The threat of natural gas drilling has made the Upper Delaware River the most endangered waterway in the nation, according to a national advocacy group.

The nonprofit American Rivers plans to announce that dubious distinction - its 25th in as many years - Wednesday at noon at a City Hall news conference.

The Upper Delaware - the stretch from Upstate New York along the Pennsylvania border and south to the New Jersey line - is believed to be an exceptionally rich area for drilling, and drilling companies have already scrambled to snap up thousands of leases from homeowners.

The group, which was founded in 1973 and claims more than 65,000 members, awards the annual designation to highlight a river system facing a major decision in the coming year.

The industry, however, disputed the dangers, saying the technology is well-established and safe if done correctly and will provide a cheap, clean fuel in the future.

The decision was not a difficult one, said Rivers' Andrew Fahlund, senior vice president of conservation.

Nominations had been solicited, and of the dozens that came in, the Upper Delaware's was "the most compelling because of the importance of the river to 17 million people, and the enormous importance of the recreation economy that's built up around a healthy, free-flowing river," Fahlund said. The Lower Delaware also includes Philadelphia's water intakes.

He added that "the nature of the threat is tremendous."

The industry views the Marcellus natural gas deposits as some of the world's richest, with the potential to add billions of dollars to the state's economy, and to the state's tax coffers.

Natural gas is seen as a cleaner alternative to petroleum because it has fewer emissions, and proponents say it could be an important "bridge fuel" to renewable energy.

But to fracture the rock that contains the natural gas reserves, companies have to drill deep and inject the formation with water - at least several million gallons per well - and other materials, including toxic chemicals.

Much of the mixture remains underground. Groups are concerned about the potential for it to migrate back to surface water supplies. The industry says this has never happened.

There have been other environmental problems, however. So far this year, the state Department of Environmental Protection has initiated 137 enforcement actions against Marcellus well drillers.

In a high-profile incident in Dimock, Susquehanna County, 14 water wells were contaminated, but not by frack water. One well exploded from leaking methane gas.

Environmental groups also say there is currently no way to adequately treat all the wastewater that comes back to the surface.

The industry is exploring alternatives and improvements and vigorously defends the technology.

"We've been relying on hydraulic fracturing technology to access those resources for more than a half century now," said Kathryn Klaber, president and executive director of Marcellus Shale Coalition, an industry lobbying group, in a prepared statement. "Those are the facts, and this organization, ideology-driven though it may be, does a disservice by choosing not to provide a full accounting of those facts."

Responding to the industry's assurances, Fahlund said: "Let's see. Who recently said that everything is safe? We've heard assurances from the oil and gas companies, and I think the American public has grown a little impatient with those assurances and rightly has to question them at this moment in time."

Maya van Rossum, the Delaware Riverkeeper, said that the Delaware River constituted "the longest stretch of anti-degradation waters in the nation," and that billions of dollars had been spent to clean it up.

"We still have lots of challenges to face," she said, adding that with "the advent of gas drilling in the Upper Delaware River watershed, we face the possibility of losing everything."

Since 2005, the state has issued 3,682 Marcellus Shale permits. Of those, 1,526 wells have been drilled. Environmental groups have asked for a statewide moratorium on drilling until new regulations now being developed are finished, but Pennsylvania has not done so.

Recently, the Delaware River Basin Commission, which has some oversight of the industry because the Delaware is designated a federal "special protection" area, instituted a moratorium in the Delaware watershed until a comprehensive environmental assessment can be done and regulations passed.

Officials said this process could take a year.

Meanwhile, people have signed more than 6,000 lease agreements with gas companies just in northeastern Pennsylvania's Wayne County.

American Rivers wants the commission to continue the moratorium. It also is urging Congress to pass legislation that would close a loophole exempting the natural gas industry from a provision of the Safe Drinking Water Act.

Fahlund said the point of listing endangered waterways was to raise awareness. He said the nation's rivers had seen "amazing improvement" in recent decades. But they now face new challenges, including the impact of natural gas extraction, suburban sprawl, and new contaminants.

Also on this year's list is the Monongahela River in Western Pennsylvania and West Virginia, also because of natural gas extraction.

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com