Lichens as monitors of radiocesium and radiostrontium in Austria

Many areas of Austria were contaminated to various degrees with radionuclides, following the reactor accident in Chernobyl. In Styria, a province of Austria, the 137Cs activity in lichens rose to over 50 kBq kg -1 from an initial value of 0.4 kBq kg -1 dry weight before Chernobyl. The 137Cs contamination of Pseudevernia furfuracea exceeded the natural radioactivity of 40K up to 430 fold. The ecological half-life of 137Cs in Pseudevernia furfuracea which grows on spruce was found to be approximately 3 years, whereas 3.8 additional years were needed to reach a fourth of the initial 137Cs activity. The half-life of 134Cs was found to be 1.3 and that of 90Sr was between 1.2 and 1.6 years. The corresponding values for the terricolous lichen Cetraria islandica were 2.5 year for 137Cs and 1.2 for 90Sr. The 137Cs levels were found to vary even within short distances. Two reasons, other than the uneven distribution of the radioactive precipitation, are given here for this observation. Pseudevernia, which grew on dead trunks, was contaminated about three times as much at the top end of the trunk as in the lower sections of the tree. This was due to the fact that the rainfall was rather vertical shortly after the Chernobyl accident, so that the upper lichens adsorbed the main part of the radionuclides. Secondly, on a mountain slope, the 137Cs level was shown to increase with altitude. This was because it rained slightly more in the higher regions shortly after the Chernobyl accident and because of the shorter vegetation period. A good indication of the high levels of contamination through various γ-ray emitting radionuclides following the reactor accident is also given here by the levels reached by those radionuclides in a soil sample from Graz, capital of Styria. In this sample the activity of all gamma emitting radionuclides was 1654 kBq m -2. 137Cs showed 2.8% of the overall radioactivity, the corresponding value for 134Cs was 1.1%.