200+ years and the water is still irradiated? doubtful, the soil would have filtered it out decades ago.
Take a look at the accident on Chernobyl, a single accident in a nuclear power plant, 24 years and there still radiation nearby.
Now, imagine The Great War, THOUSANDS of THOUSANDS of nukes destroying the whole USA.
It's just too many bombs, too many radiation, it would take much more time for nature to filter it. Radioactive dust accumulates...