Fremont, Calif. ? Launching the largest wetlands restoration project ever attempted on the West Coast, the state and federal governments have agreed to pay $100 million to buy 16,500 acres of salt ponds around San Francisco Bay.

The salt ponds will be purged of toxic deposits and turned into habitat for migratory birds and other species.

Agribusiness giant Cargill Inc. agreed to sell the land in a deal that combines the state and federal money with funding from four foundations. The deal, brokered by Sen. Dianne Feinstein, D-Calif., was announced Wednesday.

The San Francisco Bay used to have 190,000 acres of tidal marsh, but the bulk of the land has been diked, drained, filled or paved. Now, only about 20 percent of the marsh survives.

“Today we’re taking the first steps to reverse the course of history. Together we will restore an extraordinary but endangered natural resource,” Gov. Gray Davis said.

The Cargill land mostly rings the bay south of San Francisco. Of the total acreage involved, 1,400 acres are along the Napa River. The land will be owned and managed by state and federal agencies.

The Cargill land has been in salt production since the 1850s and will have to be cleaned of a highly concentrated brine that is toxic in its solid form. Purifying the land could take decades, and the cost has been put at between $200 million and $1 billion.

The federal government will contribute in $8 million. The state will cover the rest of the cost with four private partners: the William and Flora Hewlett Foundation, the David and Lucile Packard Foundation, the Gordon and Betty Moore Foundation, and the Richard and Rhoda Goldman Fund.

Diluting the ponds and restoring them to healthy marshes will rank as the largest wetlands restoration project attempted on the West Coast, rivaled only by projects to restore the Florida Everglades and Chesapeake Bay.