Characterizing the Risk to Groundwater Quality from Biosolids Beneficial Use Activities

An important risk assessment approach developed by the US Environmental Protection Agency (EPA) is the Multimedia, Multi-pathway, Multi-receptor Exposure and Risk Assessment (3MRA) technology. 3MRA enables users to conduct screening-level risk-based assessments of potential human health risks resulting from chronic exposure to pollutants. Using the 3MRA technology as the computational framework, a simple computer-based risk characterization screening tool (RCST) was developed to evaluate the effectiveness of the current regulatory pollutant limits as well as EPA recommended best management practices (BMP) to reduce the risk of groundwater quality impairment at biosolids beneficial use sites.To document the performance of the RCST, two biosolids land application sites from the state of Virginia were selected as case studies. In each case study, two specific scenarios were evaluated. In Scenario 1, the independent variables were the regulated biosolids pollutant concentrations and the biosolids application rate while, in Scenario 2, the independent variables were the regulated biosolids pollutant concentrations and the depth to groundwater.From RCST case study results, it was determined 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. At a soil depth to groundwater of only 0.5 meter, the risk to groundwater quality impairment was significant (i.e., HQ ≥ 1) at all study sites. These results suggested that, at such sites, engineering controls should be implemented (e.g., subsurface drains) to lower the water table to a sufficient depth.Alternatively, when the depth to groundwater was maintained at a distance of at least two (2) meters (i.e., ten feet), the impact of regulated pollutant concentrations up to a level equivalent to five times (5) the ceiling concentration limit was insignificant (i.e., HQ < 1) when the biosolids land application rate was held at nine metric tons per hectare (9 mt/ha) or less. These results support the EPA recommendations that a minimum soil depth to groundwater of at least two (2) meters is protective of groundwater quality at biosolids land application sites.Only under extreme operational conditions were the human health risks associated with groundwater consumption found to be significant. For example, when the biosolids application rate was increased from 90 to 900 metric tons per hectare and the pollutant concentrations were increased to a level equivalent to ten times the regulatory concentration limit, the human health risks associated with groundwater consumption were significant for all regulated pollutants evaluated. Since current beneficial use practices requires that biosolids be applied at rates no greater than agronomic rates (typically 5 to 15 metric tons per hectare) and that no biosolids can be land applied if their pollutant levels are greater than the regulatory limit, these modeling results demonstrate and underscore the effectiveness of the current Part 503 rule as well as EPA recommended BMPs in protecting public health.