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

Area of Science:

• Environmental Science
• Radiochemistry
• Geochemistry

Background:

• Primordial radionuclides are naturally occurring isotopes crucial for understanding environmental radioactivity.
• Secular equilibrium within radionuclide decay series is a key indicator of isotopic stability and ingrowth processes.
• Terrestrial environments serve as important reservoirs and pathways for natural radionuclides.

Purpose of the Study:

• To determine primordial (natural-series) radionuclide concentrations in diverse Canadian terrestrial environments.
• To investigate the degree of secular equilibrium among decay series members in soils and plants.
• To provide background concentration data for environmental risk assessment.

Main Methods:

• Literature review and field sampling across seven representative background sites in Canada.
• Analysis of soils and 162 plant samples from 38 species for various radionuclides using techniques like alpha spectroscopy, gamma spectroscopy, and ICP-MS.
• Measurement of Uranium (U), Thorium (Th), Radium (Ra), Polonium (Po), Lead (Pb), and Cesium-137 (Cs).

Main Results:

• Activity ratio of (210)Po/(210)Pb was unity in soils and lichens, but ~0.6 in annual plants, reflecting ingrowth times.
• (210)Pb in plants likely originated from atmospheric deposition of airborne (222)Rn decay products.
• (228)Th showed higher plant availability than (232)Th, attributed to ingrowth from (228)Ra and alpha recoil mobilization.

Conclusions:

• Isotope activity ratios provide insights into radionuclide behavior and ingrowth, but their applicability varies with plant tissue type and environmental conditions.
• Understanding secular equilibrium deviations is critical for accurate environmental radioactivity assessments.
• This study offers valuable background radionuclide data and highlights factors influencing their uptake and distribution in terrestrial ecosystems.