WIPP's Salt Formation is Very Low in Naturally Occurring Radioactivity

Uranium/Thorium/Potassium: The salt layers of the WIPP underground contain significantly less natural radioactivity than the rocks exposed in other mines.  In the accompanying photgraph, a miner's lamp held against the salt shows the diffuse glow of backscattered light from within the rock formation.  However, the WIPP salt contains some impurities.  Note the lighter and darker bands.  These indicate varying levels of other materials present in the salt, such as clay or wind-blown dust deposited during salt-bed formation more than 250 million years ago.  Interval-weighted composite samples from the WIPP horizon exhibit about 3-5% of the naturally occurring radioactivity that is found in average crustal rocks: