Section 3: HRS Structure

3.1 Pathways

An Hazard Ranking System (HRS) score for a site is determined by evaluating four pathways:

Ground water migration (HRS Section 3.0);
Surface water migration (HRS Section 4.0);
Soil exposure (HRS Section 5.0); and the
Air migration (HRS Section 6.0).

The surface water migration pathway is composed of two migration components; overland/flood and ground water to surface water, reflecting migration of contaminants above ground and below ground from a site into surface waters. The HRS surface water pathway score is based on the higher of these two component scores. Also, three different "threats" are evaluated for the surface water pathway: drinking water, human food chain, and environmental, reflecting the different types of threats posed by contaminated surface water. Two different "threats" are also evaluated in the soil exposure pathway: resident population threat and nearby population threat. In general, threats scores are summed in pathway calculations.

Pathway scoresheets are provided for each pathway in the HRS rule:

Ground water migration pathway (HRS rule, Table 3-1, page 51595)
Surface water migration pathway (HRS rule, Table 4-1, page 51608 and Table 4-25, page 51628)
- Drinking water migration pathway
- Human food chain threat
- Environmental threat
Soil exposure pathway (HRS rule, Table 5-1, page 51646)
- Resident population threat
- Nearby population threat
Air migration pathway (HRS rule, Table 6-1, page 51651)

3.2 Site Score

The scores of the four pathways (0 to 100) are combined by a root mean square method to give a single site score (0 to 100).

Scores are first calculated for the individual pathways and then are combined for the site using the following equation to determine the overall HRS site score:

This method places an increased emphasis on higher scoring pathways. For example, a ten point increase in a pathways score from 50 to 60 points would raise the site score by 10 points. However, the inclusion of a new pathway with a score of 10 to a site already with a pathway scoring 50 points would result in an increase of about 1 point. Thus, information collected to improve a high pathway score has a potentially greater impact on the site score than does information collected to support the scoring of lower scoring pathways. Given an existing single-pathway score (A) less than 57, the additional score required for the same pathway to reach a site score of 28.50 is:

57 - A
whereas the score required for a second pathway is given by:

(3,249 - A²)^½

For example, suppose a preliminary scoring effort resulted in a single-pathway score of 50. Within that same pathway, only (57 - 50) = 7 additional points would be required for a site score of 28.50, while in a different pathway, (3,249 - 2,500)^½ = 27.3 points would be required. Knowing the two highest pathway scores usually is sufficient to determine whether the site score is likely to be above 28.50.

One important characteristic of this method is that the site score cannot be greater than the highest pathway score. Thus, the pathway likely to achieve the highest score should be evaluated first. If this pathway score is less than 28.50, no further evaluation is necessary: the site score will be less than 28.50. The table below is from the HRS Guidance Manual (Highlight 3-3) and shows further combinations of pathway scores.

Combination of Pathway Scores That Yield Site Score of 28.50

Individual Pathway Scores	Sum of Squared Pathway Scores	Site Score
57.00 0.00 0.00 0.00	3,249	28.50
40.31 40.31 0.00 0.00	3,250	28.50
32.91 32.91 32.91 0.00	3,249	28.50
28.50 28.50 28.50 28.50	32,49	28.50

3.3 Pathway Score

Each pathway score is based on the product of three "factor categories" that are multiplied and normalized to a pathway score of 0 to 100.

The scaling factor of 82, 500 results in a pathway score of 100 when the three factor categories are at their maximum or "reasonable worst case" values of 550, 100, and 150.

3.4 Likelihood of Release

HRS Rule, Section 2.3, page 51589:

The likelihood of release factor category measures the likelihood that a hazardous substance has been or will be released to the environment. The likelihood of exposure factor category measures the likelihood that individuals will come directly into contact with hazardous substances.
Likelihood of release has the same structure in all three migration pathways.
Either an observed release is documented or the potential to release factors are evaluated.
An observed release is assigned the maximum value for likelihood of release (550 points).
An observed release can be documented either by "chemical analysis" of environmental samples or by "direct observation" of substances entering the ground water, surface water, or air.
If an observed release cannot be documented, then the potential to release factors are evaluated.
Potential to release is assigned a value of between 0 to 500 points.
Potential to release is generally evaluated based on the product of source characteristics and source containment factors.
- The source characteristics factors reflect the physical and environmental characteristics of the source and its location that influence the likelihood of release from the source in the absence of man-made barriers.
- The containment factors represent the physical characteristics of the man-made barriers at the source that lessen the likelihood of release.

3.5 Waste Characteristics

Section 2.4 of the HRS Rule, page 51589:

The waste characteristics factor category value is evaluated based on the characteristics of the "hazardous substance potentially posing the greatest hazard for the pathway" and the site hazardous waste quantity.
- The substance selected must be associated with a source with a non-zero containment factor value.
- The substance selected doesn't have to be present in each source or be the most prevalent at the site, and can be either associated with a source or in a release.
- Different substances may be identified in each pathway as the most hazardous substance for purposes of pathway scoring.
Different factors determine the identification and evaluation of the potentially most hazardous substance for a pathway.
- Ground Water Pathway: toxicity and ground water mobility
- Surface Water Pathway/Drinking Water Threat: toxicity and persistence
- Surface Water Pathway/Human Food Chain Threat: toxicity, persistence, and bioaccumulation potential
- Surface Water Pathway/Environmental Threat: ecosystem toxicity, persistence, and bioaccumulation potential
- Soil Exposure Pathway: toxicity
- Air Pathway: toxicity and air mobility
Special provisions apply to the evaluation of the toxicity and persistence of radioactive substances. HRS Rule, Section 7.2, page 51664 and 51665

3.6 Targets

Four types of targets are evaluated in each pathway:

Nearest individual
Population
Resources
Sensitive environments (Except ground water)

Targets may be subject to actual contamination or potential contamination depending the presence of an observed release and their relationship to the release.

Targets subject to actual contamination are evaluated differently depending on the magnitude of the observed release relative to media-specific benchmarks.

Targets subject to potential contamination are usually distance-weighted: potential targets further from the site are assigned lower values than targets closer to the site.

Continue to Section 4: Source Characterization