PORTLAND — A section of the Eastern Prom Trail was washed away and the East End Beach was closed Thursday after an estimated 1 million gallons of partially treated sewage spilled out of Portland’s wastewater treatment plant, according to city officials.

The washout occurred early Thursday morning when the wastewater overflowed from a treatment tank. The wastewater had already gone through part of the treatment process in which solids are removed.

Clements said this is the first time the plant has an experienced an overflow of this type.

The wastewater came from a contact tank where chlorination and dechlorination occurs at the end of the treatment process, Clements said. At least one of those tanks was drained for cleaning on Wednesday and officials will investigate if that had any connection to the overflow.

Officials estimate the overflow occurred for less than an hour beginning at 6:45 a.m.

“Our engineers are on site assessing the damage and we are reaching out to contractors to get property and trail restoration started promptly,” Clements said.

The trail is closed and barricades have been placed around the damaged section. Both the city and Portland Trails said updates on the status of the trail will be posted on social media.

It was not immediately clear if the spill would affect Saturday’s annual Peaks to Portland Swim to Benefit Kids, a significant fund-raiser for the YMCA of Southern Maine. An expected 450 swimmers, who have raised approximately $180,000 for the local YMCA, are supposed to finish a 2.4-mile swim across Casco Bay at the East End Beach.

“We’re still assessing our options,” said Meaghan Woodsome, the marketing director of the YMCA of Southern Maine. “We are still trying to gather information. We have a day or two to figure it out.”

“We know people have worked hard to be able to do this race,” Woodsome added. “We want to be able to hold it but we obviously want to make sure it’s safe for everyone.”

Clements said the sewage treatment plant is running normally Thursday.