TRANS URANIC ELEMENTS IN ARCTIC TUNDRA ECOSYSTEMS 445 



measurement system was 30% with an average background of 3 counts per 1330 min. 

 Counting data were reduced by a computer program that expressed results in picocuries 

 of ^^^Pu or ^^^•^'*°Pu per gram of sample with one standard deviation for the counting 

 statistics. 



An improved analytical procedure for the determination of ^"^'Am in large (up to 

 100 g of ash) samples became available during 1977 (Knab, 1977) and yielded the first 

 realistic results for that radionuclide in a limited number of Alaskan samples. This 

 procedure consists of DEHPP [phosphorus pentoxide, bis(2-ethylhexyl) phosphoric acid, 

 and cyclohexane] extraction of both plutonium and americium from the sample residue, 

 separation of plutonium frorfi americium by anion exchange onto a nitric acid prepared 

 column, and purification of americium by ion exchange in methanol-nitric acid and 

 ammonium thiocyanate anion columns. The eluted americium is then electrodeposited on 

 a stainless-steel planchet and counted on the same alpha-spectrometer system used for 

 plutonium. Yields were monitored by ^'^^ Am tracer. 



Several samples, particularly the animal tissues, which were analyzed for transuranic 

 elements yielded net values that were lower than the minimum detection limits (MDL) of 

 the system; for ^^^Pu extracted from 10 g of soil samples and counted for 1333 min, the 

 MDL was 0.003 pCi/g, and for "''■^^"Pu it was 0.002 pCi/g. Values of zero or negative 

 numbers are a common occurrence in environmental sampling owing to statistical 

 fluctuations in the measurements. Although a negative value for a measurement does not 

 represent a physical reality, a valid long-tenn average of many measurements can be 

 obtained only if very small or negative values are included in the population. The data 

 reported here are often averages of several samples, including those below the minimum 

 detection limit or negative numbers. Zero values were considered to represent <0.0061 

 dpm at the 95% confidence level. This procedure is consistent with data treatment at 

 other laboratories (Harley and Fisenne, 1976). Unless specifically stated, data reported as 

 ^^^Pu include the minor radioactive contribution of ^'^"^Pu. 



Results and Discussion 



Radionuclide Deposition Estimates 



Studies of transuranic elements in ecosystems are greatly aided by relating their behavior 

 to that of fallout '^"^Cs, which is easily measured and is consistently near a ratio 

 (^^^Pu/'^'^Cs) of 0.016 (Hardy, 1975). Fallout deposition on the Alaskan and Greenland 

 landscapes was calculated from data published by the U. S. Department of Energy 

 Environmental Measurements Laboratory (Hardy, 1975) with ^^ ^Cs deposition estimated 

 from the ratio ^^ "^Cs/^^Sr = 1.6 ± 0.2 (Hardy and Chu, 1967). Values were directly 

 available for Thule, Greenland, from 1959; however, the fallout deposition at Anaktuvuk 

 Pass was estimated by extrapolating the measured deposition at Fairbanks, some 500 km 

 southeast of Anaktuvuk Pass, to the study area by multiplying by 0.67, the ratio of the 

 annual precipitation rates at the two locations (21 and 32 cm/yr, respectively) (Volchok 

 and Kleinman, 1971). Similarly, fallout deposition at Bettles, about 300 km northwest of 

 Fairbanks, was estimated by multiplying by 1 .34, the ratio of their annual precipitation 

 rates (43 and 32 cm/yr, respectively). 



The correlation of worldwide fallout deposition with precipitation and the prac- 

 ticality of estimating the integrated fallout deposited in a geographic region by careful 

 soil sampling have been demonstrated by Hardy (1974; 1975) and Hardy and Krey 

 (1971). Our data indicated that ^ ^ "^Cs inventories in Greenland lichen communities were 



