Natural gas is often hailed as the most promising energy source to feed America’s power-hungry future: cleaner than coal and oil, and free of the fears surrounding nuclear power. And sitting atop one of the largest gas reserves in the world, Maryland is one of several Chesapeake Bay region states that stand to profit handsomely.

But the process of drilling for the “clean fuel” is now embattled, as the Maryland General Assembly recently sought to do what no other state in the region has done. Before a single well has been drilled, it moved to ban the practice, boldly stepping into the center of a heated conflict.

In a vote that reflects growing national concern over the practice known as hydraulic fracturing, state lawmakers in the House on Wednesday passed a bill that would essentially place a moratorium on drilling until the Maryland Department of the Environment completes a two-year study to determine whether it endangers drinking water and public health, as some environmentalists in nearby states that allow drilling charge.

“We’re not going to be like other states that drilled first and asked questions later,” said Maryland Del. Heather R. Mizeur (D-Montgomery), who drafted and sponsored the legislation. “We understand that second chances are expensive, so we should slow down and take the time to do this right the first time.”

The gas has been entombed for about 380 million years in a thick layer of rock called the Marcellus Shale, which covers 95,000 square miles from Ohio to Virginia. But the industry just recently discovered an economical way to get at it: a combination of horizontal drilling and hydraulic fracturing, or fracking, using blasts of water mixed with chemicals to fracture the shale and release the gas.

It’s a difficult and expensive way to get at the hydrocarbons, but the Marcellus Shale formation is now thought to hold as much as 500 trillion cubic feet of natural gas — with about 50 trillion cubic feet already recoverable using current technologies. Drilling could potentially bring in billions of dollars in tax revenue and jobs, along with lease payments and gas royalties from companies to property owners.

But environmentalists say fracking is a dirty business, and some government officials have listened.

The state of New York imposed a moratorium on new drilling permits in December after environmentalists raised concerns about the threat to drinking water. Drilling has launched an economic boom in Pennsylvania, but energy companies there have been hit with numerous citations for environmental violations and lawsuits from residents claiming that drilling fouled their water.

And U.S. Interior Secretary Ken Salazar said this month that he is considering federal regulations to ensure that the drilling does not damage waterways. Speaking earlier this month to the House Natural Resources Committee, Salazar said: “We are going to have a huge backlash . . . from the American public if we continue to inject chemicals and fluids into the ground without people knowing what it is that’s being injected.”

A hydraulic fracturing well is drilled vertically for up to 5,000 feet, then horizontally for about a mile, according to experts. Up to 5 million gallons of water mixed with undisclosed amounts of chemicals, including benzene, a carcinogen, and radium, a radioactive element, is fired down the well.

The hole is surrounded by concrete to keep chemicals from seeping into groundwater, and industry leaders insist there is no proof that it has. But a portion of the chemicals come back to the surface in wastewater.

The proposal passed in Maryland last week would delay the plans of two companies, Chief Oil and Gas and Samson Resources, which obtained leases worth millions of dollars from property owners to drill in Garrett County, in the state’s far west.

Under the proposed law, Maryland would levy a tax on the companies of $10 per leased acre to pay for its study, estimated to cost $1 million by its completion in August 2013. The money would help the state Department of the Environment, which lost staff to budget cuts, hire workers to conduct the study.

A companion bill in the state Senate is expected to face a tougher road to passage as gas industry lobbyists work against it. Senators are scheduled to take up the issue before the legislature adjourns April 11.

Robert M. Summers, acting state environment secretary, testified in committee on behalf of Democratic Gov. Martin O’Malley’s administration, supporting the House bill with the condition that it include a study, paid for by the industry, and allow his department to grant a permit if a company can prove beyond a shadow of a doubt that its operation would not adversely impact public health and the environment.

“Natural gas . . . is a better source of energy than coal or oil, and it is right here,” Summers said in an interview. “We have throughout the history of the country taken advantage of our resources,” but without “sometimes preventing the environmental damage that could be caused.”

Virginia officials are not as reluctant. The state Department of Mines, Minerals and Energy gave a favorable review to the permit of Carrizo Oil and Gas, allowing the company to seek a special-use permit for an exploratory well in Rockingham County. But county supervisors tabled the request indefinitely over concerns about its environmental impact.

Natural gas has potential benefits beyond being a cleaner burning fuel. A Pennsylvania State University study said gas exploration created 29,000 jobs and added $240 million to state and local tax coffers in 2008.

Revenue was expected to grow the following year, producing an economic output of nearly $4 billion, yielding $400 million in state and local taxes and creating more than 48,000 jobs. The value of additional state and local taxes from gas between 2009 and 2020 would top $12 billion, the study said.

As Maryland lawmakers argued before the 98 to 40 vote to pass the bill, Del. Michael D. Smigiel Sr.(R-Cecil), blasted the proposal, saying it would economically “handicap Western Maryland.” Smigiel said the bill was part of a “war on rural Maryland.”

Lee Fuller, vice president of government relations for the Independent Petroleum Association of America, said politics, not science, drove support for the bill.

“You don’t have to freeze the process for some unlimited amount of time,” Fuller said, noting that Pennsylvania allowed drilling and addressed environmental concerns as they arose. “They have an enormous amount of growth up there, but they’re still doing things to protect the environment.”

But Maryland lawmakers listened to environmentalists and residents complain in hearings that Pennsylvania did not do enough. Pennsylvania facilities were not prepared at first to treat the volume of contaminated wastewater, they said, and some of it was trucked to water treatment facilities outside the state and released into waterways.

The Pennsylvania Land Trust Association compiled a list of violations by Marcellus Shale drilling companies, relying on state records. According to the association, there were 91 violations of the state’s Clean Streams Law, 155 violations for discharge of industrial waste onto the ground and into waters, and 212 faulty pollution-prevention practices.

The Environmental Protection Agency and the New York Environmental Protection Agency are each engaged in environmental impact studies on hydraulic fracturing. The New York study is due within months.

There’s also the issue of where the enormous amounts of water needed for hydraulic fracturing is drawn.

“There’s big questions about where the water comes from to even do this kind of drilling,” Mizeur said. “Is it coming from local water supply or being trucked in from somewhere?”

At a state House Environmental Matters Committee hearing, a group called Clean Water Action estimated that 2 million gallons at a single well requires “549 tanker truck trips.” The Pennsylvania Department of Environmental Protection estimated that a horizontal well requires 1,000 trips by heavy trucks, often on small roads, the group said.

The truck traffic was a lightning rod for complaints at the hearing in Maryland.

Nadine Grabania, co-owner of Deep Creek Cellars winery in Garrett County, said the traffic at a well proposed by Chief Oil would hurt her business and eco-tourism. Marilyn Moors said she bought a house on 100 acres without knowing the previous owner sold the gas rights and now feels trapped.

“I will be drilled under regardless of my desires and won’t receive a penny for it,” Moors said. “Those of us who do not stand to gain anything from the . . . drilling should not be asked to bear any of the costs, in terms of decreased property value or . . . environmental damage.”

Staff writer Ann E. Marimow contributed to this report.