Water regulators told The Associated Press that wells near Hinkley are showing increased concentrations of the cancer-causing substance despite efforts to contain it.
Pacific Gas & Electric tried to stop the spread after the contamination was exposed during a 1996 case in which the utility settled with Hinkley residents for $333 million.
PG&E thought the contamination was contained after tests in 2008 showed the plume was spreading.
The Lahontan Regional Water Quality Control board on Monday ordered the utility to conduct more tests after discovering more polluted wells.
A company spokesman says PG&E is complying with the directive.