Consideration of Alternative Assumptions

8 Consideration of Alternative Assumptions. Two aspects of the example analysis for electric arc furnace waste warrant further consideration. The first is the assumption that an acceptable dose of each chemical that induces deterministic effects is 10 times its RfD. The second is the assumption that exposures would occur only once over a lifetime and for a period of 1,000 h. 

The analysis for chemicals that induce deterministic effects presented in Section 7.1.7.4 and summarized in Table 7.8 indicates that lead is the most important such constituent. Furthermore, the risk index for lead of about 0.7 is only marginally below the value of unity used to define the boundary between low-hazard and high-hazard waste. Therefore, the assumption that an acceptable dose of 

As an alternative to the assumption of a one-time exposure for 1,000 h at the time of facility closure, permanent occupancy of a disposal site following loss of institutional control could be assumed (see Section 7.1.3.4). The assumption of chronic lifetime exposure would affect the analysis for hazardous chemicals that induce deterministic effects only if estimated intakes due to additional pathways, such as consumption of contaminated vegetables or other foodstuffs produced on the site, were significant. Based on the results for lead in Table 7.8, an intake rate from additional pathways of about 50 percent of the assumed intake rate by soil ingestion, inhalation, and dermal absorption would be sufficient to increase the deterministic risk index above unity. The importance of additional pathways was not investigated in this analysis, but they clearly would warrant consideration. The increase in exposure time during permanent occupancy does not otherwise affect the analysis for chemicals that induce deterministic effects, provided RfDs are appropriate for chronic exposure, because chronic RfDs incorporate an assumption that the levels of contaminants in body organs relative to the intake rate (dose) are at steady state. 

For substances that induce stochastic effects, an assumption of permanent site occupancy would increase the lifetime risk in proportion to the increase in exposure time. The risk indexes for chemicals that induce stochastic effects and radionuclides assuming 1,000 h of exposure obtained previously are 0.02 and 0.025, respectively. If permanent occupancy of a disposal site were assumed to occur after 100 y of institutional control, the risk index for radionuclides would be reduced by a factor of 10, due to the half-life of 137Cs. Then, if exposure were assumed to occur for 4,000 h y 1 (about half of the time during a year) for a period of 30 y (see Table 7.1), the estimated stochastic risk index would be about 0.02 X (4,000/1,000) X 30 = 2.4, due mainly to the chemicals that induce stochastic effects. The 

Based on the example analysis for electric arc furnace waste, the use of different assumptions about exposure scenarios or allowable doses of chemicals that induce deterministic effects could result in a difference in the resulting classification of the waste. This example thus illustrates the importance of judgment in classifying waste. 

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