State regulators have awarded the last remaining permit needed for construction to start on a $12.7 million layover facility for Amtrak trains in Brunswick.

The Maine Department of Environmental Protection on Tuesday approved the stormwater management plan submitted by the Northern New England Passenger Rail Authority. The stormwater plan had received preliminary approval about two weeks ago.

Regulators concluded that existing berms, ditches and adjacent railroad tracks would prevent significant amounts of stormwater runoff from flowing toward abutters, and that the groundwater at the site generally flows toward a nearby stream and not to an adjacent neighborhood.

However, the group that represents the neighborhood said Tuesday it plans to appeal the decision.

There had been intense opposition to the layover facility since it was first proposed in 2012 when Amtrak’s Downeaster service was extended from Portland to Brunswick. Nearby residents wanted the project built elsewhere, arguing that the building and train activity would pollute groundwater, fill the air with diesel fumes and make too much noise. The layover building is expected to be 655 feet long, 70 feet wide and 37 feet high.

The rail authority said the facility would allow for more frequent and efficient service because trains could be stored and serviced overnight in Brunswick rather than having to return each night to Portland.

Patricia Quinn, executive director of the rail authority, said her staff will now meet with the contractor, Consigli Construction of Portland, to review DEP’s conditions and determine a construction schedule.

“This is great news for us,” she said of the permit.

In its approval order, the DEP listed 14 conditions, including a requirement that the rail authority hire an independent inspector and that it submit a revised plan for monitoring and disposing of contaminants.

John Shumadine, an attorney for the abutting neighborhood, said in a June 11 letter to the agency that the DEP approved the permit with a long list of conditions, rather than denying the application because it failed to meet groundwater standards.

“This back-door approach impermissibly insulates the applicant from public comment,” he wrote.

Bob Morrison, chairman of the Brunswick West Neighborhood Coalition, said Tuesday that the group will appeal the decision to the DEP’s board of directors, a seven-member citizen board whose members are appointed by the governor and confirmed by the Legislature. One of the board’s duties is to hear appeals of the agency’s licensing actions.

The group now has 30 days to file an appeal. If the board accepts the appeal, Morrison said, the process would be lengthy and possibly involve a public hearing.

Quinn said she believes the rail authority would be allowed to begin construction during the appeal process.

Morrison, however, said it would be “irresponsible” for the rail authority to spend public funds without knowing the outcome of the appeal.

“We don’t think the case is over,” he said, “and it involves public money, and there’s always a chance that the little people – the great unwashed here – will be able to win this case.”