By Robert Casey and Diana DeGette

While our nation copes with the disaster in the Gulf of Mexico and the multiple failures of offshore drilling regulation that led to it, another potential fossil-fuel crisis lurks onshore.

Hydraulic fracturing, also known as "fracking," injects tens of thousands of gallons of water, sand, and chemicals at high pressure into underground rock formations to release natural gas. The injected fracking fluids are known to include a variety of harmful chemicals, such as diesel fuel, benzene, methanol, and formaldehyde. Even low concentrations of these chemicals can have severe health and environmental consequences, and they are being used near drinking water supplies.

Fracking is increasingly widespread - for example, in the Marcellus Shale, a rock formation under Pennsylvania and New York that's flush with natural gas. The shale is also under a third of the Delaware River watershed, which supplies drinking water for more than 15 million people in Philadelphia, New York City, and other communities. Contamination of this watershed would be disastrous.

Alarming reports are emerging around the country of people becoming ill after fracking operations began in their communities. There have been anecdotes of drinking water that causes burning in the mouth and chest and a nurse who nearly died after treating a worker splashed with fracking fluid.

Unfortunately, though, authorities and residents investigating such incidents are rarely able to find out what chemicals drillers are using, making it impossible to say for certain what caused the contamination. This has also frustrated doctors attempting to determine what medical treatments are appropriate.

Oil and gas companies assure us their projects are safe, but they refuse to back up their assertions by disclosing the chemicals in their fracking fluids. Meanwhile, state disclosure requirements are limited. As a result, the anecdotal evidence of contamination cannot be confirmed and addressed.

As we have seen in the Gulf, poor oversight of energy exploration puts our environment and health in jeopardy. Mandating disclosure of the chemicals used in fracking is a critical step toward protecting communities and ensuring that onshore drilling is safe.

Some in the industry have said disclosure would reveal proprietary data. On the contrary, just as Coca-Cola must disclose a list of ingredients but not the details of its secret formula, our proposal would ask energy companies to share the ingredients of their fracking fluids but not the specific recipes.

Others have opposed federal regulation on the grounds that state regulations are sufficient. However, only a handful of states have fracking-fluid disclosure requirements, and some don't make the data public.

Many in the industry are beginning to see the wisdom of more openness. Last month, Range Resources said it would begin voluntarily disclosing the chemicals used in its Marcellus drilling. CEO John Pinkerton said, "It's the right thing to do morally and ethically, but it's also right for our shareholders."

One company is a good start, but every community where fracking is going on deserves similar information. We're sponsoring legislation moving through the House and Senate that would require disclosure of fracking chemicals and regulate the process to ensure it's not endangering drinking water.

The potential benefits of domestic natural-gas supplies are significant. They provide a path toward U.S. energy independence and can serve as a "bridge fuel" while alternative energy sources are developed.

But as gas drilling expands, we have a responsibility to make sure it's not harming the public. As we have seen over the past three months of unfolding devastation in the Gulf, the price of energy exploration without reasonable regulation is far too high.

Robert Casey is a Democratic U.S. senator from Pennsylvania. Diana DeGette is a Democratic U.S. representative from Colorado. They can be reached via casey.senate.gov and degette.house.gov.