Thousands of natural gas leaks reported by the state’s utility companies disappeared from the records they later submitted to the state, according to an independent analysis by a Cambridge group that says their study shows the leaks have likely not been repaired.
Nearly 5,700 potentially explosive, environmentally damaging leaks — nearly 30 percent of all leaks that went unrepaired in 2014 — vanished from the utilities’ reported count between the last day of that year and the first day of 2015, according to a study of data the companies disclosed in March, the most recent public information.
“This suggests that the utilities aren’t doing a good job tracking leaks,” said Audrey Schulman, president of the Home Energy Efficiency Team, or HEET, a Cambridge nonprofit that analyzed the data. “If they don’t have them on their books, they’re not monitoring them, and if they’re left unmonitored, leaks get worse.”
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Utility companies are spending billions of dollars to replace leaky gas pipelines across the state, and repair leaks as quickly as possible, company officials say. The leaks, which are responsible for a significant portion of the state’s greenhouse gases, are often caused by corroding cast-iron pipes or construction accidents.
The utilities cited various reasons for how leaks could have disappeared from their records without being repaired. Some leaks may no longer be emitting gas, while others may have migrated to punctures elsewhere in the pipes, they said. Some may have been temporarily plugged by snowbanks or concealed by weather that makes them hard to detect.
National Grid, the region’s largest provider of natural gas, reported 11,343 unrepaired leaks at the end of 2014 to the state Department of Public Utilities, but just 8,349 at the beginning of 2015.
It was unclear how 26 percent of unrepaired leaks could have disappeared overnight.
“It is important to note that leaks are not static, and we do not monitor each leak 365 days a year,” said Danielle Williamson, a spokeswoman for National Grid.
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It’s possible that the initial detection of some leaks came from residual gas in soil near the spot of a previously fixed leak, she said.
“Where gas readings are found may not necessarily be the exact site of the leak,” she said.
In addition, some leaks could have been taken care of by replacing gas mains, and as a result were never marked as repaired. Others may have been duplicates. There could have also been “a clerical issue, in which missing or repaired leaks had not been removed from previous years,” Williamson said.
Utilities are legally required to immediately repair leaks that pose a risk of explosion. A state law passed in 2014 requires utilities to repair minor leaks on streets that are under construction, near a school zone, or around trees that appear to be dying. The utilities used to allow those leaks to persist indefinitely because they didn’t see them as imminent threats.
Under the 2014 law, utilities must disclose the location of every leak, the date they were reported, and when they were repaired. That provision allowed Schulman to map all the leaks in the state.
Schulman said she studied the locations of the unrepaired leaks and determined that three-quarters of them are almost certainly not duplicates, nor the result of repairs to larger mains.
Her group also conducted a random survey in eight municipalities from Boston to Acton of 55 unrepaired leaks that disappeared last year, and found that more than half were still active.
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“Leaks don’t self-heal or peter out,” Schulman said. “They come from holes in ancient metal corroded pipes sitting for decades in damp soil. They are only going to get steadily worse over time.”
Officials at Eversource, which supplies gas to nearly 300,000 customers in Massachusetts, said they’re fixing leaks nearly every day and are spending $53 million this year to replace 35 miles of leak-prone gas mains. The company plans to accelerate repairs over the next few years.
Delays in the internal reporting of leaks may account for some of the discrepancy, the company said.
“This is a dynamic list that changes,” said Mike Durand, a spokesman for Eversource. “We recheck Class 3 leaks [those considered unlikely to explode] every year. If we recheck it, and it’s not there, it’s off the list.”
Sheila Doiron, a spokeswoman for Columbia Gas in Westborough, which provides gas to 310,000 customers in Massachusetts, attributed some of the missing leaks to changing weather conditions. Gas can be easier to detect on humid days, she noted, and harder when it’s windy.
“Many of the very small, or nonhazardous leaks, can be faint, merely meeting the criteria,” she said. “If we go back the following year, and that leak can no longer be detected through monitoring devices, then that leak doesn’t exist anymore.”
The discrepancy has made it difficult to determine how many leaks remain unrepaired. According to the utilities, the number of unrepaired leaks fell 19 percent from the end of 2014 to the end of 2015. But counting from the first day of 2015, the number of leaks rose by more than 21 percent.
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“It makes me wonder if the infrastructure is just crumbling faster than they can repair it,” Schulman said.
Nathan Phillips, a professor of environmental sciences at Boston University who has surveyed gas leaks around the state, said he finds the number of missing leaks disturbing.
“It illustrates how little we know about the number of leaks in the state, and how much of the gas is lost and unaccounted for,” he said. “This discovery makes us less sure about how much progress we’re making.”
David Abel can be reached at dabel@globe.com. Follow him on Twitter @davabel.