Caesium-137 Concentrations and Ecological Half-Lives in Three Epiphytic Lichen Species from Southern Greece (Megalopolis)

Abstract

The radioactive plume from the Chernobyl accident of April 26, 1986 reached Greece by May 1, substantially contaminating various regions and ecosystems of the country. Since then, the long-lived radionuclide '37Cs has been of particular concern. In the present study samples of the epiphytic lichens Anaptychia ciliaris, Lobaria pulmonaria, and Ramalina farinacea were collected from Quercus frainetto and Q. pubescens within a number of stands in the plain of Megalopolis-Peloponessos in 1988 and again in 1996, and the concentrations of the '37Cs were determined. The purpose of the study was to evaluate the level of contamination in deciduous oak forest at this southern region of Greece and to compare the ecological rate of '37Cs decline with time, the ecological half-life, and the ecological residence time in three lichen species. The mean concentrations of '37Cs in the 1988 samples were generally much lower than those reported by other workers in lichens from northern and central Greece in 1988, but significantly greater than the corresponding concentrations in the 1996 samples of this study. The concentrations of '37Cs in comparable (from the same stands) samples of 1988 and 1996 were used to estimate the ecological half-lives and the ecological residence times of '37Cs in the three species. Ramalina farinacea captures the smallest amount (952 Bq/Kg) and retains it the shortest time (ecological t? = 2.87 yr.), while Anaptychia ciliaris captures the largest amount of '37Cs (1,220 Bq/Kg) and retains it the longest time (t? = 4.78 yr.). The pH values of thallus is positively and significantly correlated to the '37Cs concentrations. Since A. ciliaris is also a common species in Greece and easy to collect, it is a good biomonitor of radionuclides. Following the accident in the nuclear power reactor at Chernobyl on 26 April 1986, the released radionuclides were first detected in northern Greece on May 1 (Assimakopoulos et al. 1987) and in southern Greece on May 2. Considerable radioactive fallout on the Greek environment mainly occurred during the period 2-7 May (Kritidis & Florou 1995). The radionuclides that caused substantial radiopollution in Greece were 137Cs, 134Cs, and 131I. Due to the short physical half-life of 131I (8 days), the environmental pollution by this radionuclide was of a temporary nature, without diminishing the importance of the damage caused to biological systems. To some extent, the same is true for 134Cs, which has a half-life of two years. On the other hand, 137Cs with a half-life of 30.18 years, has remained in the trophic webs of ecosystems many years after the accident, decreasing slowly in an exponential manner. In general, central and northern Greece received much more fallout of 137Cs (ca 23.9 KBq/m2-Papastefanou et al. 1988) than southern Greece (5-10 KBq/m2 in the Athens area-Kritidis & Florou 1995). This is apparently due to variations in the amount of rainfall. Hofmann et al. (1993) investigated this in the Austrian province of Salzburg, where heavy rains fell during the time the radioactive plume reached the area; the area also received the highest ground contamination in western Europe. Hofmann et al. (1993) also determined 137Cs concentrations in lichens collected before (1985) and after (1986) the Chernobyl accident and found about 50 fold increase in radioactivity. Great increases in radioactivity of lichens and other biocomponents have also been reported from Greece after the Chernobyl accident (Kritidis & Florou 1995; Sawidis & Heinrich 1992). Simopoulos (1989) found a remarkable geographic variability of fallout in Greece (0.3-36.2 KBq/m2), even over short distances, probably due to variations in rainfall patterns as the radioactive cloud passed. Moreover, diversions of wind, due to the mountainous terrain of the country, contributed to the variable levels of contamination from place to place. It is a well accepted view that the radiocaesium distributions observed are site specific (Belli & Tikhomirov 1996; Seaward 1992). In pre-Chernobyl studies in Poland, Kwapulinski et al. (1985a,b) found increased 137Cs concentrations in lichens with elevation, due to greater exposure and higher rainfall. Thus, a small spatial scale assessment of the contamination levels is often required, and would be 0007-2745/98/422-427$0.75/0 This content downloaded from 157.55.39.249 on Wed, 03 Aug 2016 05:55:57 UTC All use subject to http://about.jstor.org/terms 1998] RIGA-KARANDINOS & KARANDINOS: CAESIUM-137 IN LICHENS 423 TABLE 1. Caesium-137 concentrations (Bq/kg) in samples of three species of lichens collected in August, 1988 and October, 1996 from several sites in Megalopolis, Greece. Mean values of the three species followed by the same letter are not significantly different at 0.05 level. Anaptychia ciliaris Lobaria pulmonaria Ramalina farinacea 1988 Samples No. of samples (N) 31 7 6 o. of sites (n) 18 7 6 Bq/kg (Aug. 30, 1988) Mean 1220a 1014ac 952bc