There was a drastic decline in the amount of untreated wastewater that overflowed in the Connecticut River last year compared to 2023, according to a new report from the Massachusetts Department of Environmental Protection.

The report, which came out last week, showed that 543 million gallons overflowed into the the Connecticut River last year across 12 days with weather events, compared to close to 1 billion gallons in 2023.

Some communities in the Pioneer Valley constructed their sewer systems to also accept stormwater. During heavy rains, the stormwater and sewage overwhelm some wastewater treatment plants and flow — untreated — into nearby bodies of water. The event is called a combined sewer overflow (CSO), and officials issues warnings afterwards to avoid affected water bodies for 48 hours because of pollutants and bacteria.

“In 2023, we had extremely heavy rainfall over the summer and into the winter which resulted in several catastrophic floods throughout the Connecticut River watershed. In contrast, 2024 was a drought year and there was significantly less rainfall,” Ryan O’Donnell, water quality program manager at the Connecticut River Conservancy, wrote in an email.

The amount of overflow depends on the amount of rainfall and snowmelt that occurs, the MassDEP report says.

“Wetter weather conditions, particularly those with significant precipitation, increase the likelihood of CSO discharges,” the report says.

The reduction of untreated wastewater entering in the Connecticut River in 2024 because of combined sewer overflows can also be attributed to the opening of Springfield’s York Street Pump Station in 2023.

The project has “doubled the capacity of our old wastewater pump station and can pump more combined flow to the wastewater treatment plant during storm events,” said Jaimye Bartak, communications manager for the Springfield Water and Sewer Commission.

The commission spent $137 million on the pump station project.

The commission, she said, has spent more than $300 million over the last three decades to reduce and mitigate combined sewer overflow discharges into the Chicopee and Mill rivers and their parent, the Connecticut River.

The commission also has collaborated with other communities to reduce the discharges in the Connecticut River by approximately half, she said in an emailed statement.

“Much CSO investment remains to be done in the region, but we are proud of that progress and it is evident in the increased recreation on and enhanced development along the Connecticut River,” she said.

Wastewater systems across the state are required to notify the public anytime sewage discharges and overflows into Massachusetts water bodies.

Communities, like Holyoke and Chicopee, are also in consent agreements with the federal Environmental Protection Agency to completely eliminate their combined sewer overflows — a multimillion dollar undertaking.

Holyoke, which entered into a consent agreement with the U.S. Environmental Protection Agency in 2023, is working on sewer separation projects to reduce its overflows into the Connecticut River.

Joshua A. Garcia, Holyoke’s mayor, said at a groundbreaking for the River Terrace sewer separation project on Monday, that local governments have to address these issues incrementally.

Garcia said the project is very expensive, but the city can’t keep ignoring the issues. “We have to start moving toward the end goal,” he said.

Chicopee has been in a similar agreement with the EPA for over two decades.

By 2022, Chicopee had spent $225 million on its effort to separate storm drains from sewer pipes, which was a part of its EPA order that requires the city to stop dumping raw sewage into the rivers every time it rains. At the time, the project still needed $300 million to complete.

In addition to fewer discharges of untreated wastewater in the Connecticut River in 2024, there were also fewer discharges in the Mill and Chicopee rivers, the report says. The Mill River went from having 37 million gallons in discharge in 2023 to 18 million the year following. The Chicopee River went down from 23 million gallons to 11 million gallons.

Despite the local decreases, the overall volume untreated wastewater discharge increased in Massachusetts, jumping up from 7.2 billion gallons to 7.6 billion gallons of sewage that entered water bodies across the state.

Particularly, the Massachusetts Bay — on the eastern part of the state — saw an increase in combined sewer overflow discharge from 1.3 billion gallons in 2023 to 1.8 billion gallons in 2024.