A new report shows that Maine is closer than ever to eliminating underground drains that dump raw sewage and stormwater into waterways.
But a wetter climate has driven heavy downpours that are working against the state’s attempts to prevent water pollution.
"The more intense storms, the inches per hour have gone up," said Mike Riley, who handles the combined sewer overflow program at the Maine Department of Environmental Protection.
"That is a real threat to sewer systems because they can’t handle a lot of water in a short period of time, it can overwhelm them," Riley added.
Combined sewer overflows — called CSOs — are kind of a relief valve that allows raw sewage mixed with stormwater to discharge into nearby waterways in the event too much water is flowing through the system.
Maine has been eliminating these systems since the early 1990s by separating stormwater and sewage collection.
Decades of investment has brought the number of CSO outfalls from 338 in 1987 to just 108 last year, according to a recent state report. And the discharge volume has decreased substantially, from more than 6 billion gallons to around 532 million gallons.
But heavier rainstorms in recent years are complicating the state's success, according to the report. The state's climate council predicts that the state will see more precipitation in coming years as a warmer atmosphere holds increasing amounts of moisture.
"That’s working against the progress communities have been making over the past 35 years, those more intense storms," Riley said.
Winter rainstorms in particular easily overwhelm systems, since frozen ground cannot absorb the water. Almost 80% of Portland CSO discharge events happened in the winter in 2024, and nearly all discharges happened in Lewiston, Biddeford and South Portland during the same season, according to the state report.
And polluted water from CSOs caused a beach closure in Portland and impacted beaches in Bar Harbor, Biddeford and Saco, Cape Elizabeth and South Portland, according to the report.
