In a paper released Thursday, a group of scientists published the results of 13 flyovers performed during the recent Aliso Canyon natural gas leak. They conclude that the well leak had effectively doubled the methane (CH 4 ) emission rate of the Los Angeles Basin.

The researchers, who hailed from Scientific Aviation, UC Davis, UC Irvine, CU Boulder, and the National Oceanic and Atmospheric Administration, also concluded that the natural gas leak was the second-biggest failure of its kind in US history. The biggest happened in 2004 in Moss Bluff, Texas , when an underground natural gas storage facility collapsed.

Depressingly, the researchers suggested that the environmental impact from the Aliso Canyon leak would be much more damaging than the Moss Bluff collapse because "an explosion and subsequent fire during the Moss Bluff release combusted most of the leaked CH 4 , immediately forming CO 2 .” Carbon dioxide sticks around in the atmosphere longer than methane does, but methane is a far more potent greenhouse gas in the short-term.

The Aliso Canyon leak started on October 23, 2015 when SS-25, one of the 115 underground storage wells at the facility, started leaking methane (it’s still unclear what initially caused the leak). The natural gas reservoirs were originally oil-producing wells erected in 1954 which were then repurposed after they had been emptied; they now store natural gas during periods of low demand.

After the leak was discovered, Southern California Gas Company, which is owned and operated by Sempra Utilities, tried seven times to stop the high-pressure natural gas from escaping into the atmosphere. The company then endeavored to drill a relief well approximately 8,500 ft down to the gas reservoir to pump slurry into the well and seal it with cement.

That operation took several months and was not completed until February 11. In the meantime, residents of the nearby Porter Ranch community complained of headaches and nosebleeds from the leaking methane, which is treated with chemicals to make the odorless and very flammable gas more identifiable. SoCal Gas was ordered to temporarily relocate residents at its own cost.

While health officials in the LA area assured residents that the methane leaking from the natural gas was not harmful to human health, a much smaller component of natural gas is benzene, which is known to cause cancer. As such, “population exposure to benzene from the Aliso Canyon leak has received particular attention,” the researchers wrote. But benzene was found at ratios of 0.001 percent or lower compared to the methane that was leaking. "Publicly available benzene data reported in near-daily 12-hour air samples were often below the 1 nanomole/mole (part-per-billion; ppb) detection limit of the contract laboratories used for the analyses,” the paper states.

In total, the researchers estimated that the Aliso Canyon leak spewed 97,100 metric tonnes of methane into the atmosphere at rates of up to 60 metric tonnes per hour. The paper’s authors said that these estimates could be refined in coming months using different sampling methods, but they noted that their "rapid-response airborne chemical sampling method” was useful to provide a quick, independent, and accurate measure of the leak rate. Essentially, their sampling method involved flying "a chemically-instrumented Mooney aircraft,” through areas downwind of the leak, as well as doing “whole-air sampling” on the ground. These frequent samples allowed the researchers to quantify the leak rate over time.

They found that about six weeks after the blowout of the well, the leak rate started to decrease as SoCal Gas tried to withdraw gas from the high-pressure reservoir to control the rate of the leak. By January, the leak rate leveled off when SoCal Gas stopped trying to withdraw gas, as it needed to maintain a minimum amount of pressure in the reservoir.

Throughout the leak plugging process, the researchers found that “exceptionally high concentrations" of methane and ethane settled into the nearby San Fernando Valley area. In fact, the methane output of the Aliso Canyon blowout, "temporarily created the largest known anthropogenic point source of CH 4 in the US, effectively doubling the leak rate of all other sources in the Los Angeles Basin combined,” the paper notes. "Further, at its peak, this leak rate exceeded that of the next largest point source in the US—an underground coal mine in Alabama—by over a factor of two and was a factor of 10 larger than the CH 4 leak rate reported from the Total Elgin rig blowout in the North Sea in 2012.”

"The Aliso Canyon CH 4 leak rates were comparable to total CH 4 emission rates of entire oil and gas production regions in the US,” including shale operations in Barnett, Texas; Haynesville, Louisianna; and Fayetteville, Arkansas, the paper said. The methane output overall was equivalent to the annual output of energy-sector methane from medium-sized nations in Europe.

Ultimately, the paper chided the storage facility for not having a backup plan to control leaking in case of a blowout: "Single-point failures of the natural gas infrastructure can hamper deliberate methane emission control strategies designed to mitigate climate change."

Science, 2016. DOI: 10.1126/science.aaf2348 (About DOIs).