FAYETTEVILLE, N.C. — One humid day this summer, Brian Long, a senior executive at the chemical company Chemours, took a reporter on a tour of the Fayetteville Works factory.

Mr. Long showed off the plant’s new antipollution technologies, designed to stop a chemical called GenX from pouring into the Cape Fear River, escaping into the air and seeping into the ground water.

There was a new high-tech filtration system. And a new thermal oxidizer, which heats waste to 2,000 degrees. And an underground wall — still under construction — to keep the chemicals out of the river. And more.

“They’re not Band-Aids,” Mr. Long said. “They’re long-term, robust solutions.”

Yet weeks later, North Carolina officials announced that Chemours had exceeded limits on how much GenX its Fayetteville factory was emitting. This month, the state fined the company $300,000 for the violations — the second time this year the company has been penalized by the state’s environmental regulator.

GenX is part of a family of chemicals called per- and […]