It was news to Jeff Parsek that state records show there is an abandoned oil and gas well beneath his driveway. Parsek lives in a large ranch house across the street from an elementary school in a subdivision on the south side of Fort Collins.

When Parsek bought the house in 2004, he didn’t ask about oil and gas wells on the property.

“I wouldn’t have even thought to research that,” he said. But the news that there might be one didn’t faze him. While other homeowners responded to the search for potentially lost wells with curses and slammed doors – one yelled, “Thanks for ruining my afternoon!” – Parsek shrugged.

“If it started to emit something, then I might (be worried),” he said. “But to this point I’m not concerned.”

The question is whether one would know an abandoned well is emitting. When a well stops producing commercial quantities of oil and gas, companies “abandon” it, usually by placing cement plugs inside the wellbore to stop the flow of gas and fluids. The industry considers that the end of the life of a well.

“It’s not rocket science to plug these wells,” said Wyoming Oil and Gas supervisor Mark Watson. “It’s a hole in the ground that’s pretty deep. You set cement, and cement lasts a long time.”

The conventional wisdom that a well is dead once it is plugged means there’s no systematic monitoring for leaks of any of the millions of abandoned wells in the United States. Colorado has more than 35,000 abandoned wells. There are more than 50,000 in Wyoming.

“When a state sees a well is plugged, they typically put a check mark by that well in a database, or in a file somewhere, and they don’t do anything (else),” said Rob Jackson, a scientist at Stanford University in California.

Unless a well leaks fluids or a house blows up, the assumption is that everything is fine. But recent work by Jackson’s research group challenges that idea.

Methane is the main component of natural gas, and it is present in every oil and gas well. It is also a potent greenhouse gas that’s potentially explosive when it accumulates in confined spaces.

In Pennsylvania and California, Jackson and his colleagues have tested abandoned wells, both plugged and unplugged, for methane leaks. In most cases, the wells were leaking extremely small quantities of methane, but a few wells were leaking a lot – several orders of magnitude more. And some of the “super emitters” had been plugged.

Even those super leaky wells are emitting far less methane than the massive Porter Ranch leak in Southern California, but they add up. In Pennsylvania, one of Jackson’s colleagues, engineer Mary Kang, estimated that abandoned wells account for 4 to 7 percent of the state’s total man-made methane emissions.

Other studies also show that plugged wells can be leaky. The province of Alberta, the energy center of Canada, requires companies to monitor some abandoned wells. They’ve found that, on average, 7.7 percent of wells leak.

Whether those results translate to Colorado and Wyoming is hard to say, since no one is looking for leaks. Jackson thinks it’s partly an issue of states not having the resources to monitor.

“But I also think the states aren’t that interested in some cases, in many cases, in the data. I’m not sure they really want to know,” he said.

Even if they did, it’s not clear they could find the wells.

Once a well is plugged, it is cut off below-ground and covered up with dirt. Sometimes, a dead well has a marker listing its unique identifying number, but often there’s no indication at all on the surface that there was once a hole many thousands of feet deep, tapping reservoirs of oil and gas.

Inside Energy discovered as much on a recent afternoon when searching for abandoned oil and gas wells around Fort Collins. Despite having coordinates downloaded from the state oil and gas database and multiple GPS devices, finding the wells proved complicated. At one site, the supposed GPS coordinates of the well were directly underneath a giant tree.

At other sites, the wells were supposedly under roads or driveways or houses. None of them were marked, so it’s possible the wells were there, or it’s possible they were somewhere else.

In areas of the Front Range where houses and schools and shopping centers are encroaching on old oil fields, it might pose a problem.

“From a public safety perspective, even a slow leak into a building can create an accumulation in the building and then cause an explosion hazard,” said Theresa Watson, an engineering consultant and former energy regulator in Alberta.

Watson is quick to point out that there is a small risk of an abandoned well causing an explosion, but it does happen. Houses built on old wells have exploded as recently as 2007 in Trinidad and 2011 in Bradford, Pa.

“I wouldn’t move because of it I don’t think,” Watson said. “But what I would do is I would probably put a methane monitor in my house.”

Which brings us back to Jeff Parsek. Fort Collins officials acknowledge that they don’t know where the well that’s supposedly under his driveway is located. It could be where the database shows it is, or it could be under his house, or under the elementary school across the street. The well was drilled and abandoned in 1982. But it wasn’t until 2005 that Colorado started requiring precise GPS locations for active wells.

One way to prevent that kind of uncertainty is to make developers locate wells before building, which is what Watson recommended for Alberta when she was a regulator. Today, the province has a mandatory 15-foot “setback” that keeps buildings away from old wells.

Similar rules are lacking in much of the U.S., even as development continues to encroach on old oil fields. Watson, Wyoming’s oil and gas supervisor, has fielded inquiries this year from landowners in the city of Gillette and even from the Campbell County government about building on top of abandoned wells.

“We suggested to them, ‘Hey, it’s probably not a good idea to put your building on top of an abandoned well,’” he said.

But neither the city nor the county has any prohibition on it. The same is true for most municipalities in the region. The city and county of Broomfield is one exception.

“We would require that the developer or the property owner dedicate an easement over the well site, the former well site,” said Anna Bertanzetti, Broomfield’s principal planner. “It has to be at least 50 feet wide by 100 feet long and have access to public roadway.”

The idea is to leave room so that if the well ever leaks and needs to be fixed, a rig can access it. In addition to the dedicated easement, Broomfield requires property owners within 200 feet of the well be notified.

In 2013, Fort Collins adopted rules requiring a 350-foot buffer between new housing development and old wells. Longmont and Weld County also have buffers of 150 feet and 25 feet, respectively. But none of the rules address a development that’s already happened on top of abandoned wells. The patchwork of regulations can mean neighbors are governed by different standards.