Several milligrams or kilograms?
"According to the United Nations Scientific Committee on the Effects of Atomic Radiation the normal concentration of uranium in soil is 300 μg/kg to 11.7 mg/kg."
So let's say 5mg/kg. 200kg of soil per gram. How much area do you need to make 200kg of soil if you're going down 1m? I'd guess not much...
So I think OP is wrong and it's more like 'several grams'.
A cubic meter of soil is ~2 tons, maybe more, which means you only need a 10m by 10m square to get 1kg of Ur if you go 1m deep. Wow.
Looking at average suburban lot sizes, they seem to be about 7200 sqft , which is about 668m^2.
Which gives 668m^3 which gives about 4kg of uranium shockingly.
[1] https://www.reference.com/science/much-cubic-meter-soil-weig...
Google says soil concentration of Uranium is like 3ppm. But, if you do the math, 668 x 1200 x 1000 x (1/1000000) = 802g for every ppm. 3ppm Uranium would be 2.4kg by this number.
Lead is at like 15-40ppm (https://ag.umass.edu/soil-plant-nutrient-testing-laboratory/... ), so between 12 and 32 kg of lead in everyone's back yard?