Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterization• For ecological receptors exposed via surface water, risks for landfills exceed an HQ of 1for boron (HQ of 281 for unlined and 78 for clay-lined), lead (HQ of 8 for unlined), andselenium, arsenic, and barium (HQs of 2) at the 90th percentile, but 50th percentile HQsare well below 1. For surface impoundments, 90th percentile risks for severalconstituents (boron, lead, arsenic, selenium, cobalt, and barium) exceed an HQ of 1, withboron showing the highest risks (HQ over 2,000). Only boron exceeds an HQ of 1 at the50th percentile (HQ = 7 for unlined surface impoundments). The HQs over 1 for boronand selenium are consistent with reported ecological damage cases, which includeimpacts to waterbodies through the groundwater-to-surface-water pathway.• For ecological receptors exposed via sediment, 90th percentile risks exceed an HQ of 1for both landfills and surface impoundments because certain CCW constituents stronglysorb to sediments in the waterbody. Here, the 90th percentile HQ for lead was 58 forunlined landfills and clay-lined surface impoundments, and 311 for unlined surfaceimpoundments. For arsenic, HQs were 11 and 3 for unlined and clay-lined landfills, and127 and 55 for unlined and clay-lined surface impoundments. Cadmium had HQs of 5 forunlined landfills, and 30 and 9 for unlined and clay-lined surface impoundments.Antimony had an HQ of 2 for unlined landfills. Composite lined surface impoundmentsalso had risks above an HQ of 1 for lead (HQ of 4), arsenic (HQ of 31), and cadmium(HQ of 2). The 50th percentile risks are an order of magnitude or more below an HQ of 1for ecological receptors exposed via sediments.Sensitivity analysis results indicate that for most of the scenarios evaluated (over 70percent), the risk assessment model was most sensitive to parameters related to the contaminantsource and groundwater flow and transport: WMU infiltration rate, leachate concentration, andaquifer hydraulic conductivity and gradient. For strongly sorbing contaminants (such as lead andcadmium), variables related to sorption and travel time (adsorption coefficient, depth togroundwater, receptor well distance) are also important.One of the most sensitive parameters in the risk assessment (infiltration rate) is greatlyinfluenced by whether and how a WMU is lined. The 1994–2004 DOE/EPA survey results(U.S. DOE, 2006) do not include information on how many unlined facilities are still operatingtoday, but do indicate that more facilities are lined today than were in the 1995 EPRI survey dataset on which this risk assessment was based. This suggests that the risks from future CCWdisposal facilities are likely to be lower than the results presented in this report.There are uncertainties associated with the CCW risk assessment, but scenariouncertainty (i.e., uncertainty about the environmental setting around the plant) has beenminimized by basing the risk assessment on conditions around existing U.S. coal-fired powerplants around the United States. Uncertainty in environmental setting parameters has beenincorporated into the risk assessment by varying these inputs within reasonable ranges when theexact value is not known. Uncertainty in human exposure factors (such as exposure duration,body weight, and intake rates) has also been addressed through the use of national distributions.Some uncertainties not addressed explicitly in the risk assessment have been addressedthrough comparisons with other studies and data sources.April 2010–Draft EPA document. 4-59