SOME OBSERVATIONS ON SOIL TYPES
It is important to remember that this discussion does not replace a radon test. Predicting radon issues based on the prevalent soil type in the area is far from an exact science (in fact it's an impossible science); however, we have identified a couple of trends that might provide an indication of radon-potential.
Some of the highest indoor levels of radon we have seen are in areas with gravel extraction activities, and some of the lowest in areas comprising silty clay soils. This is probably due, in part, to the relatively high permeability of gravel-type soils. Soil gas (including radon) will move more easily through sand and gravel than silts or clay. This is not to say that a gravel pit in your area guarantees a radon issue, or that clay means there won’t be an issue. There are many other factors at play including the radon levels in the soil, the construction of the building, and how the building is operated.