Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterization4.0 Risk CharacterizationThis section summarizes the results of the full-scale Monte Carlo analysis andcharacterizes those results in terms of significant uncertainties and the scenarios and factors thatinfluence risks to human health and the environment. Results are presented at a high-end (90thpercentile) and typical (50th percentile) exposure for both pathways under each combination ofWMU type, ash type, and liner type.An overview of the assessment on which these results were based (e.g., wastemanagement scenarios, analysis framework) is provided in Section 2. Section 3 provides moredetails on analysis methodologies, parameter values, and assumptions. In this section, Section4.1 presents results from the human health risk assessment and includes an analysis of how linerconditions influence results. Section 4.2 presents the results from the ecological risk assessment.Tables summarizing the human and ecological results are presented in each section. Section 4.3describes the sensitivity analysis conducted for the CCW risk assessment, and Section 4.4discusses how variability and uncertainty have been addressed, including a semi-quantitativereview of the potential impact of some of the more significant uncertainties on results.The probabilistic results were based on a Monte Carlo simulation in which many modelinput parameter values were varied over 10,000 iterations of the model per waste managementscenario to yield a statistical distribution of exposures and risks. Per the Guidance for RiskCharacterization developed by the EPA Science Policy Council in 1995(http://www.epa.gov/OSA/spc/pdfs/rcguide.pdf), EPA defined the high end of the riskdistribution at the 90th percentile risk or hazard estimate generated during the Monte Carlosimulation. Thus, the 90th percentile risk results are shown in this section as the high-endestimate of the risk distribution generated during the Monte Carlo simulation of constituentrelease, fate and transport, and exposure associated with CCW disposal in landfills and surfaceimpoundments. In addition, the 50th percentile results are presented as the central tendencyestimate of that risk distribution.For exposure scenarios describing the waste management unit type (e.g., lined landfill;unlined surface impoundment), waste type (e.g., conventional CCW, ash mixed with coalrefuse), receptor (i.e., child, adult, ecological), and health endpoint (i.e., cancer, noncancer,ecological), the 90th percentile risk represents the high-end estimate of cancer or noncancer riskthat was used to help determine whether CCW disposal practices are protective of public health.To evaluate the significance of the estimated cancer risks or noncancer hazards that areattributable to CCW disposal for the exposure pathways assessed in this assessment, EPAcompared the risk estimates to a risk range (for carcinogens) or to a specific risk criterion (fornoncarcinogens) that are protective of human health and the environment:• An estimate of the excess lifetime cancer risk for individuals exposed to carcinogenic(cancer-causing) contaminants ranging from 1 chance in 1,000,000 (10 -6 excess cancerrisk) to 1 chance in 10,000 (10 -4 excess cancer risk). For decisions made to screen outApril 2010–Draft EPA document. 4-1