A few parent rocks do contain high concentrations of U, in particular black shale and phosphate rocks (3-1250ppm and 50-300 ppm respectively), and these parent rocks over geological time weather it into the soils of the land.[table 1] As far as typical soils of the relevant European region the concentrations are usually in the 2ppm - 3 ppm region. You can see this in table 3 of "Dokuchaev Soil Bulletin: Review on uranium in soil: levels, migration" https://pdfs.semanticscholar.org/526c/d0b116c0b87287493607ee...

Rockier regions of EU tend to be on the higher side, almost 3ppm, while lands flatter rolling hills tend to be near or below 2 ppm.

Quoting from table 3:

Lithuania 1.3ppm Russia 1.5ppm Poland 2.1ppm Hungary 2.3ppm

By your logic of solution by dilution we should forget about safely storing highly concentrated waste such as DU and just spread it around evenly through the globe?

DU is a waste product of Uranium mining and fuel production and the industry we've based world peace on, MAD, (since it is the only known model of balance so far). It has no place being used in a conflict zone where it will not only be dispersed from its containment, but be distributed in varying concentration in an uncontrollable unpredictable way. This substance belongs in safe depositories.

Now as far as cigarette butts, my take is that I don't like it, but that is just one slice of a much vaster pie that is plastic pollution. Here environmental damage is caused more by surface area than volume or weight.