The potential of lichens as long-term biomonitors of natural and artificial radionuclides.

Lichens were used as biomonitors of Chernobyl fallout 137Cs, of cosmogenic 7Be and of radioactive members of the natural uranium and thorium decay chains. Samples were taken from two locations in France, including lichens sampled at different distances of a coal fired power plant and close to a uranium ore processing waste disposal site. All samples were analyzed gamma-spectrometrically after equilibrium concentrations of short-lived isotopes had been attained. Activity concentrations of the members of the uranium and thorium decay chains in Parmelia sulcata sampled 1994 decrease with distance from the plant, whereas in lichens taken at the waste disposal site a decrease with time was observed. Comparison of 7Be activity concentrations measured in lichens with atmospheric deposition rates confirms that P. sulcata can be used as a quantitative biomonitor of radioactive trace substances. Retention half-lives calculated with a simple one-compartment model are 2.6 +/- 1.2 years for cesium, which was detected in all samples even more than a decade after the Chernobyl accident, and of 0.7 (+/- 0.1) to 3.3 (+/- 0.7) years for lead. Consequences of our results for model identifiability and parameter estimation of a two-compartment model are discussed.

Accumulation of 210Pb, 226Ra and radioactive cesium by fungi.

Fungi sampled in three areas in France were analyzed by gamma-spectrometry for their concentrations of 134Cs, 137Cs, 210Pb and 226Ra. In most of the samples radioactive cesium was detected with a maximum of 2860 Bq kg-1 (dry wt.). Activity concentrations of 210Pb were in the range < 1.76-36.5 Bq kg-1 (dry wt.). Activity concentrations of 226Ra were consistently lower, often by one order of magnitude. Models are developed to estimate the contributions of atmospheric 210Pb deposited onto the fruit bodies to the measured 210Pb concentrations and of the uptake of 222Rn soluted in soil pore water which subsequently decays into 210Pb. It is shown that both pathways are of only minor importance. Comparison with the soil-mushroom concentration ratios of stable lead, which were determined for some of the samples, confirmed that 210Pb in mushrooms mainly originates from direct uptake of 210Pb present in the soil. Despite of the high concentrations of 137Cs detected in most of the mushrooms, radiation doses to individuals due to mushroom consumption are dominated by 210Pb for the majority of the edible mushrooms sampled.