A probabilistic approach employing Monte Carlo simulations for assessing parameter and risks as probabilistic distributions was used in an ecological risk assessment (ERA) model to characterize risk and address uncertainty. This study addresses the following sources of uncertainty: parameter inputs in the ERA models, risk algorithms and uncertain input concentrations. To achieve this objective, both sensitivity and uncertainty analyses are being conducted. Monte Carlo simulations were used for generating probabilistic distributions of parameter and model uncertainty. All sensitivity, uncertainty, and variability analyses were coded in Visual Basic as part of the ERA model software version 2001, which was developed under the Sustainable Green Manufacturing (SGM) program. This simulation tool includes a Window's based interface, an interactive and modifiable database management system (DBMS) that addresses the food web at trophic levels, and a comprehensive evaluation of exposure pathways. To verify this model, ecological risks from Cr, Ta, Mo and DU exposure at the U.S. Army Yuma Proving Ground (YPG) and Aberdeen Proving Ground (APG) were assessed and characterized.

For the case of DU exposure to YPG terrestrial plants, the overall distributions for DU uptake for plants suggest 90% likelihood in reduction in root weight. For most terrestrial animals at YPG, the dose is less than that resulting in a decrease in offspring. At APG, DU exposure potentially poses little risk for terrestrial animals, which is no observable impact on receptor's reproduction or development. DU potentially poses lower risks to aquatic species at APG as well. The overall risk posed by the metals followed the order of Mo>Cr>Ta for both YPG and APG sits. Blacktailed-jackrabbits, lesser long-nosed bats, mule deer and cactus mice, at YPG site, are expected to have a reduction in size and weight of offspring. Terrestrial plants are likely to exhibit a reduction in root weight. For APG site, the vulnerable receptors are white-footed mice, white-tailed deer, and cottontail rabbits. For terrestrial plants, the risk result suggests a reduction in root weight. Aquatic species did not show any observable risk from Mo, Cr, and Ta in the terms of survival, growth and mortality.