The gray and rust-red hulks, some stretching between two-and-three football fields long, are anchored in rows in Suisun Bay, a shallow estuary between San Francisco Bay and the Sacramento-San Joaquin Delta.

Studies by the administration have suggested the old warships have dumped more than 20 tons of copper, lead, zinc and other metals into the estuary, a critical habitat for a number of endangered species.

"We are moving expeditiously to remove the worst polluting ships first and diligently moving to clean the rest," said David Matsuda, acting administrator of MARAD.

The settlement involving MARAD, environmental groups and state water quality regulators will see half of the ships deemed obsolete — the 25 worst polluters — removed by September 2012, with the rest gone by September 2017. In all, 52 ships eventually will be recycled at various MARAD yards or other facilities it contracts with.

The federal agency plans to keep more than a dozen of the ships anchored in the bay that are in better shape or still considered useful, including the iconic battleship USS Iowa that once served as transport to President Franklin D. Roosevelt.

Some of the removals have already begun, with four ships taken out since November 2009.

On Wednesday, tugboats dragged the SS Mission Santa Ynez toward San Francisco Bay. The ship, once a U.S. Navy oil and fuel tanker used from World War II through the late 1960s, was on its way to a dry dock in San Francisco, where it would be cleaned and prepared for the longer journey through the Panama Canal to a recycling yard in Brownsville, Texas.