362 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Materials and Methods 



The radiological safety requirements for experimentally placing transuranic elements 

 ill the field are stringent and precise. The radionuclides must be securely contained, 

 readily retrievable, and isolated to eliminate biological and physical transport away from 

 the study site. The plants in this study were grown outdoors in transuranic-contaminated 

 soil contained in small weighing lysimeters. The containers were isolated from biota by a 

 wire-mesh exclosure designed to exclude mammals and birds (Hinds et al., 1976). The 

 exclosure was situated on the Arid Lands Ecology Reserve located on the Department of 

 Energy's Hanford Site in South Central Washington. 



The containers were constructed from 1-m lengths of polyvinyl chloride (PVC) pipe 

 measuring 13.2 cm in inside diameter. Metal bale handles were attached to the open end, 

 and the other end was enclosed with a watertight end cap. These containers were placed 

 inside a slightly larger (15.7-cm-diameter) PVC pipe buried vertically in the ground so 

 that the upper end was level with the ground surface. This arrangement facilitated 

 retrieval of the contaminated soil and exposed the soil profile in the containers to realistic 

 outdoor conditions of temperature and precipitation (Hinds, 1975). 



Treatment containers were initially filled to within 35 cm from the top with 1 1.1 kg 

 of oven-dried soil. Nitrate forms of ^^^Pu, ^^^Pu, ^'^'Am, ^^"^Cm, and ^'^^Np were 

 individually added to a 3.4-kg aliquot of oven-dried soil, which was then placed in 

 separate containers in a layer 20 cm thick. An additional 1.7 kg (10 cm) of clean soil was 

 added to the top of the contaminated soil. This brought the level of soil to within 5 cm of 

 the top of the container. The surface layer of clean soil was intended to prevent the 

 spread of radionuclides by wind to the surrounding environment or their deposition on 

 the surface of the experimental plants, which would have produced erroneous uptake 

 values. 



The soil used for this study was a silt loam of the Ritzville series. The soil has a pH of 

 6.2 and a cation-exchange capacity of 22.5 meq/100 g at pH 7 (Wildung, 1977). 

 RadionucUdes were added to the soil by pipetting I ml of the 4M HNO3 solutions 

 directly onto the soil, which had been adjusted to 5% moisture content. The oxidation 

 state of the radionuclides when added to the soil was +4 for the plutonium isotopes, +3 

 for ^"^^ Am aiTd ''^'^Cm, and +5 for the ^-^''Np. Enough CaCOj was added to the soil to 

 neutralize the HNO3. The amended soil was stored for 24 hr and then thoroughly mixed 

 in a V-blender before it was transferred to the containers. Two different amounts, 1 .0 and 

 0.1 mCi/3.4 kg soil, of "^Pu, ^^^Pu, ^^'Am, and ^^^Cm were added to the soil. 

 Neptunium-237 was added to the soil only at a concentration of 0.1 inCi/3.4 kg soil. 



Control containers containing only uncontaminated soil were also prepared so that 

 the levels of contamination in the treatment vegetation attributable to external 

 deposition or root uptake of radionuclides present in the soil from fallout or other 

 sources could be determined. 



Cheatgrass {Bronius tectorum L.), an annual grass, was planted in some of the 

 containers. The only water the cheatgrass containers received came from natural 

 precipitation, which averages 16 cm/yr for the study site (Hinds and Thorp, 1971 ). Peas 

 (Pisuni sativum, var. Blue Bonnet), barley (Honlcum viilgare, var. U. Cal. Briggs), and 

 alfalfa {Medicago sativa, var. Ranger) were planted in tlie spring. The crop plant 

 containers were irrigated and weighed so that the soil moisture content was maintained at 

 about 20% by weight throughout the growing season. The peas, barley, and alfalfa plants 

 were fertihzed with NH4NO3, at the rate of 300 kg/ha, approximately halfway through 



