Officials from the U.S. Environmental Protection Agency announced on Tuesday that a massive cleanup of New Bedford’s harbor is nearing completion after nearly 40 years of work and close to $1 billion in expenses.

The bed of the Acushnet River that forms the harbor between New Bedford and Fairhaven has been polluted with dangerous concentrations of polychlorinated biphenyls, better known as PCBs, for nearly a century, according to EPA officials. PCBs are suspected to cause cancer and harm the health of people’s brains and reproductive systems.

Dave Dickerson, an EPA official who has spent nearly 30 years working on the harbor cleanup, said the people most likely to have been exposed to the pollution are recreational fishermen who ate seafood caught in New Bedford’s harbor — a cruel twist of fate for residents of the nation’s highest-grossing fishing port, which hauls in most of its catch from fishing grounds far offshore.

Environmental regulators say most of the PCB pollution in the harbor came from just two corporations that manufactured electronic components between the 1930s and 1978, the year the federal government banned the production of PCBs.

Dickerson said a series of private settlements secured through litigation against the corporate parents of the Aerovox and Cornell Dubilier companies has covered roughly half the cost of the $1 billion cleanup. The rest has been shouldered by the nation’s taxpayers.

“We're here because mistakes, knowingly and unknowingly, were made,” Congressman Bill Keating of Massachusetts’ 9th District said.

Keating was joined at a news conference Tuesday by a delegation of politicians who announced the latest $4 million settlement from Cornell Dubilier, which supplements hundreds of millions of dollars the company and Aerovox have paid in previous settlements.

EPA officials said that money will be combined with $73 million in federal funds made available through the bipartisan infrastructure bill. The money is expected to carry the harbor cleanup to completion, closing the books on a project that began in 1983 when the EPA opened investigations into factories dumping black sludge containing PCBs directly into the harbor and the city’s sewer system.

Crews have since dredged the bed of the Acushnet River and hauled close to a million cubic yards of contaminated sediments into facilities built along the shore to process the material. The sediment has either been carted away from the city on a rail spur extended to the waterfront for the project, or buried beneath the river in underwater cells capped with clean sediment.

During the next three years, EPA officials said crews will dredge the tidal marshes along the river’s edge and replace them with cleaner sediment before replanting them.

Once complete, the 40-year cleanup of New Bedford’s harbor will be the most expensive of any maritime environmental cleanup in the history of the United States’ Superfund program, which uses a mix of private and public money to address the nation’s most severe concentrations of pollution.

U.S. Senator Ed Markey said the project’s completion will place New Bedford on the cusp on a new era.

“Ultimately, it will have been close to a billion dollars which was spent to clean up this mess that had been left by a generation of industrialists who used the New Bedford harbor as a dumping ground rather than ensuring that all of the environment was protected,” Markey said.

The removal of contaminated sediment from the riverbed is expected to reduce the future costs of dredging the harbor’s main channel for fishing vessels and the larger boats expected to use the port during the construction of the nation’s first offshore wind farms.

“We can now make PCB stand for pristine, clean bay, with a clean and healthy food chain and marine ecosystem throughout the Acushnet River and Buzzards Bay,” Markey said.

But some scientists more familiar with the lasting impacts of the Acushnet River’s pollution say that day may never come.

Wendy Heiger-Bernays, a professor at Boston University who studied exposures and health risks among the roughly 100,000 people living along the harbor’s edges, said the PCB concentrations in New Bedford once reached levels unprecedented among American waterways.

Heiger-Bernays said the toxicants will continue accumulating among fish and other marine life in the river’s food chain long after the EPA formally concludes the removal of sediment along the harbor's bottom.

“Nobody should be eating the fish from this harbor,” Heiger-Bernays said. “It will remain too contaminated for that.”

Heiger-Bernays warned that the EPA must continue monitoring for PCBs in the air and should be monitoring the underwater cells closely to identify any potential release of toxic sediments, since climate change brings higher intensity storms that could affect the integrity of the cells.

“The harbor is so much cleaner than it was ever before,” Heiger-Bernays said. “It’s still not all done.”

Ben Berke is the South Coast Bureau Reporter for The Public’s Radio. He can be reached at Follow him on Twitter @BenBerke6.