The Kettleman Hills dump is one of 10 in the nation that handles the now-banned transformer fluid.
In a statement issued Tuesday, the dump's parent company, Waste Management, said extensive monitoring had confirmed the small concentrations of PCBs.
People living nearby have expressed concerns about a high rate of birth defects.
Last week, state health officials announced they have not identified a common cause for the problems.
The Associated Press contributed to this report.