Soil Radioactivity in soil results from the rock from which it is derived. It is the top 0.25 m of soil that contributes significantly to background dose. Table 5 is a summary of concentrations of major radionuclides in major rock types and soil. Background concentrations of radionuclides in soil vary because of many factors. Soil may have been produced from the weathered top layer of still-intact bedrock below or transported laterally from the same rock unit or type some distance away.
Water is the dominant transporting medium.
Glacier-derived deposits are common in the Great Lakes area, New England, and Alaska. Outwash erosion products from mountains may produce a soil surface that is more radioactive than the underlying bedrock. Wind can be a significant factor, particularly in the Southwest.