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Description
Massic activity of Cs-137 has been measured in homogenized dried fronds of the fern Athyrium filix-femina. Samples were collected in 2005 and represent 35 biomonitoring plots in coniferous (prevalently spruce) forests across the Czech Republic. We combined the results with some data from the same sites partly presented in previous publications, including contents of stable cesium (Cs-133) in fronds and substrates, and Cs-137 activity in forest-floor humus and spruce bark (the bark was sampled in 1995 and 2010 and its activities were recalculated to 2005 using effective half-lives).
Due to relatively small volume of the samples, Cs-137 was measured in a well-type gamma-ray spectrometer (sample masses were 0.50–0.74 g). The counting times ranged from 8.1 to 93.6 hours (including long repeated measurements of 9 samples). The minimum detectable activity (MDA) of Cs-137 ranged from 34.3 to 233 Bq/kg dry mass. Activities exceeded MDA in 10 samples, where they range from 84 to 891 Bq/kg when decay-corrected to September 1, 2005. These ten sites are designated „Group A“ and the others „Group B“.
In Group A, transfer factor (TF), defined as the ratio of Cs-137 activities in fronds to that in humus, is comparable on average to TF of Cs-133. Compared with Group B, Group A tends to have higher Cs-137 activity in humus (median 287 vs. 158 Bq/kg) and in bark (median 17.3 vs. 13.3 Bq/kg), higher TF of Cs-133 (median 0.75 vs. 0.13), and lower humus pH (median 3.44 vs. 3.62). Similar patterns are also observed within Group A.
Fern fronds are the first biomass at the biomonitoring sites where both Cs-133 and Cs-137 were measured, while Cs-133 was also determined in moss, grass, and needles. We discuss to what extent the Cs-137 activity in biomass can be predicted from its activity in humus and the Cs-133 uptake.