Even before a catastrophic well failure turned Aliso Canyon into an international greenhouse-gas pariah, the natural gas storage facility was one of the industry’s worst climate polluters. Carbon dioxide flowed from its heavy machinery, and methane seeped from valves and equipment.

Only two other underground fields in the United States discharged more greenhouse gases in 2014, according to an analysis of the most recent U.S. Environmental Protection Agency data.

As Porter Ranch residents prepare to return home and regulators tighten well-safety rules, the long-term environmental costs of storing pressurized gas remain.

• NEW STORY: How we did the math on greenhouse gas emissions for the storage facility near Porter Ranch

The facility in the hills above the San Fernando Valley emitted 206,268 combined tons of natural gas, carbon dioxide and other pollutants in 2014, roughly the same amount as 43,000 cars driven for a year.

“What it tells me is they are not doing all they can to control emissions during normal operation,” said Don Gamiles, an independent expert monitoring air quality in conjunction with Los Angeles city officials.

• GRAPHIC: Top 5 polluting underground natural gas storage facilities

Aliso Canyon happens to be the fifth-largest underground storage facility in the nation, providing gas to 20.5 million customers, so large emissions might be expected. Indeed, when adjusted for size, Aliso’s emissions fall below the top 10. Large facilities, though, don’t always emit more than small ones. Larger fields in Texas and West Virginia emitted substantially less than Aliso Canyon, according to an analysis by the Los Angeles News Group and inewsource.

• MAP: Greenhouse gas emissions at underground storage facilities in the US

That Aliso Canyon was a major source of pollution on any given day, even without the massive leak, rings true with some Porter Ranch residents.

“That’s confirmation of all that smell already coming prior to this,” said Marco Maldonado, 37. “How come they didn’t notify the public?”

Residents have been complaining of nosebleeds and other health problems for years, Maldonado said. Wracked by health concerns, he and his wife are wary, with their first child due in April.

“We’re kind of scared of the uncertainty, and my wife is very stressed,” he said, “as to how we’re going to bring our child into this, because we don’t trust them any more.”

The Southern California Gas Co., a subsidiary of San Diego-based Sempra Energy, declined to comment for this story. But it is making improvements. Three aging natural gas turbine-powered compressors are being replaced with 22,000 horsepower electric compressors, a project that is expected to reduce carbon dioxide.

Impacts near and far

The emissions at such sites matter for several reasons. They are made up partly of methane or natural gas, the same gas used for heating and gas stoves, so leaks mean waste. Methane also is an especially potent climate change gas. It absorbs heat in the atmosphere and holds it close to the earth, where it affects oceans, land temperatures and weather.

“Twenty-five percent of the warming our planet experiences right now is due to methane, and the largest industrial source of methane is oil and gas,” said Mark Brownstein, vice president of the Climate and Energy Program at the Environmental Defense Fund, which has sponsored more than a dozen studies of methane emissions in the oil and gas sector.

Climate-warming gases aren’t the only reasons for concern. High emissions from a storage site could also indicate that people living nearby are being exposed to other, ride-along contaminants such as benzene or toluene, said David Lyon, another greenhouse gas expert with the defense fund.

At “a facility like Aliso Canyon, if it has higher than average carbon dioxide emissions,” Lyon said, “it is possible that there are a lot of emissions of toxic air pollutants from the compressors as well.”

Sources of emissions

Picture the labyrinthine pipes and heavy machinery needed to pressurize and inject gas more than a mile underground. These compressor stations pollute from two main sources: Carbon dioxide pours from internal combustion engines — similar to those in a car — that drive the compressors. Then methane gas escapes or is deliberately vented as it wends through the system.

Topping the list

While the SoCalGas Aliso Canyon field ranks third for total emissions, one Houston-based company owns several storage fields in the top five, either in terms of absolute pollution or when adjusted for reservoir size.

Boardwalk Pipeline Partners, LP (NYSE: BWP) controls Hattiesburg Gas Storage and Petal Compressor, a tiny reservoir that in 2014 emitted the second highest total amount in the nation. It also owns ranking facilities in Louisiana and Texas.

• GRAPHIC: Top 5 polluting underground natural gas storage facilities in the United States

Boardwalk said the emissions from its compressor stations are typical for the industry. “The facilities you inquired about are manned 24-7 by qualified operators,” said Molly Ladd Whitaker, director of corporate communications. “We operate our facilities safely and reliably, in compliance with all applicable laws.”

Controlling the flow

Over the past 20 years, many companies have made sharp reductions in pollution by replacing leaky components, said R. Subramanian, a researcher with Carnegie Mellon University who has measured emissions at gas storage sites.

Natural gas has been so abundant at these underground fields that it has been used instead of air to simply maintain pressure in some equipment, Subramanian said. Many companies have now switched these so-called pneumatic devices to electric compressed air.

SoCalGas expected to spend more than $23 million over three years to maintain, replace and upgrade aging and obsolete compressors at its four underground storage facilities in Southern California, according to a regulatory filing. The company would need state approval for any customer rate increases, otherwise costs could come out of shareholders’ pockets.

Operators, though, can be reluctant to fix equipment if it results in downtime and cuts into profits, Subramanian said.

“It is a question of whether the company wants to spend that money,” he said.

Scientists also are working on new technology to detect leaks. Today, gas companies might hire a specialist to search for leaks using cameras, while in the future ongoing monitoring could spot leaks automatically, said Lyon of the Environmental Defense Fund.

Room for oversight

Since the uncontrolled methane flow at Aliso Canyon began, many have called for greater oversight and cleaner operations — both to prevent catastrophic leaks underground and the slower, ongoing equipment emissions.

In mid-January, both U.S. senators from California called for a legal analysis of whether federal agencies were making full use of their authority to regulate gas storage fields. Congressman Brad Sherman, D-Sherman Oaks, also said he has been in discussions with the Pipeline and Hazardous Materials Safety Administration, a unit of the Department of Transportation that has come in for particularly pointed questions.

“What we need is strong nationwide regulations,” Sherman said.

Earlier this month, federal regulators issued a government advisory “to remind all owners and operators” to look for leaks and corrosion, fix them and make sure pressures on equipment don’t exceed design pressure.

The American Petroleum Institute, in response, noted that it had already issued its own best practice guidelines aimed at storage operators in September 2015, before the Aliso Canyon well failure.

The undercount

While the EPA reports of pollution from gas storage areas show there are serious emissions from dozens of facilities, they don’t come close to counting all the pollution, say scientists who have made systematic measurements at many sites.

“It’s a huge underestimate,” Lyon said.

Studies indicate climate emissions from storage areas may be 260 percent of those reported to the federal government. Here are some of the holes in the government data:

• The data do not include most of the unexpected leaks. For instance, the Aliso Canyon leak will not be included in Southern California Gas Co.’s EPA filings next year. It’s not required. The forms on which companies report their emissions don’t even have a place for giant, uncontrolled leaks.

• The data do not include the methane leaking from pressurized equipment if it is in standby mode.

• The data only include emissions from storage operators who have more than 25,000 tons of carbon dioxide equivalent emissions per year. Yet the agency uses an outdated factor that undercounts carbon dioxide equivalency.

Coming home

These past months have been a trial for Aliso Canyon’s neighbors in the San Fernando Valley. About 4,500 families had to leave their homes, waiting for the failed well to stop flowing. As they return, some will be more attuned to the site’s everyday emissions.

“I think the residents of Porter Ranch, including myself, are going to be concerned about the slow leak(s), having seen the effect of the big leak,” Sherman said. “It’s always been a good idea to eliminate leaks at Aliso Canyon. Now, with the whole community concerned, it becomes a real imperative.”

Mike Reicher is a reporter with the Los Angeles News Group. Ingrid Lobet works for inewsource, an investigative journalism nonprofit that is partnering with the Los Angeles News Group to cover the methane well breach.