The MWRA's board voted unanimously last week on a new plan to upgrade the city’s sewer system. But advocates say the plan doesn't do enough to minimize sewage dumps into the river.

“From our perspective, it’s still not good enough,” said Emily Norton, the executive director of the Charles River Watershed Association (CRWA), a nonprofit dedicated to preserving the health of the river for the past 60 years, said in a phone call.

“They were looking at four different options. Why wouldn’t you do the most aggressive one? That would mean the least amount of sewage going into the river.”

Boston has a combined sewer system, in which raw sewage pipes overlap with stormwater drainage pipes that connect to the storm drains at street level. Normally, this causes little issue.

But if there is a rainstorm, the pipes are flooded with stormwater, and to prevent an overload, excess water is dumped from the system into the Charles. Because the storm drain pipes connect to the sewage system, that water is contaminated with untreated sewage. This is called a Combined Sewer Overflow (CSO). And under the Clean Water Act, the state must undertake upgrades to prevent these sewage dumps.

In October, the MWRA presented its first draft of the new plan, which would have cut sewage discharge into the river in half by 2050. But the water quality of the Charles would have been changed from temporarily allowing sewage overflows to permanently allowing them. After massive public outcry, led in large part by river advocate groups like the CRWA, the state delayed its vote until last week, when it presented a new proposal.

“ This one, they’re calling zero CSOs in a 2050 typical year,” Norton said. “That makes it seem like there will be no CSOs in 2050, and that’s really not the case. It’s engineerspeak for a modeled year of what the weather could be like around 2050, so it’s more like an average. Some years, even MWRA acknowledges, there would be CSOs, and some years there wouldn’t.”

The MRWA Board’s approved plan would complete infrastructure upgrades that control for up to three inches of precipitation within a 24-hour period. Precipitation over that limit would likely result in overflows. In 2025, Boston had at least one such storm, when a late-season May Nor’Easter brought 3.55 inches of rain.

The “most aggressive” version of the plan, in contrast, controlled for CSOs up to in cases of extreme weather events that have a likelihood of happening only once every 25 years.

“There still would've been CSOs in the largest of storms, but that would be pretty rare,” Norton said. “It’s something I think we could all live with.”

The MWRA has controlled up to a 25-year storm for other projects. In 2011, it constructed a storage tunnel in South Boston to prevent sewage from spilling into the Boston Harbor during storms, which controls up to a 25-year storm. But, the MWRA argued, the expense for doing the same for the river is not justifiable. The plan the board adopted would cost about $954 million; the 25-year storm plan would cost about $2.9 billion.

The CRWA published calculations last week using the authority’s estimates to show that the difference between the approved plan and the 25-year control plan is less than $4 in taxes per household per month in 2026 dollars.

“ They throw a lot of excuses out, including financial, including impacts on the community from construction,” Norton said. “Have you ever been in Greater Boston? We deal with construction all the time. That’s part of what you do to keep your built environment in good working condition. Everyone we’ve heard from says, ‘Just do it. Of course it’s worth it.’”

The board’s approval is not the final step; now, the MWRA’s plan will go to the Massachusetts Department of Environmental Protection and the U.S. Environmental Protection Agency. That process includes a five-month public comment period, which Norton said the CRWA would soon begin organizing for.