Aliso Canyon was US' largest methane release

THE Aliso Canyon gas well blowout in southern California, US, released more than 100,000 t of methane before it was sealed on 11 February, the largest methane leak in US history.

Researchers from the University of California, Irvine (UCI) and the National Oceanic & Atmospheric Administration (NOAA) say in the first study published on the leak that the total emissions during the 112-day event were equivalent to a quarter of the annual methane pollution from all other sources in the Los Angeles Basin.

Donald Blake, atmospheric chemist at UCI, said: “The methane releases were extraordinarily high, the highest we've seen.”

The team set up air-quality sensing equipment around the Aliso Canyon site and in the surrounding areas of Porter Ranch – from where over 11,000 residents were evacuated – and the heavily populated area of the San Fernando Valley. They correlated the data with satellite images to establish the accuracy of their measurements.

The team’s measurements found “above normal” levels of potentially dangerous compounds present in natural gas, including benzene, toluene and xylenes. These compounds are volatile, and could have long-term human health effects.

The well operator, the Southern California Gas Company (SoCal), assured residents who chose to remain in the affected area during the gas leak that there were no long-term effects to human health from methane, and insisted any symptoms would be temporary.

The analysis showed the environmental impact of the leak was the equivalent to the emissions of 500,000 cars. At its peak, the blowout doubled the rate of methane emissions from the Los Angeles Basin and was temporarily the largest known human-caused source of methane in the US, approximately twice the emissions of the next largest source, a coal mine in Alabama.

The disaster has substantially impacted California’s ability to meet its greenhouse gas emissions targets this year. The researchers have highlighted the problem of unintended emissions from natural gas production, processing, pipelines and storage infrastructure in the US, and have called for stricter gas control efforts to be implemented by the federal government.

“Our results show how failures of natural gas infrastructure can significantly impact greenhouse gas control efforts,” said Thomas Ryerson, research chemist for the NOAA.

Science DOI: 10.1126/science.aaf2348