ORGANIC CONTAMINANTS IN MUNICIPAL BIOSOLIDS - RISK ASSESSMENT, QUANTITATIVE PATHWAYS ANALYSIS, AND CURRENT RESEARCH PRIORITIES

Basic research and monitoring of the fate and potential effects of PCB s and other xenobiotic organics in biosolids (municipal sewage sludge) used on cropland have identified specific Pathways by which the xenob iotic organics in biosolids can reach and cause exposure to humans, li vestock, plants, soil biota, wildlife, etc. In order to provide the sc ientific basis for the Clean Water Act Regulations (40 CFR 503) on lan d application of biosolids in the U.S., a Pathway Approach to risk ass essment was undertaken. Pathways included general food production; gar den food production; soil ingestion by humans, livestock, and wildlife ; human exposure through livestock tissues where the livestock were ex posed through crop contamination or biosolids/soil ingestion; wildlife exposure through soil organisms; release to surface and groundwater; volatilization into inhaled air, or dust generated by tillage, Two Pat hways were found to comprise the greatest risk from persistent lipophi lic organic compounds such as PCBs: (1) adherence of biosolids to fora ge/pasture crops from surface application of fluid biosolids, followed by grazing and ingestion of biosolids by livestock used as human food ; and (2) direct ingestion of biosolids by children, Each Pathway cons iders risk to Highly Exposed Individuals (HEIs) rather than to the gen eral population who seldom have appreciable exposure to biosolids or f oods grown on biosolid-amended soils. Because present (1995) biosolids contain very low levels of PCBs in countries which have prohibited ma nufacture and use of these compounds, the estimated increase in lifeti me cancer risk to HEIs from biosolids-borne PCBs applied to cropland o r gardens was much less than 10(-4). Low biosolids PCBs and low probab ility of simultaneously meeting all the constraints of the HEI indicat e that HEIs have less than 10(-7) increase in lifetime cancer risk fro m biosolids-borne PCBs; this provides even higher protection to the ge neral population. We conclude that quantitative risk assessment for po tentially toxic constituents in biosolids can be meaningfully conducte d because research has provided transfer coefficients from biosolids a nd biosolid-amended soils to plants and animals needed to assess risk for many organic compounds.