Between 1951 and 92 there were 1,021 nuclear detonations at the Nevada Test Site, 65 miles from Vegas. 921 of the detonations were underground, some as deep as 5,000 feet. A large chamber was formed by the detonations, leaving a cavity filled with radioactive rubble. About a third of the tests were conducted directly in aquifers, and others were hundreds of thousands of feet below the water table. When testing ended in 1992, the Dept. of Energy estimated that more than 300 million curies of radiation remained in the environment at that time, making the site one of the most radioactively contaminated locations in the U.S. In the worst-affected zones, the concentration of radioactivity in affected groundwater reaches millions of picocuries per liter. (The federal standard for drinking water is 20 picocuries per liter.) Although radiation levels in the water continue to decline over time, the longer-lived isotopes could pose risks to workers or future settlers on the NNSS for tens of thousands of years. Other than that it seems Vegas tap water drinking supply should be perfectly fine to drink.