Primordial radionuclides in Canadian background sites: secular equilibrium and isotopic differences

A literature review and field sampling were done to obtain information on primordial (natural-series) radionuclide concentrations in terrestrial environments in diverse locations across Canada. Of special interest was the degree of secular equilibrium among members of decay series. The analytes measured in soils and plants were natU by neutron activation-delayed neutron counting, 228Th, 230Th, 232Th, 226Ra and 210Po by alpha spectroscopy, 210Pb by gas flow proportional counting, 228Ra by beta counting and 137Cs by gamma spectroscopy. In addition, ICP-MS was used to obtain concentrations of about 50 analytes including elemental U, Pb, and Th. Samples were from seven representative background sites with a total of 162 plant samples from 38 different species. These data were supplemented by a review that gathered a large portion of the similar data from published sources. The sites chosen were semi-natural, far from any nuclear industry, although several were specifically located in areas with slightly elevated natural U concentrations. As might be expected, there were many cases of non-detectable concentrations. However, certain trends were evident. The activity ratio 210Po/210Pb was unity in soils and non-annual plant tissues such as lichens. It was about 0.6 in annual plant tissues. These results are consistent with the time required for ingrowth of 210Po to reach secular equilibrium. There was evidence from several sources that 210Pb in plants came predominantly from deposition of 210Pb from air after the decay of airborne 222Rn. This was expected. Somewhat unexpected was the observation that 228Th seemed to be much more plant available than 232Th, even though both are in the same decay series and should be chemically similar. The difference was attributed to the combined effects of ingrowth from 228Ra in the plant and effects of alpha recoil in mobilizing 228Th in the soil. In general, the results of this study will benefit risk assessment, both in providing background concentrations, but also some indication of where isotope activity ratios can and cannot be used to estimate concentrations.