The U.S. Environmental Protection Agency on Monday designated nearly four miles of the Neponset River as a Superfund site, a status that initiates a long-term response to clean up hazardous material in the river through Milton and the Boston neighborhoods of Hyde Park, Mattapan and Dorchester.

For decades, that lower stretch of the river has festered in pollution from elevated levels of polychlorinated biphenyls, a group of human-made chemicals that the EPA considers to be "probable human carcinogens" and have been shown to cause cancer and other serious health issues in animals.

EPA Regional Administrator David Cash described the designation as a “win-win” for local communities and the environment "because we now have a mechanism to address the contamination and the sediment that has plagued the river for decades." He made the announcement at a Mattapan park along the river, where he was joined by Sen. Elizabeth Warren, Rep. Ayanna Pressley and Boston Mayor Michelle Wu.

An investigation of the site is scheduled before the end of this year, and cleanup could begin in late 2023 or 2024, according to the EPA.

"Dangerous chemicals do not belong in our water,” Wu said. “They do not belong in our communities.”

In 2015, state environmental officials lobbied for the 3.7-mile stretch of the river to be included in the EPA’s National Priorities List. Three years later, a preliminary assessment was completed, and in 2019, the EPA inspected the site.

Pressley expressed gratitude for the grassroots efforts of advocacy groups, who fought for the review and removal of contamination in a river that runs through several low-income neighborhoods.

Roughly two-thirds of the nation’s Superfund sites are within one mile of public housing, “posing serious health and reproductive risks to Black, brown and low-income communities, communities like Dorchester, Hyde Park and Mattapan — that by no coincidence are home to many of our lowest-income Black and Brown siblings,” she said. "This is what we mean when we say environmental racism.”

A bipartisan Infrastructure law passed last year invests $3.5 billion in the Superfund Remedial Program.

“We know that to combat decades of environmental racism we must prioritize environmental justice, which is intrinsically linked to racial, health and economic justice,” Pressley added.