Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk CharacterizationZero values reflect the liner leakage rates in the empirical data set used to developcomposite landfill liner infiltration rates used in this risk assessment (from U.S. EPA, 2002b; seeSection 3.4.2), which are mostly zero values or very low in terms of infiltration rate. Althoughthese infiltration rates are based on the best data available to EPA, these data are not specific toCCW facilities. This represents an uncertainty in the analysis (see Sections 3.4.2 and 4.4.3.2).4.1.1.5 Modeled Peak Concentration Arrival TimesArrival times for the peak well concentrations used to calculate groundwater to drinkingwater risks for selected CCW constituents (arsenic, boron, cobalt, selenium, and thallium) areplotted as cumulative distributions for surface impoundments and landfills in the figures inAppendix L. These constituents were selected to represent the chemicals with the highest risksand to span the range of mobility in the subsurface. Table 4-7 summarizes these time of travelresults by showing selected percentiles from these distributions for each WMU/linercombination modeled in the risk assessment.As can be seen in Table 4-7, the peak arrival times for most constituents in unlinedsurface impoundment is less than 100 years (i.e., peak concentration occurs before or shortlyafter surface impoundment closure). The 10th percentile ranges from 70 years (for arsenic III,boron, and selenium VI) to 76 years (for selenium IV). The 50th percentile arrival times remainunder 100 years for most constituents, with only the less mobile forms of arsenic and seleniumhaving 50th percentile arrival times later than 100 years.Arrival times for unlined landfills are much longer, ranging up to thousands of years. Forboron and selenium IV, the 50th percentiles are 2,000 and 10,000 years respectively. However,even at the 10th percentile, arrival times ranged from 300 years (for boron) to 4,600 years (forselenium IV).At the higher percentiles, arrival times shown as greater than 10,000 years indicate thatthe contaminant plume did not reach the well before the simulation ended. Although the plumemight eventually reach the well in these cases, EPA does not believe that extending thesimulation beyond 10,000 years would have captured any significant risk beyond what wascaptured by the selection of the 90th percentile values, which reflect cases where the plume didreach the well. In other words, the 90th percentile values would not be influenced by whetherlower percentile concentrations were zero or the concentration at a peak beyond 10,000 years.Table 4-7. Time to Peak Well Concentration by WMU and Liner Type as ModeledLiner PercentileLandfills (all waste types)UnlinedArsenicIIITime to Peak (years) a,bArsenicIV Boron CobaltSeleniumIVSeleniumVIThallium c10 400 2,000 300 1,200 4,600 400 58030 1,100 7,100 880 4,100 9,400 1,000 1,10050 2,800 9,700 2,000 7,800 10,000 2,600 2,30070 6,400 10,000 4,300 10,000 >10,000 5,500 4,40090 >10,000 >10,000 9,400 >10,000 >10,000 10,000 9,700(continued)April 2010–Draft EPA document. 4-11