Human and Ecological Risk Assessment - Earthjustice

Section 2.0Problem Formulationliner conditions have changed as CCW facilities were built or expanded since 1995. The 56WMUs surveyed in the U.S. DOE (2006) study were commissioned between 1994 and 2004.Although the actual number of WMUs that were established in that timeframe cannot be verified,based on proxy data (i.e., CCW available for disposal in those states with identified new WMUsand coal-fired power plant generating capacity), the sample coverage was estimated to be at least61 to 63 percent of the total population of the newly commissioned WMUs. 7 With the exceptionof one landfill, the newly constructed facilities are all lined, with either clay, synthetic, orcomposite liners. The single unlined landfill identified in the recent DOE report receives bottomash, which is characterized as an inert waste by the state, and therefore, a liner is not required.There has been a marked trend away from unlined WMUs in favor of lined units, with a distinctpreference for synthetic or composite liners. A comparison of the 26 coal combustion plants inboth the EPRI survey and the DOE/EPA survey (U.S. DOE, 2006) showed that although most ofthose facilities (17 of 26) were using unlined WMUs in 1995, all 26 are now placing wastes innew or expanded landfills or surface impoundments that are lined with clay, synthetic, orcomposite liners. However, it is likely that the older unlined units were closed with wastes inplace, and that these wastes therefore still pose a threat through groundwater pathways. Inaddition, the available data cannot be used to determine the number of unlined units thatcontinue to operate in the United States. See further discussion of the uncertainty posed by theuse of the EPRI liner data in Section 4.4.1.The full-scale assessment was conducted using several modeling components: (1) EPA’sComposite Model for Leachate Migration with Transformation Products (EPACMTP; U.S. EPA,1997a) groundwater model, (2) a simple steady-state surface water and aquatic food web model,and (3) a multipathway exposure and risk modeling system.2.4.1 Data CollectionFor the sites representing each WMU and waste type combination selected for analysis,the Monte Carlo analysis began with input files that contain, for each Monte Carlo realization,the following variables collected at and around each of the 181 modeled sites:• WMU area, depth, and capacity• WMU liner status (no liner, clay liner, composite liner)• Waste type (conventional CCW, CCW codisposed with coal refuse, FBC wastes)• Soil texture (for vadose zone properties and infiltration rates)• Soil pH and organic carbon• Aquifer type• Groundwater temperature• Climate center (for infiltration rates)• USGS Hydrologic Region (for surface water quality data)• Surface water type and flow conditions.7 For additional details as to how these estimates were derived, the reader is referred to the DOE study, pages S-2to S-3 of the Summary Section and Section 3.1.2.April 2010–Draft EPA document. 2-13