Fallout Radionuclide Distribution in Lichen Communities Near Thule

Page 1

Fallout Radionuclide Distribution ABSTRACT. Concentrations of the worldwide fallout radionuclides manganese-54. occurring potassium-40 were measured in 80 lichen species collected near Thule. Greenland, during July-August 1968. Emphasis was placed on 1:i7Cs for detailed comparison of radionuclide distribution within the 6 major and 5 minor lichen communities of the 1,500 km.- area studied. Mean 1:i7Cs concentrations in major lichen species ranged from 22 picocuries per gram standard dry weight in Stereocaulon paschale to 50 pCi/g. in Cornicularia divergens, with an overall average of 33 pCi/g. Significant differences were noted between species, habitats and world RESUME. Distribution des radionuclei de retombee dans les associations lichenique pres de Thule. En juillet-aout 1968, on a mesure les concentrations mondiales cerium-praseodymium-144, et du potassium-40 naturel, dans 80 especes de lichens recueillis pres de Thule, Greenland. L’etude a surtout porte sur le 137Cs, pour une comparaison detaillee de la distribution des radionuclei a travers les 6 associations 7 licheniques majeures et les 5 associations mineures de 1’aire etudiee (1,500 km-). Les concentrations moyennes de 137Cs dans les especes licheniques majeures variaient de 22 picocuries par gramme de poids normal a sec dans Stereocaulon paschale, a 50 pCi/g dans Cornicularia divergens, avec une moyenne generale de 33 pCi/g. On a note des differences significatives entre les especes, les habitats et les regions du monde. During July and August 1968 an investigation of fallout radionuclides in the environs of Thule, Greenland, was made by a Danish-American expedition that studied environmental consequences of a United States Air Force bomber crash there (Aarkrog 1970). Terrestrial aspects of that investigation included collection and analyses of lichen samples for various radionuclides because of their demon strated abilities to retain appreciable amounts of such material (Gorham 1959; iDepartment of Radiology and Radiation Biology, Colorado State University, Fort Collins,

Page 2

270 FALLOUT RADIONUCLIDE DISTRIBUTION Hanson 1967a). It also afforded a unique opportunity to obtain information that could be compared with radiation ecology studies conducted in northern Alaska and other circumpolar regions. Eighty samples of foliose and fruticose lichens were collected from 27 sampling sites in the Thule region during the period 26 July to 26 August 1968. Ten sam ples were sufficiently uniform to permit collection on an areal basis (0.25 to 0.40 sq. m.) and thus to allow estimates of productivity and total fallout radionuclide retention. Samples were air dried at Thule and returned to the home laboratory, where they were moisturized and meticulously hand-separated into the various lichen species, vascular plants, plant debris, and soil that normally comprise a lichen community. This was necessary to make interspecific comparisons at various sampling locations and to remove extraneous material that would modify true weights and radionuclide determinations. The components were dried to constant weight at 70°C. for 48 hours to determine percentage of the total community. The lichens were then separately ground in a Wiley mill and aliquots processed according to standard procedures (AOAC 1965) to obtain standard dry and ash weights. Gamma-emitting radionuclides were measured in a nominal 23 x 28 cm. Na(Tl) crystal with a 7.7 x 15 cm. well, connected to a 400-channel gamma' analyser-spectrometer. Data were reduced by an electronic computer program designed to yield results for naturally-occurring potassium-40 and seven radio nuclides commonly encountered in worldwide fallout. All lichen species from each community sample were then mixed and again measured for gamma-emitters. The term community included all lichen species (populations) within a sample, which was then designated by the dominant lichen species at that location. The dominant species contributed an average of 90 per cent of the lichen biomass in the 80 sam ples obtained and the remainder consisted of varying amounts of several species. Cetraria nivalis was the most commonly observed fruticose lichen community of the Thule region and served as a standard for comparison with other lichen species and as an indicator of radionuclide distribution in the environment. It was collected at 22 of 27 sampling locations (Fig. 1). Five other major lichen com munities were represented in the region; one or more samples were obtained of Stereocaulon paschale at 10 locations, Alectoria ochroleuca at 7, Cetraria cuculata at 6, and Cetraria delisei and Cornicularia divergens each at 5 locations. In addi tion, 2 samples of Cornicularia nigricans and single samples of Neuropogon sulphureus, Thamnolia vermicularis, Umbilicaria spp. and Peltigera sp. were col cesium-137 and cerium-praseodymium-144 and the naturally radioactive potas sium-40 were present in nearly all lichen samples.

Page 3

FALLOUT RADIONUCLIDE DISTRIBUTION 271 GREEMAUO ICE CAP fig. 1. Map of Thule District, North Green land, showing lichen Using mCs as a standard for comparison of radionuclide ratios, the following ranking was observed: Strontium-90, a beta emitter, was present in 79 lichen samples at a concentration of 0.24 that of 137Cs. These ratios were consistent with results obtained in north ern Alaska (Hanson and Eberhardt 1971, in press) and in southwestern Green land (Aarkrog and Lippert 1969) near the same time. This indicated a normal complement of fission products from stratospheric fallout and provided con venient tags for radiation ecology studies. Emphasis was placed on 137Cs for detailed examination because it has been extensively studied in circumpolar Greatest 137Cs concentrations were usually found in Umbilicaria, Cornicularia and Alectoria communities, median values in Cetraria and Neuropogon species, and consistently lower values occurred in Stereocaulon, Peltigera and Thamnolia TABLE 1. Comparison of mean 137Cs concentrations in 8 lichen genera. Genera No. Samples Mean 137Cs Concentration pCi/g. standard dry weight

Page 4

272 FALLOUT RADIONUCLIDE DISTRIBUTION Statistical analysis of 1:!TCs levels in multiplicate samples of 6 lichen species was performed by normalizing the data by logarithmic transformation (Eberhardt 1964) and determining significant differences by analysis of variance. Duncan's multiple range test was used to compare the means of the various species (Table 2). TABLE 2. Comparison of mean 1:i7Cs concentrations in major pCi/g standard dry weight No. Geometric .95 Confidence Species Samples Mean * Limits Cetraria delisei 6 36.6 AB 23.9 56.0 *Duncan’s multiple range test results; means with different letters are significantly dilferent at Similar examination of 137Cs concentration in C. nivalis, all other lichen species, and plant debris from various habitats in the Bylot Sound region showed signifi cantly higher amounts on Saunders and Wolstenholme Islands compared to-sur rounding shoreline and mainland habitats (Table 3). Lowest values, about one-third the maximum concentrations, were associated with the dry, rocky moraine at Nunatarssuaq (Location 10) and on the arid hillsides near Narssarssuk and Kap Atholl (Locations 13, 14 and 19). Strontium-90 concentrations showed much the same geographic distribution as 1:!7Cs values. The difference between the habitats is partly explained by the greater occurrence of S. paschale communi ties, low in 1H7Cs concentrations, in the mainland habitats; whereas a majority of the C. divergens and A. ochroleuca communities, with high 1:!7Cs concentrations, were associated with the islands. There was no apparent rationale to explain the higher values on the islands, unless the substantial winds of the Thule region significantly affected the fallout deposition by rearranging the snowfall. The greater concentrations of fallout material in plant debris than in the lichens from which it was separated was attributed to dead material containing fallout materials depos ited in earlier years when fallout deposition was greater, the slow breakdown of the litter, and the more dynamic translocation of elements by the lichens. Plant debris was uniformly distributed through the samples at a mean mass of 0.16 that of the lichen component. Broken lichen parts made up a substantial portion and the remainder was composed of mosses (Lycopodium spp. and Selaginella spp.) or leaves of willow (Salix arctica), Labrador tea. (Ledum palustre), intergrifolia), and sedges (Carex spp.). Much of the vascular plant debris appeared to have been accumulated over considerable time, and formed a cushion near the base of the lichen communities. Samples of vascular plants were not collected

Page 5

FALLOUT RADIONUCLIDE DISTRIBUTION 273 except for one large sample (2 m.-) of the 1968 seasonal growth of arctic timothy (Alopecurus alpina) from the Eiderduck Islands located between Saunders and one-tenth the 1 ITCs concentration and about ten times as much 4"K, compared Standing crops of lichens in 10 places that permitted realistic areal sampling yielded 380 ± 30 (S.E.) grams standard dry weight per square metre in the C. ties. Single samples of C. delisei and 5. paschale yielded 580 and 1,000 g./m.-, respectively. Total radionuclide content of these lichen community components are shown in Table 4. The ratios of the radionuclides are near those quoted earlier for the numerous individual samples, but with limited confidence in the single values from the C. delisei and S. paschale communities. The fruticose lichen communities of the Thule region were discontinuously distributed over the landscape and this precluded the intensive sampling desired; however, the data illustrate the important influence of ecological parameters upon radionuclide concentrations. The lichen communities (samples) individually con sisted of 6 to 10 lichen species growing in concert and which contained variable amounts of radionuclides as they competed for fallout and other materials. Vascu lar plant and lichen debris represented 3 to 36 percent as much mass as the lichens, and about half of the samples contained appreciable amounts of wind blown soil and rocks that contributed variable mass and radionuclide concentra tions. The samples, therefore, represented a complex melange that warranted the tedious separation of the components so that realistic comparisons could be made with lichen samples obtained in northern Alaska as part of our long-term radia tion ecology studies (Hanson et al. 1967; Hanson 1967b) and those in northern Scandinavia (Liden and Gustafsson 1967; Miettinen and Hiisanen 1967). The use of data from unseparated bulk samples would have precluded many of the com parisons and evaluations of ecological significance, such as those made in Tables TABLE 3. Comparisons of ia7Cs concentrations in C. nivalis, all other lichens, and plant debris from various habitats of the Thule, Greenland region (G.M. = Geometric Mean; C.L. = Confidence Limits in units of picocuries 137Cs per gram standard dry weight). Wolstenholme Islands AH Mainland *Mean significantly different from corresponding Islands mean at .05 level. **Mean significantly different from corresponding Islands mean at .01 level.

Page 6

274 FALLOUT RADIONUCLIDE DISTRIBUTION TABLE 4. Fallout radionuclide content of Thule, Greenland lichen community components, summer 1968. Several environmental factors obviously exert great influence on the lichen productivity and distribution in the Thule region. Most lichens were found imills, stone stripes and nets or other irregular surface features that provided moisture, sites for organic matter accumulation and shelter from the substantial winds. Moisture appeared to be one of the more apparent limiting factors for both plant growth and fallout deposition. Annual precipitation at Thule during the period 1960 to 1967 averaged 13 cm., compared to 32 cm. at Fairbanks, Alaska (Hardy and Rivera 1969) which serves as the data source for our studies in that region. This disparity in precipitation was especially important to radionuclide deposition during and immediately following the 1961-1962 period of atmospheric nuclear weapons tests. The data are expressed in terms of !,llSr deposition, which is the standard of measurement for fallout deposition, but may be converted to l:;TCs deposition by the use of the ratio 1:!TCs: !,"Sr= 1.9 ± 0.2 (Hardy and Chu 1967). The amount of 117Cs retained in Thule lichen communities (Table 4) was about half that in northern Alaskan Cladonia-Cetraria mats (28 nCi/m.-) during the summer of 1968. This is partially explained by the greater fallout deposition and biomass of lichens in Alaska (1,600 g./m.1’) compared to the Thule Cetraria and Alectoria communities (380 and 340 g./m.-, respectively). Vascular plants and debris in the Thule lichen communities contained about 20 per cent as much 1;;'Cs as the lichen components, compared to 2 to 3 per cent in Alaskan samples, emphasizing the low biomass per unit area and the importance of separating the components for individual examination. Total radionuclide content of the 10 lichen communities for which areal sampling was suitable averaged 44 nCi/m.1’, of which 83 per cent was concen trated in the lichen component, 17 per cent in the vascular plants and debris, and < 1 per cent in the soil and rocks. Contributions of the 4 radionuclides listed in Table 3, within the lichen and vascular plant fractions were; 44 per cent from

Page 7

FALLOUT RADIONUCLIDE DISTRIBUTION 275 KiTCs, 37 per cent from U4Ce-Pr, 8 per cent from J"Sr, 6 per cent from “'"Zn and 4 per cent from r'4Mn. Coefficients of variation for these radionuclides were gen erally about 0.15 for !’"Sr, 1:,7Cs and 144Ce-Pr, but varied from 0.25 to 0.60 for :,4Mn and 6r>Zn. Eberhardt (1964) reported an average C.V. of about 0.35 for i:,7Cs in an extensive series of arctic plant and animal samples. The lower values in this report may be due to the small number of samples compared. Structural differences and ecological niche of lichens were evidently important factors in determining radionuclide concentration ranking of the various species. Cornicularia and Alectoria had the highest 1:!7Cs concentrations and are richly branched species that often colonized ridges and formed simpler communities than the Cetraria species. Cetraria delisei, the so-called “snowpatch lichen” be cause of its association with drift areas, may have accumulated greater amounts of 1:i7Cs released from melting snow than were available to other Cetraria species. Stereocaulon and Thamnolia species contained the least concentrations of 1:i7Cs and tend to be structurally simpler and less branched than other lichens studied. These observations pose problems for interpretation and offer areas of inquiry for Lichen communities within a 1,500 km.- area surrounding Thule, Greenland, during the summer of 1968 contained worldwide fallout radionuclides similar in nature and ratios to those found in northern Alaska. Total amount on an areal basis was one-half that in Alaska, primarily because of less fallout deposition and biomass of lichens per m.- in northern Greenland. Significantly different 1:!7Cs concentrations among 6 lichen species and 2 major habitats emphasized the im portance of climatic factors, ecological niche and structural diversity in influencing the radionuclide distribution within the Thule lichen communities. Olson for statistical analysis of data. Taxonomic determinations of several lichens were made by Dr. J. W. Thomson, Department of Botany, University of Wisconsin. This work was performed under United States Atomic Energy Commission Con aarkrog, a. 1970. Radioecological investigations. USAF Nuclear Safety, 65(2): 74-9. -------------and J. lippert. 1969. Environmental radioactivity in Greenland in 1968. Danish Atomic Energy Commission Research Establishment Risb Report No. 203. pp. 11-12. association of official agricultural chemists. 1965. Official methods of analysis, plants and animals. Nature, 204: 238-40. gorham, e. 1959. A comparison of lower and higher plants as accumulators of radioactive fall-out. Canadian Journal of Botany, 37: 327-29.

Page 8

276 FALLOUT RADIONUCLIDE DISTRIBUTION hanson, w. c. 1967a. Radioecological concentration processes characterizing arctic eco systems. In: B. Aberg and F. P. Hungate, eds. Radioecological concentration processes, Oxford: Pergamon Press, pp. 183-91. fallout radionuclides in Alaskan arctic ecosystems. In: B. Aberg and F. P. Hungate eds. Radioecological concentration processes. Oxford: Pergamon Press, pp. 233-45. in northern Alaskan lichen communities. Third National Radioecology Symposium, May 10-12,1971. Oak Ridge, Tenn., U.S.A. (in press). Fallout Program Quarterly Summary Report, USAEC Report HASL-182. New York: U.S. …