Radon Maps to Aid with Testing
This page gives the geological perspective, with particular attention to Michigan. Click on the thumbnails to enlarge.
A "Radon Potential" map from the US Geological Survey. Click to view the map on their site. Note the purple prong running NE through Hillsdale, Lenawee, Washtenaw, Livingston and Oakland counties.
US map of gamma radiation levels at one meter above ground. Michigan looks pretty clean, but the same prong is visible. There are also some hot spots around the thumb.
Glacier map, showing maximum overall boundaries. Note the boundary between the Huron-Erie and Huron lobes. That is what caused the prong that shows up so prominently in the other maps on this page.
A more detailed look at the moraines. The boundary between H/HE lobes is quite striking.
moraine: n. from French dialect (Savoy) morêna Date: 1789 : an accumulation of earth and stones carried and finally deposited by a glacier.
Surface geology of SE Michigan. The pink areas (the prong) are the primary Michigan radon belt. This area is primarily glacial outwash sand & gravel, from 1-20 meters thick. Uranium in the gravel creates the radon, and the loose soil allows it to migrate. The blue band (clay & silt) running through Monroe, Wayne & Macomb counties has very low radon levels.
Midwest US average radon level by county. There's that glacial prong in Michigan again.
Midwest US percentage of homes above EPA Action Level, by county. Just another way of showing where the problems are.
As suggested by the previous comments, surface soil type is a good predictor of radon problem areas. Here are soil maps of SE Michigan Counties. Click to enlarge, but be patient while they download.
Washtenaw (171k). Any home in the light green areas is very likely to have a radon problem, dark green areas less likely, and yellow sometimes, sometimes not.
Oakland (109k). Unfortunately, each county was independently mapped at different times, and they do not mesh at the borders or use the same color coding. At this level of detail, take everything with a grain of salt.