(Above) The Neponset River. The photo was taken during a MassDEP investigation of possible sediment contamination along the river in 2008. (Below) A sign posted along the Neponset River during the same time period.

For years, pollution in the Neponset River has limited recreational access to the body of water.

While activities like boating are allowed on the river, which traces Boston’s southern border with Milton, officials warn against swimming and wading. Public health guidance discourages eating fish from the river, though catch-and-release fishing is permitted.

Now, the Environmental Protection Agency has announced its plan to clean up part of the Lower Neponset River.

The agency’s selection, in an official document called an “action memorandum,” is the most comprehensive of four official options that the EPA described over the summer in an effort to address contaminants in the Lower Neponset River, which was declared a Superfund site in 2022.

Under that designation, which marks it as a contaminated area without a responsible party to clean up the mess, the EPA was equipped to step in and work on clearing the river of years of industrial contamination.

The sediment in the river’s bed is polluted with polychlorinated biphenyls, or PCBs, man-made chemicals that were part of industrial production in the area from the late 1920s until 1979, when they were banned by U.S. law.

Exposure to PCBs, which bind to sediments in the river, can lead to a host of health concerns, including an increased risk of developing cancer.

The EPA has identified other potentially harmful contaminants in the river as well, which would also be targeted for removal.

Together, those contaminants present concerns for human health, as well as potential impacts to sensitive ecosystems in and around the river. A changing climate and aging infrastructure in the area leave officials worried that those pollutants could move around if not addressed.

In June, the EPA released its Engineering Evaluation and Cost Analysis, or EE/CA for the site, which covers the first mile-long stretch of the river from its confluence with Mother Brook, a smaller waterway in Dedham and Hyde Park that is part of the Neponset watershed, to the Tileston and Hollingsworth Dam between Hyde Park and Milton.

Later work will cover the second phase of the river — from the Tileston and Hollingsworth Dam to the Baker Dam in Lower Mills. Already, the EPA has done work to remove contaminated soil from Riverside Square and the former Lewis Chemical Facility.

The about-450-page EE/CA outlined three paths forward for cleaning up the river — as well as a fourth, officially required, “do-nothing” option for comparison.

In that report, the EPA identified the most intensive option, effectively, what is outlined in the action memorandum, as their preferred solution. In the EE/CA the EPA cited a level of permanence that yielded the best cost-effective strategy — other potential solutions likely would have required more cleanup work later, while this strategy is likely to get the bulk of the contamination out in one go.

Community members, too, voiced support for the most intensive solution.

In official comments submitted to the EPA, over 100 submissions expressed support for efforts to clean up the river. Many of those, according to the action memorandum, expressed specific support for the recommended removal action.

“It’s not as disturbing to the community if they’re going to only be on site in this one-mile stretch for a few years, but just for one period of time, as opposed to doing it for a couple years, leaving and then coming back to do more cleanup,” said Andres Ripley, Greenways program director at Neponset River Watershed Association. “And of course, they’re going to be cleaning up more contamination.”

Those community comments also highlighted concerns around air monitoring during the process — real-time air monitoring will be conducted during the clean up — as well as communication with residents and future public use of the river.

Under the plan outlined in the action memorandum, the EPA said it intends to remove contaminated sediment where PCBs occur at more than one part per million. The agency said it expects that process to require the removal and replacement of the top three feet of sediment in the riverbed, though they may go deeper if they find further contamination.

After removing the contaminated sediment, the EPA will “cap” the stretch of the river, a process which will involve adding a new layer of material over the riverbed’s sediment to prevent the movement of any remaining contaminants and to stabilize the river channel and provide habitat for organisms in the ecosystem.

The specific design of the capisn’t currently established — many of the specifics of the project are subject to a design phase that is expected to start in the next few months or early next year — but according to the memorandum, the design of the cap may vary across the river’s length based on specific need.

The proposed plan also includes the removal of the Tileston and Hollingsworth Dam, infrastructure from the 1960s, that has fallen into disrepair.

According to the action memorandum, contaminated sediment has built up against the dam. If the aging dam were to fail, that sediment — which has in part remained there but has been gradually leaking downstream — would surge down the river. The memorandum called that outcome a “potential uncontrolled and catastrophic release of hazardous substances” downriver.

For the EPA’s work, removing the dam, as well as the contaminated sediment, is expected to limit the risk of future pollution if the dam fails. But for the Neponset River Watershed Association, removal of the Tileston and Hollingsworth Dam also represents an exciting step that moves broader efforts around improving the health of the river.

Ripley said the group is generally in favor of removing dams where possible to provide better water quality and habitat restoration. For many at the watershed association, removing the Tileston and Hollingsworth has been an ongoing conversation for more than a decade.

Its removal would also be one step closer to allowing fish to migrate up and down the river again.

Historically, the waterway was a migratory pathway for diadromous fish — those that migrate between salt and freshwater — such as alewife, smelt and white perch. As dams were added along the river, often for industrial purposes, it prevented the fish from being able to make the journey.

Now, Ripley said, the Tileston and Hollingsworth Dam, plus the Baker Dam, are the only barriers to those migrations in the stretch of the river that lines the border between Boston and Milton, where the EPA’s work is focused.

The Baker Dam, too, has been the subject of conversation regarding removal, but in previous discussions, community members have been more hesitant to see it taken out — Ripley said concerns have generally centered on sentimental connections to the dam, which is a remnant of the Walter Baker Chocolate Factory that once operated there, as well as its more aesthetic appearance.

But Ripley said more conversation is needed and shifting attitudes, as well as new experiences as the river faces shifts due to climate change, may have shifted community willingness to see part or all of the dam removed. He called that step the next goal for the Neponset River Watershed Association.

“We’ve seen a lot more flooding, specifically regarding the people who live in the Baker Condos there, who, at one point were in favor of [keeping] it,” Ripley said. “There’s a different perspective on climate change and flooding than I think there was 20 years ago.”

For the EPA’s work on the river, the release of the action memorandum means working toward a concrete design for the clean-up work along the river, and starting the process of trying to get the work funded.

The work comes with a price tag of $78.6 million.

Through the Superfund process, potentially responsible parties — individuals or groups that might be liable for the cleanup — may pay for the work off the bat, or the EPA will front the money and pursue the money from those parties later through lawsuits.

Once the project is designed, the EPA said it expects the work to take three years and 10 months, with construction potentially starting in late 2026 or early 2027.

During that time, Ripley said the project’s Community Advisory Group, a collection of residents and advocates who provide feedback to the EPA, will continue to keep an eye on the process, especially around how it plans to tackle the capping efforts to make sure it stands up to climate change and fosters a healthy ecosystem.

The 2022 Superfund designation of the site was announced, it was heralded by neighbors and local river advocates as an important step forward in cleaning up the long-polluted river.

The release of the action memorandum represents more forward motion.