DISTRIBUTION OF PLUTONIUM IN ECOSYSTEMS 377 



Plutonium Inventories in the Floodplain and Canyon Ecosystems 



Total plutonium present in different ecosystem components is referred to as a static 

 inventory or budget. For each ecosystem the inventory was calculated by multiplying the 

 mass values of Table 2 and the plutonium concentrations (picocuries per gram) in the 

 respective components (Table 3). Results on areal distribution of soil plutonium (Fig. 1) 

 were integrated to provide an estimate of the plutonium inventory for the entire 3-ha 

 floodplain. Total soil plutonium inventory in the top 20 cm, based on the summation of 

 subinventories of four different concentration zones, was 0.5 Ci (8 g). The 100- and 

 150-pCi/g zones contained 88% of the plutonium, but they occupied only 46% of the 

 area of the 3-ha floodplain*. Because only 50% of the soil determinations included both 

 the topsoil (0 to 10 cm) and subsoil (10 to 20 cm) and because the zones of 

 concentration indicated by the isopleths in Fig. 1 are interpolated between sampling 

 stations, the soil inventory is provisional and does not represent high resolution of 

 plutonium distribution in the floodplain. 



The inventory of plutonium for the 0- to 20-cm soil depth in Mortandad Canyon was 

 calculated for the 3 X 1500-m segment (intensive study sites I and II) of stream channel 

 immediately below the effluent outfall. Nearly all the plutonium inventory is present in 

 this segment of the canyon (Hakonson and Bostick, 1975). The estimated plutonium 

 inventory was 0.054 Ci as of 1974, which corresponded closely to the estimated input of 

 0.051 Ci based on treatment-plant release records. 



A summation of component inventories shows that, by far, the majority of the 

 plutonium resides in soil. More than 30 yr after the initial deposit, less than 0.1% of the 

 total plutonium is present in living components of the Oak Ridge floodplain. For 

 Mortandad Canyon, the inventory of plutonium in biotic components of the canyon 

 ecosystem was 0.00015 Ci in the 0- to 1500-m segment, or 0.28% of the total plutonium 

 present in the canyon ecosystem. 



Inventory Ratios for Plutonium in the Floodplain and Canyon Ecosystems 



One approach used to understand the significance of relative distributions of plutonium 

 in ecosystems is to relate the plutonium inventory of the receptor to the plutonium 

 content of the donor or source component in terms of inventory ratios (IR's): 



P _ Activity/unit area in receptor 

 Activity/unit area in source 



Contaminated soil is the major source of plutonium for all biotic components of both the 

 floodplain and canyon ecosystems; consequently soil has been used as the denominator to 

 calculate IR's (Table 3). Components other than soil may contribute plutonium to certain 

 other components, e.g., wood is a likely source of plutonium in tree leaves. The reader 

 may choose data from Table 3 to derive other IR's not provided by our analysis. 



The IR results (Table 3) demonstrate the importance of soils as the reservoir for 

 environmental plutonium. Over 99% of the plutonium of both study sites was associated 

 with soils. Biota serve as an incidental, although potentially important, receptor for 

 plutonium in the environment, but only a small fraction of the total plutonium is present 

 in the biotic components. Root and Utter components of the floodplain had an IR of 

 approximately lO""^ compared with a value of 10~^ for litter at Los Alamos. Inventory 

 ratios for tree and aboveground vegetative components ranged from 10~^ to 10~^ on the 



