PHILADELPHIA - The boom in shale natural gas drilling has raised hopes the United States will be able to rely on the cleaner-burning fuel to meet future energy needs. But concerns about its impact on water quality could slow the industry's ability to tap this bountiful resource.

New York City urged a ban on natural gas drilling in its watersheds on Wednesday.

Some questions and answers:

WHY ARE ENVIRONMENTALISTS CONCERNED ABOUT SHALE GAS DRILLING?

Critics of the U.S. boom in shale gas drilling say the practice contaminates the aquifers where many rural residents get their domestic water supplies; pollutes the air around gas rigs and compressor stations, and scars the landscape with drilling pads and new roads.

The natural gas industry says the drilling technique known as hydraulic fracturing, or fracking, is entirely safe, citing research that has yet to prove any link between fracking and water contamination that could cause illness.

WHAT'S THE PROBLEM WITH WATER SUPPLIES?

Fracking chemicals are escaping into groundwater, critics say, and in several states there have been reports of fouled water and increased illness since drilling began. In addition, naturally occurring toxic substances such as arsenic are released from underground by fracturing and have been found at elevated levels near some drilling operations.

There are more than 200 introduced chemicals used in fracturing but details of how they are used are not published by energy companies. They are not required to disclose it because of an exemption to a federal clean water law granted to the oil and gas industry in 2005. That exemption has made it hard for critics to prove their case.

Drilling chemicals may cause many illnesses including cancer, fertility problems and neurological disorders, critics say.

HAS ANYONE ACTUALLY FOUND TOXIC CHEMICALS IN WATER WELLS NEAR GAS DRILLING?

Yes. The U.S. Environmental Protection Agency found 14 contaminants of concern in 11 private wells in the central Wyoming farming community of Pavillion, an area with about 250 gas wells. The August report did not identify the source of the contamination but is conducting more tests and is expected to reach a conclusion by spring 2010. In Pennsylvania, at least two privately conducted water tests near gas drilling have also found chemical contamination. One set of tests is being used in a lawsuit by a landowner against the gas company.

HOW DOES THE INDUSTRY RESPOND TO THESE CLAIMS?

Companies argue that the fracturing chemicals are heavily diluted, and are injected through layers of steel and concrete into the shale a mile or more underground and thousands of feet below aquifers, so they cannot mingle with drinking water. Industry officials say there has never been a documented case of water contamination from gas drilling. Some fracturing chemicals are also used in household products, which may explain their presence in water tests, energy companies say.

WHAT'S THE EXPERIENCE OF PEOPLE WHO LIVE NEAR GAS DRILLING?

Residents complain of water that is discolored, foul-smelling, bad-tasting, and in some cases even black. Some say drinking it causes sickness and bathing in it causes skin rashes. In a few cases, water has become flammable because methane has migrated from the drilling operations to water wells, a fact that has been confirmed by regulators in Pennsylvania. Many low-income people who live near gas rigs drink bottled water, and some have their water supplied by the gas company.

IS THERE A PROBLEM WITH WASTE WATER?

Yes. Around a third of the millions of gallons of water used in fracturing comes back to the surface where it is either reused or trucked to treatment plants. In Pennsylvania, where the industry is rushing to exploit the massive Marcellus Shale formation, critics say there isn't enough capacity to remove toxic chemicals from waste water. As a result, some waste gets pumped into rivers and creeks with little or no treatment, critics say. Some residents have accused tank trucks of dumping waste water on rural roads.

(Reporting by Jon Hurdle; Editing by Daniel Trotta)

Pennsylvania Energy