Basic research and monitoring of sludge utilization programs have identified specific Pathways by which potentially toxic constituents of sewage sludge can reach and cause toxicity to livestock, humans, plants, soil biota, wildlife, etc. In the process of preparing a new regulation for land application of sewage sludge in the US, a Pathway approach to risk assessment was undertaken. Two Pathways were found to comprise the greatest risk from persistent lipophilic organic compounds such as PCBs: (1) direct ingestion of sludge by children; and (2) adherence of sludge to forage/pasture crops from surface application of fluid sludge, followed by grazing and ingestion of sludge by livestock used as human food. Each Pathway considers risk to Most Exposed Individuals (MEIs) who have high exposure to sludge. Because 1990 sewage sludges contain very low levels of PCBs, the estimated risk level to MEIs was less 0.0001, low sludge PCBs and low probability of simultaneously meeting all the constraints of the MEI indicate that MEIs are at less 0.0000001 lifetime risk. The authors conclude that quantitative risk assessment for potentially toxic constituents in sewage sludge can be meaningfully conducted because research has provided transfer coefficients from sludges and sludge-amended soils to plants and animals needed for many organic compounds.

Sponsoring agency: Risk Reduction Engineering Laboratory, Office of Research and Development, U.S. Environmental Protection Agency. "Presented at and to be published in the proceedings from the EEC Symposium on the Treatment and Use of Sewage Sludge and Liquid Agricultural Wastes (Athens, September 1990)." "PB91-182758." "EPA/600/D-91/062." Includes bibliographical references (p. 15-18).