The Merrimack Valley, the area north of Boston that was shaken by dozens of natural gas explosions Thursday, is served by some of the nation's oldest and most leak-prone pipes.
Investigators have yet to determine what caused the gas explosions that burned at least 39 homes in the towns of Lawrence, Andover and North Andover. But the gas utility that serves the area has more miles of old, cast-iron gas mains than all but 15 utilities in the nation, according to a USA TODAY analysis of federal safety data.
In its most recent filing with federal pipeline safety regulators, Columbia Gas reported 471 miles of cast-iron and wrought-iron gas distribution lines – the kind of old gas pipe that the government and safety experts have been pushing utilities to replace for more than a decade.
Fire investigators and state emergency management officials said Thursday night that the devastating chain of fires might have been caused by over-pressurization in the gas lines. Federal, state and local investigations continue today. Columbia Gas, which serves 300,000 customers in the Merrimack Valley, has not commented on the potential cause.
One man was killed and at least 10 other people were injured in the fires.
Whatever the cause, the area's bare-metal gas mains have been a well-documented safety concern for government regulators and the gas company itself for years, federal and state records show.
USA TODAY Investigation: Danger lurks underground from aging gas pipes
The old cast-iron pipes prevalent in the Boston-area systems – some installed well more than 50 years ago – are susceptible to rust and corrosion, which can lead to leaks and explosions.
The old pipes have been blamed by fire investigators and the National Transportation Safety Board in many deadly gas explosion disasters in recent years.
The Pipeline and Hazardous Materials Safety Administration has been pushing gas utilities for more than a decade to replace aging pipes with more resilient materials such as plastic, but it's not required by law.
The gas industry has replaced thousands of miles of pipe, but a daunting amount of work remains. It can cost $1 million per mile or more to replace aging pipe. The costs are typically passed on to customers.
The vast majority of the nation's 1,000-plus gas utilities use very little or none of the most vulnerable cast-iron mains. An investigation by USA TODAY in 2014 found that the largest share of the old pipe is concentrated in heavily populated areas, where the risk of catastrophic consequences in greater.
More than 80 percent of the cast-iron mains are concentrated in 10 states, mostly in the Northeast, according to a USA TODAY review of the most recent federal safety data, from 2017. About a third of the old mains are buried in and around just three metro areas: New York, Boston and Detroit.
In the Boston area, Boston Gas in the metro area and Columbia Gas in the Merrimack Valley combine to use more than 2,200 miles of old iron mains. That's more iron gas pipe than is still in use in 45 other states combined.
In its latest update to its gas line improvement plan with Massachusetts regulators, Columbia Gas reported this spring that its cast iron and bare-steel gas distribution pipes make up about 15 percent of its distribution network. That’s more than twice the average that USA TODAY calculated for natural gas utilities nationwide.
In that Gas System Enhancement Plan petition, the gas company said it had replaced about 50 miles of “leak prone” gas lines in 2017. The company said it had plans to replace another 500 miles of the aging, leak-prone gas mains – but the work would take until 2033.
In April, Columbia Gas of Massachusetts applied to the state’s Department of Utilities for a rate hike it said would increase monthly bills for a typical residential customer by 2.3 percent during winter and 11 percent during summer.
The company said it needed the hike to fund “a multi-year effort to continuously improve the company’s organizational and operational processes consistent with the evolving regulatory landscape focused on more stringent enforcement of pipeline-safety regulations to assure the integrity of the distribution system.”
The application is winding through the regulatory review process.
Columbia Gas of Massachusetts reported at least 700 known leaks in its system in 2017, including hundreds it deemed hazardous. More than 200 of those were attributed to corrosion, failed welds and other problems with the pipes.
Federal data show cast-iron mains historically have been involved in a disproportionate share of significant gas leaks. About 2 percent of gas mains are made of cast iron, but they were implicated in about 11 percent of the most severe leaks from distribution pipes from 2005 to 2017 – and 41 percent of the fatalities.
A cast-iron gas main installed in 1927 was blamed for a Brooklyn apartment building explosion in January that injured four people.
In 2016, a gas pipe installed in Shreveport, Louisiana, in 1911 leaked under pressure, killing one person.
In 2015, a cast-iron pipe installed in the Detroit area in 1923 cracked almost in two; the resulting leak killed one person.
And in one of the deadliest incidents in recent years, eight people were killed and 48 were injured when a gas explosion leveled part of a New York City block in East Harlem in 2014. The iron gas main servicing the buildings was installed in 1887.
Twenty states have rid their gas systems of the cast-iron and wrought-iron pipes altogether, but the ancient infrastructure still dominates systems in older parts of the country, most notably in the Midwest and Northeast.
More than half of the gas pipes under Massachusetts cities was installed before 1970. One out of every 4 miles of gas mains in the state was put in the ground before 1940.
Utilities in those communities face a more difficult task in replacing their aging mains than elsewhere. Pipes in crowded cities are harder to retrofit because the cost and disruption to everyday life is greater.
The aging pipes are a high-risk example of the nation's struggle to replace its crumbling infrastructure.
The danger remains hidden beneath the ground until a pipe fails or is struck by something and a spark ignites a monstrous blast.
Natural gas is piped into 67 million homes and at least 5 million businesses and schools across the country, with gas distribution and service lines snaking beneath most American neighborhoods.
In USA TODAY's 2014 investigation, data analysis showed that nine of the 10 utilities that use the most iron pipe far exceeded the average rate for hazardous gas leaks in 2013. Three utilities, serving the New York and Philadelphia regions, reported hazardous leak rates greater than 10 times the national average.
The federal Pipeline and Hazardous Materials Safety Administration has reported progress in replacing the old pipes in many states. The agency posts updated figures each year showing how much troublesome pipe remains in each utility's system, along with dire warnings about the need to remove and replace it all.
The agency enforces a federal law that requires the gas utilities to file Distribution Integrity Management Plans with state regulators, in which they outline the greatest threats to their networks and what they're doing to make sure gas leaks don't threaten people or property.
The agency doesn't have the authority to require replacement by a deadline.
Contributing: Kevin McCoy