Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysisapproximately 300 coal-fired power plants identified by EIA data as having onsite wastemanagement of conventional CCW and CCW codisposed with coal refuse throughout the UnitedStates. For FBC wastes, these 177 sites include only 3 FBC landfills. EPA was able to add 4additional FBC landfill sites to better represent FBC waste management, for an overall total of181 sites in this analysis.The conceptual site layouts applied to each of the sites are described and pictured inSection 2.2.2. Two site layouts were used to define the relationship between a landfill or surfaceimpoundment and (1) a drinking water well (for human risk via the groundwater-to-drinkingwaterpathway) and (2) a surface water body (for human and ecological risk via thegroundwater-to-surface-water pathway). In each case, the receptor point (well or waterbody) wasassumed to lie within the boundaries of the groundwater contaminant plume. The distance fromthe edge of the WMU to the well or waterbody was varied for each model run based on nationaldistributions, with well distance taken from a national distribution for Subtitle D municipallandfills (U.S. EPA, 1988a) and distance to surface water taken from a set of measured distancesfor CCW landfills and surface impoundments developed for this assessment. Appendix Cpresents additional details on these distributions.The temporal framework was mainly defined by the time of travel from the modeledWMU to the well or waterbody, which can be up to one mile away from the edge of the unit, andthe exposure duration over which risks were calculated. The subsurface migration of some CCWconstituents (e.g., lead) may be very slow; therefore, it may take a long time for the contaminantplume to reach the receptor well or nearest waterbody, and the maximum concentration may notoccur until a very long time after the WMU ceases operations. This time delay may be on theorder of thousands of years. To avoid excessive model run time while not missing significantrisk at the receptor point, the groundwater model was run until the observed groundwaterconcentration of a contaminant at the receptor point dropped below a minimum concentration(10 -16 mg/L) or until the model had been run for a time period of 10,000 years. The minimumconcentration used for all fate and transport simulations (10 -16 mg/L) was at least a million timesbelow any risk- or health-based criteria.For the groundwater-to-drinking-water pathway (human health risk), risks werecalculated based on a maximum time-averaged concentration around the peak concentration ateach receptor well. The exposure duration (which varies from 1 to 50 years) 10 was appliedaround the peak drinking well concentration to obtain the maximum time-averagedconcentration.For the groundwater-to-surface-water pathway, the groundwater model produced surfacewater contaminant loads (based on groundwater concentration and flow) for a stream thatpenetrates the aquifer. Because the surface water model is a steady-state model, there is notemporal component to it and the receptor is exposed to the same concentration over the entireexposure duration. For human health risk, the loadings from groundwater to surface water wereaveraged over the exposure duration, bracketing the time of the peak groundwater concentration.10 Distributions of exposure duration and other exposure variables were obtained from the Exposure FactorsHandbook (U.S. EPA, 1997c,d,e) as described in Section 3.8.2 and Appendix F.April 2010–Draft EPA document. 3-14