Improving risk assessment of land-applied biosolids with probabilistic approaches

Land application of municipal and industrial wastewater biosolids for use as a fertilizer or soil conditioner is a common practice in the United States. Regulations are in place to reduce pathogens, minimize disease vectors, and limit concentrations of nutrients and some metals, but extensive assessment of the risk of biosolid-associated chemicals to human health and environmental systems is uncommon. Recently, the USEPA developed the Biosolids Tool (BST) to facilitate more comprehensive chemical risk assessment of land-applied biosolids based on a deterministic approach that utilizes conservative model inputs without regard for the variability and uncertainty inherent in environmental exposures. Management decisions based on probabilistic risk assessment (PRA), in which variability and uncertainty are quantified and risk is linked to specific population segments, may provide a more accurate understanding of risk. We examined the sediment risk assessment literature and explored the application of probabilistic model inputs within the BST to better understand how deterministic risk assessment and PRA methods compare for characterizing risk. The BST model results for noncancer and cancer risk outcomes associated with total ingestion of aluminum and benzo(a)pyrene in biosolids applied to pastureland for an adult and child indicated that PRA provides a more nuanced understanding of risk than the traditionally used deterministic approach. Receptor-specific risk patterns, model sensitivity, and risk drivers are discussed. Findings underscore the need for incorporating probabilistic methods into regulatory frameworks to improve the accuracy and reliability of risk assessments for biosolids land application.