Dynamic model for radionuclide uptake in lichen

Abstract

Samples of atmospheric particulate material and terrestrial plants, including lichens, were collected in New Brunswick, Canada between 1980 and 1983 and analyzed for a wide range of artificial and naturally-occuring radionuclides, including fission products ( 141Ce, 144Ce, 103Ru, 106Ru, 95Zr and 137Cs) derived from the 16 October 1980 Chinese nuclear test. Activity ratios of some of the short-lived fission products in air particulates and lichens are in reasonable agreement with those predicted from fission product yields for nuclear weapons tests, indicating that only minor fractionation occurred for these radionuclides during their transport through air particulate and lichen environmental phases. The 7Be inventories measured in a suite of lichen (Cladonia rangiferina) samples were used to calibrate each lichen plant for its collection efficiency for atmospheric particulates and fallout radioactivity. A lichen model has been developed to predict lichen inventories of radioactivity for different lichen growth functions and bio-elimination rates. Assuming that lichen growth results in a linear increase in surface area with time, the experimental results yield biological residence times of 1–2 years for 210Pb and Pu and 5–8 years for 137Cs. The more efficient retention of 137Cs is probably due to its physiological uptake in lichen plants as a proxy for potassium, as evidenced by an observed, inverse relationship between 137Cs and 40K activities in lichen.