WINTHROP – At first glance, it looked like any other day Thursday on Annabessacook Lake. Birds chirped from the trees across the water. A red-winged blackbird flitted in the tall grass next to the lake.

But there were ominous black patches here and there on the surface of the water. There were long strips of absorbent white pads around the edge of the water and red pads 300 feet out. Earlier, an oil-stained muskrat was found dead at the edge of the lake. A pair of turtles came to a similar fate.

“There is an environmental impact,” said Peter Blanchard, an oil and hazardous materials specialist with the state Department of Environmental Protection. “We’re disappointed and frustrated that it happened again.”

At issue was No. 6 heating oil seeping from a stream into the lake. The oil has been buried in sediment and possibly bedrock for years, leakage from the former Carleton Mill on Main Street. It has created problems in the past.

Two years ago, beginning in April 2005, the DEP and other crews spent three months digging out the oil leaking into Mill Stream from beneath an old boiler room at the defunct mill. They tore up the tank and the pipes that were leaking the oil. They set up booms to trap the oil, tore up buildings and hunted the noxious substance in various nooks and crannies. At the time, it was one of the biggest cleanup operations ever in state history.

“We thought we had it,” Blanchard said, standing next to Lake Annabessacook as a new rescue and cleanup effort was under way.

Heavy rains earlier this month as well as melting snow caused a surge of water from Mill Street to the lake.

“When that rush of water comes through, it hydraulically pushed the oil out,” Blanchard said.

It was not clear just how much oil had seeped into the lake this time. A neighbor reported dark, glimmering spots on the water over the weekend and the environmental groups got right to work. They put more than a thousand feet of absorbent and containment booms in the water, cleaned up oil-contaminated debris from the shoreline, and prepared to return to the lake over the next week. The operation involves a crew of a dozen from the DEP and from other agencies contracted to help with the cleaning.

“The guys have worked really hard on this,” Blanchard said. “They’ve done a great job.”

Besides the dead muskrat and turtles, there is evidence of others that have come in contact with the oil. There were loons and blue herons with dark, oily patches on them moving in and around their lake home.

“We could see the oil on them, but they were still taking off or swimming away,” Blanchard said. “They didn’t want anything to do with us.”

The worst of the oil had been cleaned up and further problems were not expected. But with oil still trapped in sediment and possibly in the bedrock beneath Mill Stream, Blanchard said his crews would continue to search around Carleton Mill, which was the source of leaks in the 1960s, three times in 1990 and again in 1993.

Carleton Mill is a cluster of brick buildings on Main Street in Winthrop’s downtown. It is mostly office space utilized by the town commerce center, and it’s surrounded by other businesses. There’s an auto garage and a sandwich shop across the street and the post office next door.

The mill closed in 2001, yet the oil leaked in 2005 and now, 2007.

Meanwhile, the Maine Department of Inland Fisheries and Wildlife was called in to assess the impact on the local animal population and the flora around the lake. A game warden took away more than a half dozen turtles that had survived oil contamination. By late Thursday, no more dead animals had surfaced. Blanchard said the crews would be back next week to search for more of the elusive, buried oil.

“We’re going to redouble our efforts,” Blanchard said. “We’ll do everything we can to keep it from happening again.”