City departments are working on figuring out where oil is coming from that is entering the Nashua River by Notre Dame Park.

Nashua Deputy Chief Michael O'Brien said on Feb. 23 a passerby first reported a "rainbow sheen" on the surface of the river near the park, off Water Street, and called the fire deparment.

Absorbant booms were set in place to take care of what appeared to be a small amount of oil. However, with more rainwater on Wednesday morning, the outflow from the culvert drain increased and, again, more oily residue was seen on top of the water's surface Feb. 27, O'Brien said.

"I don't feel we have a catastrophic leak, but we are working with the City of Nashua, wastewater, and Public Works, and also with DES, to try and find the exact cause," O'Brien said. "So we don't yet know the origin, but it's not a gusher."

O'Brien said it could be a slow leak of some kind, or even possibly the result of someone illegally emptying motor oil into the drainage system.

He said the booms placed in the water on Saturday were still working and just needed some adjustment, but absorbant pads were added Wednesday. The fact that the booms were still effective four days later indicate that the flow of oil is minimal, O'Brien said.