410 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



The depth of plutonium transport into channel and bank soil profUes in the Los 

 Alamos canyons is much greater than that at Trinity. In areas where permanent surface 

 water exists (i.e., Mortandad Canyon), elevated plutonium concentrations are found at 

 depths of 100 cm in the channel and at depths of 50 cm in the stream bank. Plutonium 

 concentrations in channel soils do not show any consistent patterns with sampling depth, 

 whereas decreasing concentrations with depth are evident in bank soils. In downstream 

 areas, which are dry except during periods of storm runoff, plutonium occurs at depths of 

 at least 30 cm (Nyhan, Miera, and Peters, 1 976a). 



The transport of plutonium into the channel alluvium and stream-bank soil has been 

 rapid, as shown by the presence of elevated ■^ ^^Pu at the lower sampling depths. Elevated 

 ^^^Pu was observed at soil depths of 30 cm in Mortandad Canyon in 1972, about 4 yr 

 after the first significant release of this element to the canyon (Hakonson and Bostick, 

 1976). In contrast, fallout 239,240p^ -^ Trinity soils Syr after the bomb test was 

 confmed to the upper 2.5 cm of soU (Olafson, Nishita, and Larson, 1957). 



A common feature of plutonium distribution in soils from both locations was that in 

 1974 less than one-half the total plutonium in the soil column was present in the surface 

 2.5 cm (Table 4) despite differences in soils and source of plutonium. In Acid— Pueblo 

 Canyon lOyr after the decommissioning of those facilities for waste disposal, an average 

 of 67% of the soil column inventory was below the 12.5-cm depth, which reflects 

 depletion of plutonium from the surface layers by vertical and horizontal transport 

 processes. Previous studies in the canyons have shown that horizontal transport of soil 

 during storm runoff events is an important mechanism in the downstream transport of 

 plutonium (Purtymun, 1974; Hakonson, Nyhan, and Purtymun, 1976). 



The depletion of plutonium from the soil surface decreases the probability of 

 horizontal transport by wind and water but may increase the probability of uptake by 

 vegetation during the time that the element is distributed within the plant rooting zone. 

 However, over long periods of time, continued movement of plutonium into the soil 

 profile may remove the element from the biologically active zone of the soil. 



Particle Size Relationships. Tlie highest concentrations of plutonium in soil at the Los 

 Alamos and Area 21 locations were associated with the silt-clay fraction, whereas this 

 fraction at Area GZ, 1 km from the crater, contained the lowest concentrations of 

 plutonium (Table 5) (Nyhan, Miera, and Neher, 1976b; Nyhan, Miera, and Peters, 1976c). 

 At the GZ location, the highest concentrations were measured in the 1- to 2-mm soil 

 particles, which perhaps reflects the physical characteristics of the fallout debris near the 

 detonation site and/or depletion of the plutonium from smaller size fractions by wind or 

 water transport vectors. Decreasing plutonium particle sizes with increasing distance from 

 the crater were also noted at weapons test sites in Nevada (Romney, 1977). 



The inventory of plutonium among the various soil size fractions in surface soils at 

 the Los Alamos and Area GZ Trinity study sites was similar in that the silt— clay size 

 fraction (<53 jim) comprised less than 10% of the soil mass and contained less than 15% 

 of the plutonium (Table 5), whereas over 80% of the plutonium was associated with soil 

 particles greater than 53 /am (Nyhan, Miera, and Neher, 1976b; Nyhan, Miera, and Peters, 

 1976c). The reverse was true at Area 21, Trinity Site, where the <53-/im fraction 

 comprised 36% of the soil mass and contained over 70% of the soil plutonium inventory. 



