed, some observers say.

Already, shale drilling has increased from 1 percent of U.S. gas production in 2000 to 10 percent today, according to a report by consulting firm PFC Energy. A recent MIT study estimates there's enough natural gas to supply the entire nation for 90 years.

Much of that would have to be fracked out of shale deposits located in over 20 states. 

Rep. Henry Waxman (D-Ca.), chair of the House Energy and Commerce Committee, and Rep. Edward Markey (D-Mass.), chair of the Subcommittee on Energy and Environment, are now demanding that oil and gas companies disclose the types of fluids being injected into wells.

EPA is holding a public hearing process as part of a $1.9 million peer-review study it will conduct in 2011 to investigate the impact of fracking on water quality.

See also:

Shale Gas Booming Globally, Despite Chemical Dangers

Gas Drilling Ban in NY Still a Possibility in 2010

Congressional Members Intensify Hydraulic Fracking Probe 

Natural Gas Boom Not Worth Costs and Risks, Study Warns

Cities Push Back as EPA Begins Study of Fracking's Impact on Water

Congress Considers Fracking Regulation Amid Hodgepodge of State Drilling Rules

Exxon CEO Defends Fracking on Capitol Hill

'Fracking' Accidents Prompt Calls for Oversight