SACRAMENTO — The state Supreme Court added millions of dollars to five insurers’ tabs on Thursday when it upheld the “stacking” of multiple policies covering the Stringfellow Acid Pits, the notorious hazardous waste site in Riverside County.

The unanimous court said the insurance company-defendants in State of California v. Continental Insurance, S170506, must pay “all sums” due on each policy issued over a 12-year period, a figure restricted only by individual policy limits. The insurers, backed by a trial court ruling, argued that they should only be asked to cover a proportionate share for a single policy period.