314 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Design of the Model 



The model was conceived to describe plutonium cycUng in a deciduous forest. Studies 

 from a plutonium-contaminated forest adjacent to White Oak Creek on the U. S. 

 Department of Energy's Oak Ridge reservation were used as much as possible to design 

 the model and estabhsh its parameter values. In 1944, during the Manhattan Project, the 

 White Oak Creek floodplain was contaminated with 2 3 9,2 4 0p|j ^^^ mixed fission 

 products. After 33 yr a deciduous forest dominated by white ash {Fraxinus americana, 

 eight trees per 100 m^) and sycamore {Plantanus occidentalis, three trees per 100 m^) has 

 developed on the site (Van Voris and Dahlman, 1976). 



I 



Fig. 1 Diagrammatic model of plutonium transfers in a deciduous forest ecosystem 

 showing abstracted compartments and annual transfer coefficients. The basic model 

 included compartments linked by solid lines. Dashed lines indicate transfers and 

 compartments coupled after calibration of the basic model. 



Initially, a six-compartment model with 10 transfers was set up to represent 

 plutonium dynamics in soil and vegetative components of the ecosystem (Fig. 1). Average 

 annual biomass values (grams of dry weight per square meter) and plutonium 

 concentrations (picocuries per gram of dry weight) were multiplied to arrive at the 

 amount of plutonium (picocuries per square meter) in each compartment of the forest 

 (Table 1). A majority of the transfers in the model were calculated on the basis of 

 biomass flux (grams per square meter per year) from the donor compartment. In lieu of 

 site-specific data, fluxes were derived from data collected for eastern deciduous forests in 

 the Oak Ridge area during the IBP (Harris, Goldstein, and Henderson, 1973; SoUins, 

 Reichle, and Olson, 1973; Harris et al., 1975). Values for annual transfer coefficients and 

 their derivation are given in Table 2. Parameter values for the six-compartment model 

 were arrived at independent of model performance. Later, some parameter adjustment 

 (Table 2) was necessary to caUbrate the predicted amount of plutonium (picocuries per 

 square meter) in the forest after a 30-yr computer simulation; the calculated inventory 

 was based on field data. 



