The U.S. Environmental Protection Agency ordered the defense contractor to address groundwater contamination from the former Benchmark Technology facility in California.
Northrop Grumman has spent $10 million in cleanup operations at the former facility based on a 1989 order from the Los Angeles Regional Water Quality Control Board.
Groundwater at the site, the EPA said, was contaminated with volatile organic compounds used for degreasing, metal cleaning and other purposes tied to the work of more than 60 different companies.
The EPA notes groundwater in the area is used to supply more than 1 million people with 90 percent of their drinking water.
EPA Regional Administrator Jared Blumenfeld said the action against Northrop Grumman shows polluters must pay.
"Today's order will protect the essential drinking water supplies in the San Gabriel Valley," he said in a statement.
The treatment system will be used to prevent contaminated groundwater from migrating, he explained.