Human and Ecological Risk Assessment - Earthjustice

Executive SummaryHuman and Ecological Risk Assessment of Coal Combustion WastesFor ecological receptors exposed viasurface water, risks for landfills exceeded anHQ of 1 for boron and lead at the 90thpercentile, but 50th percentile HQs werewell below 1. For surface impoundments,90th percentile risks for several constituentsexceeded the risk criteria, with boronshowing the highest risks (HQ = 2,000).Only boron exceeded an HQ of 1 at the 50thpercentile (HQ = 7). Exceedances for boronand selenium are consistent with reportedecological damage cases, which includeimpacts to waterbodies through thegroundwater-to-surface-water pathway.For ecological receptors exposed viasediment, 90th percentile risks for lead,arsenic, and cadmium exceeded the riskcriteria for both landfills and surfaceimpoundments because these constituentsstrongly sorb to sediments in the waterbody.The 50th percentile risks were generally anorder of magnitude or more below the riskcriteria.Sensitivity analysis results indicate thatfor more than 70 percent of the scenariosevaluated, the risk assessment model wasmost sensitive to parameters related to thecontaminant source and groundwater flowand transport, including WMU infiltrationrate, leachate concentration, and aquiferhydraulic conductivity and gradient. For thegroundwater-to-surface water pathway,another sensitive parameter is the flow rateof the waterbody into which thecontaminated groundwater is discharging.For strongly sorbing contaminants (such aslead and cadmium), variables related tosorption and travel time are also important(adsorption coefficient, depth togroundwater, and receptor well distance).Although the best available data andtechniques were used, there were severaluncertainties associated with the CCW riskassessment. The major types of uncertaintywere as follows:• Scenario Uncertainty includes theassumptions and modeling decisions thatare made to represent an exposurescenario.• Model Uncertainty is associated withall models used in a risk assessmentbecause mathematical expressions aresimplifications of reality thatapproximate real-world conditions andprocesses.• Parameter Uncertainty occurs whenthere is a lack of data about the valuesused in the equations, data available arenot representative of the instance beingmodeled, or parameter values have notbeen measured precisely because oflimitations in technology.Scenario uncertainty has been minimizedby basing the risk assessment on conditionsaround existing U.S. coal-fired power plantsaround the United States. Uncertainty inenvironmental setting parameters has beenincorporated into the risk assessment byvarying these inputs within reasonableranges when the exact value is not known.Uncertainty in human exposure factors (suchas exposure duration, body weight, andintake rates) has also been addressedthrough the use of national distributions.Some uncertainties not addressedexplicitly in the risk assessment have beenaddressed through comparisons with otherstudies and data sources. These include theappropriateness of the leachate data used forlandfills, concentrations of mercury incurrent CCW, and the potential impacts offuture mercury regulations.Other uncertainties are not as easilyaddressed as the ones above. These includeissues such as receptor well distance, linerconditions, ecological benchmarks,ecological receptors at risk, and synergisticrisks. Detailed discussion of all the riskApril 2010–Draft EPA document.ES-9