In the last 150 years, prospectors and energy companies have drilled as many as 12 million holes across the United States in search of oil and gas. Many of those holes were plugged after they dried up. But hundreds of thousands were simply abandoned and forgotten, often leaving no records of their existence.

 

Government reports have warned for decades that abandoned wells can provide pathways for oil, gas or brine-laden water to contaminate groundwater supplies or to travel up to the surface. Abandoned wells have polluted the drinking water source for Fort Knox, Ky., and leaked oil into water wells in Ohio and Michigan. Similar problems have occurred in Texas, New York, Colorado and other states where drilling has occurred.

 

In 2008, gas from an abandoned well leaked into a septic system in Pennsylvania and exploded when someone tried to light a candle in a bathroom, killing the person, according to a 2009 draft report by the state's Department of Environmental Protection. That report also documented at least two dozen other cases of gas seeping from old wells, including three where the drilling of new wells "communicated" with old wells, leaking gas into water supplies and forcing the evacuation of a home.

 

In February, methane from an old well made its way into the basement of a house in West Mifflin, Pa., triggering a small explosion. Two families were evacuated and have not yet returned home.

 

Such incidents rarely receive much attention outside the states and neighborhoods they affect. But as the nation's latest drilling boom continues, abandoned wells have begun attracting more attention, particularly in states where the earth is already pock-marked with holes left by earlier waves of extraction. New wells sometimes disturb layers of rock and dirt near fragile old wells, leading to new cases of contamination.

 

The most recent effort to count the nation's unplugged wells was a survey published in 2008 by the Interstate Oil and Gas Compact Commission, a multistate agency made up of regulators and industry representatives. It found that states had located nearly 60,000 wells that needed to be plugged — and estimated that as many as a million more may be out there. In Pennsylvania alone, regulators estimate that 184,000 wells were drilled before records were kept. Many of those wells were plugged with stumps, rocks or nothing at all.

 

"The fact that there are thousands of these out there that need to be addressed, it's a problem, and it's a problem common to all states" with a history of drilling, said Bradley Field, who heads New York's Division of Mineral Resources.

 

The task of finding, plugging and monitoring old wells is daunting to cash-strapped state governments. A shallow well in good condition can sometimes be plugged with cement for a few thousand dollars. But costs typically run into the tens of thousands, and a price tag of $100,000 or more isn't unusual.

 

In the last decade, New York has managed to plug only about 125 of its estimated 40,000 deteriorating wells. It has taken Kentucky more than two decades to plug about 4,000 wells — and it has a waiting list of almost 13,000 more. Even Texas, which has invested heavily in abandoned wells, is years away from plugging all of its open holes. Since 1984, it has plugged more than 30,000 wells. But almost 10,000 are still open, and more are found and added to the list all the time.

 

Some regulators fear that the number of abandoned wells will grow when the current drilling boom runs its course. Last year, oil and gas operators drilled almost 45,000 new wells across the United States, and that number is expected to hold steady or increase as the nation tries to wean itself from foreign oil. If even a small fraction of those wells is eventually abandoned, states will be left with the bill, just as they were when the last boom ended in the mid-1980s.

 

To prevent that from happening, states require energy companies to post bonds before they begin building their wells. But the bonds are often so low that it can be more economical for a company to forfeit its bond rather than plug its wells. In Pennsylvania, for instance, an energy company can cover hundreds of wells with a single $25,000 bond.

 

John Hanger, who until January headed Pennsylvania's Department of Environmental Protection, called the bonds "scandalously low."

 

"There are some choices you shouldn't put in front of even good companies," Hanger said. "I'd like to think the companies would do the right thing, but we know that just isn't always the case."

 

One of Pennsylvania's worst cases of gas migration occurred in the Borough of Versailles, a small, working-class community just outside Pittsburgh. From 1919 through 1921, more than 175 gas wells were drilled in the town. Residents put wells in their backyards to heat their homes, packing them into the 25-by-100-foot lots.

 

The boom dried up when most of the wells proved unproductive. But in the 1960s, pockets of gas began leaking into homes. Some houses were condemned and demolished, and Versailles eventually became a case study for federal scientists trying to locate old wells.

 

Researchers studied old maps and walked the grounds with magnetometers, which detect the magnetic field from metal casings in the wells. If casings were never installed or had been removed, they could test the soil for hydrocarbons that might be leaking to the surface.

 

Some of the old wells were plugged. But more often vents were installed to direct gas away from the homes. Today, dozens of pipes pop out of the ground in yards, behind garages and through houses, slowly leaking methane and hydrogen sulfide so the explosive gases don't accumulate. In 2009, Versailles received a $368,600 federal grant to maintain its aging vents. About 50 methane alarms have also been installed in the town.