Protecting Groundwater Quality at Biosolids Land Application Sites Using EPA's Multimedia, Multi-Pathway, Multi-Receptor Exposure and Risk Assessment (3MRA) Technology

During development of the 40 Code of Federal Regulations Part 503 biosolids rule, the United States Environmental Protection Agency (EPA) evaluated a number of fate and transport pathways by which humans and ecological receptors could become exposed to biosolids pollutants. While EPA originally determined that the human health risks associated with exposure of biosolids pollutants through groundwater ingestion was insignificant, recent improvements in EPA risk assessment methodology has generated regulatory and public interest in reevaluating the underlying assumptions and results from the groundwater exposure pathway risk assessment.One of the most important risk assessment tools recently released by the EPA is the Multimedia, Multi-pathway, Multi-receptor Exposure and Risk Assessment (3MRA) technology, a tool that allows rapid screening-level risk-based assessment of potential human and ecological health risks resulting from chronic (long-term) exposure to pollutants released from land-based waste management units including biosolids land application sites. Using the 3MRA technology as the computational framework, a simple computer-based risk characterization screening tool (RCST) has been developed to evaluate the effectiveness of best management practices (BMPs) to reduce the risk of groundwater quality impairment at biosolids land application sites. The RCST output consists of a non-carcinogenic risk characterization screening represented by a hazard quotient (HQ) whose value is based on the ingestion of groundwater associated with a biosolids land application activity.Case study results indicated that the depth to groundwater, regulated pollutant concentration and biosolids application rate can be modified by biosolids decision-makers to reduce the risk of groundwater quality impairment. For example, when the depth to groundwater was maintained at a distance of only 0.5 meters, the impact of land applied biosolids on groundwater quality was negligible even when pollutant concentrations were equivalent to ten times (10) the ceiling concentration limit. These results support EPA recommendations that a minimum soil depth to groundwater of two meters is protective of groundwater quality at biosolids land application sites.