408 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Vegetation at both study locations contains the highest plutonium concentrations of 

 any biotic component yet examined (Hakonson and Bostick, 1976). Plutonium 

 concentrations in native grasses and forbs ranged from 0.08 to 76 pCi/g (dry weight) at 

 Los Alamos and from 0.002 to 0.37 pCi/g in the Trinity fallout zone; levels in vegetation 

 generally do not exceed those in corresponding soil samples. Additionally, the highest 

 plutonium concentrations were associated with plants growing closest to the ground 

 surface; taller growth forms, such as shrubs and trees, contained the lowest concentra- 

 tions (Hakonson and Bostick, 1976; Hakonson and Johnson, 1974). 



Plutonium concentrations in rodents, as representatives of the primary consumer 

 trophic level, reflect the low physiological availability of the element. Pooled samples of 

 internal organs from rodents generally do not contain measurable levels of plutonium, 

 even though habitat soils may contain up to a few hundred picocuries per gram. 

 Plutonium concentrations in whole rodents ranged from analytical detection limits of 

 about 5 fCi/g to a few hundred femtocuries per gram; most of this radioactivity was 

 associated with samples of pelt and gastrointestinal (GI) tract and contents. 



Plutonium concentration variability, as characterized by the coefficient of variation in 

 soils, plants, and animals, was uniformly higli at all study sites. It commonly varied up to 

 2.0, with extreme values approaching 3.0 (Hakonson and Bostick, 1976; Nyhan, Miera, 

 and Neher, 1976b). Variability of this magnitude has been observed at several 

 environmental plutonium study sites in the United States (Little, 1976; Gilbert and 

 Eberhardt, 1976) and results in the need for very large sample sizes in field experiments 

 (Gilbert and Eberhardt, 1976; White and Hakonson, 1978). 



Soils 



Horizontal Distribution. Horizontal plutonium concentration gradients are evident in 

 both study areas, reflecting the dispersion from point sources of plutonium. Concentra- 

 tions in the Los Alamos stream channels decrease one to two orders of magnitude in a 

 predictable fashion (Nyhan, Miera, and Peters, 1976a; Hakonson and Bostick, 1976) 

 within 10 km of th€ effluent sources, whereas similar differences occur over much greater 

 distances at Trinity and do not necessarily decrease with distance. For example, 

 plutonium concentrations in Trinity soils gradually increase from a minimum just outside 

 the crater to a maximum at about 50 km from the crater (Nyhan, Miera, and Neher, 

 1976b; Larson etal., 1951). 



Liquid effluent radionuclides at Los Alamos have been transported laterally into the 

 stream banks as well as to downstream areas. Stream-bank soils accumulate radionuclides 

 to levels equivalent to adjacent channel soil (Anonymous, 1977), and they serve as a 

 long-term source of these materials to stream-bank biota. The stream banks, wliich are 

 heavily vegetated, retard the downstream movement of radionuclides since they are not 

 subject to the severe erosion encountered in the channel. 



Although plutonium concentrations average much higher in the canyons than at 

 Trinity, the extent of the contamination in the canyons is confined to less than 0.1 km^, 

 whereas the low-level contamination at Trinity Site covers several thousand square 

 kilometers. Consequently the ecosystems at risk at Los Alamos are exposed to higher 

 concentrations of plutonium than those at Trinity; however, the areas involved are 

 smaller, and corrective action could be taken more easily should it ever be necessary. 



