The largest methane leak ever at a Southern California underground natural gas storage facility in 2015-16 was caused by preventable groundwater-induced microbial corrosion on a seven-inch storage well casing, a final root cause analysis has indicated.

According to a third-party state-commissioned investigation issued Friday, Southern California Gas Co. could have prevented the disaster at its 86 Bcf Aliso Canyon storage field, but it was not required historically to perform the extensive leak monitoring and maintenance that California now requires.

The Sempra Energy gas utility, the nation's largest, and the Division of Oil, Gas and Geothermal Resources (DOGGR) emphasized that the steps taken since the four-month-long Aliso leak addressed most of the recommendations from consultant Texas-based Blade Energy Partners.

SoCalGas was cited for not conducting any follow-up investigative work following more than 60 casing leaks in Aliso storage wells over a four-decade period back to 1979, Blade told the California Public Utilities Commission (CPUC) and DOGGR.

CPUC and DOGGR are still conducting parallel investigations and will incorporate Blade's work, which includes an indication that "updated well safety practices and regulations address most of the root causes identified."

A public meeting in which Blade will present its findings is to be scheduled soon, said CPUC spokesperson Terrie Prosper. The parallel probes by the two state agencies are focusing on "overall well and field operations," and they are expected to be completed later this year, using technical assistance from Lawrence Berkeley, Lawrence Livermore and Sandia National Laboratories.

SoCalGas officials said "Aliso Canyon today is safe to operate and Blade's report indicates the industry leading safety enhancements and regulations put in place will prevent this type of incident from occurring again."

"California has taken aggressive steps to prevent future gas leaks through stringent regulations for all [12] gas storage fields in the state," said Department of Conservation spokesperson Teresa Schilling. "These regulations put the emphasis on proactively preventing leaks, and they are the strongest protocols for gas storage in the United States."

Schilling listed nine areas covered by the new regulations, including well construction standards, real-time data-gathering systems, requirements for monitoring and inspections, and protocols for decommissioning of underground gas storage facilities.

Ultimately, the CPUC plans to make a determination on the long-term future of the Aliso storage facility, which covers 3,600 acres of a former oilfield that closed in the 1950s. The commission has an ongoing proceeding "to determine the feasibility of minimizing or eliminating the use of Aliso Canyon while still maintaining energy and electric reliability for the greater Los Angeles region."

Blade's report confirms "gross negligence" by SoCalGas in failing to conduct basic safety inspections. "Due to the company's neglect by putting profit over prudent inspections and maintenance, thousands of families were sickened, relocated and lost loved ones," said Food & Water Watch’s Alexandra Nagy, California director.