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; November 1988
BftCKGROCJtO INFORMATION
PKDPOSKD REVISIONS TO HAZAED RANKING SYSTEM
The U.S. Environmental Protection Agency (EPA) is proposing revisions to
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 op toe NPL are eligible for
long-term "remedial action" financed under the Comprehensive Environmental
Response, Conpensation, and Liability Act of 1980 (CERCLA), as amended by SARA.
SARA authorizes a "Hazardous Substances Superfund" totaling $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.
SARA requires EPA to revise the HRS to assure that, to the maximum extent
feasible, it accurately assesses relative risk. Specifically, EPA is to:
p Assess how surface water contamination affects the human food chain and
recreational use of surface water.
o Consider potential contamination of ambient air as well as actual
contamination.
o Give a high priority to sites which have contaminated principal
drinking water supplies.
o Consider the toxicity, quantity, and concentrations of hazardous
constituents in fly ash wastes.
The current HRS, adopted in 1982, evaluates the relative threat of a site
over five pathways. The HRS score is based on the evaluation of three migra-
tion pathways—ground water/ surface water, and air. The two other pathways,
direct contact and fire/explosion, are evaluated to determine the need for
immediate removal (emergency) action. HRS scores range from 0 to 100. Sites
scoring 28.50 and above on the current HRS are eligible for the NPL.
The proposed revisions (Figure 1-4) retain the same basic approach as the
current HRS, while incorporating SARA requirements as well as improvements
identified as-necessary by EPA. The revisions propose to retain the ground
water, surface water, and air pathways, drop the direct contact and
fire/explosion pathways, and add a fourth pathway, onsite exposure, which is
similar to direct contact.
Proposal of the HRS revisions is followed by a 60-day public comment
period. EPA will review and respond to all comments, and change the proposed
revisions where appropriate. The revised HRS is expected to be published in
the Federal Register in early 1990.
Until that time, the current HRS is in effect. EPA will continue to
evaluate potential sites with the current HRS and place them on the NPL if they
meet the listing requirements. SARA does not require that sites now on the NPL
be re-evaluated with the new HRS. Sites already proposed on the basis of the
current HRS can be placed on the final NPL without re-evaluation until the
revised HRS takes effect. Based""on the current HRS, 797 sites are on the final
NPL (2 sites were deleted in September 1988) and an additional 378 have been
proposed.
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THE MAJOR REVISIONS TO TOE HRS PROPOSE TO:
o Add a fourth pathway, onsite exposure, similar to the direct contact-
pathway of the current HRS. EPA experience indicates exposure to
onsite contaminated soils or wastes is often important in selecting
remedial action for a site.
o Modify the surface water pathway to:
— consider contamination of. the aquatic human food chain
— consider exposure of recreational users
^-- evaluate the potential risk of flooding of the site
o Modify the air pathway to include the potential of a site to release
contaminants to the air. In the current HRS, the air pathway is
scored only if an observed release can be documented.
o Allow the flexibility to use data on concentrations of hazardous
constituents in wastes, if available, to calculate the hazardous waste
quantity factor. (The current HRS uses only the quantity of hazardous
waste as deposited.) A tiered system is proposed that will use
constituent data, waste quantity data, volume, and area, providing
greater accuracy by allowing use of the best available data.
o Change the toxicity factor in all four pathways, basing it not only on
acute toxicity but carcinogenic and chronic non-carcinogenic toxicity
as well.
o Add mobility factors to the ground water and air pathways to evaluate
the ability of specific substances to migrate and reach potential
targets. Mobility, in combination with toxicity, should provide a
more accurate assessment of the relative risks posed by specific
substances.
o More accurately assess target populations and sensitive environments'
by giving greater weight in the ground water and surface water
pathways to:
— those exposed to documented contamination from the site than
those potentially exposed. (The current HRS treats potential and
actual contamination equally.)
and
— those exposed to contamination above health-based benchmarks
(for example, Federal drinking water standards) or ecologically-
based benchmarks
o Weight target populations and sensitive environments in the ground
water, air and onsite pathway based on distance so that the people and
environments closest to the site receive the highest score, with scores
decreasing as distance from the site decreases.
o Increase the number of sensitive environments evaluated and the weights
given them in the surface water., air, and onsite pathways.
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Figure 1
Ground Water Migration Pathway
Current HRS
Release
Waste Characteristics
Q
Observed Release
or a
Route Characteristics
Q Depth to Aquifer of Concern
Q Net Precipitation
Q Permeability of Unsaturated Zone
Q Physical State
Q Containment
Hazardous Waste Quantity
Toxicity/'Persistence
Targets
Q Ground Water Use
Q Distance to Nearest
We///Population Served
Revised HRS
Likelihood of X
Release
Observed Release
or
Potential to Release
Q Depth to Aquifer/
HYDRAULIC
CONDUCTIVITY
Q Net Precipitation
Q SORPTIVE CAPACITY
Q Containment
Waste Characteristics X
Q Hazardous Waste Quantity*
Q Toxicity/MOBILITY
Targets
Q Ground Water Use*
Q Population*
Q MAXIMALLY EXPOSED
INDIVIDUAL
Q WELLHEAD
PROTECTION AREA
Items in italic under Current HRS have been dropped or replaced.
Items in caps under Revised HRS are new. Most items not in caps have been
revised significantly.
'Factor based on several sub-factors.
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Figure 2
Surface Water Migration Pathway
Current MRS
Release
Waste Characteristics
argets
Observed Release 3
or Q
Route Characteristics
Q Facility_Slope/
Intervening Terrain
Q One Year, 24 Hour
Rainfall
Q Physical State
Q Distance to Nearest Surface Water
Q Containment
Hazardous Waste Quantity
Toxicity/Persistance
Surface Water Use
Population Served/
Distance to Nearest
Intake Downstream
Distance to a Sensitive
Environment
Revised MRS
Likelihood of
Release
Drinking Water Threat
Waste Characteristics X
Observed Release
or
Potential to Release
OVERLAND FLOW
Q Containment
0 RUNOFF'
Q Distance to Surface Water
POTENTIAL TO RELEASE BY FLOOD
Q CONTAINMENT (FLOOD)
Q FLOOD FREQUENCY
Q Hazardous Waste Quantity*
Q Toxicrty/Persistence
Targets
Q' Surface Water Use*
Q Population*
Q MAXIMALLY EXPOSED
INDIVIDUAL
Likelihood of
Release
(same as above)
Human Food Chain Threat
Waste Characteristics X
Q Hazardous Waste Quantity*
Q Toxicrty/Persistence/
BIOACCUMULATION
Targets
Q FISHERY USE
Q POPULATION*
Likelihood of
Release
(same as above)
Recreational Threat
Waste Characteristics X
Q Hazardous Waste Quantity*
Q Toxicity/Persistence/DOSE
ADJUSTING FACTOR
Targets
Q POPULATION*
Likelihood of
Release
(same as above)
Environmental Threat
Waste Characteristics X
Q Hazardous Waste Quantity*
3 ECOSYSTEM
TOXICITY/Persistence
Targets
Sensitive
Environments
Items in italic under Current HRS have been dropped or replaced.
Items in caps under Revised HRS are new. Mo$t items not in caps have been
revised significantly. -
"Factor based on several sub-factors.
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Figure 3
Air Migration Pathway
Current HRS
Release .. X
Observed Release
Waste Characteristics X Targets
rJ Hazardous Waste Quantity Q
Q Toxicity Q
Q Reactivity and
Incompatibility Q
Land Use
Population Within
4-Mile Radius
Distance to Sensitive
Environment
Revised HRS
Likelihood of X
Release
Observed Release
or
POTENTIAL TO RELEASE
Q SOURCE TYPE
Q SOURCE MOBILITY*
Q SOURCE CONTAINMENT
Waste Characteristics X
Q Hazardous Waste Quantity*
Q Toxicrty/MOBILITY*
Targets
Q Land Use
Q Population
Q MAXIMALLY
EXPOSED INDIVIDUAL
Q Sensitive
Environments
Items in Italic under Current HRS have been dropped or replaced.
Items in caps under Revised HRS are new. Most items not in caps have been
revised significantly.
'Factor based on several sub-factors.
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Figure 4
ONSITE EXPOSURE PATHWAY
REVISED MRS*
LIKELIHOOD OF X
EXPOSURE
Q OBSERVED
CONTAMINATION
RESIDENT POPULATION THREAT
WASTE CHARACTERISTICS X TARGETS
Q TOXICITY
Q HIGH RISK POPULATION
Q TOTAL RESIDENT
POPULATION
Q TERRESTRIAL SENSITIVE
ENVIRONMENTS
LIKELIHOOD OP X
EXPOSURE
Q ACCESSIBILITY/
FREQUENCY OF USE
Q HAZARDOUS WASTE QUANTITY
NEARBY POPULATION THREAT
WASTE CHARACTERISTICS X
Q TOXICITY
TARGETS
Q POPULATION WITHIN
1 MILE
'The current MRS includes a direct contact pathway, but that pathway is not used in calculating the
overall HRS migration score.
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SARA REQUIRKffitfTS
SARA requires that EPA modify the HRS so that, "to the maximum extent
feasible, tit] accurately assesses the relative degree of risk to human
health and the environment posed by sites." Several specific requirements
are spelled out.
Section 105 requires EPA to:
o 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.
o Evaluate damage to natural resources that may affect the human food
chain.
o Assess contamination or potential contamination of ambient air.
Section 118 requires EPA to:
o 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.
Section 125 requires EPA to;
o 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
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DEVELOPMENT PROCESS
EPA'c Office of Emergency and Remedial Response (OERR), which is
responsible for revising the HRS, undertook a number of activities in the
course of developing these revisions:
o Analvzed specific issues identified since the current KR3 was adopted
'in 1982.
o Established an EPA-wide Work Group consisting of representatives from
Regional and Headquarters offices, as well as State agencies.
o Published an Advance Notice of Proposed Rulemaking on April 9, 1987
(52 FR 11513). EPA received 46 comments totaling 600 pages in the
30-day comment period after publication.
o Held a public meeting in Arlington, Virginia, on May 7-8, 1987, to hear
comments on revising the HRS. Formal comments were presented by the
American Mining Congress and New York State.
o Worked with a committee from EPA's Science Advisory Board (SAB), a
public advisory group structured to provide a balanced expert
assessment of scientific issues related to problems facing EPA. The
SAB committee held six 2-day public meetings on technical aspects of
the current HRS, including three specific issues:
— to develop options for revising the way toxicity of hazardous
substances is evaluated and scored in the HRS
— to determine if the HRS is biased against sites with mining wastes
and other high-volume wastes, including issues related to the use of
data on concentrations of hazardous substances instead of just data
on total waste quantities
— to determine the appropriate target distance over which population
exposure to air pollution is assessed
o Developed a revised HRS option that was subsequently refined by the
Work Group and incorporated SAB's recommendations.
CURRENT HRS
The current HRS uses a structured value analysis approach to scoring
sites. This approach assigns numerical values to factors that relate to or
indicate risk based on conditions at the site. The factors are grouped in1"?
three categories—observed release/route characteristics, waste characteris-
tics, and targets—and are combined to obtain category scores. Each category
has a maximum value, as does each component factor.
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The category scopes in the current HPS are then multiplied together within
each of the migration pathways (ground water, surface water, and air) and
normalised to obtain a pathway score. Finally, the scores for the three
pathways (gw, sw, a) are combined using a root-niean-square approach. Tne final
HRS score is the square root of the sura of the squares of the pathway scores
divided by a factor, 1.73, which puts all final scores on a scale of 0-10(2:
o o
HRS Score = / S^ + SS
gw T u sw
V 1.73
If all migration pathway scores are low, the HRS score will be low.
However, the HRS score can be relatively high even if only one pathway score is
high. This is an irrportant requirement for HRS scoring because some extremely
dangerous sites pose threats through only one migration pathway. For example,
buried leaking drums of hazardous substances can contaminate drinking water
wells but — if the drums are deep enough and the substances not very volatile
— not air or surface water.
PROPOSE) HRS REVISIONS
The proposed HRS retains the three migration pathways. An EPA analysis of
remedial actions at NPL sites indicates that some significant risks from direct
contact may not have been completely addressed by removal actions, and these
risks should be of concern in determining priorities for remedial action.
Therefore, a fourth pathway, "onsite exposure," is now included in the total
site score. The pathway assesses direct human exposure to hazardous substances
or contaminated soil. The fire/explosion pathway has been deleted.
The essential structural features of the proposed HRS generally remain
the same as those of the current HRS—that is, relative risks continue to be
evaluated using pathways, factor categories, and factors—and the root-mean-
square method is retained. However, every factor has been revised or is new in
the proposed revisions. A few factors are eliminated, either because they do
not discriminate among sites or because they are replaced by more accurate
measures.
Revisions Affecting Multiple Pathways
A number of major proposed revisions to the HRS involve more than one of
the four migration pathways. They are summarized before the individual
ways are discussed.
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Observed Release. The current HRS scores an observed release if the
measured concentration of the hazardous substance is significantly abova the
background level and if that concentration can reasonably be attributed to the
site. EPA is retaining this approach to scoring observed releases in all four
pathways but is proposing 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
current HRS was developed, EPA judged that the cost during initial
investigations (preliminary assessments and site inspections) of reliably
determining the amount of hazardous constituents within the hazardous waste is
prohibitive and, in some cases, not feasible. Therefore, the current HRS uses
the total quantity of waste containing hazardous substances (as defined in
CERCLA Section 101),. excluding any wastes that are contained so that they
cannot migrate.
After considering a number of alternatives to the current method and
presenting several to SAB, EPA is proposing a tiered approach to determining
the hazardous waste, quantity factor. Hazardous constituent concentration
data, quantity of wastes as deposited, volume of waste, or surface area or
volume of the source could be used. This approach provides the flexibility to
use the best data available.
Toxicity. Toxicity, a factor in the waste characteristics category for
all four proposed pathways, is intended to represent the relative potential of
a substance to cause adverse health effects.
The current HRS.assigns 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 emphasize acute toxicity of
a substance. However, EPA's experience has been that adverse health effects at
hazardous waste sites may result from carcinogenic and chronic noncarcinogenic
exposures as well as acute exposures.
After evaluating a number of methods to characterize and score toxicity,
EPA presented several options to SAB. The option chosen for the proposed HRS
is based on three measures of toxicity:
o Carcinogenicity, based on two factors that EPA's Carcinogen Assessment
Group has developed for a variety of substances:
— cancer potency factors derived from experimental animals or human
epidemiologic data, if available
— qualitative weight-of-evidence—that is, the overall strength of the
data indicating potential carcinogenicity
o Chronic noncarcinogenic toxicity, based on verified Reference Do^s
(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 undergo a formal EPA-wide review and verification.
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o Acute toxicity, based, on the LD50 or 1^50 (lethal dose or lethal
concentration at which 50 percent of experimental animals exposed die.)
Targets (People and Sensitive Environments). In the current MRS, the
people actually exposed to contamination do .not count more than those
potentially exposed, nor is the level of exposure considered. To more
accurately assess risks, EPA is giving^greater weight to actual exposures. EPA
proposes to:
o Add factors to the ground water, surface water, and air pathways
reflecting risks to the maximally exposed individual (MEI) -- that is,"
the person who is closest to the site and so is expected to be exposed
to the highest concentration of contaminants.
o Give greater weight to people whose drinking water is contaminated (or,
for the onsite pathway, people living or going to school on contaminat-
ed soil). The evaluation of exposed target populations in both the
ground water and surface water pathways would include a weighting
factor based on the Federal primary drinking water standards, or some
other health-based benchmark if no standard exists.
o Give greater weight to actual contamination in the human food chain and
recreational subpathways of the surface water pathway.
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.
Environmental Threats. In developing the current HRS, EPA decided, given
the need to set priorities for spending of limited monies, to place greater
weight on sites that posed threats to public health rather than to the environ-
ment. EPA's past experience, however, suggests that a number of sites posing a
serious threat to the environment ate not scoring high enough to be on the NPL,
and that some of the most serious threats clearly warrant remedial action.
Therefore, the proposed HRS gives greater weight than the current HRS to
impacts on sensitive environments (wetlands, for example) in the surface water
and air pathways. Sensitive environments are also considered in the onsite
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.
Ground Water Pathway
The ground water migration pathway in both the current 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 (under-
ground formations holding usable amounts of water) and any drinking water wells
that draw on those aquifers.
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The proposed revisions provide the same general structure as the current
HRS. Ho-.MSver, every factor of the ground vvater pathway is revised. The most
significant revision assigns weights to the target population based on distance
from the site to account for dilution in the aquifer. In addition, the area
(target distance limit) in which drinking water wells are considered is
expanded. A new factor, sorptive capacity, is added to the potential--to -
release calculations. In the waste characteristics category, EPA is. proposing
to consider the mobility of each hazardous substance rather than persistence,
as in the current ground water pathway.
The current HSS does not consider the direction of ground water flow in
determining which populations or environments may be affected by the migration
of hazardous substances at the site. The targets category gives equal weight
to the entire population in drawing water within 3 miles of the site.
After evaluating several options for considering ground water or
contaminant flow direction, EPA is proposing to retain the current system
based on cost and technical considerations. Accurately determining local flow
within the target distance would require 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 contamination, the target populations are weighted higher than those only
potentially exposed. Thus, the proposed revisions indirectly consider
direction of substance migration by assigning weights to people drinking water
contaminated either above or below health-based benchmarks and by using the MEI
factor.
Likelihood of Release. The proposed potential-to-release to ground water
is comparable to the route characteristics/containment portion of the current
HRS. EPA is proposing a number of changes in how potential releases are
scored. In the current HRS, values for depth to aquifer, net precipitation,
permeability, and physical state are added, then multiplied by the value of a
fifth factor, containment. The proposed HRS uses four factors:
o Depth to aquifer/hydraulic conductivity, which provides a measure of
the time required for a contaminant to reach the underlying aquifer.
o Net precipitation, which indicates the amount of water available to
infiltrate into ground water.
o Sorptive capacity, which measures the potential of geologic materials
to slow the migration of contaminants to aquifers.
o Containment, which measures the means taken to minimize or prevent
releases of contaminants at a site into ground water.
The potential to release is the sum of the values of the first three
factors multiplied by the value for containment.
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Waste Characteristics. The current waste characteristics category
includes hazardous waste quantity and toxicity/persistence factors. The
method used to evaluate persistence, however, is based on biodegradability and
is generally not applicable to ground water. In addition to the changes in
waste quantity and toxicity, the revised MRS 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 proposed revisions expand the target distance limit from 3 to 4
miles. Within that limit, four factors (instead of two) are considered:
ground water use, population, maximally exposed individual (MEI), and Wellhead
Protection Area.
The ground water use factor in the proposed HRS revisions has been
expanded to consider more uses than in the current HRS.
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 current HRS, all the people who drink water from wells within
3 miles of the site are counted equally. The total population is then
combined in a matrix with distance to the nearest well to assign a single
value. The proposed revisions separate these factors to more clearly reflect
individual risks and resource value/population risk. Population served is
divided into four groups:
o People exposed to contamination above health-based benchmarks—for
example, Federal drinking water standards.
o People exposed to contamination not above health-based benchmarks
but significantly above background within a certain range.
o People exposed to contamination not above health-based benchmarks but
significantly above background within a lower range than in the
previous group.
o People potentially exposed, weighted for distance.
The population factor is the sum of the four values.
The MEI is a new factor in the targets category and is evaluated by
measuring the distance to the nearest drinking water well. In the current HRS,
the person using the nearest well—which is, roughly comparable to the MEI—was
considered in a matrix with population. The two are now separate factors.
. The presence of a Wellhead Protection Area. (WHPA), as designated un£er
Section 1428 of the Safe Drinking Water Act, is a new factor in the targels
category score. This revision addresses SARA Section 118, which requires a
high priority for sites affecting principal drinking water supplies. WHPAs
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.
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Surface
The surface water migration pathway in both the current and revised HRS
(Figure 2) evaluates the likelihood that runoff containing hazardous substances
from a site could move through surface water and affect people or the environ-
s-vent. The proposed HRS replaces the route characteristics portion of the
current relaass category with two new groups of factors — overland flow and
potential to release by flood. These two factors are added to obtain a score
for likelihood of release in each of the four subpathways.
The proposed revisions would divide the surface water pathways into four
subpathways representing threats to drinking water, the human food chain,
recreational use, and the environment. The total surface water pathway score
is the sum of the scores of the four subpathways. This proposed change in
structure provides a relatively simple way to account for the different
substances and targets that may be important for the different types of
potential exposure in the four subpathways.
In the current HRS, the distance to the targets at risk, is measured from
the probable point where hazardous substances enter the surface water to a
point 3 miles downstream of the farthest observed contamination (1 mile in
static water). The proposed revisions extend this target distance to 15 miles
from the source in all four subpathways. The target values are modified by
dilution weighting — that is, a lower value is assigned a larger body of water
because the substance is more diluted.
Drinking Water Threat. The drinking water threat in the revised HRS
retains the waste quantity and toxic ity/persistence factors of the current HRS
but evaluates them differently. Persistence is no longer based solely on
biodegradation but on four additional decay processes as well (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 quantity, in calculating the waste characteristics score.
The drinking water targets category reflects the human population poten-
tially at risk from an actual or potential release of hazardous substances
into surface water.
The drinking water threat in the proposed HRS retains the use and
population factors of the current HRS but substantially modifies them.
Instead of the four uses in the current 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 current HRS is replaced with an MEI factor that is
evaluat-ed separately and is based on dilution at the nearest intake. The
population served is evaluated in four groups based on actual and potential
exposure, and is the same as in the revised ground water pathway. The
population potentially exposed to contamination is weighted based on dilution.
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SARA Section 105(a} (3) (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 determinGd that the iTiost
significant, measurable food chain risks involved contamination of the aquatic
food chain. Therefore, the proposed surface water 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,
toxicityr and persistence, from among those known to be present at the site and
available to the surface water 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
pathway.
Recreational Threat. SARA Section 105(c)(2) requires EPA to assess health
risks from recreational activities in contaminated surface water. EPA
concluded that the risks may be significant at some sites and decided to add a
separate recreational subpathway to the surface water pathway to address these
concerns.
The recreational threat assigns a dose adjusting factor in evaluating
toxicity/persistence. The factor estimates the dose an individual receives
through the skin (for example, during swimming or wading) inhalation, and
ingestion, compared to the dose from drinking the same water.
The population factor consists of two elements:
o The estimated number of visits to an area that is actually
contaminated. This number is based on the populations at different
distances from the area (0-5 miles, 5-10 miles, 10-15 miles),
accessibility/attractiveness (waterfront parks, boat ramps, designed
swimming beaches, etc.), and a recreational dose adjusting factor.
o The estimated number of visits to an area threatened by contamination.
This value is derived as above, adjusted by a dilution weighting
factor.
The highest scoring area is used as the population factor value for the
recreational threat.
Environmental Threat. In the surface water pathway of the current HRS,
sensitive environments are 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
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using EE-A 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 environ-
ments targets are weighted into groups based on ecologically-based benchmarks
where sensitive environments are contaminated; otherwise, dilution factors are
applied.
Air Pathway
The proposed air pathway (Figure 3) has the same three categories as in
the current MRS air pathway, but each is revised. The current air pathway is
evaluated only if an observed release of hazardous substances can be
documented, As required by SARA Section 105(a)(8)(A), the revised MRS considers
characteristics of the site to assess the potential for releases if no release
has been documented. The likelihood of release is determined, as well as how
many people and nsitive environments could be exposed to hazardous
substances earn-i in the air and the inherent hazard associated with potential
exposures. The potential to release is assigned a value based on:
o Source type—containers (including tanks), contaminated soil (including
land treatment), fire sites, landfills, surface impoundments, and waste
piles.
o Source mobility, which reflects the relative tendency of hazardous
substances contained in a source to migrate as a gas or particulates.
o Source containment, which assesses the ability of natural or contructed
barriers to inhibit the escape of hazardous substances.
In addition to the changes to waste quantity and toxicity in the waste
characteristics category discussed earlier, the reactivity and compatibility
factors 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 proposed HRS, the three target factors in the current MRS—land
use, population, and distance to a sensitive environment—are modified, and a
factor added to reflect the risk to the MEI. The 4-mile limit for population
in the current HRS is retained; the limit for sensitive environments evaluated
would be extended from 2 to 4 miles. In both cases, distance weighting factors
are used to represent the reduced concentrations farther away from the site.
Onsite Exposure Pathway
The onsite exposure pathway evaluates the possibility that people or
sensitive environments will have direct, physical contact with hazardous-wastes
or contaminated soil. It is similar to the direct contact pathway, which ic
scored in the current HRS but is not used to determine if a site should be on
the NPL. The revised HRS evaluates the onsite threat by looking at two groups
potentially at risk—those living on property with hazardous wastes or contami-
nated soils and those living nearby with access to the property. The1 resident
population is evaluated based only on presence of contamination and not on
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release potential, as in the other pathways, because contaminants do not have
to migrate offsite for exposure to occur. Three targets are evaluated in the
resident population:
o High-risk population—children under 7, including those living or
going to school or day care on contaminated property.
o Total population, excluding those counted in the high-risk group.
o Sensitive terrestrial environments (aquatic environments are considered
in the surface water pathway).
The nearby population is evaluated on the basis of:
o Accessibility/frequency of use, which evaluates the likelihood
of exposure, not its severity.
o Hazardous waste quantity expressed as total areal extent of the site
because this factor also evaluates the likelihood of.exposure.
o Population within a 1-mile travel distance of the site.
Additional Considerations
In the preamble to the proposed revisions to the MRS, EPA has also re-
quested comment on two issues:
o The cutoff score for proposing sites for the NPL.
o The policy of scoring sites based on current conditions.
These issues are discussed below.
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
required by CERCLA, not because the EPA had determined that 28.50 represented a
threshold of unacceptable risks. Believing that the current cutoff score has
been a useful management tool, EPA is proposing that the cutoff score for the
revised HRS be functionally equivalent to the current cutoff. However, the
rather substantial revisions of the proposed HRS make it necessary 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 is examining several approaches for defining "equivalent to
28.50":
o Use statistical analysis to determine what revised HRS score best
corresponds to 28.50 on the current HRS.
o Select a cutoff that would yield an NPL of the same size as would the
current cutoff score.
o Identify the quantitative risk levels that on the average correspond
to a current HRS score of 28.50 and then determine what revised HRS
score best corresponds to that risk level.
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EPA intends to evaluate various cutoff score analyses ap.d will announce
the cutoff score in the final HRS rale.
Scoring on the Basis of Current Conditioas. Under the current HRS, EPA
generally scores the three migration pathways based on the conditions at the
site before any response action has been taken, rather than based on current
conditions at the site. In revising the MRS, EPA decided that it may be
appropriate to evaluate sites based on current conditions and to consider prior
responses in calculating an HRS score. EPA is requesting public comment on
consideration of prior response actions.
The existing policy of evaluating sites based on original conditions was
based on concerns that it might:
o Encourage private parties to only take action sufficient to lower the
score so the site would not be eligible for the NPL.
o Discourage public agencies from taking early actions that could lower
the score/ preventing the site from being on the NPL and therefore
eligible for Superfund monies.
EPA is considering two approaches to incorporate current site conditions
in the HRS score. Under either approach, EPA would only consider removals
prior to a site inspection to avoid continuously updating the score to reflect
ongoing cleanup activity. The two approaches are:
o Consideration of current conditions for certain pathways or factors
where appropriate.
o Consideration of current conditions routinely, but identifying and
exempting situations where current conditions will not lead to a more
accurate assessment of risks.
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