A massive natural gas leak at a storage facility in Southern California was caused by microbial corrosion of well equipment, according to a new independent report from analysis firm Blade Energy Partners. The report blames the storage facility owner, Southern California Gas (SoCalGas) for failing to conduct follow-up inspections of equipment, despite knowing about 60 smaller leaks at the facility that had occurred since the 1970s.

The final leak—which spewed 109,000 metric tons of methane into the air over five months between 2015 and 2016—was the biggest methane leak in US history. (A larger loss of methane occurred in 2004 in Texas, but a corresponding fire immediately combusted the methane into carbon dioxide.) But the California leak at the Aliso Canyon Natural Gas Storage Field was particularly devastating because methane, unfortunately, is a far more potent greenhouse gas than carbon dioxide.

The new report (PDF) was commissioned three years earlier to find the root cause of the leak. According to a press release from the California Public Utilities Commission (CPUC), Blade Energy Partners found that the leak came from a seven-inch outer well casing which had corroded due to exposure to microbes from groundwater. The natural gas storage facility at Aliso Canyon is made up of dozens of vast underground caverns which were previously filled with oil before they were pumped and emptied decades ago. Since then, the caverns have been used to store natural gas to supply the Southern California area.

The indirect cause of the leak, according to Blade, was a failure on the part of SoCal Gas to conduct detailed inspections of the well equipment. "Blade identified more than 60 casing leaks at Aliso Canyon before the October 2015 incident going back to the 1970s, but no failure investigations were conducted by SoCalGas," the CPUC wrote. The company also failed to do any risk assessment focused on well failure.

In its own statement , SoCalGas said that the report confirmed that it violated no well or safety regulations that existed at the time. "The leak was an industry-changing event resulting in the development and implementation of enhanced safety regulations and practices," SoCalGas wrote. "Today, Aliso Canyon is safe to operate, and Blade's report indicates the industry-leading safety enhancements and new regulations put in place after the leak should prevent this type of incident from occurring again."

In 2018, CPUC mandated that natural gas storage facilities under its jurisdiction comply with well construction standards and undergo "mechanical integrity testing." Natural gas storage must also make use of "real-time pressure monitoring." The CPUC also dictates how withdrawals of gas can occur, to minimize risk of a blowout.

In February, a judge approved a $119.5 million settlement between SoCalGas and the California Air Resources Board related to the Aliso Canyon natural gas leak. A large portion of that money will go to build methane digesters at 12 dairy farms in California's San Joaquin Valley.

According to The New York Times, there are more than 36,000 people suing the company in civil court. Many residents of the nearby neighborhood of Porter Ranch were moved from their homes for five months as SoCalGas attempted to fix the leak, and some of those who didn't leave reported headaches and nosebleeds from the nearby leak.