Of all the things that come to mind when you think of oil and gas fracking, radioactive cancer-causing sludge may not necessarily be at the top of the list. But award-winning investigative DeSmog journalist Justin Nobel is working to change that with his new book Petroleum-238, which exposes an unsettling secret at the heart of the industry (read an excerpt here). Oil and gas companies across the U.S. are producing billions of gallons of radioac…