386 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Soils used in the glasshouse studies were obtained from the surface layer (0 to 20 cm) 

 of the South Field, where crops were grown. The soil had an average total plutonium 

 content of 1.96 pCi/g (dry weight) with 21 a % ^^^Pu. The soils (~8 kg) were potted and 

 fertilized, and the same varieties of wheat, soybeans, and corn were grown to maturity. 



In the selection of locations for sampling soil and resuspendible particles, 18 grid 

 points were located in the South Field and 12 in the North Field. Grids were composed 

 of samipling blocks placed 3, 15, and 27 m from the southwestern margin of the fields on 

 transects across the short axis of the fields. The transects originated at 30.4-m intervals on 

 the long axis of each field. Each sampling block was 3 m by 10 m and contained ten 1- by 

 3-m plots. A randomly chosen plot was permanently marked in each block. Thus there 

 were 18 sampling locations in the South Field and 12 in the North Field. 



Soil cores of 3.8-cm diameter were taken with a split-barrel sampler and divided into 

 0- to 5-cm, 5- to 15-cm, and 15- to 30-cm fractions for plutonium analysis. For 

 unexplained reasons plutonium concentrations in the 0- to 5-cm samples collected before 

 tillage were 50% lower than in samples collected after tillage; therefore these samples 

 were replaced with samples that had been collected at the same depth before tillage with 

 a hand soil auger. 



The resuspendible particles on the soil surface from areas where aboveground 

 vegetation had been removed were collected by drawing a nearly laminar flow of air 

 (velocity = 6 m/sec) across a 232-cm^ area under a 1 -cm-tall stainless-steel hood. The 

 resuspended materials were collected in the paper bag of a small electric vacuum cleaner. 

 The interior of the plastic compartment holding the paper bag was wiped to collect 

 materials passing through the bag, and these materials were included in the sample. 



Samples of soil and of resuspendible particles were collected at three different times: 

 (1) before tillage, (2) after soil preparation for wheat, and (3) after wheat harvest. 



All samples were ashed before deterniination of plutonium contents. Plant, grain, and 

 resuspension samples were ashed at 550°C. Soil samples were ground to a particle size of 

 <500 Mm and ashed at 500°C. 



Actinide elements were leached from a <10-g aliquot of the sample ash with hot 8M 

 HCl, and valences were adjusted to ensure formation of Pu(IV) and Np(lV). Plutonium, 

 neptunium, and uranium were extracted into 10% triisooctylamine in xylene, and the 

 plutonium was separated from the neptunium and uranium by reducing Pu(IV) to Pu(III) 

 with NH4I and extracting into 8Af HCl. This solution was evaporated to dryness and 

 oxidized to destroy residual organic matter. The plutonium was taken up in 8M HNO3, 

 and the valence was adjusted to Pu(IV). Final purification was accomplished by adsorbing 

 the Pu(IV) onto an anion-exchange column and removing any residual iron, uranium, or 

 other contaminants from the column with 4MHNO3. The Pu(IV) was reduced to Pu(lII) 

 and eluted from the column with H2SO3. Following purification, the plutonium was 

 electrodeposited on platinum plates, and the amounts of '^"'^Pu and 239,240p^ ^^^^ 

 determined by alpha spectrometry with low-background high-resolution surface-barrier 

 detectors. Counting times ranged from 2 to 7 days, depending on the plutonium 

 concentrafion of the sample. An internal standard of ^^^Pu was used to determine 

 recovery efficiencies. 



After the concentrations of ^^^Pu and 23 9,24 0pjj j^^ ^ sample had been determined, 

 the rafio of the ^^^Pu concentration to the 2 3 9,24 0pu concentrafion was computed. 



