Two small oil leaks - one accidental, one apparently intentional - have created a visible slick in the harbor waters off Boston Street in the Baltimore neighborhood of Canton, state environmental officials say.

The monthlong cleanup has taken longer than expected in part because rain and melting snow have washed trash out of storm drains, said Richard McIntire, spokesman for the Maryland Department of the Environment.

"People don't realize everything that goes down the storm drain actually ends up at the harbor," he said.

On Dec. 1, a boiler at Highlandtown Elementary School broke, sending 500 gallons of heating oil into storm drains and, eventually, the harbor. Containment booms were put in place to capture the oil, and a cleanup firm was hired.

Then, McIntire said, someone apparently poured oil down a drain. That too wound up in the water.

"It takes a long time for this stuff to work its way through the system," he said.

Crews have not seen the heating oil in more than a week and may remove the containment booms soon.