A century after oil barons scoured Texas for prime plots from which to extract black gold, another boom is underway: the plugging of thousands of abandoned oil wells. It’s an oil rush in reverse, spurred by the promise of federal money.

In 2021, President Biden signed the Bipartisan Infrastructure Law, which released $4.7 billion to states and federal agencies for plugging fallow oil and gas projects known as “orphan wells” if they lacked an owner.

“There has never been federal money made available to plug these wells,” said Adam Peltz, a lawyer with the Environmental Defense Fund, an advocacy group.

Each leaky well could pose a grave environmental danger to surrounding areas in the form of a methane plume or groundwater contamination. Yet closing a single orphan well can cost tens of thousands of dollars.

One federal agency that is beginning to resolve this problem is the National Park Service, which has started using the funding to build a four-member team of orphan-well detectives. Its mandate is to track down the dirtiest orphan wells on more than 84 million acres of federal lands the agency oversees and plug them — which had previously been a pipe dream.

In January, the service’s inaugural project began: to plug 10 wells spread throughout a labyrinth of bayou canals in Jean Lafitte National Historical Park and Preserve in southern Louisiana.

The work is expensive. Forrest Smith, a petroleum and environmental engineer at the agency, estimates that each well in this park will cost about $100,000 to close. With $9.8 million in funding for current projects — pulled from the billions allocated broadly to state governments and federal agencies — and millions more on the way, his team is eyeing several dozen more wells across the country for closure. It’s the first dent in a list of about 2,000 wells on the federal lands under the stewardship of the park service.

The grand tally of U.S. wells that have been abandoned or that do not have an owner is propagating like an algae bloom. In 2018, the Interstate Oil and Gas Compact Commission recorded just over 60,000 orphan wells nationwide. By 2021, that number had surpassed 130,000.

And that’s probably nowhere near the true total. Between the patchwork of databases, the inconsistent quality of records and the varying definitions for categorizing wells, even ballpark numbers are difficult to establish. The interstate commission estimates there may be 800,000 undocumented orphan wells. “There’s still this huge uncertainty,” said Mary Kang, a professor at McGill University who monitors the efforts to quantify orphan wells.

The park service’s tally of 2,000 wells was not easy to come by. In 2019, agency officials were aware of only 500 or so. After reviewing databases not included in the agency’s count, Mr. Smith discovered an additional 1,500.

“My supervisor’s eyes got really big,” Mr. Smith recalled. “He was like, ‘Are you joking?’”

Beyond confirming a well’s existence, databases typically provide little additional information. That’s when the detective work begins. Clues may emerge in the shape of battery tanks or drilling pads captured with satellite imaging. For one mysterious well in the Hoh Rain Forest on the northwestern tip of Washington state, a random post on an online hiking forum provided the team with a specific location.

Next, an inspector ventures out with a metal detector, a gas sniffer and a list of questions: Where is the well, exactly? Is it plugged? Is it causing environmental damage? Could a hiker bump into it?

With any luck, the agency will find an owner able to afford the costs of plugging a well. But often, that’s not the case.

Federal lands supervised by the National Park Service are a daunting showcase of American ecological diversity encompassing deserts, swamps, mountains and forests, and orphaned oil wells can be found all over them.

“The actual sites are more remote in public lands and national parks, which poses challenges,” said Winnie Stachelberg, the infrastructure coordinator at the Interior Department. That may be an understatement.

One orphan well scheduled to be plugged rests on an arid mesa in Glen Canyon National Recreational Area in Utah.

“The nearest road is over eight miles away,” said Cheyney Clopton, an inspector with the agency. The job may require a helicopter, but the obscurity of the site makes even finding a pilot difficult. “That might be my biggest challenge so far, actually,” she said.

A well-plugging expedition would not be considered a casual jaunt. Lugging oil rigs, construction equipment and bivouac gear into the wilderness to plug a well involves logistical acrobatics, a feat that Mr. Smith described as “90 percent planning, 10 percent execution.”

Those in charge of the work at Jean Lafitte in Louisiana described how piping from some of the park’s orphaned wells poked up near the waterline, threatening boats. And because their operators had abandoned them, the wells might not be properly sealed, carrying the risk of releasing toxins into the water.

“Just kills everything in its path,” said Mr. Peltz of the Environmental Defense Fund. “Salting-the-earth style.”

A platoon of contractors, along with state and park representatives, has been traversing the park’s swamps via barge. On site, the workers decapitate exposed piping and pump cement into well pipes, sealing them.

Depending on the location of other wells, technological odd couples form to tackle a single mission. For example, plugging a well site in Guadalupe Mountains National Park in Texas will entail an agency airdrop of big equipment, like drilling machinery and skid steers, while workers and lighter gear will reach the site by mule train. Because the only road to the well was washed away five decades ago, it will need to be repaired before any well work can happen.

Once those tasks are completed, a single well will be plugged. As the number of abandoned wells continues to climb, even billions of dollars may seem inadequate for the project. But just three years ago, this environmental issue was so peripheral that few were even measuring it.

“Would I have liked it to have been a $47-billion stimulus instead of a $4.7 billion stimulus? Sure,” Mr. Peltz said. “Is it amazing that there was $5 billion? Yes, because that’s a whole lot more than zero.”