Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk CharacterizationTime to Peak Well Concentration by WMU and Liner Type as Modeled (continued)Time to Peak (years) a,bLiner PercentileArsenicIIIArsenicIV Boron CobaltSeleniumIVSeleniumVI Thallium c10 400 1,900 550 1,400 8,100 400 57030 1,400 8,200 1,400 5,900 >10,000 1,300 1,200Compacted50 4,000 10,000 5,600 10,000 >10,000 5,100 4,300clay70 >10,000 >10,000 10,000 >10,000 >10,000 10,000 10,00090 >10,000 >10,000 >10,000 >10,000 >10,000 >10,000 >10,00010 10,000 >10,000 9,600 >10,000 10,000 9,000 >10,000Synthetic or30 >10,000 >10,000 >10,000 >10,000 >10,000 >10,000 >10,000composite50 >10,000 >10,000 >10,000 >10,000 >10,000 >10,000 >10,000(clay andsynthetic) 70 >10,000 >10,000 >10,000 >10,000 >10,000 >10,000 >10,00090 >10,000 >10,000 >10,000 >10,000 >10,000 >10,000 >10,000Surface Impoundments (all waste types)10 70 73 70 71 76 70 N/A30 73 97 72 78 610 72 N/AUnlined 50 78 220 74 97 4,400 74 N/A70 91 890 80 190 10,000 80 N/A90 170 6,500 110 970 >10,000 110 N/A10 75 95 75 86 81 75 N/A30 86 350 80 140 3,000 80 N/ACompacted50 110 1,300 90 270 7,900 90 N/Aclay70 150 5,000 110 690 10,000 110 N/A90 340 10,000 150 3,100 >10,000 150 N/ASynthetic orcomposite(clay andsynthetic)10 1,300 10,000 960 9,500 1,900 990 N/A30 3,900 >10,000 2,800 >10,000 6,900 2,800 N/A50 8,600 >10,000 4,400 >10,000 >10,000 4,600 N/A70 >10,000 >10,000 7,000 >10,000 >10,000 7,300 N/A90 >10,000 >10,000 10,000 >10,000 >10,000 10,000 N/Aa Arrival times have been rounded to two significant digits.b >10,000 indicates that the contaminant plume did not reach the receptor well during the modeled period.c N/A = Not Applicable. Thallium was not modeled for surface impoundments (see Section 4.1.1 above).As with the higher constituent concentrations that are characteristic of surfaceimpoundments, the shorter arrival times for surface impoundments are primarily due to thehydraulic head of the waste liquids in the unit; by contrast, landfill leaching is driven solely byinfiltration of precipitation through the cap and liner of the unit and the peak concentration takesmuch longer to reach the well.The arrival times presented in Table 4-7 correspond to the arrival of the maximumestimated risks for each model run. However, for model runs where the risk range or HQcriterion was exceeded, the first exceedence would sometimes occur earlier than the maximumrisk arrivals reported in Table 4-7. This is consistent with the appearance of damage casesdescribed in U.S. EPA (2007), which were sometimes observed sooner than the time-to-peakestimates in Table 4-7.April 2010–Draft EPA document. 4-12