&EPA
United States
Environmental Protection
Agency
Office of
Solid Waste and
Emergency Response
Publication 9320.7-03FS
November 1990
Ranking System:
Backoround Information
Office of Emergency and Remedial Response
Hazardous Site Evaluation Division (OS-230)
Quick Reference Fact Sheet
The U.S. Environmental Protection Agency (EPA) has revised the Hazard Ranking System (HRS) in response
to the Superfund Amendments and Reauthorization Act of 1986 (SARA). The HRS is the scoring system EPA uses
to assess the relative threat associated with the release or potential release of hazardous substances from a waste
site. The HRS score is the primary criterion EPA uses to determine whether a site should be placed on the National
Priorities List (NPL). The NPL identifies sites that warrant further investigation to determine if they pose risks
to public health or the environment. Sites on the NPL are eligible for long-term "remedial action" financed under
the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA), as amended
by SARA. SARA authorizes a "Hazardous Substances Superfund" totalling $8.5 billion over 5 years to pay costs
not assumed by those responsible for problems at a site. The HRS uses data that can be collected relatively quickly
and inexpensively, thus allowing most Superfund resources to be directed to remedial actions at sites on the NPL.
Summary of Revisions
The revised HRS retains the same cutoff score
and basic approach as the original HRS, while incor-
porating SARA requirements as well as improve-
ments identified as necessary by EPA and the public.
The revised HRS retains the ground water, surface
water, and air pathways, drops the direct contact
and fire/explosion pathways, and adds a fourth
pathway, soil exposure.
Several key provisions of the revised HRS make it
more comprehensive. They:
Evaluate new exposure pathways or threats
that assess direct contact of people with
contaminated soils, and contamination of
the aquatic food chain.
Expand how toxicity is evaluated, consider-
ing not only acute health effects, but also
carcinogenic and chronic noncarcinogenic
effects.
Increase the sensitive environments consid-
ered from just wetlands and endangered
species to environments designated by vari-
ous Federal and State agencies.
Evaluate the potential for air to be contami-
nated and for contaminated ground water
to enter surface water.
Other provisions make the revised HRS more
accurate. They:
Allow use of concentration data to deter-
mine the quantity of waste at a site.
Assign higher scores when people are actu-
ally 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.
The complexity and scope of the issues involved in
revising the HRS required EPA to get widespread
input. EPA sought information from a number of
sources such as its Science Advisory Board and, on
three occasions, requested public comment: before
drafting the revisions, after proposing the revisions
in the Federal Register, and after publishing a Field
Test report describing how the revisions scored
actual hazardous waste sites. These procedures gen-
erated over 2,500 comments (from approximately
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145 commenters). The majority of the commenters
believed that the revised HRS represented an im-
provement over the original HRS. Other commen-
ters, however, believed that the data required were
too extensive for a screening tool and raised numer-
ous technical issues. EPA made significant changes
based on these comments, as well as on the Field
Test. The result is a revised HRS that is a practical
and effective tool in identifying the nation's worst
hazardous waste sites.
Sara Requirements
SARA required that EPA modify the HRS so that,
"to the maximum extent feasible, [it] accurately
assesses the relative degree of risk to human health
and the environment posed by sites." Several spe-
cific requirements were spelled out.
Section 105 required EPA to:
Assess human health risks associated with
contamination or potential contamination
of surface waters, either directly or as a
result of run-off. This assessment should
take into account the use of these waters for
recreation and the potential migration of
any contaminant through surface water to
downstream sources of drinking water.
Evaluate damage to natural resources that
may affect the human food chain.
Assess contamination or potential contami-
nation of ambient air.
Section 118 required EPA to:
Give a high priority to sites where contami-
nation has resulted in the closing of drink-
ing water wells, or has contaminated a prin-
cipal drinking water supply.
Section 125 required EPA to:
Revise the HRS to assure appropriate con-
sideration of sites that contain substantial
volumes of wastes described in Section
3001(b)(3)(A)(i) of the Solid Waste Dis-
posal 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 com-
bustion of coal or other fossil fuels.
assessment must consider:
The
© Quantity, toxicity, and concentra-
tions of hazardous constituents
present in such wastes.
Extent of, and potential for, release
of such constituents into the envi-
ronment.
Degree of risk to human health and
the environment posed by such con-
stituents.
Original HRS
The original HRS used a structured value analy-
sis approach to scoring sites. This approach as-
signed numerical values to factors that relate to or
indicate risk based on conditions at the site. The
factors were grouped into three categories ~ ob-
served release/route characteristics, waste charac-
teristics, and targets ~ and were combined to obtain
category scores. Each category had a maximum
value, as did each component factor.
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 com-
bined using a root-mean-square approach. The final
HRS 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.
HRS =
gw
1.73
If all migration pathway scores were low, the HRS
score was low. However, the HRS score could be
relatively high even if only one pathway score was
high. This was an important requirement for HRS
scoring because some extremely dangerous sites
pose threats through only one migration pathway.
For example, buried leaking drums of hazardous
2 The HR5:
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substances could contaminate drinking water wells
but - if the drums were deep enough and the sub-
stances not very volatile not surface water or air.
Revised MRS
A number of major changes from the original
HRS involve more than one of the four pathways.
They are summarized before the individual path-
ways are discussed.
Structure. The revised HRS retains the three mi-
gration pathways. An EPA analysis of remedial
actions at NPL sites indicates that some significant
risks from direct contact may not have been com-
pletely addressed by removal actions, and these
risks should be of concern in determining priorities
for remedial action. Therefore, a fourth pathway,
soil exposure (named onsite exposure in the pro-
posed revisions), is now included in the total site
score. The pathway assesses direct human exposure
to hazardous substances or contaminated soil. The
direct contact and fire/explosion pathways have
been deleted.
The essential structural features of the revised
HRS generally remain the same as those of the
original HRS - that is, relative risks continue to be
evaluated using pathways, three factor categories
(likelihood of release, waste characteristics, and
targets), and factors -- and the score is calculated
similarly.
HRS
Every factor has been revised or is new in the
revised HRS. A few factors have been eliminated,
either because they did not discriminate among sites
or because they were replaced by more accurate
measures.
Key changes were made in the waste characteris-
tics factor category, the hazardous waste quantity
factor is now multiplied by toxicity and other factors,
instead of being added as they were in the original
HRS. This is one of several changes that make the
revised HRS more consistent with risk assessment
principles.
Observed Release. The original HRS scored an
observed release if the measured concentration of
the hazardous substance was significantly above the
background level and if that concentration could
reasonably be attributed to the site. EPA is retaining
this approach to scoring observed releases in all
four pathways but has incorporated criteria for
determining when a release is significantly above
background.
Hazardous Waste Quantity. Hazardous wastes,
in addition to including hazardous substances, almost
always include nontoxic substances. When the origi-
nal HRS was developed, EPA judged that the cost
during initial investigations (preliminary assess-
ments and site inspections) of reliably determining
the amount of hazardous constituents within the
hazardous waste was prohibitive and, in some cases,
not feasible. Therefore, the original HRS used the
total quantity of waste containing hazardous sub-
stances (as defined in CERCLA Section 101), ex-
cluding any wastes that were contained so that they
could not migrate.
The revised HRS uses a tiered approach to deter-
mine the hazardous waste quantity factor. Hazard-
ous constituent concentration data, mass of waste as
deposited, volume, or surface area of the source can
be used. This approach provides the flexibility to use
the best data available.
Toxicity. Toxicity, a factor in the waste character-
istics category for all four pathways, is intended to
represent the relative potential of a substance to
cause adverse health effects.
The original HRS assigned a toxicity factor value
from 0 to 3 based on the toxicity ratings developed by
N. I. Sax or the National Fire Protection Association
rating scheme. Both ratings primarily emphasized
acute toxicity of a substance. However, EPA's expe-
rience has been that adverse health effects at hazard-
ous waste sites may result from carcinogenic and
chronic noncarcinogenic exposures as well as acute
exposures.
The revised HRS evaluates three measures of tox-
icity in a tiered approach that uses acute data only
when the other data are not available. The three
measures are:
Cancer risks, based on two factors that
The HRS: Background Information 3
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EPA's Carcinogen Assessment Group has
developed for a variety of substances:
Cancer potency factors (also re-
ferred to as slope factors) derived
from experimental animals or
human epidemiologic data, if avail-
able.
Qualitative weight-of-evidence that
is, the overall strength of the data
indicating potential carcinogenicity.
Noncancer effects of chronic exposure, based
on verified Reference Doses (RfDs), the
estimated amount of a substance to which
the human population (including sensitive
subgroups) can be exposed on a daily basis
over a lifetime without an appreciable risk
of harmful noncancer effects. RfDs un-
dergo a formal EPA-wide review and verifi-
cation.
Acute toxicity, based on the LD^ or LC
(lethal dose or lethal concentration at which
50 percent of experimental animals exposed
die).
Targets (People and Sensitive Environments). In
the original HRS, the people actually exposed to
contamination did not count more than those poten-
tially exposed, nor was the level of exposure consid-
ered. To assess risks more accurately, the revised
HRS gives greater weight to actual exposures by:
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 con-
centration of contaminants.
Giving greater weight to people whose drink-
ing water is contaminated (or, for the soil
exposure pathway, people living, working,
or going to school on contaminated soil).
The evaluation of exposed target popula-
tions 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 bench-
mark if no standard exists.
Giving greater weight in the surface water
pathway to actual contamination of the
aquatic human food chain.
4 The HRS: Background Information
Where no actual exposure has been documented,
the people potentially exposed are distance .weighted
in the ground water and air pathways and dilution
weighted in the surface water pathway.
The revised HRS also replaces the use factor of
the original HRS with a more comprehensive re-
sources factor that considers recreational and other
uses in the ground water, surface water, and air
pathways.
Environmental Threats. In developing the origi-
nal HRS, EPA decided, given the need to set priori-
ties 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 expe-
rience 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 envi-
ronments are also considered in the soil exposure
pathway. Relative risks to human health, however,
are still weighted more heavily than sensitive envi-
ronments. In addition, the revised HRS expands
significantly the types of sensitive environments
evaluated at a site.
Radionudides. The revised HRS includes a spe-
cial section (Section 7) on scoring radionuclides that
allows for a parallel evaluation of radionuclides.
Ground Water Migration Pathway
The ground water migration pathway in both the
original and revised HRS (Figure 1) evaluates the
likelihood that hazardous substances at a site or
facility will migrate through the ground below and
contaminate aquifers (underground formations
holding usable amounts of water) and any drinking
water wells that draw on those aquifers.
The revised HRS ground water pathway has the
same general structure as in the original HRS.
However, every factor has been revised. The most
significant revision assigns weights to the target
population based on distance from the site to ac-
count for dilution in the aquifer. In addition, the
area (target distance limit) in which drinking water
wells are considered has been expanded. A new
factor, travel time, has been added to the potential-
to-release calculations. In the waste characteristics
category, the mobility of each hazardous substance
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is considered, rather than persistence as in the
original HRS.
The original HRS did not consider the direction
of ground water flow in determining which popula-
tions or environments could be affected by the mi-
gration of hazardous substances at the site. The
targets category gave equal weight to the entire
population drawing water within 3 miles of the site.
After evaluating several options for considering
ground water or contaminant flow direction, EPA
decided to retain the original system, based on cost
and technical considerations. Accurately determin-
ing local flow within the target distance would re-
quire considerable expenditure of time and public
funds, which EPA believes is justified only at the
nation's highest priority sites-that is, those already
on the NPL. However, where there is known con-
tamination, the target populations are weighted higher
than those only potentially exposed. Thus, the re-
vised HRS indirectly considers direction of sub-
stance migration by assigning weights to people
drinking water contaminated either above or below
health-based benchmarks and by usbg the nearest
exposed individual factor.
Likelihood of Release. The potential-to-release to
ground water is comparable to the route character-
istics/containment portion of the original HRS. EPA
has made a number of changes in how potential
releases are scored. In the original HRS, values for
depth to aquifer, net precipitation, permeability,
and physical state were added, then multiplied by the
value of a fifth factor, containment. The revised
HRS uses four factors:
Containment, which measures the means
Figure 1
Ground Water Migration Pathway
Original HRS
Likelihood of Release x Waste Characteristics x Targets
Observed Release
or
Route Characteristics:
Depth to Aquifer of Concern
Net Precipitation
Permeability of Unsaturated
Zone
Physical State
Containment
Toxicity/Persistence
Hazardous Waste Quantity
Ground Water Use
Distance to Nearest Well/
Population Served
Revised HRS
Likelihood of Release x Waste Characteristics x Targets
Observed Release
or
Potential to Release:
Containment
Net Precipitation
Depth to Aquifer
Travel Time
Toxlclty/Mobility
Hazardous Waste Quantity
Nearest Well
Population
Resources
Wellhead Protection Area
The HRS: Background Information 5
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taken at a site to minimize or prevent re-
leases of contaminants into ground water.
Net precipitation, which indicates the amount
of water available to infiltrate into ground
water.
Depth to aquifer, which provides a measure
of the time required for a contaminant to
reach the underlying aquifer.
Travel time, which measures the potential
of geologic materials to slow the migration
of contaminants to aquifers.
The potential to release is the sum of the values of the
first three factors multiplied by the value for con-
tainment.
Waste Characteristics. The waste characteristics
category of the original HRS included toxicity/per-
sistence and hazardous waste quantity factors. The
method used to evaluate persistence, however, was
based on biodegradability and was generally not ap-
plicable to ground water. In addition to the 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 popula-
tion potentially at risk from an actual or potential
release of hazardous substances from the site to an
aquifer. The revised HRS expands the target dis-
tance 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 cate-
gory 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 expo-
sure 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 indi-
6 The HRS: Background Information
vidual 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 dis-
tance.
The resources factor, a more comprehensive
measure, has replaced the ground water use factor in
the original HRS.
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 Protec-
tion 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.
Surface Water Migration Pathway
The surface water migration pathway in both the
original and revised HRS (Figure 2) evaluates the
likelihood that runoff containing hazardous sub-
stances from a site can move through surface water
and affect people or the environment. The revised
HRS differs from the original HRS in several ways.
The revised HRS:
Replaces route characteristics with two po-
tential-to-release components overland
flow/flood and ground water to surface water.
If both components are scored, the pathway
score is the higher of the two scores.
Divides the surface water pathway into three
subpathways representing threats to drink-
ing water, the human food chain, and the en-
vironment. The surface water migration
pathway score is the sum of the scores of the
three subpathways. This change in struc-
ture provides a relatively simple way to
account for the different substances and
targets that may be important for the differ-
ent types of potential exposure in the
subpathways.
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Surface
Figure 2
Water Migration Pathway
Original MRS
Likelihood of Release
Observed Release
or
Route Characteristics
Facility Slope/Intervening
Terrain
1-Year, 24-Hour Rainfall
Distance to Nearest Surface
Water
Physical State
Containment
x Waste Characteristics x Targets
Toxicity/Persistence
Hazardous Waste Quantity
Surface Water Use
Distance to Sensitive
Environment
Population Served/Distance
to Nearest Intake
Downstream
Revised MRS
Likelihood of Release:
Overland/Flood
Component
Observed Release
or
Potential to Release
By Overland Flow:
Containment
Runoff
Distance to Surface
Water
By Flood:
Containment
Flood Frequency
or
Ground Water to
Surface Water
Component
Observed Release
or
Potential to Release
Containment
Net Precipitation
Depth to Aquifer
Travel Time
Drinking Water Threat
Waste Characteristics x Targets
Toxicity/Persistence/Mobility*
Hazardous Waste Quantity
Nearest Intake
Population
Resources
Human Food Chain Threat
Waste Characteristics x Targets
Toxicity/Persistence/
Bioaccumulation/Mobility*
Hazardous Waste Quantity
Food Chain Individual
Population
Environmental Threat
Waste Characteristics x Targets
Ecosystem Toxictty/Mobility*/ Sensitive Environments
Persistence/Bioaccumulation
Hazardous Waste Quantity
Component;'. ^/iy,\j,^/'^^-1,1'yty,^
The MRS: Background Information 7
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Extends the distance to the targets at risk
from the probable point where hazardous
substances enter the surface water to a
point 15 miles from the source (versus 3
miles downstream of the farthest observed
contamination, or 1 mile in static water, in
the original MRS). The target values are
modified by dilution weighting -- that is, a
lower value is assigned to a larger body of
water because the substance is more di-
luted.
Drinking Water Threat. The drinking water
threat in the revised HRS retains the waste quantity
and toxicity/persistence factors of the original HRS
but evaluates them differently. Persistence is no
longer based solely on biodegradation but on four
additional decay processes (hydrolysis, photolysis,
volatilization, and free-radical oxidation). For each
hazardous substance in (or likely to be in) surface
water, a persistence value is assigned that reflects
the time the substance remains in the surface water.
The substance with the highest toxicity/persistence
value is used, along with the hazardous waste quan-
tity, in calculating the waste characteristics score.
The drinking water targets category in the revised
HRS retains the use and population factors of the
original HRS but substantially modifies them. In-
stead 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 hi three groups based on actual and
potential exposure. The population potentially ex-
posed 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 hu-
man food chain based on potential or observed con-
tamination 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
8 The HRS: Background Information
persistence, from among those known to be present
at the site and available to the surface water migra-
tion 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 fish-
ing-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 path-
way of the original HRS, 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 dam-
age and expands the types of environments consid-
ered. 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.
Soil Exposure Pathway
The soil exposure pathway (Figure 3) evaluates
the potential threats posed by direct, physical con-
tact with hazardous wastes or contaminated soil. It
is similar to the direct contact pathway, which was
scored in the original HRS but was not used to
determine if a site should be on the NPL. The revised
HRS evaluates the threat by looking at two groups
potentially at risk those living on property with
hazardous wastes or contaminated soils and those
living nearby with access to the property. The
resident population is evaluated based only on pres-
ence of contamination within the site boundary and
within 200 feet of the boundary. The resident popu-
lation is not evaluated on release potential, as in the
other pathways, because contaminants do not have
to migrate offsite for exposure to occur. Five targets
are evaluated in the resident population:
Resident individual ~ a person living on, or
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Figure 3
Soil Exposure Pathway
(Revised MRS Only)
Resident Population Threat
Likelihood of Exposure x Waste Characteristics x Targets
Observed Contamination
Toxiclty
Hazardous Waste Quantity
Resident Individual
Resident Population
Workers
Resources
Terrestrial Sensitive
Environments
Nearby Population Threat
Likelihood of Exposure x Waste Characteristics x Targets
Attractiveness/Accessibility
Area of Contamination
Toxicity
Hazardous Waste Quantity
Population Within 1 Mile
Nearby Individual
of:
going to school or day care on, contami-
nated property.
Resident population ~ people living on or
going to school or day care on contaminated
property.
Workers ~ people working on contaminated
property.
Resources - contaminated property used
for commerce, agriculture, silviculture, live-
stock production, or livestock grazing.
Terrestrial sensitive environments on con-
taminated property - aquatic environments
are considered in the surface water migra-
tion pathway.
The nearby population is evaluated on the basis
Attractiveness/accessibility and area of con-
tamination, which evaluate the likelihood of
exposure.
Population within a 1-mile travel distance
of the site.
Nearby Individual.
Air Migration Pathway
The air migration pathway of the revised MRS
(Figure 4) has the same three categories as the origi-
nal HRS, but each is revised. The original air
pathway was evaluated only if an observed release of
hazardous substances could be documented. As
required by SARA Section 105(a)(8)(A), the revised
HRS considers characteristics of the site to assess
the potential for release if none has been docu-
mented. The likelihood of release is determined, as
well as how many people and sensitive environments
could be exposed to hazardous substances carried in
the air and the inherent hazard associated with
potential exposures. The potential to release by
gases and particulates is evaluated separately based
on:
Containment, which assesses the ability of
natural or constructed barriers to inhibit
the escape of hazardous substances from a
source.
The HRS: Background information 9
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Source type ~ for example, containers (in-
cluding tanks), contaminated soil (includ-
ing land treatment), fire sites, landfills, sur-
face impoundments, and waste piles.
Migration potential, which reflects the rela-
tive tendency of hazardous substances con-
tained in a source to migrate.
In addition to the changes to waste quantity and
toxicity in the waste characteristics category dis-
cussed earlier, the reactivity and compatibility fac-
tors in the original HRS have been deleted because
they have proved not to be applicable to the vast
majority of NPL sites; mobility has been added. All
hazardous substances at a site are evaluated for gas
mobility. Particulate mobility is evaluated based on
the local climate. The two values are combined in a
matrix to determine the mobility factor.
In the revised HRS, the three target factors in the
original HRSland use, population within a 4-mile
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 re-
tained; the limit for sensitive environments evalu-
ated has been extended from 2 to 4 miles. In both
cases, distance weighting factors are used to repre-
sent the reduced concentrations farther away from
the site.
Figure 4
Air Migration Pathway
Original HRS
Likelihood of Release
Observed Release
x Waste Characteristics x Targets
Toxicity
Reactivity and Incompatibility
Hazardous Waste Quantity
Land Use
Population Within 4-Mile
Radius
Distance to Sensitive
Environment
Revised HRS
Likelihood of Release
Observed Release
or
Potential to Release
Gas
Gas Containment
Gas Source Type
Gas Migration Potential
Particulate
Particulate Containment
Particulate Source Type
Particulate Migration
Potential
x Waste Characteristics x Targets
Toxicity/Mobility
Hazardous Waste Quantity
Resources
Population Within 4-Mile
Radius
Nearest Individual
Sensitive Environments
10 The HRS: Background Information
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In the preamble to the proposed revisions to the
HRS, EPA requested comment on two issues:
O The cutoff score for proposing sites for the
NPL.
The policy of scoring sites based on current
conditions.
Cutoff Score. EPA chose an HRS score of 28.50 as
a cutoff for placing sites on the NPL because it
yielded an initial NPL of at least 400 sites as sug-
gested by CERCLA, not because EPA had deter-
mined that 28.50 represented a threshold of unac-
ceptable risks. Believing that the current cutoff
score has been a useful management tool, EPA
proposed that the cutoff score for the revised HRS be
functionally equivalent to the original cutoff. How-
ever, EPA wanted to evaluate the practical effects of
keeping the cutoff score at 28.50 - that is, will that
score continue to provide an appropriate set of
priorities for management purposes. EPA examined
several approaches for defining "equivalent to 28.50".
These approaches included:
A statistical analysis to determine what
revised HRS score best corresponds to 28.50
on the original HRS.
A determination of the percentage of poten-
tial 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 per-
centage.
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.
Based on an analysis of 110 test sites, scored with
both the original and revised HRS, EPA has decided
not to change the cutoff score at this time because the
analysis did not point to a single number as the
appropriate cutoff. The field test data show that few
sites score in the range of 25 to 30 with the revised
HRS. EPA believes that this range may represent a
true breakpoint in the distribution of site scores and
that the sites scoring above the range of 25-30 are
clearly the types of sites that should be captured with
a screening tool.
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 distin-
guished. 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 £).50
in the past have been shown to present risks.
ScoringontheBasisofCiuremtConditions. Under
the original HRS, EPA generally scored the three
migration pathways based on the conditions at the
site before any response action had been taken,
rather than based on current conditions at the site.
In revising the HRS, EPA decided that it may be
appropriate to evaluate sites based on current con-
ditions and to consider prior responses in calculat-
ing an HRS score.
The policy of evaluating sites based on current
conditions raised concerns that it might:
Encourage private parties to only take ac-
tion sufficient to lower the score so the site
would not be placed on the NPL.
Discourage public agencies from taking early
actions that could lower the score, thus pre-
venting the site from being on the NPL and
therefore eligible for Superfund monies.
EPA examined two approaches to incorporate
current site conditions in the HRS score. Under
either approach, EPA would only consider actions
prior to a site inspection, which provides most of the
data used to score a site. Because response action at
sites may be an ongoing process, it would be burden-
some to recalculate scores continually to reflect such
actions. The two approaches were:
Consideration of current conditions for
certain pathways or factors where appro-
priate.
A Consideration of current conditions rou-
tinely, but identification of situations where
initial conditions more accurately reflect
risks.
EPA decided to consider response actions prior
to a site inspection because it will provide increased
incentives for rapid response.
The HRS: Background Information 11
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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 NFL. This approach would also pro-
vide less incentive for a rapid response action, which
could lower the contamination risk in all pathways.
EPA is placing these limitations on what re-
sponse 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 re-
maining onsite, a minimum value will be
assigned to the hazardous waste quantity
factor.
For Further Information, Contact:
Hazardous Site Evaluation Division
Office of Emergency and Remedial Response
Mall Code OS-230 :
U.S. Environmental Protection Agency
401 M Street, SW
Washington, DC 20460
Or
The Superfund Hotline, (800) 424-9346 in the
continental U.S. or (202) 382-3000 in the
Washington, DC, area.
12 The HRS: Background Information
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