Debate remains about how toxic dioxin is to humans, but the Department of Toxic Substances Control says there is no safe level of exposure.

Kimi Klein, a human health toxicologist with the department, said that although the dioxin on the mill site was not the most toxic dioxin out there, there was “very good evidence” that chronic exposure to dioxin caused cancer and “it is our policy to say if any chemical causes cancer there is no safe level.”

Fort Bragg must clean the dioxin-contaminated coastline this year or risk losing a $4.2 million grant from the California Coastal Conservancy for a coastal trail. Its options: haul the soil in a thousand truckloads to a landfill about 200 miles away, or bury it on site in a plastic-lined, 1.3-acre landfill.

Alarmed by the ultimatum, residents called in Paul E. Stamets, author of “Mycelium Running: How Mushrooms Can Help Save the World.”

Typically, contaminated soil is hauled off, buried or burned. Using the mushroom method, Mr. Stamets said, it is put in plots, strewn with straw and left alone with mushroom spawn. The spawn release a fine, threadlike web called mycelium that secretes enzymes “like little Pac-Mans that break down molecular bonds,” Mr. Stamets said. And presto: toxins fall apart.

In January, Mr. Stamets came down from Fungi Perfecti, his mushroom farm in Olympia, Wash. He walked the three-mile coastline at the site, winding around rocky coves on wind-swept bluffs where grass has grown over an airstrip but barely conceals the ash piles. It was “one of the most beautiful places in the world, hands down,” he said.

Quick to caution against easy remedies  “I am not a panacea for all their problems”  he said he had hope for cleaning up dioxin and other hazardous substances on the site. “The less recalcitrant toxins could be broken down within 10 years.”