Boston University assumed full responsibility for an oil spill last week that spread more than a half a mile over the Charles River, school officials said yesterday.

After receiving an anonymous call on January 25, state investigators discovered an oily sheen on the Charles River near the Boston University Bridge, according to Agustin F. Carbo, the on-site coordinator for the Environmental Protection Agency (EPA).

By Wednesday, the Massachusetts Department of Environmental Protection had traced the source of the spill to a leaking underground heating oil tank which belonged to B.U., according to Carbo.

EPA officials said that although the volume of the spill is known to be under 1,000 gallons, they cannot determine the exact amount spilled.

"It's difficult to tell, because when oil spreads into the water, it can look like a lot," Carbo said.

According to John G. Palfrey '94, an official in the EPA's regional administration, last week's rains made the spill even more difficult to measure.

Boston University officials said that by measuring the volume of oil left in the tank, they have determined that less than 10 gallons of oil leaked into the river.

"It is fully under control," said B.U. spokesperson Colin Reilly.

After draining the tank, investigators discovered a "pinhole leak," Reilly said.

The oil leaked from an underground tank used to store heating oil for Boston University's School of Fine Arts. Although the tank was capable of holding 20,000 gallons, it contained only 11,000 at the time of the leak, Reilly said.

EPA officials praised B.U. for helping with the clean-up.

"B.U. is cooperating very well, and we will be working with them in the future," Carbo said.

B.U. has contracted Clean Harbors Inc.--one of the nation's largest environmental cleanup corporations--to clean up the spill. According to Clean Harbors Vice President of Government Relations William J. Geary, Clean Harbors handles almost all spills into the Charles River.

Clean-up workers drained the tank Thursday and prepared yesterday to excavate and remove it. B.U. has installed a temporary above-ground tank to store heating oil for the Fine Arts school.

The University faces civil penalties for violating the Clean Water Act, according to Palfrey. He said the EPA will investigate the school's facilities to make sure the school is in full compliance with its spill prevention plan.

Swimmable River

"Protection of the Charles is at the top of our list of priories at EPA-New England," EPA Administrator for New England John P. DeVillars said in a press release Thursday.

"While the spill is not likely to have an adverse long-term impact on the river, we need to be absolutely positive that it doesn't happen again," DeVillars said. "Mistakes happen, but many of them are preventable. We've set a goal of making the Charles fishable and swimmable by the year 2005 and EPA is committed to achieving it.