Purpose of Waste Classification

3 In the United States, it is deemed unacceptable to dilute a waste that poses a high-hazard for the purpose of reducing the hazard, including doing so by combining wastes of different hazard, unless such combination eliminates an inherently hazardous characteristic e.g., ignitability, corrosivity). 

4 It is desirable to identify and segregate exempt wastes at the earliest possible time to avoid unnecessary expenditure of resources to further manage them and to avoid subsequent contamination with hazardous materials. 

In summary, a hazardous waste classification system is needed because (1) disposal of the composite unclassified waste would be prohibitively expensive and (2) the differences in timing between waste generation (now) and the development of treatment and disposal facilities (the future) require that wastes be segregated in anticipation of cost-effective means of waste management and disposal. Waste classification also allows consistent communication of the information needed to develop adequate treatment and disposal capacity and to develop appropriate regulations. 

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The recommendations on waste classification should focus on concepts, principles, and approaches to implementation. Recommendations on approaches to using assumed limits on risk or dose to establish quantitative boundaries of waste classes expressed as limits on concentrations of hazardous substances would be presented, and precedents that could be used to define the assumed limits on risk or dose and to assess risk or dose for purposes of waste classification would be discussed. However, specific recommendations on values of any such limits and many of the considerations involved in establishing them would not be given, because this is properly the role of policy makers and regulatory authorities. 

NCRP believes that a conceptually simple composite risk index for mixtures of hazardous substances can be developed that provides an adequate representation of risk for the purpose of waste classification. The composite risk index is written in terms of separate risk indexes for substances that induce stochastic (s) and deterministic (d) effects as ... 

Thus, for purposes of waste classification, it appears possible to assess the risk from the intrusion and atmospheric release pathways at a generic hazardous waste disposal site. However, a generic assessment of risk from the water release pathway normally would incorporate assumptions that would be extremely conservative for many sites (e.g., the amount of water infiltration and travel times of hazardous substances to a nearby well). 

Recommendations on Selection of a Measure of Response. For purposes of waste classification, NCRP believes that it would be desirable, in principle, to use the same measure of response for all hazardous substances, essentially because this approach would help give equal weight to all such substances in classifying waste. 

Estimated thresholds for deterministic responses in virtually all humans based on lower confidence limits of benchmark doses or NOAELs, as described above, would be used as points of departure in establishing allowable doses of chemicals that induce deterministic responses for purposes of waste classification. NCRP s recommendations on the magnitude of safety and uncertainty factors that should be applied to benchmark doses or NOAELs in classifying waste are described in Section 6.3.1.1. 

Assumptions about exposure scenarios are required in developing a risk-based waste classification system. These scenarios would be used to calculate potential risks posed by hazardous wastes for purposes of waste classification. An exposure scenario essentially is a set of assumptions about events and processes that could result in exposure of humans. 

Developing Exposure Scenarios for Purposes of Waste Classification... 

In general, calculation of the risk or dose from waste disposal in the numerator of the risk index in Equation 6.2 or 6.3 involves the risk assessment process discussed in Section 3.1.5.1. As summarized in Section 6.1.3, NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites should be used in calculating risk or dose for purposes of waste classification. Implementation of models describing exposure scenarios for inadvertent intruders at waste disposal sites and their associated exposure pathways generally results in estimates of risk or dose per unit concentration of hazardous substances in waste. These results then are combined with the assumptions about allowable risk discussed in the previous section to obtain limits on concentrations of hazardous substances in exempt or low-hazard waste. 

Calculation of the composite risk index for the purpose of waste classification based on the simplified Equation 6.8 is illustrated using the hypothetical data given in Table 6.1. Consistent with the form of the risk index in Equations 6.3 and 6.8, risk indexes for individual hazardous substances in Table 6.1 are expressed as the ratio of a... 

Finally, NCRP emphasizes that calculated risk indexes for substances that induce deterministic or stochastic responses are not intended to be used as predictors of the probability of a response for any actual or hypothetical exposure situation. The risk index is nothing more than a simple, dimensionless representation of the risk posed by hazardous substances in waste to be used for purposes of waste classification. 

The use of MLEs of probability coefficients for radionuclides but UCLs for chemicals that induce stochastic responses is the most important issue that would need to be resolved to achieve a consistent approach to estimating risks for the purpose of waste classification. For some chemicals, the difference between MLE and UCL can be a factor of 100 or more. The difference between using fatalities or incidence as the measure of response is unlikely to be important. Use of the linearized, multistage model to extrapolate the dose-response relationship for chemicals that induce stochastic effects, as recommended by NCRP, should be reasonably consistent with estimates of the dose-response relationship for radionuclides, and this model has been used widely in estimating probability coefficients in chemical risk assessments. The difference in the number of organs or tissues that are taken into account, although it cannot be reconciled at the present time, should be unimportant. 

In many respects, the foundations and framework of the proposed risk-based hazardous waste classification system and the recommended approaches to implementation are intended to be neutral in regard to the degree of conservatism in protecting public health. With respect to calculations of risk or dose in the numerator of the risk index, important examples include (1) the recommendation that best estimates (MLEs) of probability coefficients for stochastic responses should be used for all substances that cause stochastic responses in classifying waste, rather than upper bounds (UCLs) as normally used in risk assessments for chemicals that induce stochastic effects, and (2) the recommended approach to estimating threshold doses of substances that induce deterministic effects in humans based on lower confidence limits of benchmark doses obtained from studies in humans or animals. Similarly, NCRP believes that the allowable (negligible or acceptable) risks or doses in the denominator of the risk index should be consistent with values used in health protection of the public in other routine exposure situations. NCRP does not believe that the allowable risks or doses assumed for purposes of waste classification should include margins of safety that are not applied in other situations. 

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