BOSTON (AP) — Wildlife rescuers were rushing Monday to tend to dozens of geese and ducks that were soaked in oil after a spill in a Boston river.

Police and fire departments were dispatched to the Muddy River on the border of Boston and the town of Brookline a little after noon on Sunday following reports of a possible oil leak, Brookline police said. Responders found there was some kind of leak into the waterway that impacted wildlife — including numerous ducks and geese, police said.

State environmental officials and wildlife rescuers responded and were still on the job Monday, officials said.

Preliminary findings show the oil leak appears to be coming from a storm drain under a nearby condominium complex, said Danielle Burney, spokesperson for the Massachusetts Executive Office of Energy and Environmental Affairs. The Massachusetts Department of Environmental Protection estimates less than 100 gallons (379 liters) of oil was released, Burney said.

The DEP was still investigating the exact source of the spill, which is being contained and managed, Burney said.

“The New England Wildlife Center is on-site to collect affected waterfowl for treatment and rehabilitation at their facility. They will continue to monitor the area to ensure the safety and well-being of the local wildlife,” Burney said.

Rescuers said it would take up to a month for the birds to be treated and released back into the wild. Twenty birds were accounted for by Monday afternoon, but dozens more were expected to need care, said Katrina Bergman, president of New England Wildlife Center, which was responding to the animals.

Bergman said most of the birds were Canada geese and mallards, which are a common sight alongside the Muddy River. The river is a popular site for walkers and joggers. The spill happened in the area of the Longwood train station, about a mile from Fenway Park.

Caring for the animals is difficult because they need to be cared for without stressing them out, said Zak Mertz, chief executive officer of New England Wildlife Center.

Giving the injured wildlife room to let rescuers tend to them was critical, officials said.

“We don't want to cause them to do any extra activity, especially if they have oil in the mouth and nose, that could do more damage,” Mertz said.

Wildlife rescue involves proper training that most members of the public lack, police said.

“We know that members of the community are concerned for the well being of the impacted wildlife and were interested in what they could do to help. On scene for something like this it’s important that we only use people with proper PPE and training,” Brookline police said in a statement.