ON THE USE OF UNCERTAINTY ANALYSES TO TEST HYPOTHESES REGARDING DETERMINISTIC MODEL PREDICTIONS OF ENVIRONMENTAL PROCESSES

This paper illustrates the use of Monte Carlo parameter uncertainty an d sensitivity analyses to test hypotheses regarding predictions of det erministic models of environmental transport, dose, risk and other phe nomena. The methodology is illustrated by testing whether Pu-238 is tr ansferred more readily than Pu-239+240 from the gastrointestinal (GI) tract of cattle to their tissues (muscle, liver and blood). This illus tration is based on a study wherein beef-cattle grazed for up to 1064 days on a fenced plutonium (Pu)-contaminated arid site in Area 13 near the Nevada Test Site in the United States. Periodically, cattle were sacrificed and their tissues analyzed for Pu and other radionuclides. Monte Carlo parameter uncertainty analyses were conducted rising the P u data in a deterministic model of GI-to-tissue transfer to obtain a s ubjective probability density function (pdf) of the ratio of predicted Pu-238 over Pu-239+240 fractional transfers. This subjective pdf was used to test, conditional on the assumed deterministic model and param eter Uncertainties, the hypothesis that Pu-238 was transferred more re adily from the GI tract to tissues than Pu-239+240. Differences in fra ctional transfers for Pu-238 and Pu-239+240 were indicated using this methodology. Conditional sensitivity analyses of the model predictions were also conducted. These analyses indicated that Pu cattle tissue c oncentrations had the largest impact of any model parameter on the pdf of predicted Pu fractional transfers. Issues that arise in conducting uncertainty and sensitivity analyses of deterministic models are disc ussed This research was supported by the Basic Environmental Complianc e and Monitoring Program (BECAMP), US Department of Energy, Nevada Fie ld Office, Las Vegas, Nevada.