Contaminated plumes of groundwater under the Nevada Test Site, where 828 underground nuclear tests were conducted between 1956 and 1992, may travel beyond the borders of the site toward populated areas in as little as 10 years, some scientists say. Federal scientists knew when they conducted the tests that the area’s groundwater would be tainted, but they believed at the time that the water barely moved and that radioactive particles would be sealed into cavities by the blasts or absorbed by underground rock. However, recent studies have found that radioactive particles can travel with water and that the water beneath the site is flowing more rapidly than previously thought. For nearby residents, the talk about groundwater dangers is compounding fears about the federal government’s plan to build a repository for nuclear waste at Yucca Mountain, near the Nevada Test Site.