In Canada and the United States approximately 82% of the annual total dose
of radiation to any person comes from terrestrial and cosmogenic sources. T
he soil is also the main source of radon exposure to humans. Therefore it i
s important to know the background levels of natural and anthropogenic radi
oactivity in soils. In this study the radionuclides K-40, Ra-226, Th-232 an
d (CS)-C-137 were measured in the soils of southwestern Ontario in order to
determine the background levels and to understand the soil profile distrib
ution. Clay content is the most important variable affecting the background
levels of K-40, Ra-226, and Th-232 in the soil. The profile distribution o
f these radionuclides is further influenced by pedogenic processes, particu
larly carbonate leaching from the solum and clay illuviation from the Ae to
the Bt horizons. The lateral and vertical distribution of bomb-fallout Cs-
137 is influenced by soil management such as tillage, biopedoturbation by s
oil animals such as earthworms and groundhogs, soil erosion and soil organi
c matter cycling. Multiple regression analysis showed that clay content, sa
nd content, percent CaCO3, pH and organic carbon content had variable influ
ences on each of the radionuclide contents in the soil.