In this part of the country, the potentially enormous natural gas play of the Marcellus Shale has many residents lining up to lease their land to gas prospectors. Estimates vary on the precise size of the Marcellus Shale, which stretches from West Virginia across much of Pennsylvania and eastern Ohio and into the Southern Tier of New York. But by any estimate, the gas deposit is huge  perhaps as much as 500 trillion cubic feet. (New York State uses a little over 1.1 trillion cubic feet of natural gas each year.)

An industry-financed study published this week suggested that as much as $6 billion in government revenue and up to 280,000 jobs could be at stake in the Marcellus Shale region.

Fracking has been around for decades, and it is an increasingly prominent tool in the effort to unlock previously unreachable gas reserves. The oil and gas industry estimates that 90 percent of the more than 450,000 operating gas wells in the United States rely on hydraulic fracturing.

Roughly 99.5 percent of the fluids typically used in fracking, the industry says, are just water and sand, with trace amounts of chemical thickeners, lubricants and other compounds added to help the process along. The cocktail is injected thousands of feet below the water table and, the industry argues, can’t possibly be responsible for growing complaints of spoiled streams and wells. But critics say that the relationship between fracking fluids and groundwater contamination has never been thoroughly studied  and that proving a link has been made more difficult by oil and gas companies that have jealously guarded as trade secrets the exact chemical ingredients used at each well.

Image Terry Greenwood, a farmer in Daisytown, Pa., at a hearing with the Environmental Protection Agency. Credit... Jeff Swensen for The New York Times

Several other concerns linger over fracking, as well as other aspects of gas drilling  including the design and integrity of well casings and the transport and potential spilling of chemicals and the millions of gallons of water required for just one fracking job.

The recent string of accidents in the oil and gas industries  including the gulf spill and a blowout last month at a gas field in Clearfield County, Pa., that spewed gas and wastewater for 16 hours  has unnerved residents and regulators.