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.