Natural gas has been found in unusual concentrations in the ground near the site of an explosion that killed seven people in New York City last week, a National Transportation Safety Board (NTSB) official said.

Con Edison crews performed at least 50 tests between 18-24 inches deep in soil around the blast site, according to NTSB’s Robert Sumwalt.

“In at least five holes they found the presence of natural gas in the ground, and the gas concentration was up to 20% in five locations,” Sumwalt said during a press conference Friday. “It ranged from 5% to 20%.”

Con Edison and New York City officials have said there were no complaints about possible gas leaks from residents of the two buildings destroyed by the explosion and fire in the months preceding the incident, other than one called in just minutes before the blast (see Daily GPI, March 13). Con Edison received a call about a possible gas leak in an adjacent building at 9:13 a.m. EDT last Wednesday and dispatched a crew to investigate two minutes later. The explosion occurred at 9:31 a.m., before the utility’s employees arrived on the scene. That was the only call reporting a potential gas leak at the location going back nearly 10 months, officials said during a press conference Thursday. The city also had not received calls about gas leaks at the buildings at its emergency 911 and non-emergency 311 numbers in the days prior to the explosion.

At least seven people have been confirmed dead and dozens were treated for injuries following the explosion and collapse of two buildings in the East Harlem section of New York City. The explosion is believed to have been the result of a gas leak (see Daily GPI, March 12).

The kind of natural gas concentrations found near the demolished buildings are unusual on Manhattan island, Sumwalt said.

“It’s not like we’re talking about the Bakken Shale, where there’s gas inherently in the soil. Normally, the soil in New York City 18-24 inches down into the ground would have zero concentration of natural gas…that further leads to the hypothesis that this may well have been a natural gas leak. We still need to analyze that, but that’s giving us a pretty good indication,” he said.

Edward Foppiano, Con Ed senior vice president of gas operations, has said some of the pipes in the area were up to 126 years old, but none were considered to have deteriorated enough to have warranted inclusion in the utility’s next three-year pipe replacement program. Con Ed officials have said that the gas main serving the buildings required service only twice in the past 10 years. In 2011, Con Edison replaced about 70 feet of piping in conjunction with a water service replacement, and in 2004 the utility repaired a leak on a coupling on the main. And two surveys of the cast iron portion of the gas main last month identified no leaks, they said.

The 8-inch diameter cast iron pipe that served the two buildings was a half-inch thick when it was installed in 1887, and the copper service lines connecting the main to the buildings were installed in 1991, Sumwalt said.

“Over the next several days we will be conducting pressure testing of the lines. We will do at least two pressure tests on at least two areas of the pipeline. The testing will actually be done by Con Ed under the supervision of the NTSB. The purpose of the pressure testing is to determine if there is a leak in a particular line and, if there is, to determine the location of the leak.”