The Westbrook Fire Department has isolated a large fuel oil spill spanning about a half-mile in the Presumpscot River and through the city’s storm drain system.

Because the oil was seeping into the river from the storm drains, it’s hard to determine the source of the spill, Westbrook Fire Chief Steve Sloan said in a call Friday morning. It could have been a fuel delivery truck or leaky oil tank in a basement, he said.

The department responded to reports of an oil sheen on the river at about 8:20 a.m. Friday along Brown Street, between Cumberland and North streets, the chief said. As of about 10:30 a.m., the department had placed protective booms on the river and at the storm drain outlets to contain the spill.

The Westbrook Fire Department is working with the Department of Environmental Protection to investigate the cause, but Sloan said it will be difficult because of the amount of branches in the drain system.

He said it is “imperative” for people with fuel tanks in their basement to take care of them when they are leaking.