Where? I'm not sure which way the wind blows on the east coast, but anywhere but out to sea would afferct millions.
Not really. The blast itself not only turns a portion of the material into energy, but basically throws the rest of it all over the place, as particles. This would definitely have an effect on the biosphere, but remember that the core of a fission weapon is rather small. It's not that much hazardous material, spread out over thousands (maybe millions) of square miles. Higher cancer rates near ground zero, perhaps slight increases in outlying areas. We wouldn't have hordes of mutants, though. Chernobyl was entirely different. That sucker was irradiating steam and weather like crazy (IIRC).