More than one in five of the 15,000 active underground natural gas storage wells in the U.S. appear to be at risk for serious leaks due to obsolete well designs, according to a study by Harvard T.H. Chan School of Public Health researchers. The wells are similar in design to that of the Aliso Canyon gas storage facility in California that leaked for about four months in 2015-16 and is considered the largest single accidental release of greenhouse gases in U.S. history.

The study was published May 24 in Environmental Research Letters.

The study, led by Dr. Drew Michanowicz, postdoctoral research fellow in the Department of Environmental Health, and senior author Dr. Aaron Bernstein, associate director of the Center for Health and Global Environment at Harvard, identified more than 2,700 largely unregulated underground storage wells across 19 states with the same design as Aliso Canyon and that were not originally designed for gas storage. These repurposed production wells have a median age of 74 years, and some were constructed more than 100 years ago.

“Underground storage of natural gas is a critical part of the U.S. supply chain, and many portions of the country rely on this infrastructure for heating and increasingly for electricity generation. As Aliso Canyon and other incidents have demonstrated, the vulnerability of a single well presents a major risk to energy security, greenhouse gas emissions, and to the safety and health of people who live near it,” Dr. Michanowicz said in a press release.

The obsolete wells operate in 19 states across 160 underground natural gas storage facilities and encompass 51 percent of the total working gas capacity in the nation. The oldest of these wells are located in Ohio, Pennsylvania, New York, and West Virginia.

“Partly because no federal safety regulations apply to natural gas storage wells or their operations (now pending), very little aggregate information was available regarding these mid-stream systems. After we identified this data gap, we realized we needed to build our own database to begin to assess this previously inapparent hazard,” said study co-author Dr. Jonathan Buonocore, research associate in the Center for Health and Global Environment, in a press release.