CALAIS, Maine — Authorities say tides and fast flowing water will take care of any problems stemming from a spill of 950,000 gallons of sewage on Monday into the St. Croix River.

A main sewer line broke Monday morning at a sewage treatment plant along the St. Croix River in St. Stephen, New Brunswick, located across the river from Calais. The St. Croix forms part of the U.S. border with Canada.

Derek O’Brien, chief administrative officer for the town of St. Stephen, said the break took place early in the day and was fixed as quickly as possible, after which the plant was put back online.

Officials from the Maine Department of Marine Resources and the Maine Department of Environmental Protection said Thursday that there is nothing more to be done, though they will monitor water quality in the area and down river.

Department of Environmental Protection spokesman David Madore said he believes the contamination from the spill will flush out because of tides and the heavy rains falling Wednesday and Thursday.

“We need to see how this plays out before we make the determination,” he said.

Department of Marine Resources spokesman Jeff Nichols said Wednesday that an area from Calais to Hinton Point in neighboring Robbinston already has been closed to shellfish harvesting since the early 2000s.

In the prohibited area, it is “unlawful to dig, take or possess clams, quahogs, oysters or mussels from the shores, flats and waters,” according to a notification from the Department of Marine Resources.

That existing closure is “sufficient” to deal with the current spill, Nichols said.

Kohl Kanwit, director of the Department of Marine Resources’ Bureau of Public Health, said the area has been closed for years because of the nearby location of the Calais wastewater treatment plant and four “overboard discharges,” which she described as mini treatment plants for single homes or small clusters of homes.

State and federal environmental agencies do not consider the area polluted, but the Department of Marine Resources has had it closed off as a precaution out of concern for the possible pollution of sensitive shellfish, she said. Shellfish are filter feeders, and even minimal pollutants can build up in their bodies and be passed on to people.

Officials in Canada also are waiting to see what happens with the 950,000 gallons of sewage spilled on Monday.

O’Brien said the tides and fast flow of the river will do much to correct the problem.

“Nature tends to do its thing very well,” he said.

Canadian officials will be monitoring coves and other areas of concern to make sure sewage runoff doesn’t build up there, he said.

Canada’s Department of the Environment will let St. Stephen officials know if further action is needed, O’Brien said.

Jennifer Graham, director of communications for the New Brunswick Department of Environment and Local Government, said the sewage plant is required to keep a screen in place at the end of the discharge pipe to make sure solids aren’t discharged.

This “significantly reduces impacts to the environment in the event of a bypass. That’s why, in this case, environmental impacts are minimal,” she said.

The department will be working with St. Stephen to determine what went wrong and how to prevent a recurrence, she said.

Calais City Manager Jim Porter said he learned about the spill by reading a Bangor Daily News article on it.

“The city of Calais hasn’t gotten any official word,” he said.

But he also said he believes heavy rains and tides will take care of the problem.

The U.S. Coast Guard also has been monitoring the situation, but its regulatory authority with sewage is “somewhat limited,” Petty Officer Alex Olbert said Tuesday.

“If it comes from a boat, we have a lot more authority to take action,” he said.

Heather Almeda, manager of the St. Croix International Waterway Commission, said the organization has no specific involvement with cleanup, but she was disappointed to learn of the recent “contamination” of the watershed.

“Nothing can undo what happened on Monday. However, I sincerely hope the town, the province and the state will take measures to mitigate the risk this poses to the fish, wildlife and people who depend on the St. Croix,” she said. “Furthermore, exploring the cause of this unfortunate accident will be essential in preventing it from recurring.”