Ecological Relationships of Plutonium 

 in Southwest Ecosystems 



T. E. HAKONSON and J. W. NYHAN 



A comprehensive summary of results was prepared on plutonium distribution and 

 transport in Los Alamos and Trinity Site study areas. Despite differences in ecosystems 

 and plutonium source, there are several similarities in plutonium distribution between Los 

 Alamos and Trinity Site study areas. First, the soils I sediment component contains 

 virtually all the plutoniufn inventory, with vegetation and rodents containing less than 

 0.1% of the total in all cases. 



Plutonium has penetrated to considerable soil depths at both locations, although it 

 has occurred much more rapidly and to a greater degree in the alluvial soil at Los Alamos 

 than in the arid terrestrial system at Trinity Site. However, in all cases less than 50% of 

 soil-column plutonium inventories was found in the surface 2.5 cm. The plutonium 

 penetration depth appears to correspond to the moisture penetration depth at Trinity 

 Site. This is probably the governing factor at Los Alamos, although storm runoff and 

 accompanying turbulent mixing processes complicate the process. In Acid-Pueblo 

 Canyon, the bulk of the soil column inventory lies in the lower profiles, an indication of 

 the loss of the plutonium from surface layers due to sediment transport. 



Soil plutonium, in most cases, was associated with relatively coarse-size fractions. The 

 silt-clay (<53 ym) fraction contained relatively little (<15%) of the plutonium: this 

 reflects the small amounts of this size fraction in study area soils. An exception was in 

 Area 21 at Trinity, where the <53-im72 soil-size fraction contained about 73% of soil 

 plutonium inventories. The importance of these distributional differences was demon- 

 strated for Trinity Site, where Bagnold dust samples from Area 21 contained 54% 

 silt-clay material and samples from Area Ground Zero (GZJ contained less than 10% of 

 this material 



Concentrations in herbaceous vegetation were generally related to those in soils from 

 all sites. Our belief, although unsubstantiated, is that external contamination of the plant 

 surfaces is the major contaminating meclmnism in these arid systems. Vie plutonium 

 concentrations in certain rodent tissues from all study areas were related to corresponding 

 soil concentrations. Over 95% of the plutonium body burden in rodents was associated 

 with pelt and gastrointestinal tract samples, indicating the dominance of physical 

 processes as the contaminating mechanism 



Horizontal transport of soil plutonium is dominated by physical processes. At Los 

 Alamos water governs the downstream transport of soil plutonium, and indications are 

 that wind is a relatively more important transport vector at Trinity Site. 



In no case was there evidence for trophic-level increase due to physiological processes 

 as plutonium passes from the soil to vegetation to the rodents, although food habits of 

 rodents are not hiown sufficiently to preclude a trophic-level increase. We believe, 

 however, that rodents most likely come into contact with environmental plutonium 

 directly from the soil and not through a food-web intermediary. 



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