Note: This is a U.S. EPA text, reproduced in its entirety for availability on the Internet: "The Revised Hazard Ranking System: Background Information," U.S. EPA, Office of Solid Waste and Emergency Response, Publication 9320.7-03FS, November 1990.

• Allow use of concentration data to determine the quantity of waste at a site.
• Assign higher scores when people are actually exposed to contamination than when they are potentially exposed.
• Assign higher scores to potentially exposed people and sensitive environments closest to a site, with scores decreasing as distance from a site increases.

• Give a high priority to sites where contamination has resulted in the closing of drinking water wells, or has contaminated a principal drinking water supply.

• Revise the HRS to assure appropriate consideration of sites that contain substantial volumes of wastes described in Section 3001(b)(3)(A)(i) of the Solid Waste Disposal Act, also known as the Resource Conservation and Recovery Act (RCRA). These wastes include fly ash, bottom ash, slag, and waste from control of flue gas emissions, all generated primarily by combustion of coal or other fossil fuels. The assessment must consider:

Quantity, toxicity, and concentrations of hazardous constituents present in such wastes.

Extent of, and potential for, release of such constituents into the environment.

Degree of risk to human health and the environment posed by such constituents.

The category scores in the original HRS were then multiplied together within each of the migration pathways (ground water, surface water, and air) and normalized to obtain a pathway score. Finally, the scores for the three pathways (gw, sw, a) were combined using a root-mean-square approach. The final HRS score (or overall site score) was the square root of the sum of the squares of the pathway scores divided by a factor, 1.73, which put all final scores on a scale of 0-100.

• Adding factors to the ground water, surface water, and air pathways reflecting risks to the nearest exposed individual -- that is, the person who is closest to the site and so is expected to be exposed to the highest concentration of contaminants.
• Giving greater weight to people whose drinking water is contaminated (or, for the soil exposure pathway, people living, working, or going to school on contaminated soil). The evaluation of exposed target populations in both the ground water and surface water pathways includes a weighting factor based on the Federal primary drinking water standards, or some other health-based benchmark if no standard exists.
• Giving greater weight in the surface water pathway to actual contamination of the aquatic human food chain.

The revised HRS also replaces the use factor of the original HRS with a more comprehensive resources factor that considers recreational and other uses in ground water, surface water, and air pathways.

Environmental Threats. In developing the original HRS, EPA decided, given the need to set priorities for the spending of limited monies, to place greater weight on sites that posed threats to public health rather than to the environment. EPA's experience since then, however, suggested that a number of sites posing a serious threat to the environment were not scoring high enough to be on the NPL, and that some of the most serious threats clearly warrant remedial action. Therefore, the revised HRS gives greater weight than the original HRS to impacts on sensitive environments (wetlands, for example) in the surface water and air pathways. Sensitive environments are also considered in the soil exposure pathway. Relative risks to human health, however, are still weighted more heavily than sensitive environments. In addition, the revised HRS expands significantly the types of sensitive environments evaluated at a site.

Radionuclides. The revised HRS includes a special section (Section 7) on scoring radionuclides that allow for a parallel evaluation of radionuclides.

Waste Characteristics. The waste characteristics category of the original HRS included toxicity / persistence and hazardous waste quantity factors. The method used to evaluate persistence, however, was based on biodegradability and was generally not applicable to ground water. In addition to these changes in waste quantity and toxicity, the revised HRS replaces persistence with a mobility factor reflecting the rate at which a substance migrates. Combining mobility with the revised toxicity factor allows for discrimination among highly toxic substances that migrate at very different rates.

Targets. The targets category reflects the population potentially at risk from an actual or potential release of hazardous substances from the site to an aquifer. The revised HRS expands the target distance limit from 3 to 4 miles. Within that limit, four factors (instead of two) are considered: nearest well, population, resources, and Wellhead Protection Area.

The nearest well is a new factor in the targets category and is evaluated by measuring the distance to the nearest drinking water well. In the original HRS, the person using the nearest well was considered in a matrix with population. The two are now separate factors.

The second factor, population, indicates the number of people actually or potentially at risk from exposure to hazardous substances in drinking water wells. In the original HRS, all the people who drank water from wells within 3 miles of the site were counted equally. The total population was then combined in a matrix with distance to the nearest well to assign a single value. The revised HRS separates these factors to more clearly reflect individual risks and resource value/population risk. Population served is the sum of three groups:

• People exposed to contamination above health-based benchmarks -- for example, Federal drinking water standards.
• People exposed to contamination not above health-based benchmarks but significantly above background.
• People potentially exposed, weighted for distance.

The presence of a Wellhead Protection Area, as designated under Section 1428 of the Safe Drinking Water Act, is a new factor in the targets category score. This revision addresses SARA Section 118, which requires a high priority for sites affecting principal drinking water supplies. Wellhead Protection Areas are defined as areas around a well or well field supplying a public water system through which potentially harmful contaminants are likely to move toward and reach the well or well field.

The drinking water targets category in the revised HRS retains the use and population factors of the original HRS but substantially modifies them. Instead of the four uses in the original HRS use factor, with only the highest assigned a value, two uses (drinking water and other uses) are assigned values, providing a better evaluation of the risk to the resource. The distance to a surface water intake in the original HRS has been replaced with a nearest intake factor that is evaluated separately and is based on dilution at the nearest intake. As in the revised ground water pathway, the population served is evaluated in three groups based on actual and potential exposure. The population potentially exposed is weighted based on dilution.

Human Food Chain Threat. SARA Section 105(a)(8)(A) requires EPA, in revising the HRS, to consider the effects of hazardous waste sites on the human food chain. In developing the revisions, EPA determined that the most significant, measurable food chain risks involved contamination of the aquatic food chain. Therefore, the revised surface water migration pathway includes evaluation of the human food chain based on potential or observed contamination of aquatic food chain organisms.

In evaluating waste characteristics (and targets as well), a single hazardous substance is selected, on the basis of bioaccumulation potential, toxicity, and persistence, from among those known to be present at the site and available to the surface water migration pathway. Persistence is determined based on the same five decay processes as in the drinking water threat.

The targets category reflects the threat to people from consumption of fish and shellfish taken from the surface water migration pathway. Fishery use -- for example, commercial, subsistence, or sport fishing -- is evaluated to give an estimate of resource value. Population is calculated by estimating food chain products harvested from the contaminated surface water. Population is the sum of actual and potential contamination, and is determined based on bioaccumulation and annual production of each fishery in the surface water migration pathway.

Environmental Threat. In the surface water pathway of the original HRS, the sensitive environments were assigned a value in the targets category on the basis of distance to a particular type of sensitive environment - wetlands, for example. The revised HRS places more emphasis on environmental damage and expands the types of environments considered. Ecosystem toxicity is determined using EPA chronic water quality criteria for the protection of aquatic life (or other measures if the criteria are not available). Ecosystem persistence is evaluated as it is for the drinking water subpathway. The sensitive environments targets are weighted into groups based on ecologically-based benchmarks where sensitive environments are contaminated; otherwise, dilution factors are applied.

• Attractiveness/accessibility and area of contamination, which evaluate the likelihood of exposure.
• Population within a 1-mile travel distance of the site.
• Nearby Individual.

• Containment, which assesses the ability of natural or constructed barriers to inhibit the escape of hazardous substances from a source.
• Source type - for example, containers (including tanks), contaminated soil (including land treatment), fire sites, landfills, surface impoundments, and waste piles.
• Migration potential, which reflects the relative tendency of hazardous substances contained in a source to migrate.

In the revised HRS, the three target factors in the original HRS -- land use, population within a 4-miles radius, and distance to a sensitive environment -- have been modified, and a factor has been added to reflect the risk to the nearest individual. The 4-mile limit for population in the original HRS has been retained; the limit for sensitive environments evaluated has been extended from 2 to 4 miles. In both cases, distance weighting factors are used to represent the reduced concentrations farther away from the site.

• Statistical analysis to determine what revised HRS score best corresponds to 28.50 on the original HRS.
• A determination of the percentage of potential sites in CERCLIS (EPA's inventory of potential hazardous waste sites) that score above 28.50 on the original NPL and the setting of a cutoff that yields the same percentage.
• An identification of risk levels that on the average correspond to an original HRS score of 28.50 and a determination of what revised HRS score best corresponds to that risk level.

Because the HRS is intended to be a screening tool, EPA has never attached significance to the cutoff score as an indicator of a specific level of risk from a site, nor has EPA intended to imply that "risky" and "nonrisky" sites can be precisely distinguished. Nevertheless, the cutoff score has been a successful screening tool that has allowed EPA to set priorities and to move forward with studying and, where appropriate, to clean up hazardous waste sites. The vast majority of sites scoring above 28.50 in the past have been shown to present risks.

• Consideration of current conditions for certain pathways or factors where appropriate.
• Consideration of current conditions routinely, but identification of situations where initial conditions more accurately reflect risks.

EPA decided not to limit consideration of current conditions to certain pathways (for example, the soil exposure pathway) because this would overstate the risk at sites where removal of wastes has eliminated threats in all pathways and could lead to their being placed on the NPL. This approach would also provide less incentive for a rapid response action, which could lower the contamination risk in all pathways.

EPA is placing these limitations on what response actions will be considered:

• Response actions will be considered only if they actually remove waste from the site and dispose or destroy it in a RCRA-permitted facility. This approach should discourage private parties from taking only the action needed to lower the score below the HRS cutoff.
• Where EPA cannot adequately determine the amount of hazardous constituents remaining onsite, a minimum value will be assigned to the hazardous waste quantity factor.