Bomb (137)Cs in modern honey reveals a regional soil control on pollutant cycling by plants

(137)Cs is a long-lived (30-year radioactive half-life) fission product dispersed globally by mid-20th century atmospheric nuclear weapons testing. Here we show that vegetation thousands of kilometers from testing sites continues to cycle (137)Cs because it mimics potassium, and consequently, bees magnify this radionuclide in honey. There were no atmospheric weapons tests in the eastern United States, but most honey here has detectable (137)Cs at >0.03 Bq kg(−1), and in the southeastern U.S., activities can be >500 times higher. By measuring honey, we show regional patterns in the biogeochemical cycling of (137)Cs and conclude that plants and animals receive disproportionally high exposure to ionizing radiation from (137)Cs in low potassium soils. In several cases, the presence of (137)Cs more than doubled the ionizing radiation from gamma and x-rays in the honey, indicating that despite its radioactive half-life, the environmental legacy of regional (137)Cs pollution can persist for more than six decades.