The Environmental Protection Agency approved a plan by Maryland, Pennsylvania, Virginia and the District of Columbia to clean up the Chesapeake Bay, the nation’s largest estuary. Halting the bay’s decline and restoring its living resources will cost millions of dollars, said EPA Administrator Lee M. Thomas. The bay, fed by 150 rivers, creeks and streams, drains a 64,000-square-mile watershed. Its fish, shellfish and waterfowl habitat have been a major economic resource since prehistoric times, but heavy human settlement and industrialization have poured tons of pollutants into the bay. State officials said federal participation eased their apprehension that federal support of the bay programs might be weakened by new strains caused by the growing budget deficit.