Characterizing Human Health Risks Associated with Biosolids Land Application Practices

Using the United States Environmental Protection Agency's (EPA) Multimedia, Multi-pathway, Multi-receptor Exposure and Risk Assessment (3MRA) technology, researchers have developed a computer-based biosolids groundwater risk characterization screening tool (RCST). The RCST, which generates a non-carcinogenic human health risk estimate (i.e., hazard quotient or HQ), has the ability to conduct screening-level risk-based characterization of potential human risks associated with the chronic (long-term) exposure to pollutants released from biosolids land application sites. Application of the RCST to biosolids land application sites located in Yakima County (Washington State) demonstrated that the soil depth to groundwater, regulated pollutant concentration and biosolids application rate were the site-specific parameters found to have the greatest potential impact on managing the risk of groundwater quality impairment.In all scenarios investigated, the heavy metal selenium was found to be the most mobile of regulated biosolids pollutants. However, when the soil depth to groundwater was limited to 0.5 meter (and the application rate was held at 9 metric tons per hectare — mt/ha), the impact of biosolids pollutants on human health risks associated with groundwater consumption from wells located 1000 meters from the land application site was minimal. The HQ value was found to vary from 2.00 × 10−10 (for nickel) to 3.26 × 10−5 (for selenium) when the concentrations of biosolids pollutants were raised to 10X the ceiling concentration limit. Only under extreme biosolids applications and pollutant concentration conditions were public health risks characterized as significant. For example, when the biosolids application rate was increased from 90 to 900 mt/ha and the biosolids pollutant concentrations were increased to a level equivalent to 10X the ceiling concentration limit, the HQ values varied from 1.44 (for zinc) to 324.00 (for selenium).Since current regulatory practices requires that biosolids be applied at rates no greater than the nitrogen-based agronomic rate and that no biosolids can be land applied having pollutant concentrations above the ceiling limits, these modeling results demonstrate and underscore the effectiveness of the current Part 503 rule as well as EPA recommended best management practices in protecting public health from pollutants associated with land applied biosolids.