In a come-from-behind win for Royal Dutch Shell plc and other oil giants, an appeals court has ruled that the U.S. government must reimburse the companies for the cost of cleaning up a World War II–era toxic dump site in California. The amount of damages remains to be determined, but it could top $100 million.

Reversing a prior ruling, the U.S. Court of Appeals for the Federal Circuit concluded that the government needs to take responsibility for petroleum refinery waste that the oil companies dumped on 22 acres of land in Fullerton, Calif., in the 1940s. The Federal Circuit remanded the case to the trial judge, Thomas Wheeler of the U.S. Court of Federal Claims, to figure out exactly how much reimbursement the oil companies are entitled to for cleanup efforts they undertook in the 1990s.