Exposure scenarios, inadvertent intrusion

NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at disposal sites should be used in classifying waste. This recommendation is based on two considerations. First, scenarios for inadvertent intrusion can be applied to an assumed type of disposal system at any site, whereas scenarios for exposure of members of the public due to release and transport of hazardous substances to locations beyond the boundary of a disposal facility are highly site-specific and, thus, are not appropriate for use in generally classifying waste. 

An important characteristic of acceptable disposal facilities for high-hazard waste is that inadvertent intrusion into a facility, such as by drilling, must be unlikely. Therefore, assessments of risk or dose to hypothetical inadvertent intruders based on exposure scenarios that are assumed to occur do not provide a suitable basis for determining acceptable disposals in facilities located well below the ground surface. 

The use of an acceptable (barely tolerable) risk to classify nonexempt waste can be justified, in part, on the following grounds. Disposal facilities for exempt and low-hazard waste both are located near the ground surface, and many scenarios for inadvertent intrusion into municipal/industrial landfills for nonhazardous waste also would be credible occurrences at disposal sites for low-hazard waste. However, these types of scenarios should be less likely to occur at hazardous waste sites, compared with sites for disposal of nonhazardous waste, given the intention to maintain institutional control and records of past disposal activities for a considerable period of time after closure of hazardous waste sites and the possibility that societal memory of disposal activities will be retained long after institutional control is relinquished. Thus, the risk to future inadvertent intruders at dedicated hazardous waste disposal sites, taking into account the probability that exposures according to postulated scenarios would actually occur, should be comparable to the risk at disposal sites for nonhazardous waste. 

Exposure Scenarios for Classifying Exempt Waste. Based on the definition of exempt waste as any waste that would be generally acceptable for disposal in a municipal/industrial landfill for non-hazardous waste, scenarios for inadvertent intrusion appropriate to this type of facility should be used in determining whether a waste would be classified as exempt. 

Establish generic exposure scenarios for inadvertent human intrusion into a municipal/industrial landfill for disposal of exempt waste and intrusion into a dedicated near-surface facility for disposal of low-hazard waste. 

In addition, the reliance on generic scenarios for inadvertent intrusion in classifying waste cannot, by definition, represent site-specific risks. However, this is not a serious shortcoming because such scenarios have been used in establishing subclasses of low-level radioactive waste for disposal in near-surface facilities (NRC, 1982a). Furthermore, as emphasized in this Report, establishment of a risk-based waste classification system using particular exposure scenarios does not obviate the need to perform site-specific risk assessments for the purpose of establishing waste acceptance criteria at each disposal site. 

In implementing the risk-based waste classification system developed in this Report, the selection of exposure scenarios appropriate to waste disposal is an important technical issue that must be addressed. NCRP believes that scenarios for inadvertent intrusion into near-surface disposal facilities are appropriate in classifying waste for purposes of disposal and, further, that scenarios involving permanent occupancy of disposal sites after loss of institutional control would be appropriate (see Section 6.1.3) such scenarios are commonly used in regulating near-surface disposal of low-level radioactive waste and in risk assessments at hazardous waste sites subject to remediation under CERCLA. 

Alternative Exposure Scenarios. In the scenarios for inadvertent intrusion at a radioactive waste disposal facility considered in Sections 7.1.3.1 through 7.1.3.3, intrusion is assumed to be a onetime event occurring at 100 y after disposal. This Section considers alternative scenarios and their impacts on classification of the Hanford waste. 

Approach to Example Analysis. Similar to the previous examples involving radioactive wastes, these residues were assumed to be placed in a typical near-surface disposal facility having a RCRA Subtitle C permit. In this example, it is assumed that an inadvertent intruder excavates an area of the disposal site of approximately 200 m2. This excavation is sufficient to reach the waste, and the exposure pathways considered involve inhalation of resuspended waste, ingestion of waste, and dermal absorption. The intrusion is identified and halted prior to any structures being constructed on the disposal site and before any farming activity can be developed. As in the similar scenarios used in the radioactive waste examples, exposure is assumed to continue for 1,000 h. 

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