704 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



(~2.5 X 10^ )uCi/m^). Such toxicity was expressed as a decline in microbial numbers. 

 Since microbes perform functions in soil that are important to plant growth, indirect 

 effects to higher plants and animals could be elicited through microbial perturbations 

 from plutonium in soil. 



It appears that large quantities of a transuranic nuclide would be required in the field 

 to cause obvious ecological effects. Two very crude calculations, one for liigher plants and 

 one for animals, illustrate the approximate levels of ^^^Pu required to produce, for 

 instance, detectable mortality. 



Plant Communities 



Assumptions: A grassland plant community requires a dose rate of about 40 rad/day to 

 show measurable changes in diversity (Whicker and Fraley, 1974); the effective decay 

 energy for ^^^Pu is 53 MeV/d, considering an RBE of 10 (International Commission on 

 Radiological Protection, 1960); a concentration ratio (CR = activity per gram of 

 plant -i- activity per gram of soil) of lO""* is assumed (Energy Research and Development 

 Administration, 1976); the '^^Pu is assumed to be uniformly distributed within plant 

 tissues and uniformly distributed in the upper 3 cm of soil, which has a bulk density of 

 1 .2 g/cm'' . Surficial contamination is neglected. 

 Calculations: 



Required ^^^Pu concentration in plant tissue 



1.5 X 10"^ inCi/g 



(40 rad/day )(6.25 X 10' MeV/g-rad) _ , , „ ,^_2 



(53 MeV/d)(3.2 X 10^ d/day-MCi) 



Required ^■'^Pu concentration in soil 

 ^ l.5xlO"'MCi/g 



Required -^^^Pu application to soil 



= (150MCi/g)(1.2g/cm^)(3cm3/cm2)(10'* cm^/mM=5.4 x 10*^ A/Ci/m^ 



This value is within an order of magnitude of the soil plutonium levels that appear to 

 evoke some toxic effects in plants under greenhouse conditions (R. E. Wildung and T. R. 

 Garland, Battelle— Pacific Northwest Laboratories, personal communication). 



Animals 



Assumptions: Inhalation of suspended soil is considered the critical route of entry; 

 human and experimental animal data and standards are used; the maximum permissible 

 "^ ^^Pu human lung burden of 1 .6 x 10^-'' /^Ci/g is achieved with a mean air concentration 

 of 10^^ ixQilnv' (International Commission on Radiological Protection. 1950); the 

 critical concentration of ^^^Pu in the lung required for subacute death is 1 x 10^' M^i/g 

 (Bair, 1974); and a mean resuspension factor of lO~^/m is assumed. 



