The Environmental Protection Agency presented the findings of its study Wednesday night at a public meeting.
Technicians collected 3,735 soil samples from a corner of the 2,850-acre hilltop lab where most of the testing was done. The study found 10 percent of soil samples collected from the Santa Susana Field Laboratory contained radioactive concentrations exceeding background levels.
Most of the contaminated soil was found in areas like the materials handling facility that were previously cleaned, but a EPA project managers said isolated spots in those areas were missed.
The pollution occurred in restricted areas of the lab and environmental officials said there was no immediate threat to the community because the site is secure.
The findings are a part of a $42-million study launched by the EPA to investigate radioactive pollution at the site.
The deadline to clean up the site is 2017.
The Associated Press contributed to this report.