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 PCBs and other xenobiotic organics in biosolids (municipal sewage sludge) used on cropland have identified specific Pathways by which the xenobiotic organics in biosolids can reach and cause exposure to humans, livestock, plants, soil biota, wildlife, etc. In order to provide the scientific basis for the Clean Water Act Regulations (40 CFR 503) on land application of biosolids in the U.S., a Pathway Approach to risk assessment was undertaken. Pathways included general food production; garden food production; soil ingestion by humans, livestock, and wildlife; human exposure through livestock tissues where the livestock were exposed 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 Pathways were found to comprise the greatest risk from persistent lipophilic organic compounds such as PCBs: (1) adherence of biosolids to forage/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 considers risk to Highly Exposed Individuals (HEIs) rather than to the general population who seldom have appreciable exposure to biosolids or foods grown on biosolid-amended soils. Because present (1995) biosolids contain very low levels of PCBs in countries which have prohibited manufacture and use of these compounds, the estimated increase in lifetime cancer risk to HEIs from biosolids-borne PCBs applied to cropland or gardens was much less than 10−4. Low biosolids PCBs and low probability of simultaneously meeting all the constraints of the HEI indicate that HEIs have less than 10−7 increase in lifetime cancer risk from biosolids-borne PCBs; this provides even higher protection to the general population. We conclude that quantitative risk assessment for potentially toxic constituents in biosolids can be meaningfully conducted because research has provided transfer coefficients from biosolids and biosolid-amended soils to plants and animals needed to assess risk for many organic compounds.