Monte Carlo analysis of the U.S. EPA model of human exposure to cadmium in sewage sludge through consumption of garden crops.

The derivation of the recently promulgated U.S. EPA regulations for land application of sewage sludge considered a model of human exposure to cadmium (Cd) from consumption of garden crops grown on sludge-amended soil. The EPA approach to the calculation of the maximum acceptable loading rate of Cd to soil from this pathway employs single point estimates for each input variable. It also considers Cd uptake slopes for crops grown in sludge-amended soils with pH from 4.4-8.4. The complexity and variability inherent in this pathway argue against obtaining a realistic assessment of this pathway with point estimates of input parameters. Furthermore, the use of uptake slopes from alkaline soils is likely to lead to underestimating Cd uptake from acidic soils prevalent in the eastern and southern United States. A re-analysis of this pathway is presented employing Monte Carlo probabilistic analysis of the exposure variables with Cd uptake slopes restricted to soils with pH < or = 6.5. Despite uncertainties in the derivation of some probabilistic inputs, this approach appears to present a more realistic assessment of this pathway.