DU is just as likely to turn up here -sometimes the wind dumps matter, sand from the Sahara for example, here, did so heavily just a few weeks ago, so of course it’s circulating far and wide, as Paul B commented up there. Human activity and nature disturbs it all the time. And don’t assume from the name it is just Uranium, or that it’s pure, it’s full of lots of other stuff, as much if not nastier too, about 8% of it. It’s in the air, water and soil, everywhere.
Clark just makes these evidence-free pronouncements all the time, especially on nuclear-related matters, knee-jerk downplaying the lethality and danger to all life. He’s hilarous, but dangerous too.