SEOUL — North Korea has long maintained that all six of its nuclear weapons tests were conducted safely. But on Tuesday, a Seoul-based human rights group warned that radioactive contamination may have spread through groundwater from the North’s Punggye-ri nuclear test site, potentially jeopardizing the health of people in North Korea and neighboring countries. The Transitional Justice Working Group said in its report that radioactive materials could have affected tens of thousands of North Koreans living near Punggye-ri and spread to China, South Korea and Japan through mushrooms and other…