HELENA, Mont. — Residents living within a Montana Superfund site can move forward with their efforts to get the Atlantic Richfield Co. to pay for a better cleanup of arsenic left on their property after a century of copper smelting, the Montana Supreme Court ruled Friday.

The ruling sets the stage for a trial in state court in a 2008 lawsuit brought by about 100 residents from the Opportunity area in southwestern Montana.

Arco sought to have the lawsuit thrown out, arguing it was already complying with a federal cleanup plan approved by the Environmental Protection Agency.

The EPA says arsenic levels below 250 parts per million are safe for residential soils. Arco argued federal law prohibited state lawsuits from interfering with an ongoing cleanup.

The Montana Supreme Court, in its 6-1 ruling, found that Superfund law does not prevent private landowners from seeking damages from another private party for the purpose of restoring their own private property.

The residents want their property restored to the normal amount of arsenic in area soil — about 25 parts per million.

Attorneys Mark Kovacich and Justin Stalpes, who represent the Opportunity residents, tell The Montana Standard that the decision sets a precedent that could affect all Superfund cleanups.