Submitted to: International Symposium on Biogeochemistry of Trace Elements Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: April 1, 2001
Publication Date: July 28, 2001
The USEPA's 1993 standards for the use or disposal of sewage sludge (40 CFR Part 503) derived risk-based numerical values for Mo based on the limiting pathway of biosolids to land to plants to ruminant animals which are most sensitive to chronic excessive Mo. Following legal challenge, most Mo numerical standards were withdrawn, pending additional field-generated data using modern biosolids (of < ceiling Mo concentrations of 75 mg kg-1) and pathway reassessment. This paper represents a re-evaluation of biosolids-Mo data, refinement of the algorithms used in the risk assessment, and reassessment of the molybdenosis risk from land application of biosolids. Forage Mo uptake coefficients were derived from field studies utilizing modern biosolids applied to a range of plants, soil types, and soil pH values. Typical cattle diet scenarios were used to calculate a diet-weighted UC value that realistically represents forage-Mo exposure to cattle. Recent biosolids use data were employed to estimate the fraction of animal forage (FC) likely to be impacted by biosolids applications. Long-term field data were used to estimate long-term Mo leaching, and a leaching correction factor (LC) used to adjust cumulative biosolids application limits (not included in the initial EPA calculation). The revised proposed limit would be 40 kg Mo/ha. We regard the modifications to algorithms and parameters and calculations as conservative, and believe the risk of molybdenosis from biosolids-Mo is small. Providing adequate Cu mineral supplements for ruminant livestock is a standard procedure in herd management, adds further conservatism to the new risk assessment.