A week after the ruptured natural gas well in Aliso Canyon was finally declared sealed, we have a full account of the environmental damage — and it doesn’t look good. A new paper published in the journal Science declared it to be one of the largest environmental disasters in US history. In total, 97,100 metric tons of methane were released into the atmosphere over the course of 112 days, equal to the greenhouse gas emissions of over half a million cars.

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The study drew its measurements from a series of research aircraft flights carried out between November 7, 2015, about two weeks after the leak was first reported, and February 13, 2016, two days after the leak was plugged. Scientific aviation specialist Stephen Conley of the University of California, Davis, was asked by the California Energy Commission to conduct the first flyover early on, before anyone realized just how serious the gas leak really was.

The levels of airborne methane were so high that the Conley’s team assumed there was something wrong with their instruments — the background concentration of methane in the air is typically about 2 parts per million, but the aircraft registered concentrations as high as 50-60 ppm. It wasn’t until researchers tested levels on the ground using a different instrument that they realized the frightening readings were accurate.

Related: BREAKING: Massive Los Angeles methane gas leak declared permanently sealed

Once it became clear just how bad the problem was, Conley began conducting regular flyovers to track the movement of the methane plume as it moved downwind. At its peak in November, about 60 metric tons of methane was gushing from the well every single hour. SoCalGas was able to slow the rate a bit by drawing natural gas from the well to relieve pressure, and delivering it to energy consumers.

Though the well has been declared “permanently sealed” by a plug of cement, small amounts of gas are continuing to seep out of the ground. It’s unclear exactly where the methane is coming from — it could be due to gas trapped in the soil, the wellhead, or even potentially an imperfect seal. Conley will have to continue monitoring the site until the flow rate stabilizes. Despite the small amount of leakage, the 5,000 households that were evacuated from nearby Porter Ranch should still be safe to return home.

Related: Abandoned oil and gas wells are leaking methane across the USA

Perhaps the most worrying finding is this: the Aliso Canyon leak is unlikely to be an isolated incident.  The leak in question began due to an old, 1950s-era pipe that had never been upgraded to incorporate a safety valve. It was an entirely avoidable failure, and one that could happen again — the Aliso Canyon facility alone contains 115 other wells which could potentially fail, and there are around 400 other underground natural gas storage facilities across the US, many of which also contain old, deteriorating pipes. If we want to prevent another disastrous gas leak like the one that happened in California, the federal government is going to have to step up and start regulating the industry.

Via Gizmodo

Images via Earthworks and Cal OES