PLUTONIUM DYNAMICS IN A DECIDUOUS FOREST ECOSYSTEM 515 



TABLE 1 Standing-Crop Biomass, Plutonium Concentrations, and 



Areal Amounts of Plutonium in a 30-yr-old Contaminated 



Deciduous Forest at Oak Ridge, Tennessee 



*Mass is based on a 20-cm soil depth and a soil density of 1.3 g/cm^ ; 

 concentration is the floodplain average. 



t Biomass is estimated from mensuration data (Van Voris and Dahlman, 

 1976) and regression equations (Harris, Goldstein, and Henderson, 1973); 

 concentrations are based on field measurements. 



:[: Biomass and concentration data are based on field measurements. 



After the six-compartment model had been cahbrated, plutonium transfers to animal 

 components of the ecosystem were simulated. The complexities associated with transfers 

 of plutonium to consumers and soil fauna in the model necessitated our simpHfying 

 assumptions to arrive at parameter estimates. For example, pathways for resuspension of 

 plutonium-contaminated soil to atmosphere and subsequent inhalation by animals were 

 not represented in the model. Resuspension factors (Anspaugh et al., 1975) range from 

 10~^° to 10~^ ^ m~^ for the forest, based on plutonium concentrations measured in soil 

 and air (Dahlman and McLeod, 1977). Since this resuspension factor is in the lower range 

 of values measured in natural environments [i.e., 10~^ to 10~^^ m~* (Hanson, 1975)], 

 food-chain transfer to animals is modeled as the chief transport pathway. 



The forest is modeled as a closed system since inputs of plutonium (e.g., fallout 

 resulting in a cumulative plutonium concentration in soil of 0.02 pCi/g) are negligible 

 relative to the existing soil contamination (plutonium in soil ranges from ~25 to ~150 

 pCi/g). Forest outputs include surface mnoff, erosion, and groundwater seepage, but 

 these processes are beyond the scope of this model. We believe that solution-phase 

 transport is negligible as an output from the ecosystem because plutonium is strongly 

 sorbed to soil (Bondietti, Reynolds, and Shanks, 1976). In addition, the downward 

 movement of plutonium in soil is not considered important over the time frame of the 

 simulations. Although there is evidence of a downward movement of plutonium in soil 

 over time (Bennett, 1976; Jakubick, 1976), soil fauna could promote redistribution of 

 plutonium from the subsoil to the soil surface. Reichle et al. (1973) calculated a 44-yr 

 turnover time for the top 25 cm of forest soil due solely to earthworm activity. 



Since the purpose of the model is descriptive, we have not made millennium 

 predictions of plutonium dynamics in the forest. Loucks (1970) points out that there is a 

 tendency in forests toward perturbation at time intervals ranging from decades to 

 centuries. Therefore simulations v^th the model were limited to time spans of less than 

 500 yr. Over this period of time, decreasing plutonium concentrations from radioactive 

 decay have a negligible effect on model predictions. 



