The polar blast in Texas earlier this year revealed a dirty secret in the most prolific U.S. oil field: Two under-the-radar natural gas plants that are a persistent source of pollution.

Natural disasters in the state often turn into environmental disasters, and February’s cold wave was no exception. Stricken by power outages and mechanical failures, industrial facilities burned off or released huge quantities of hazardous gases as they shut down. The worst culprits, however, weren’t the vast petrochemical complexes on the Gulf Coast but the two Permian Basin facilities that take raw gas from wells and purify it into sales-quality fuel.

The Wildcat and Sand Hills plants, both run by Houston-based Targa Resources Corp., accounted for almost 20% of the state’s total pollution during the freeze and nearly four times the amount emitted by the country’s biggest refinery, according to an analysis of state records that was carried out by Air Alliance Houston, Environment Texas and Environmental Defense Fund and reviewed by Bloomberg.

The winter storm wasn’t a one-off: The two plants have released hazardous gases above permitted levels more than 400 times since the beginning of 2019, according to filings with the Texas Commission on Environmental Quality. That’s equivalent to about once every two days. Targa didn’t respond to multiple requests for comment.

That two mid-sized processing plants can become super emitters underscores how the vast U.S. gas network is only as strong as its weakest link. Environmental goals adopted by producers and refiners count for little if the facilities holding the system together are vulnerable to breakdowns. The crisis triggered by the freeze has spurred notoriously regulation-averse Texas into action, with state lawmakers weighing bills that may require gas plants and pipelines to be winterized.

“When you have hundreds of emission events a year, is it really an upset or is it just usual operating conditions?” said James Doty, a former TCEQ mobile monitoring manager. “You would obviously expect more emissions from the biggest oil and gas facilities in the world than you would from these types of plants.”

State filings likely don’t even capture the scope of the pollution released during the storm. Companies are required to report hazardous compounds like benzene, hydrogen sulfide and sulfur dioxide but not greenhouse gases such as carbon dioxide and methane.

Targa suffered major outages as a result the storm, with its operations at about half capacity during the 10-day period, Chief Executive Officers Matthew Meloy told investors on a Feb. 23 earnings call. He declined to provide any more detail and didn’t specify whether the problems were caused by the cold, power outages or both.

One reason for the high volume of emissions from gas processing plants is that the facilities are in short supply in the Permian, where supplies of the fuel have swelled as a byproduct of drilling for more-valuable oil in the past decade. As a result, the plants operate under extreme pressure and one mechanical fault can cause outages that lead to flaring, or burning off gas that has nowhere to go.