A Government Accountability Office report found the Environmental Protection Agency collected $4 billion in Superfund clean-up funds from 1990 to 2000, but half of those funds have not been obligated.

The rest of the funds will be obligated over the next decade, according to the GAO report.

The obligation amounts vary site by site, said Carolyn Yocom, a director in GAO's health care practice, in an interview with The Federal Drive with Tom Temin.

"One of the challenges EPA has with these Superfund sites is each one is very unique and has a different set of issues, and as [EPA] begins to look into what's going on at a particular site, they may have to change course partway," Yocom said.

GAO previously issued a report that showed that EPA's costs to clean up many sites are likely to exceed the available funding.

"I don't think there are concerns about over-collection," she said.