The Hazard Ranking System And The National Priority List

The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) of 1980, better known as Superfund, became law "to provide for liability, compensation, cleanup and emergency response for hazardous substances released into the environment and the cleanup of inactive hazardous waste disposal sites." CERCLA was intended to give the EPA authority and funds to clean up abandoned waste sites and to respond to emergencies related to hazardous waste.

If a site poses a significant threat, the EPA uses its Hazard Ranking System (HRS) to measure the relative risk. Based upon this ranking system, sites warranting the highest priority for remedial action become part of the National Priority List (NPL).

The HRS ranks the potential threat posed by facilities based upon containment of hazardous substances, route of release, characteristics and amount of substances, and likely targets. HRS methodology provides a quantitative estimate of the relative hazards posed by a site, taking into account the potential for human and environmental exposure to hazardous substances. The HRS score is based on the probability of contamination from three sources— groundwater, surface water, and air—on the site in question. The HRS score assigned to a hazardous site reflects the potential hazards relative to other sites (Hallstedt, Puskar & Levine 1986).

SM is the potential for harm to humans or the environment from migration of a hazardous substance to groundwater, surface water, or air; it is a composite of scores of each of the three routes SFE is the potential for harm from flammable or explosive substances

SDC is the potential for harm from direct contact with hazardous substances at the site

The score for each of these hazard modes is obtained from a set of factors characterizing the facility's potential to cause harm as shown in Table 11.7.1. Each factor is assigned a numerical value according to the prescribed cri teria. This value is then multiplied by a weight factor, yielding the factor score.

The factor scores are then combined: scores within a factor category are added together, then the total scores for each factor category are multiplied together. SM is a composite of the scores of three possible migration routes:

Figure 11.7.1 shows a typical worksheet for calculating the score for groundwater. Other worksheets are included in 40 CFR Part 300, Appendix A (1987).

Use of the HRS requires considerable information about the site, its surroundings, the hazardous substances present, and the geology in relation to the aquifers. If the data are missing for more than one factor in connection with the evaluation of a route, then that route score becomes 0, and there is no need to assign scores to factors in a route set at 0.

The factors that most affect an HRS site score are the proximity to a densely populated area or source of drinking water, the quantity of hazardous substances present, and toxicity of those hazardous substances. The HRS methodology has been criticized for the following reasons:

There is a strong bias toward human health effects, with only slight chance of a site in question receiving a high score if it represents only a threat or hazard to the environment.

Because of the human health bias, there is an even stronger bias in favor of highly populated affected areas. The air emission migration route must be documented by actual release, while groundwater and surface water routes have no such documentation requirement. The scoring for toxicity and persistence of chemicals may be based on site containment, which is not necessarily related to a known or potential release of toxic chemicals.

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