Document ID: EPA-HQ-SFUND-2010-1086-0104
Agency: epa
Document Type: Rule
Title: Addition of a Subsurface Intrusion Component to the Hazard Ranking System
Posted Date: 2017-01-09T05:00Z

[Federal Register Volume 82, Number 5 (Monday, January 9, 2017)]
[Rules and Regulations]
[Pages 2760-2807]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2016-30640]

[[Page 2759]]

Vol. 82

Monday,

No. 5

January 9, 2017

Part III

 Environmental Protection Agency

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40 CFR Part 300

Addition of a Subsurface Intrusion Component to the Hazard Ranking 
System; Final Rule

  Federal Register / Vol. 82, No. 5 / Monday, January 9, 2017 / Rules 
and Regulations  

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 300

[EPA-HQ-SFUND-2010-1086; FRL-9956-58-OLEM]
RIN 2050-AG67

Addition of a Subsurface Intrusion Component to the Hazard 
Ranking System

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: The U.S. Environmental Protection Agency (EPA) is adding a 
subsurface intrusion (SsI) component to the Hazard Ranking System 
(HRS), which is the principal mechanism that EPA uses to evaluate sites 
for placement on the National Priorities List (NPL). The NPL is a list 
of national priorities among the known or threatened releases of 
hazardous substances, pollutants or contaminants throughout the United 
States. Sites on the NPL are priorities for further investigation to 
determine if further response actions are warranted. The subsurface 
intrusion component (this addition) expands the number of available 
options for EPA and state and tribal organizations performing work on 
behalf of EPA to evaluate actual and potential threats to public health 
from releases of hazardous substances, pollutants, or contaminants. 
This addition enables EPA to directly consider human exposure to 
hazardous substances, pollutants, or contaminants that enter regularly 
occupied structures through subsurface intrusion in assessing a site's 
relative risk, and thus, enable sites with subsurface intrusion 
contamination to be evaluated for placement on the NPL.

DATES: This final rule is effective February 8, 2017.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-SFUND-2010-1086. All documents in the docket are 
listed on the http://www.regulations.gov Web site. Although listed in 
the index, some information is not publicly available, e.g., CBI or 
other information whose disclosure is restricted by statute. Certain 
other material, such as copyrighted material, is not placed on the 
Internet and will be publicly available only in hard copy form. 
Publicly available docket materials are available electronically 
through http://www.regulations.gov or in hard copy at the EPA Docket 
Center Reading Room (see https://www.epa.gov/dockets/epa-docket-center-reading-room for more information).

FOR FURTHER INFORMATION CONTACT: Terry Jeng, phone: (703) 603-8852, 
email: jeng.terry@epa.gov, Site Assessment and Remedy Decisions Branch, 
Assessment and Remediation Division, Office of Superfund Remediation 
and Technology Innovation (Mail Code 5204P), U.S. Environmental 
Protection Agency, 1200 Pennsylvania Avenue NW, Washington, DC 20460; 
or the Superfund Hotline, phone (800) 424-9346 or (703) 412-9810 in the 
Washington, DC metropolitan area.

SUPPLEMENTARY INFORMATION: The information presented in this preamble 
is organized as follows:

I. Statutory Authority for Regulatory Change
II. Background
    A. The Hazard Ranking System
    B. Site Assessment and the Superfund Remedial Process
    C. Impact of the SsI Addition on Current Cleanup Programs, 
Resources and Cost
    D. Impact of the Subsurface Intrusion Addition on the Hazard 
Ranking System
III. Overview of the Final Rule
    A. HRS Structure With the Subsurface Intrusion Component
    B. SsI Component Addition
    1. New Definitions
    2. Delineation of Areas of Subsurface Intrusion
    a. Area of Observed Exposure (AOE)
    b. Area of Subsurface Contamination (ASC)
    3. Likelihood of Exposure
    a. Observed Exposure
    b. Potential for Exposure
    c. Calculation of the Likelihood of Exposure Factor Category 
Value
    4. Waste Characteristics
    a. Toxicity/Degradation
    b. Hazardous Waste Quantity
    c. Calculation of the Waste Characteristics Factor Category 
Value
    5. Targets
    a. Identification of Eligible Targets
    b. Exposed Individual and Levels of Exposure
    c. Population
    d. Resources
    e. Calculation of the Targets Factor Category Value
    6. Calculation and Incorporation of the SsI Component Score Into 
the HRS Site Score
    a. Calculation of the SsI Component Score
    b. Incorporation of the SsI Component Score Into the Soil 
Exposure and Subsurface Intrusion Pathway Score
    c. Incorporation of the Soil Exposure and Subsurface Intrusion 
Pathway Score Into a Site Score
    C. Testing the SsI Component
    1. Conceptual Site Model/Sensitivity Analysis
    2. Test Site (Tier 1) Summaries
    3. Pilot Study
IV. Summary of Changes to the HRS
    A. Changes Since Proposal
    B. Summary of Updates to the HRS (Sections 2, 5, 6, and 7)
V. Discussion of Major Comments
    A. Responses to Comments on EPA Questions Posed in the Proposed 
Rule
    B. Major Comment Theme Summaries and Responses
VI. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act (PRA)
    C. Regulatory Flexibility Act (RFA)
    D. Unfunded Mandates Reform Act (UMRA)
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From 
Environmental Health Risks and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    K. Executive Order 12580: Superfund Implementation
    L. Congressional Review Act (CRA)

I. Statutory Authority for Regulatory Change

    EPA has revised the HRS, the principal mechanism for placing sites 
on the NPL, to add a component for evaluating the threat or potential 
threat posed by subsurface intrusion to protect human health and the 
environment. Without an evaluation of threats posed by subsurface 
intrusion contamination, the HRS is not a complete assessment because 
it omits a known pathway of human exposure to contamination. The 
addition of subsurface intrusion to the HRS is compliant with 
Comprehensive Environmental Response, Compensation, and Liability Act 
(CERCLA) Section 105(a)(8)(A), which requires EPA to prioritize sites 
based on ``the population at risk, the hazard potential of hazardous 
substances at such facilities, the potential for contamination of 
drinking water supplies, the potential for direct human contact [and] 
the potential for destruction of sensitive ecosystems. This addition to 
the HRS also improves the agency's ability to identify priority sites 
for further investigation and enhances EPA's ability, in dialogue with 
other federal agencies and the states and tribes, to determine the most 
appropriate state or federal authority to address sites. For 
information on alternatives to this rulemaking that were considered for 
addressing subsurface intrusion contamination, please see the

[[Page 2761]]

preamble to the proposed HRS SsI Addition [81 FR 10372, February 29, 
2016].
    Additionally, the Government Accountability Office (GAO) stated in 
its May 2010 report \1\:
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    \1\ EPA's Estimated Costs to Remediate Existing Sites Exceed 
Current Funding Levels, and More Sites are Expected to Be Added to 
the National Priorities List, GAO Report to Congressional 
Requesters, GAO-10-380, May 2010.

    EPA may not be listing some sites that pose health risks that 
are serious enough that the sites should be considered for inclusion 
on the NPL. While EPA is assessing vapor intrusion contamination at 
listed NPL sites, EPA does not assess the relative risks posed by 
vapor intrusion when deciding which sites to include on the NPL. By 
not including these risks, states may be left to remediate those 
sites without federal assistance, and given states' constrained 
budgets, some states may not have the ability to clean up these 
sites on their own . . . However, if these sites are not assessed 
and, if needed, listed on the NPL, some seriously contaminated 
hazardous waste sites with unacceptable human exposure may not 
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otherwise be cleaned up.

    The authority for these technical modifications to the HRS is in 
section 105(a)(8)(A) of CERCLA enacted in 1980. Under CERCLA, the 
National Oil and Hazardous Substances Pollution Contingency Plan (NCP) 
(40 CFR 300) must include criteria for determining priorities among 
releases or threatened releases for the purpose of taking remedial or 
removal actions. Section 105(a)(8)(A) of CERCLA required EPA to 
establish:

    [C]riteria for determining priorities among releases or 
threatened releases [of hazardous substances] throughout the United 
States for the purpose of taking remedial action and, to the extent 
practicable, taking into account the potential urgency of such 
action, for the purpose of taking removal action. Criteria and 
priorities . . . shall be based upon relative risk or danger to 
public health or welfare or the environment. . .taking into account 
to the extent possible the population at risk, the hazard potential 
of hazardous substances at such facilities, the potential for 
contamination of drinking water supplies, the potential for direct 
human contact [and] the potential for destruction of sensitive 
ecosystems. . . .

    To meet this requirement and provide criteria to set priorities, 
EPA adopted the HRS as Appendix A to the NCP (47 FR 31180, July 16, 
1982). The HRS was last revised on December 14, 1990 (55 FR 51532) to 
include the evaluation of additional threats to ensure a complete 
assessment of the relative risk that a site may pose to the public. 
Section 105(a)(8)(B) of CERCLA requires that the statutory criteria 
described in section 105(a)(8)(A) be used to prepare a list of national 
priorities among the known releases, or threatened releases throughout 
the United States. The NPL is Appendix B of the NCP (40 CFR 300, 
Appendix B).
    In 1986, Congress passed the Superfund Amendments and 
Reauthorization Act (SARA) (Pub. L. 99-499), which added section 
105(c)(1) to CERCLA, requiring EPA to amend the HRS to assure ``to the 
maximum extent feasible, that the hazard ranking system accurately 
assesses the relative degree of risk to human health and the 
environment posed by sites and facilities subject to review.'' In 
addition, CERCLA section 115 authorizes EPA to promulgate any 
regulations necessary to carry out the provisions of CERCLA.
    Furthermore, the Congressional Conference Report on SARA included 
the absolute standard against which HRS revisions could be assessed:

    This standard is to be applied within the context of the purpose 
for the National Priorities List; i.e., identifying for the States 
and the public those facilities and sites which appear to warrant 
remedial actions. * * * This standard does not, however, require the 
Hazard Ranking System to be equivalent to detailed risk assessments, 
quantitative or qualitative, such as might be performed as part of 
remedial actions. The standard requires the Hazard Ranking System to 
rank sites as accurately as the Agency believes is feasible using 
information from preliminary assessments and site inspections * * * 
Meeting this standard does not require long-term monitoring or an 
accurate determination of the full nature and extent of 
contamination at sites or the projected levels of exposure such as 
might be done during remedial investigations and feasibility 
studies. This provision is intended to ensure that the Hazard 
Ranking System performs with a degree of accuracy appropriate to its 
role in expeditiously identifying candidates for response actions. 
[H.R. Rep. No. 962, 99th Cong., 2nd Sess. at 199-200 [1986]]

    When the HRS was last revised in 1990, the technology to detect and 
evaluate subsurface intrusion threats was not sufficiently developed. 
For example, there were no health-based benchmark concentration values 
for residences or standardized technologies for sampling indoor air, 
precision of analytical equipment prior to computerization was limited, 
and associations between contaminated ground water and soil vapors were 
not well understood. However, it is now possible for subsurface 
intrusion threats to be evaluated in a more comprehensive manner. 
Therefore, it is now appropriate, given the potential that subsurface 
intrusion presents for direct human contact, to add to the HRS the 
consideration of threats due to subsurface intrusion.
    This final rule ensures the HRS does not omit a known pathway of 
human exposure to contamination due to subsurface intrusion of released 
hazardous substances and provides a mechanism for assessing subsurface 
intrusion threats and identifying sites for placement on the NPL. 
Furthermore, these sites are now eligible for Superfund-financed 
remedial actions.

II. Background

    The HRS is a crucial part of the agency's program for determining 
which sites are a priority for further remedial investigation and 
possible cleanup under CERCLA. To understand the importance of this 
rulemaking it is necessary to understand the role of the HRS in 
identifying sites for the NPL, the role of the HRS in the overall site 
assessment and Superfund remedial process, and this final rule's 
impacts on current and future Superfund activities. In addition, it is 
also necessary to understand the impact of adding the SsI component to 
the HRS.

A. The Hazard Ranking System

    The HRS is a scoring system used to assess the relative risk 
associated with actual or potential releases of hazardous substances 
from a site based on the information that can be collected in a 
preliminary assessment (PA) and site inspection (SI). The HRS is not a 
tool for conducting a quantitative risk assessment and was designed to 
be a measure of relative risk among sites rather than absolute site-
specific risk. As required by CERCLA, EPA has designed the Superfund 
program to focus its resources on the priority sites. Consequently, the 
initial studies--the PA and SI--which are performed on a large number 
of sites, are relatively modest in scope and cost compared to the 
remedial investigations and feasibility studies subsequently performed 
on NPL sites.
    Because of the need to expeditiously perform PAs and SIs, Congress 
placed certain constraints on the data requirements for an HRS 
evaluation. The required HRS data should be information that, for most 
sites, can be collected during a screening level site inspection or 
that are already available. Thus, the HRS does not rely on data that 
require extensive sampling or repeated sampling over extended periods 
of time. However, EPA allows for the expansion of the typical SI to 
allow for additional data collection for more complex sites that cannot 
be adequately characterized using standard SI methodologies. The HRS 
has also been designed so that it can be applied consistently to a wide 
variety of sites, enabling sites to be

[[Page 2762]]

ranked relative to each other with respect to actual or potential 
hazards.
    Based on the state of the science, site specific data may be 
collected beyond that which is normally available after a typical site 
inspection. In these situations, the HRS in general, and the SsI 
component, can incorporate that data into the HRS evaluation. For 
example, the SsI component can use site-specific data as follows:
     Determination of the Hazardous Waste Quantity Factor 
Value--If the mass of all hazardous substances can be adequately 
determined (i.e., is known or can be estimated with reasonable 
confidence), the HRS requires this estimate (identified as a Tier A 
estimate) be used to assign the hazardous waste quantity for all 
regularly occupied structures in an area of exposure (AOE) for which 
this information is available. See section 2.4.2 and 5.2.1.2.2 of the 
HRS.
     Determining the extent of an ASC--If sufficient data are 
available and state of the science shows there is no unacceptable risk 
due to subsurface intrusion into a regularly occupied structure located 
within an ASC, that structure or subunit can be excluded from the ASC. 
Therefore, such structures would not be included in the evaluation of 
the Hazardous Waste Quantity Factor or in the determination of other 
factors evaluated based on structures or subunits within an ASC. See 
section 5.2.0 of the HRS.
     Populations within the ASC--If sufficient structure-
specific concentration data is available and state of the science shows 
there is no unacceptable risk of exposure to populations in a regularly 
occupied structure in an ASC, those populations are not included in the 
evaluation of the Targets Factor Category. See section 5.2.1.3 of the 
HRS.
    EPA notes that if other site-specific information is available that 
clearly demonstrates that the site does not pose an unacceptable risk 
to human health via subsurface intrusion, there are points during the 
PA or SI process, where further evaluation of the site for the 
subsurface intrusion threat by the Superfund program can be terminated. 
Please see section B. of this preamble for further information on the 
Site Assessment process.
    As EPA explained when it originally adopted the HRS, ``the HRS is a 
means for applying uniform technical judgment regarding the potential 
hazards presented by a facility relative to other facilities. It does 
not address the feasibility, desirability, or degree of cleanup 
required.'' (47 FR 31220, July 16, 1982).
    The HRS uses a structured value analysis approach to scoring sites. 
This approach assigns values to factors related to or indicative of 
risk. The basic elements of the HRS are factors that are based on 
information that can be collected in a limited screening assessment. A 
scale of numerical rating values is provided for each factor and a 
value is assigned to each factor based on conditions at the site. 
Individual values are then weighted. The factors are grouped into three 
factor categories--observed release/route characteristics, waste 
characteristics, and targets--and are combined to obtain factor 
category scores. Each factor category has a maximum value, as does each 
of the component factors within the category. The relevant factor 
category scores are multiplied together within each pathway and 
normalized to obtain a pathway score. The pathway scores are combined 
using a root-mean-square approach to calculate the overall site score; 
that is, the final HRS score is the square root of the sum of the 
squares of the pathway scores divided by the square root of the number 
of HRS pathways. If all pathway scores are low, the HRS score will be 
low. However, the final score will be relatively high even if only one 
pathway score is high. EPA considers this an important requirement for 
the HRS scoring because some extremely dangerous sites pose threats 
through only one migration mode. For example, at a site, leaking drums 
of hazardous substances may be contaminating drinking water wells, 
thereby posing a significant threat via the groundwater migration 
pathway. But if the drums are buried deeply enough and the hazardous 
substances are not very volatile, the drums may not release any 
hazardous substances and not pose a threat to the air or to surface 
water.
    EPA emphasizes that the HRS score is a number between 0 and 100, 
which reflects relative risk amongst candidate NPL sites. An HRS site 
score is not a measure of actual site-specific risk.

B. Site Assessment and the Superfund Remedial Process

    EPA's Superfund remedial site assessment process evaluates sites to 
ascertain if further investigation is needed for determining whether an 
unacceptable risk is present.
    The majority of sites evaluated through the EPA's site assessment 
program do not meet the criteria for possible placement on the NPL and 
are ``screened out'' of the Superfund Remedial process. (See Figure 1. 
Status of EPA's Site Assessments). Since EPA adopted the HRS, 52, 859 
sites have been assessed under EPA's Superfund program. Of those sites, 
1,782 were placed on the NPL, as of September 2016.
Site Assessment Strategy
    The site assessment process is structured as a series of limited 
investigations which may include: (1) A Pre-CERCLA screening 
assessment; (2) a preliminary assessment; and (3) a site inspection or 
expanded site inspection (Figure 2. Site Assessment Process, below, 
illustrates this process). If a site progresses through the site 
assessment process for further investigation, the requirements for 
documenting risk become increasingly rigorous. The following includes a 
summary of the major phases of the site assessment process.
     A Pre-CERCLA Screening is an initial review of existing 
information on a possible Superfund site. If a release of a hazardous 
substance has occurred or if the potential of a hazardous substance to 
release exists the site may be eligible for further remedial evaluation 
under CERCLA authority. If further evaluation is warranted the site 
should be entered into the remedial assessment active site inventory 
for further assessment.
     The PA decision process parallels an HRS analysis, but 
makes environmental ``worst-case'' assumptions of possible significant 
risk regarding transport of contamination to receptors based on minimal 
available information and professional judgment.
     The SI collects information to confirm the accuracy of the 
PA assumptions. The information should be sufficient to support an HRS 
evaluation with minimal further investigation.
     If placement on the NPL is pursued, the information 
collected during the SI provides the basis for supporting the HRS 
scoring scenario.

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[GRAPHIC] [TIFF OMITTED] TR09JA17.058

    The following discussion provides further information on each of 
these phases.
Pre-CERCLA Screening Assessment
    A Pre-CERCLA Screening is used to establish whether:
     A release or potential release of a hazardous substance 
has occurred at a site;
     The site is eligible for further remedial assessment under 
CERCLA authority;
     The site needs further attention under Superfund or 
another cleanup program; and
     The site warrants entry into the federal Superfund 
program's active site inventory for further assessment or response.
    Determining whether releases of hazardous substances, pollutants, 
or contaminants can be addressed by CERCLA requires the application of 
site-specific facts to CERCLA statutory requirements and EPA policy. 
The initial determination as to whether a site warrants further 
investigation is based on three site-specific facts including: (1) 
Evidence of an actual release or potential to release; (2) targets 
impacted by a release of contamination at the site; and (3) 
documentation that a target has been exposed to a hazardous substance 
released from the site. Examples of targets include populations, 
drinking water wells, drinking water surface intakes, municipal wells, 
fisheries and sensitive environments.
Preliminary Assessment
    A PA uses readily available data to determine if there is evidence 
of a release that poses an unacceptable possible threat as specified in 
the NCP (40 CFR 300.420).
     The PA is a limited-scope investigation performed by 
States and/or EPA on every CERCLA site
     The PA may include the collection of readily available 
information and an on- or off-site reconnaissance may be conducted
     The PA distinguishes, based on already existing 
information, between sites that appear to pose little or no threat to 
human health and the environment and sites that require further 
investigation to determine if the threat to human health and the 
environment is unacceptable.

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[GRAPHIC] [TIFF OMITTED] TR09JA17.059

    If based on the results of a PA, EPA determines that a site 
warrants further screening under the CERCLA remedial program, the 
agency initiates a site inspection
Site Inspection
    The purpose of the SI is to collect the data necessary to perform 
an HRS evaluation. An SI determines if a release of a hazardous 
substance poses an actual or potential threat to human health or the 
environment, to determine if there is an immediate threat to people or 
the environment in the area, and to collect sufficient data to enable 
the site to be scored using the HRS. EPA may expand the site inspection 
scope as needed. This expanded site inspection (ESI) collects 
additional data beyond what is collected in the standard site 
inspection to evaluate sites for HRS scoring. ESIs are reserved for 
more complex sites that cannot be adequately characterized using 
standard site inspection methods.
     SI investigators typically collect waste and environmental 
samples to determine the substances present at a site and whether they 
are being released to the environment, as well as other information to 
perform an HRS evaluation.
     EPA distinguishes, based on the information collected 
during the SI, between sites that appear to pose little or no threat to 
human health and the environment and sites that require further 
investigation to determine if the threat to human health and the 
environment exists.
     If the information indicates a threat, EPA determines the 
best approach for addressing the threat, which can be placement on the 
NPL or use of an alternative authority.
    If at any time in this site assessment process, EPA determines that 
sufficient information indicates the site poses no unacceptable risk, 
or if it can be addressed under alternative authorities it can be 
removed from the process. Also, if an imminent or substantial 
endangerment to public health is identified, EPA can initiate CERCLA 
removal actions.

[[Page 2765]]

The NPL Rulemaking Process
    The NPL is a list of national priorities for further investigation 
amongst the known or threatened releases of hazardous substances, 
pollutants or contaminants throughout the United States. The list, 
which is appendix B of the NCP (40 CFR part 300), is required under 
section 105(a)(8)(B) of CERCLA, as amended. Section 105(a)(8)(B) 
defines the NPL as a list of ``releases'' and the highest priority 
``facilities'' and requires that the NPL be revised at least annually. 
The NPL is intended primarily to guide the EPA in determining which 
sites warrant further investigation to assess the nature and extent of 
public health and environmental risks associated with a release of 
hazardous substances, pollutants or contaminants. The NPL is of only 
limited significance, however, as it does not assign liability to any 
party or to the owner of any specific property. Also, placing a site on 
the NPL does not mean that any remedial or removal action necessarily 
need be taken.
    For purposes of listing, the NPL includes two sections, one of 
sites that are generally evaluated and cleaned up by the EPA (the 
``General Superfund section'') and one of sites that are owned or 
operated by other federal agencies (the ``Federal Facilities 
section''). With respect to the Federal Facilities sites, these sites 
are generally being addressed by other federal agencies. Under 
Executive Order 12580 (52 FR 2923, January 29, 1987) and CERCLA section 
120, each federal agency is responsible for carrying out most response 
actions at facilities under its own jurisdiction, custody or control, 
although the EPA is responsible for preparing a Hazard Ranking System 
(``HRS'') score and determining whether the facility is placed on the 
NPL and having oversight authority at the sites for further actions.
NPL Site Selection Process
    The NPL is required to be revised annually and it is intended 
primarily to guide EPA in determining which sites warrant further 
investigation to assess the nature and extent of public health and 
environmental risks associated with a release of hazardous substances, 
pollutants or contaminants. This selection process is illustrated in 
figure 3, below. Sites with HRS scores of 28.50 or greater are eligible 
for placement on the NPL. Only non-Federal Facility sites on the NPL 
are eligible for Superfund-financed remedial actions. Once a site is 
determined to be NPL-caliber and a decision has been made that the 
federal Superfund program should manage the site cleanup, EPA regions 
apply a strong initial presumption in favor of placement on the NPL.
    Once the site is proposed for the NPL (i.e., announced in the 
Federal Register), a 60-day comment period is initiated to allow the 
public to comment on the proposal. EPA responds to all public comments, 
and depending on the results of the public comment period, the site 
could be removed from consideration for placement of the NPL; re-
proposed in the future due to public comments; or placed on the NPL. 
Once the site is placed on the NPL, the rulemaking can be challenged in 
court under the Administrative Procedure Act (APA). If no challenge is 
made or if the court finds the rulemaking consistent with APA 
requirements, it is then eligible for further investigation under the 
Superfund remedial program. (Figure 3. Process for Placing a Site on 
the NPL).
[GRAPHIC] [TIFF OMITTED] TR09JA17.060

C. Impact of the SsI Addition on Current Cleanup Programs, Resources 
and Cost

    This SsI addition to the HRS will have the most significant impact 
on EPA's Superfund cleanup program. This regulatory change expands 
available options for EPA and organizations performing work on behalf 
of EPA (state and tribal partners) to evaluate actual and potential 
threats to public health and the environment from subsurface intrusion 
contamination. This modification to the HRS, by itself, only augments 
the criteria for applying the HRS. It has no effect on small 
businesses.
    This final rule will not affect the status of sites currently on or 
proposed to the NPL. Sites that are currently on or proposed to the NPL 
have already been evaluated under another pathway (i.e., ground water 
migration, air migration, surface water migration, or soil exposure) 
and have been shown to or are projected to qualify for placement on the 
NPL. The method selected for including the SsI evaluation in the HRS 
site score can only result in an increase in a site score, Therefore, 
all sites qualifying for the NPL based on its HRS site score prior to 
this final rule will continue to do so. It is consistent with section 
105(c)(3) of CERCLA, as amended, that these sites will not be re-
evaluated. This final rule will not disrupt EPA's placement of sites on 
the NPL.

[[Page 2766]]

    The possible impact on federal agencies other than EPA performing 
Superfund actions will be less than that on private sites being 
addressed by EPA. Federal agencies currently address subsurface 
intrusion issues as part of their environmental programs and 
authorities. Executive Order 12580 delegates broad CERCLA authority to 
federal agencies for responding to actual and potential releases of 
hazardous substances where a release is either on, or the sole source 
of the release is from, any facility or vessel under the jurisdiction, 
custody, or control of the federal agency. Federal agencies are 
required to exercise this authority consistent with the requirements of 
CERCLA section 120, as amended, and implement regulations under the 
NCP, for both NPL and non-NPL sites. Therefore, federal agencies are in 
a position to proactively identify and respond to risks posed by 
subsurface intrusion of hazardous substances into regularly occupied 
structures for all populations who live and work in areas where the 
subsurface environment may create exposures. If it is determined that 
releases of hazardous substances pose immediate threats to public 
health and the environment, EPA fully expects that the appropriate 
federal agency will continue to undertake response actions to address 
such threats. Many federal agencies, including EPA, have developed or 
are developing new or updated agency-specific policy and guidance 
documents to address subsurface intrusion threats.
    As a result of federal agency existing environmental programs and 
authorities, this rulemaking is not anticipated to have a significant 
impact to the resources and costs to federal cleanup programs.
    Since EPA's overall appropriated Superfund budget as well EPA's 
cooperative agreement budget for performing site assessments will 
continue to remain relatively steady, EPA anticipates that this final 
rule will not result in additional site assessments nor the placement 
of more sites on the NPL during any particular interval, but rather a 
shift in the make-up of the type of sites included on the NPL. EPA will 
continue to review sites as part of Superfund remedial site assessment 
to determine whether sites are eligible for further remedial evaluation 
under CERCLA authorities and prioritize sites that pose the highest 
risk. This is not a change to how EPA currently evaluates and 
prioritizes sites for the NPL. Because the level of effort required to 
evaluate a site, regardless of pathway, varies on a site-by-site basis, 
depending on the size and extent of contamination at the site, it 
cannot be predicted with any certainly that there will be an increase 
in cost or level of effort for any particular site due to this 
rulemaking.
    This rulemaking, which could lead to the inclusion of a site on the 
NPL that did not qualify for the NPL previously, does not itself impose 
any costs on outside parties; it does not establish that EPA will 
necessarily undertake response actions, nor does it require any action 
by a private party or determine liability for site response costs. 
Costs are limited to screening relevant sites for subsurface intrusion 
contamination during site inspections and the resulting HRS evaluation 
and documentation record preparation. Costs that arise from site 
remedial responses are the result of site-specific decisions made post-
listing, not directly from the act of listing itself. These costs are a 
result of a release of hazardous substances and would not be incurred 
if hazardous substances had not been released.
    Later Superfund-related decisions that consider information 
collected under the HRS SsI Addition could separately have specific 
economic costs and benefits (e.g., remediation costs and reduced risk), 
but these impacts are contingent upon a series of separate and 
sequential actions after listing a site on the NPL. Therefore, addition 
of subsurface intrusion to the HRS is several regulatory steps removed 
from imposing costs on private entities.
    This rulemaking does not impose any requirements on small entities, 
and therefore can be certified as no Significant Economic Impact on a 
Substantial Number of Small Entities (SISNOSE). With the exception of 
other federal agencies, site assessments are performed by EPA and on 
behalf of EPA by states and tribes in cooperative agreement 
partnerships with EPA. Under section 601 of the Regulatory Flexibility 
Act, federal agencies do not fit under the definition of small 
business, small entity, small organization or small governmental 
jurisdiction.

D. Impact of the Subsurface Intrusion Addition on the Hazard Ranking 
System

    This final rule, with the addition of a subsurface intrusion 
component, does not change the purpose of the HRS, its fundamental 
structure or its application. It does not change the balance between 
the pathways or calculation of the overall HRS site score and the same 
cutoff score to qualify a site for the NPL is maintained. The current 
approach for scoring the ground water, surface water, and air migration 
pathways is not being altered by the addition of a subsurface intrusion 
component. EPA added the subsurface intrusion threat as a component to 
the present soil exposure pathway because its structure already focuses 
on populations actually or potentially coming into direct contact with 
hazardous substances. The re-structured pathway is called the ``Soil 
Exposure and Subsurface Intrusion'' pathway and now allows for the 
consideration of the threat posed by subsurface contaminant intrusion. 
The Soil Exposure and Subsurface Intrusion pathway retains the existing 
two soil exposure threats (resident population and nearby population) 
in the pathway as one component, with subsurface intrusion as the 
second component.
    The narrow technical modifications resulting from this Final Rule 
reflect the agency's actions to encompass additional risks posed by 
releases of hazardous substances and to address the SARA statutory 
requirement that EPA amend the HRS to assure ``to the maximum extent 
feasible, that the HRS accurately assesses the relative degree of risk 
to human health and the environment posed by sites subject to review.'' 
Thus, the fundamental purpose and structure of the HRS approach has not 
changed with this amendment to the HRS to include the consideration of 
subsurface intrusion.

III. Overview of the Final Rule

    This final rule revises the 1990 HRS to include a component for 
evaluating the threats posed from subsurface intrusion. The following 
sections discuss the structure of the HRS, the subsurface intrusion 
component within the HRS, the major factors of the subsurface intrusion 
addition, and how the evaluation will be performed using a structure 
consistent with the other threats, components, and pathways in the HRS, 
but taking into account the unique parameters impacting the probability 
of exposure to subsurface intrusion. All sites that qualified for the 
NPL under the 1990 HRS, would still qualify for the NPL under this 
revised HRS. For a more comprehensive description and rationale of 
changes, see the February 29, 2016 Proposed Rule [81 FR 10372, February 
29, 2016].

A. HRS Structure With the Subsurface Intrusion Component

    EPA added the evaluation of the relative risk posed by subsurface 
intrusion of hazardous substances into regularly occupied structures by 
restructuring the soil exposure pathway from the 1990 HRS to include 
subsurface intrusion. The soil exposure pathway has been renamed the 
soil exposure and subsurface intrusion pathway to reflect both 
components of

[[Page 2767]]

the new pathway. No changes are included in the other three HRS 
pathways, with the exception of the use of a reference concentration 
instead of a reference dose to determine a hazardous substance's 
health-based benchmark in the air migration pathway. See Figure 4 for a 
depiction of how the promulgated addition fits into the HRS structure.
[GRAPHIC] [TIFF OMITTED] TR09JA17.061

    As explained in the preamble to the proposed HRS SsI addition, the 
subsurface intrusion component is added as a new component of the soil 
exposure and subsurface intrusion pathway. The soil exposure pathway 
included in the 1990 HRS is retained as one component of the Soil 
Exposure and Subsurface Intrusion pathway. The scoring of the soil 
exposure component remains unaltered, but the score is assigned as the 
soil exposure component score, not the pathway score. (See section 5.1 
of the HRS). As discussed in greater detail below, the SsI component 
has the same basic structure, scoring, and weighting as other parts of 
the HRS.
    The score for the soil exposure and subsurface intrusion pathway is 
based on a combination of the two component scores--soil exposure and 
subsurface intrusion but the pathway score is capped at the same value 
as other HRS pathways. The soil exposure component score is added to 
the subsurface intrusion component score to determine the pathway 
score. The two component scores are additive to reflect that 
populations may be exposed via both routes: The soil exposure component 
reflects exposures to people when outside a structure and focuses on 
ingestion, and the subsurface intrusion component reflects exposures 
inside a structure and focuses on inhalation. Hence, the addition of 
the two component scores reflects the potential cumulative risk of 
multiple exposure routes and is not double counting the same relative 
risk.
    A maximum pathway score is not contingent on scoring both the soil 
exposure and subsurface intrusion components. It is possible for a site 
to have only one component evaluated and still reach the maximum 
pathway score. Because the scoring of the soil exposure component is 
not being altered, this component would contribute the same score to 
the overall site score absent the addition of subsurface intrusion.

B. SsI Component Addition

    The structure of the HRS is fundamentally the same for all 
individual pathways, components, and/or threats. The design of the HRS 
reflects a conceptual understanding of how hazardous substance releases 
from CERCLA sites can result in risks to public health and welfare and 
the environment. The risk scenario at these sites is a function of:
     The probability of exposure to (or releases to a medium in 
a migration pathway of) hazardous substances,
     The expected magnitude and duration of the releases or 
exposures,
     The toxicity or other potential adverse effects to a 
receptor associated with a target from the releases,

[[Page 2768]]

     For the three migration pathways, the probability that the 
release will reach a target and the expected change in the 
concentration of hazardous substances during the movement from the 
location of the contamination to the targets. For the exposure pathway, 
the probability a receptor will be exposed at the target location,
     The expected dose to the receptor, and
     The expected number and type of the receptors.
    The above considerations are addressed in three factor categories: 
Likelihood of exposure (or release), waste characteristics, and 
targets.
    The following subsections describe the structure of the subsurface 
intrusion component and how this structure is consistent conceptually 
with the existing structure of the other HRS pathways and components: 
(1) New definitions, (2) delineation of areas of subsurface intrusion, 
(3) likelihood of exposure, (4) waste characteristics, (5) targets, and 
(6) calculating and incorporating the subsurface intrusion component 
score into the HRS site score.
1. New Definitions--See Section 1.1 of the HRS \2\
---------------------------------------------------------------------------

    \2\ For references to a specific section of the HRS addition, 
please refer to the regulatory text of the rulemaking.
---------------------------------------------------------------------------

    EPA has added 15 new definitions to the HRS, section 1.1, along 
with updated nomenclature to existing definitions. EPA received no 
comments on the 14 proposed new definitions to the rule; therefore, EPA 
is finalizing the new definitions as proposed with the following 
change: The term surficial ground water has been changed to shallow 
ground water for clarity. In addition, EPA has added the term non-
aqueous phase liquid (NAPL) to the definition section because EPA added 
consideration of NAPLs to the assignment of degradation factor values 
and the weighting of targets in the area of subsurface contamination 
(ASC).
2. Delineation of Areas of Subsurface Intrusion--See Section 5.2.0 of 
the HRS
    EPA has included in the subsurface intrusion component evaluation 
two areas in which exposure due to subsurface intrusion contamination 
exists or is likely to exist: (1) Areas of observed exposure--areas in 
which contaminant intrusion into regularly occupied structures has been 
documented, and (2) areas of subsurface contamination--areas in which 
subsurface contamination underlying regularly occupied structures (such 
as in shallow ground water or soil vapor) has been documented, but at 
which either sampling of indoor air has not documented that subsurface 
contamination has entered a regularly occupied structure or no sampling 
of indoor air has been undertaken.
a. Area of Observed Exposure (AOE) (See Section 5.2.0 of the HRS)
    An area (or areas) of observed exposure at a site is identified 
based on the location of regularly occupied structures with a 
documented significant increase in hazardous substance concentrations 
above background levels resulting at least in part from subsurface 
intrusion attributable to the site being evaluated. The area 
encompassed by such structures constitutes the area of observed 
exposure (AOE). Other regularly occupied structures within this 
encompassed area (or areas) are also inferred to be in the AOE unless 
available information indicates otherwise.
b. Area of Subsurface Contamination (ASC)--See Section 5.2.0 of the HRS
    An area (or areas) of subsurface contamination is identified as an 
area outside that of the AOE, at which subsurface contamination has 
been documented at levels meeting observed release criteria 
(contamination at levels significantly above background and the 
significant increase can be attributed at least in part to the site). 
The contamination would be present in subslab or semi-enclosed or 
enclosed crawl space samples or in a subsurface sample. (See section 
2.3 of the HRS for observed exposure criteria.) In addition, EPA is 
limiting the delineation of an ASC to be based on the location of 
subsurface contamination meeting the criteria for observed exposure or 
observed release and has a vapor pressure greater than or equal to one 
torr or a Henry's constant greater than or equal to 10-5 
atm-m\3\/mol. The populations in an ASC are assigned a weighting value 
ranging from 0.1 to 0.9 depending on such factors as the distance of 
subsurface contamination to a regularly occupied structure's 
foundation, the sample media, and the presence of a non-aqueous phase 
liquid (NAPL).
3. Likelihood of Exposure--See Section 5.2.1.1 of the HRS
    A key factor considered in the HRS relative risk ranking is whether 
any exposure to a hazardous substance via subsurface intrusion has 
occurred, or if not, whether there is a probability that exposure could 
occur in a regularly occupied structure. This is termed the likelihood 
of exposure for the subsurface intrusion component.
a. Observed Exposure--See Section 5.2.1.1.1 of the HRS
    For HRS purposes, an observed exposure is established if it can be 
documented that a hazardous substance from the site being evaluated has 
moved through the subsurface and has entered at least one regularly 
occupied structure.
b. Potential for Exposure--See Section 5.2.1.1.2 of the HRS
    When an observed exposure has not been established, the potential 
for exposure can be determined for any regularly occupied structure 
located in an ASC.
    The evaluation of the potential for exposure for the subsurface 
intrusion component uses the same concept and framework used to 
estimate the potential to release in other pathways. This involves 
predicting the probability of exposure in an area of subsurface 
contamination based on structural containment features of the regularly 
occupied structure and a hazardous substance's physical and chemical 
properties and the physical subsurface properties that influence the 
probability that intrusion is occurring. These factor values include:

 Structure Containment
 Depth to Contamination
 Vertical Migration
 Vapor Migration Potential

    Consistent with potential to release determinations in the HRS, the 
potential for exposure for this component is calculated by summing 
depth to contamination, vertical migration and vapor migration 
potential factor values and multiplying the sum by the containment 
factor value to determine a potential for exposure factor value.
c. Calculation of the Likelihood of Exposure Factor Category Value--See 
Section 5.2.1.1.3 of the HRS
    As in all HRS pathways and components, the likelihood of exposure 
factor category value is assigned based on the higher of the observed 
exposure (or release) value or the potential for exposure (or release) 
value. The maximum value assigned for the likelihood of exposure factor 
category is 550 and is assigned if observed exposure is documented. If 
observed exposure is not documented, the value assigned when evaluating 
potential for exposure ranges between 0 and 500.

[[Page 2769]]

4. Waste Characteristics--See Section 5.2.1.2 of the HRS
    The waste characteristics factor category is based on factors that 
are related to the relative risk considerations included in the basic 
HRS structure. The factors considered in determining the waste 
characteristics factor category value are the toxicity of the hazardous 
substances, the ability of the hazardous substance to degrade, and an 
estimate of the quantity of the hazardous substances to which occupants 
could be exposed.
a. Toxicity/Degradation--See Section 5.2.1.2.1 of the HRS
    The combined toxicity/degradation factor includes consideration of 
both the toxicity and the possibility for degradation of hazardous 
substances being evaluated for HRS purposes. The toxicity factor in the 
overall HRS structure reflects the toxicity of a hazardous substance 
associated with a source, release or exposure at a site, and is 
assigned the same factor value for all the pathways and components in 
the HRS. Any hazardous substance identified in an observed exposure 
within the AOE or meeting the observed release criteria in either the 
AOE or ASC will be assigned a toxicity factor value.
    The degradation factor represents the possibility for a substance 
to degrade in the subsurface prior to intruding into a regularly 
occupied structure. The subsurface intrusion component evaluates 
degradation based on the substance being evaluated, the depth to 
contamination, and the presence of a NAPL. It also assumes the presence 
of biologically active soil unless information indicates otherwise. If 
it has been documented that a hazardous substance has been found to 
have entered a regularly occupied structure, regardless of the 
substance or the site conditions, the degradation value is assigned to 
reflect the likelihood that the substance is not significantly 
degrading in the subsurface. Additionally, any eligible hazardous 
substance present in the subsurface below an AOE or ASC as a NAPL at 
depth less than 30 feet is assigned a degradation value to reflect the 
likelihood that the substance will not significantly degrade in the 
subsurface environment.
    The toxicity and degradation factors are multiplied together to 
assign a combined factor value. If multiple substances are present, the 
highest combined factor value is selected for use in determining the 
waste characteristics factor category value, as discussed below.
b. Hazardous Waste Quantity--See Section 5.2.1.2.2 of the HRS
    The waste quantity factor value for this component reflects only 
the amount of hazardous substances that people are exposed to, that is, 
the amount in regularly occupied structures. EPA has retained a four-
tiered hierarchical approach consistent with the HRS as well as minimum 
waste quantity factors. The estimation of waste quantity for the 
subsurface intrusion component considers the regularly occupied 
structures located within the AOE and ASC. For sites at which the 
component waste quantity (the sum waste quantities for all regularly 
occupied structures in the AOE and ASC) is below 10, a minimum factor 
of 10 would apply, the same as in other pathways and components. The 
minimum waste quantity factors are included because of insufficient 
information at many sites to adequately estimate waste quantity with 
confidence.
c. Calculation of the Waste Characteristics Factor Category Value--See 
Section 5.2.1.2.3 of the HRS
    As in all HRS pathways and components, the waste characteristics 
category value is the product of the waste characteristics factor 
values (e.g., toxicity/degradation factor value) for the SsI component 
and the hazardous waste quantity factor value, all of which are scaled 
so as to be weighted consistently in all pathways. Similar to the 
likelihood of exposure factor category, the waste characteristics 
factor category is subject to a maximum value to maintain the balance 
between factor categories. This approach is consistent with the 1990 
HRS structure.
5. Targets--See Section 5.2.1.3 of the HRS
    The targets factor is based upon estimates of the expected dose to 
each receptor associated with a target and the number and type of 
receptors present at each target. In assessing human risk, it is 
critical to understand the nature and extent of exposure to 
individuals, populations, and resources.
a. Identification of Eligible Targets--See Section 5.2.1.3 of the HRS
    The soil exposure and subsurface intrusion pathway uses the same 
target categories used in the HRS soil exposure pathway, including 
exposed individual, resident populations, workers, and resources. 
However, unlike the HRS soil exposure pathway, workers are to be 
evaluated as exposed individuals and as part of the population within 
an area of subsurface contamination instead of being evaluated under a 
separate worker factor value.
b. Exposed Individual and Levels of Exposure--See Section 5.2.1.3.1 of 
the HRS
i. Identifying Levels of Exposure and Benchmarks for Subsurface 
Intrusion
    In the SsI component, targets in the AOE are considered actually 
contaminated, whereas, those in the ASC are considered potentially 
contaminated. The targets in an AOE are further divided into Level I 
and II, based on whether the hazardous substance concentrations are at 
or above identified health-based benchmarks.
    The targets within an ASC are categorized based on the type of 
sample (e.g., gas, soil, water), the distance of the sample from the 
targets (e.g., the depth of the sample below the structure), and 
whether a NAPL is present. Weighting factors ranging from 0.1 to 0.9 
are then assigned accordingly.
ii. Exposed Individual--See Section 5.2.1.3.1 of the HRS
    The evaluation of exposed individuals in the SsI component includes 
individuals living, attending school or day care, or working in a 
regularly occupied structure. Individuals in the eligible target 
population are expected to be exposed to the highest concentration of 
the hazardous substance in question for a significant time.
c. Population--See Section 5.2.1.3.2 of the HRS
    The population factor for the SsI component includes all 
populations qualifying as exposed individuals, including residents, 
students, workers, and those attending day care. Workers are weighted 
slightly differently than other exposed individuals to reflect that a 
worker's exposure is limited to the time present in a workplace. The 
number of workers present in a structure or subunit is adjusted by an 
appropriate factor reflecting whether or not they are a full-time or 
part-time worker.
i. Weighting of Targets in the Area of Observed Exposure (AOE)--See 
Sections 5.2.1.3.2.1 and 5.2.1.3.2.2 of the HRS
    Consistent with the weighting of populations throughout the HRS, 
the subsurface intrusion component will weight targets in an AOE 
subject to Level I contaminant concentrations by a factor of 10 and 
weight targets subject to Level II contaminant concentrations by a 
factor of 1. Eligible populations include individuals living, working, 
and

[[Page 2770]]

attending school or day care in regularly occupied structures.
    Within the AOE, those populations in regularly occupied structures 
for which observed exposures have not been established but the 
structures are surrounded by regularly occupied structures in which 
observed exposures have been identified, are also considered as 
actually contaminated unless evidence indicates otherwise. Targets 
inferred to be exposed to this contamination will be weighted as Level 
II as there are no actual sample results to compare against benchmarks.
    In the case of multi-story/multi-subunit structures, all regularly 
occupied subunits on a level with an observed exposure and all levels 
below are considered to be within an AOE, unless available information 
indicates otherwise. For multi-story/multi-subunit structures located 
within an AOE, but where an observed exposure has not been documented, 
only those regularly occupied spaces on the lowest level are considered 
to be within an AOE, unless available information indicates otherwise.
ii. Weighting of Targets in the Area of Subsurface Contamination 
(ASC)--See Section 5.2.1.3.2.3 of the HRS
    Due to the variability in subsurface intrusion rates, the potential 
weighting factor values for targets within an ASC range from 0.1 to 0.9 
and depend on where the subsurface contamination has been found and 
whether a NAPL is present.
    Potential targets are weighted to reflect the distance to or the 
depth at which contamination is found and whether a NAPL is present. 
The weighting factors applied to populations being evaluated based on 
the presence of subsurface contamination containing a NAPL reflects 
greater subsurface source concentrations and an increased probability 
that contaminant intrusion into a regularly occupied structure from the 
subsurface will result in a concentration significantly above 
background levels for the site. In the case of multi-story/multi-
subunit structures, all regularly occupied subunits on a level above 
one where an observed exposure has been documented or inferred, or 
where a gaseous indoor air sample meeting observed release criteria is 
present, are considered to be located within an ASC, unless available 
information indicates otherwise. For multi-story/multi-subunit 
structures located only within an ASC, only those regularly occupied 
subunits within the lowest level are considered in an HRS evaluation.
    Eligible populations in an ASC include individuals living in, 
attending school or day care, and working in regularly occupied 
structures. However, the number of workers is adjusted to reflect that 
their exposure is limited to the time they are in a workplace.
d. Resources--See Section 5.2.1.3.3 of the HRS
    Resources for this component include regularly occupied structures 
that are located within a defined AOE or ASC and in which populations 
may be exposed to contamination due to subsurface intrusion. Libraries, 
recreational facilities, and religious or tribal structures used by 
individuals may qualify as eligible resources.
e. Calculation of the Targets Factor Category Value--See Section 
5.2.1.3.4 of the HRS
    The Target Factor Category Value is the sum of all the Target 
Factor values.
6. Calculation and Incorporation of the SsI Component Score Into the 
HRS Site Score
    The following subsections summarize the calculation of the 
subsurface intrusion component score, how the component score is used 
in the calculation of the soil exposure and subsurface intrusion 
pathway score, and how, in turn, the pathway score is subsequently 
incorporated into the HRS site score.
a. Calculation of the SsI Component Score--See Section 5.2.2 of the HRS
    The SsI Component score is the product of the likelihood of 
exposure factor category value, the waste characteristics factor 
category value, and the targets factor category value; that value is 
divided by a weighting factor so that it has equal magnitude to other 
component scores (subject to a maximum value).
b. Incorporation of the SsI Component Score into the Soil Exposure and 
Subsurface Intrusion Pathway Score--See Section 5.3 of the HRS
    The Soil Exposure and Subsurface Intrusion pathway score is a 
combination of the two component scores.
c. Incorporation of the Soil Exposure and Subsurface Intrusion Pathway 
Score Into a Site Score--See Section 2.1.1 of the HRS
    EPA did not change the methodology used to assign an overall site 
score due to the addition of the subsurface intrusion component to the 
soil exposure pathway and renaming that pathway the soil exposure and 
subsurface intrusion pathway. The overall site score remains a function 
of four pathway scores and the same weighting is given to each pathway 
score as in the 1990 HRS.

C. Testing the SsI Component

    The SsI component was tested extensively throughout the development 
of this rule, using multiple methods. The main goals of testing the 
component included:
     Ensuring the addition of the SsI component to the soil 
exposure pathway did not change relative contribution to the site score 
as the other HRS pathways and maintained the same relative risk of a 
site with a similar threshold for qualifying for the NPL.
     Ensuring the number of targets subject to actual 
contamination needed to achieve a site score sufficient for NPL 
proposal remained consistent across pathways.
     Ensuring that applying the SsI component as part of an HRS 
evaluation would not result in identification of sites with a low level 
of risk or would not identify sites with a high level of risk.
    These goals were met by using conceptual simulations to project the 
effectiveness and appropriateness for factor values, by developing and 
testing numerous example site scenarios to refine the model and by 
applying the model to test sites to determine its efficacy. The 
following information provides details on the approaches used to test 
the SsI component.
1. Conceptual Site Model/Sensitivity Analysis
    Sensitivity analyses were performed during development of the rule 
to test the SsI component and identify and assign the relative 
magnitude of the factors having the greatest impact on the HRS site 
score. The analyses illustrated the types of sites that would qualify 
for the NPL considering subsurface intrusion contamination, and sites 
that would qualify for the NPL considering the contribution of 
subsurface intrusion contamination to other pathways. The scenarios 
illustrate different site characteristics and different factor value 
weightings. An initial conceptual site scenario evaluation was 
developed with varying likelihood of intrusion levels, zone of 
contamination, waste characteristics and levels of contamination. The 
conceptual site scenario evaluation was varied to reflect possible 
ranges in the factors considered in the HRS evaluation.
    The first phase of testing estimated site scores based on options 
considered

[[Page 2771]]

for identifying eligible targets and delineating target areas. The 
testing was conducted using factor values, factor category values, and 
scoring algorithms consistent with other parts of the HRS. This ensured 
relative risk was evaluated and consistently weighted among pathways. A 
second phase was conducted for identifying target areas delineated by 
AOEs and ASCs of various site scenarios to test the HRS addition and to 
illustrate the features of sites that would qualify for the NPL 
considering vapor intrusion contamination. To illustrate the subsurface 
intrusion component and contribution of weighting of factor values, 
three comprehensive site scoring scenarios were evaluated: A site would 
not qualify for placement on the NPL (score below 28.50), a site would 
marginally qualify for the NPL (score of or about 28.50), and a site 
would exceed the scoring criterion for the NPL (site score considerably 
above 28.50). Based on this final rule, the results revealed that sites 
without areas of observed exposures and a typical waste characteristic 
value would require a minimum of 685 receptors living, working or 
attending school or daycare above an area of subsurface contamination 
to receive a score of 28.50 based on shallow subsurface sampling. Sites 
with documented subsurface intrusion into an occupied structure, a 
typical waste characteristic value and indoor air samples below health-
based benchmarks would require a minimum of 223 receptors to receive a 
score of 28.50. This illustrates that this final rule will not result 
in a large number of sites qualifying for the NPL as it is unlikely 
this number of receptors in an area of subsurface contamination will 
commonly occur. This is the similar number of receptors needed for a 
site to qualify for the NPL in other pathways.
2. Test Sites (Tier 1)
    To support the final rulemaking, EPA conducted a screening-level 
assessment of sites with identified subsurface intrusion threats. As a 
first step in collecting the list of sites potentially affected by the 
final rule, EPA consulted with site assessment experts that work in 
Superfund to identify potential site candidates. EPA also reached out 
to state counterparts, in particular to state programs that were known 
to have taken a more thorough investigation of the subsurface intrusion 
pathway at sites. Through this process, EPA identified approximately 
1,073 sites. These sites are not currently on the NPL, and all have a 
potential or identified SsI threat. Within the group of sites 
potentially affected by the HRS SsI Addition, EPA defined four 
categories:
    1. Tier 4: Sites identified as having a suspected SsI threat based 
on EPA's Superfund database and Agency for Toxic Substances and Disease 
Registry keyword searches, as well as EPA or state self-identification, 
but for which no sampling data were obtained;
    2. Tier 3: Sites identified as having characteristics or evidence 
that indicate SsI may have occurred or will occur;
    3. Tier 2: Sites identified as having an SsI threat documented by 
subslab, crawl space, or indoor air samples, but insufficient HRS-
required evaluation factors to qualify for the NPL; and
    4. Tier 1: Sites identified as having an SsI threat with documented 
actual exposure of a sufficient number of targets with enough other 
HRS-required evaluation factors to suggest the site may qualify for the 
NPL.
    EPA selected the Tier 1 sites for use in testing the SsI component 
evaluation process. The 11 Test Sites had documentation of indoor 
contamination due to subsurface intrusion based on actual sampling data 
and other typically HRS-required data. Of the 11 sites scored, 9 were 
projected to score 28.50 or higher using only the SsI component. 1 site 
was projected to score 28.50 or higher only by including both the 
scores from the SsI component evaluation and the ground water migration 
pathway evaluation in the site score. It was unknown whether these 
sites would qualify for the NPL when they were chosen as Test Sites, as 
the SsI scoring process had not been developed. The Test Site with a 
projected score below 28.50 did not qualify for the NPL even though the 
site was located in a mixed-used residential and industrial area, 
illustrating that not all sites in an urban area will qualify for the 
NPL.
    That 10 of the 11 Test Sites have a projected HRS site score of 
28.50 or greater using the SsI component is not an indication that the 
addition of the SsI component will result in a large number of SsI 
sites qualifying for the NPL; this would be a possible projection if 
the Test Sites were chosen randomly so as to represent a typical SsI 
site. The Test Sites were not randomly chosen, but instead were 
specifically chosen because they have a documented subsurface intrusion 
threats at the sites and sufficient available data to test all parts of 
the SsI component. The Test Sites all had areas of observed exposure; 
most had more than 38 structures at the site (some with hundreds of 
structures), and all but two Test Sites had at least 50 targets (more 
than half had over 100 targets). Each site was also associated with 
volatile hazardous substances that are considered hazardous to human 
health at low concentrations. Appendix B of the Technical Support 
Document (TSD) for this final rulemaking provides a summary of these 
scoring evaluations.
3. Pilot Study
    The main purpose of the Pilot Study was to identify sites currently 
being evaluated for SsI by the EPA regions with a suspected subsurface 
intrusion threat and determine whether an SI would provide enough 
information to score a site under the new component. Additional goals 
of the Pilot Study were to gather data and determine if design of the 
SsI model is practical and gives expected results; identify a range for 
the cost of a projected SsI site assessment; and assist in developing 
future guidelines for SsI assessments. A total of 10 sites were 
identified across 5 of the 10 EPA Regions. The pilot studies were not 
intended to identify sites for placement on the NPL, and not all sites 
considered for the pilot studies achieved an HRS score greater than (or 
equal to) 28.50. However, collecting actual data for the purposes of 
generating an SsI component score, ensured the HRS was considering 
subsurface intrusion threats appropriately. Ultimately, the pilot 
studies were used to proof the concept and validate the SsI component 
in terms of the application of selected weighting factor values and the 
efficacy for accurately identifying sites with significant relative 
risk.

IV. Summary of Changes to the HRS

    Comments on the Proposed Rule were received from 15 organizations/
individuals. The commenters included state and federal agencies, 
industry associations, community groups, consultants, and private 
citizens. No major conceptual or structural changes were necessary 
based on comments received during the public comment period. While many 
of the comments focused on the structure of the SsI component, there 
was not sufficient rationale for making major changes to the basic 
structure of the SsI component. There were minor revisions made based 
on comments, which help refine the mechanics of assigning an HRS site 
score. As a result, the SsI component better reflects current science 
and better aligns with underlying concepts in the OSWER Technical Guide 
for Assessing and Mitigating the Vapor Intrusion Pathway from 
Subsurface Sources to Indoor Air (VI Guide). These changes had no 
impact on the overall structure of the SsI component and do not impact 
the relative weighting among the HRS

[[Page 2772]]

pathways or the level of risk required to qualify for the NPL.

A. Changes Since Proposal

1. Consideration of Contaminated Ground Water Intrusion
    Section 5.2 was revised to clarify that areas of subsurface 
contamination are only delineated based on the presence of hazardous 
substances meeting the criteria for observed exposure or observed 
release and have a vapor pressure greater than or equal to one torr or 
a Henry's constant greater than or equal to 10-5 atm-m\3\/
mol. However, if samples indicate intrusion of liquids containing 
hazardous substances has occurred into regularly occupied structures, 
the samples of that liquid are still used in delineating an Area of 
Observed Exposure to reflect the threat to targets. These revisions 
were made to correct a seeming inconsistency in wording between the 
discussion in the preamble to the proposed rule and the proposed 
regulatory language.
2. Consideration of Non-Aqueous Phase Liquids (NAPLs) in Weighting of 
Targets in an ASC
    Table 5-21, Weighting Factor Values for Populations within an Area 
of Subsurface Contamination, of the HRS was revised to include 
consideration of the presence of NAPLs identified in an area of 
subsurface contamination. These additions increase the weighting of the 
population in an area of subsurface contamination to the SsI component 
score. These revisions were in response to comments that the proposed 
addition did not reflect the magnitude of contaminant concentrations in 
the evaluation of targets in the area of subsurface contamination. 
While EPA considers it unlikely that the actual aerial distribution and 
magnitude of contaminant concentrations can be determined in an area of 
observed contamination during a site inspection, if NAPLs are 
identified as present, EPA agrees that there is a greater risk to 
receptors than if no NAPL is present.
3. Modifications to the Determination of Degradation Factor Values
    Section 5.2.1.2.1.2 of the HRS was revised to make it easier for 
the reader to determine degradation factor values and to add 
consideration of the presence of NAPLs. Commenters asserted that the 
text was difficult to follow and that the presence of NAPLs was a major 
factor in the impact of degradation. A new table, Table 5-18 of the 
HRS, simplifying the assignment of degradation factor values based on 
the depth to contamination and a substance's half-life was inserted to 
replace proposed text. Additionally, if no half-life information is 
available for a hazardous substance and the substance is not already 
assigned a degradation factor value of 1, a value of 1 will be 
assigned. This modification further simplifies the degradation 
evaluation and is also protective of human health, for if no half-life 
information is available for a hazardous substance, EPA cannot assume 
that degradation will occur. In addition, parent-daughter relationships 
between substances are no longer considered in the assignment of the 
degradation factor value, in part to simplify the assignment and in 
part to reflect the variation in rates of degradation due to site-
specific subsurface conditions. Even if degradation occurs, if a 
contaminant is at high enough concentration to exist as a NAPL at 
depths less than or equal to 30 feet, it is more likely to pose a 
threat to populations in overlying structure.
4. Modifications Made to Section 5.2.1.1.2.1, Structure Containment and 
Table 5-12
    Section 5.2.1.1.2.1 and Table 5-12 of the HRS were revised in 
response to comments on the rationale for assigning containment values 
to individual structures. The assignment of a structure containment 
factor value assigned to structures in Table 5-12 with vapor mitigation 
systems or other response actions was revised. These revisions were 
made in response to a comment questioning why response actions taken by 
federal, state, and tribal authorities are treated differently than 
those taken by private entities in determining containment for a 
structure. The language regarding treatment of removals by federal, 
state, and tribal authorities has been removed from Table 5-12 and the 
corresponding containment value was assigned a 1. This change allows a 
consideration of public and private removal actions to be evaluated in 
a consistent manner.
    Section 5.2.1.1.2.1 and Table 5-12 of the HRS was also revised to 
remove from the table the direction of the assignment of a structure 
containment value for a regularly occupied structure with unknown 
containment features. This direction, which assigns a value of 
``greater than zero'' to this situation, was moved to the text in 
section 5.2.1.1.2.1 of the HRS. This revision was made in response to a 
comment questioning the rationale for the various containment values 
and was made to improve the continuity of the table, which directs the 
assignment of values when containment features of the structure are 
known. A structure with a containment factor value of greater than zero 
cannot be used in assigning a potential for exposure factor value. EPA 
considers it appropriate that the potential for exposure factor value 
should be based on actual field observations. However a structure with 
a structure containment value of greater than zero allows the structure 
to be evaluated for assigning waste characteristics values (e.g., a 
hazardous waste quantity factor value) and for assigning target factor 
values. EPA considers the inclusion of structures with unknown 
containment features in the calculation of waste characteristics and 
targets values appropriate as it reflects that very few structures are 
built to be sufficiently air tight to prevent subsurface intrusion.
5. Consideration of Hydraulic Conductivity in Vertical Migration
    Table 5-14 of the HRS was revised to allow assignment of an 
effective porosity/permeability factor value based on site-specific 
measurements of hydraulic conductivity, if known. This addition was 
made in response to a comment suggesting the rule be modified to allow 
use of site-specific information for this purpose when available.
6. Changes to Definitions
    The term surficial ground water was re-named shallow ground water 
and was changed to be consistent with current EPA usage.
    EPA has added the term non-aqueous phase liquid (NAPL) to the 
definition section. EPA added consideration of the identification of 
concentrations of hazardous substances high enough to indicate the 
presence of NAPLs in the subsurface during a site inspection to the 
assignment of degradation factor values and the weighting of targets in 
the ASC. The presence of NAPLs in the subsurface demonstrates the 
hazardous substances will be present at high concentrations for a 
significant time period at that location and the high concentration is 
not a transient situation.

B. Summary of Updates to the HRS (Sections 2, 5, 6, and 7)

1. Addition of an SsI Component to the HRS (Sections 2, 5, and 7)
    a. The addition of a subsurface intrusion component is added to the 
1990 Soil Exposure pathway as section 5.2 in Chapter 5 of the 2016 
Revised HRS. The new pathway name is the soil exposure and subsurface 
intrusion pathway. The existing method for evaluating the soil exposure 
threat will remain unchanged.
    b. Chapter 2: Evaluations Common to All Pathways is updated to 
reflect the

[[Page 2773]]

addition of the subsurface intrusion component to the renamed the soil 
exposure and subsurface intrusion pathway. The evaluations for the 
migration pathways and the soil exposure component remain unchanged. A 
parallel structure was added for the subsurface intrusion component.
    c. Chapter 7: Sites Containing Radioactive Substances is updated to 
reflect how radioactive substances are evaluated using the added 
subsurface intrusion component.
2. Terminology Updates Affecting Specific Sections of the HRS (Sections 
2, 5 & 6)
    The following terms are updated to reflect current terminology and 
procedures used by EPA in performing risk assessments.
    a. Ambient Water Quality Criteria: Ambient Water Quality Criteria 
(AWQC) are now identified also as National Recommended Water Quality 
Criteria (NRWQC). In addition, the acute AWQC are now identified as the 
Criterion Maximum Concentration (CMC) and the chronic criteria are 
referred to as the Criterion Continuous Concentration (CCC). (See 
section 1.1 of the HRS.) These criteria are used to determine the level 
of threat to environmental targets.
    b. Reference Concentrations: For inhalation exposures, EPA is 
adopting the use of Reference Concentrations (RfCs) instead of 
Reference Doses (RfDs) when determining non-cancer-related risk levels. 
RfCs are used in determining the level of threat to human targets due 
to possible inhalation and when determining the toxicity of the 
substances.
    c. Cancer Unit Risk: For inhalation exposures, EPA is adopting the 
use of Inhalation Unit Risk (IUR) instead of cancer slope factors in 
determining cancer-related risk levels. IURs are used in determining 
the level of threat to human targets due to possible inhalation and 
when determining the toxicity of the substances.
    d. Weight-of-Evidence Groupings: The 2005 EPA weight-of-evidence 
groupings supporting the designation of a substance as a human 
carcinogen have been incorporated into the HRS algorithm for assigning 
the toxicity factor value. (The former EPA weight-of-evidence 
categories included as part of the 1990 HRS have been retained as EPA 
has not yet completed assigning all substances to the revised 
categories and are doing so at the time the EPA substance literature 
reviews are updated.)

V. Discussion of Major Comments

    Comments on the Proposed Rule were received from 15 organizations/
individuals. The commenters included state and federal agencies, 
industry associations, community groups, consultants, and private 
citizens. This section discusses the major issues raised by commenters, 
which are summarized, and EPA's summary of responses. In addition, EPA 
solicited and received input from commenters on three technical 
questions posed in the Preamble to the Proposed Rule.
    A support document, Response to Comments on the 2016 Revisions to 
the Hazard Ranking System (HRS), that includes all issues raised during 
the public comment period, comments received on the questions posed in 
the preamble to the proposed rule and EPA's more comprehensive response 
to each issue, is available in the docket for this rulemaking.

A. Responses to Comments on EPA Questions Posed in the Proposed Rule

    Question 1: Is there a way to determine the presence and extent of 
biologically active soil at a site during a limited site investigation? 
If so, what soil characteristics should EPA consider to determine 
whether biologically active soil is documented to be present?
    EPA received multiple comments in response to this question. One 
commenter suggested that this activity is beyond the scope of the site 
assessment process, while another commenter suggested that EPA consider 
measuring specific compounds or other factors reflecting biological 
activity when conducting soil vapor analysis. A third commenter 
remarked that half-lives faster than 100 days are presumably due to 
aerobic biodegradation and that most vadose zone soils that are not 
grossly impacted are considered biologically active. A commenter also 
suggested using soil characteristics reflected in soil surveys to 
reflect the possibility that biologically active soil could be present. 
No commenter suggested practical methods to determine site-specific 
biological activity throughout a site or over time.
    The HRS SsI addition was revised to clarify the assumption of the 
presence of biologically soil in evaluating the degradation factor 
unless evidence indicates otherwise (see section 5.2.1.2.1.2 of the 
HRS).
    Question 2: How could EPA further take into account the difference 
in dilution and air exchange rates in large industrial buildings as 
compared to smaller residential and commercial structures when 
calculating the hazardous waste quantity for the HRS SsI Addition?
    EPA received multiple comments in response to this question. One 
commenter suggested developing intrusion screening values based on 
exposure scenarios for ``most sensitive individual'' and ``industrial'' 
models. One commenter indicated that there is not a dependable way to 
account for the differences between large commercial/industrial 
structures and smaller residential/commercial structures. Another 
commenter noted that there are several parameters (e.g., building 
energy efficiency) that would impact the differences in dilution and 
air exchange rates and which are generally unavailable during an 
initial assessment. A commenter discussed developing a sliding scale 
based on the size of the building and the building's general use to 
account for the differences in contaminant clearance rates.
    EPA did not make any changes to the final rule based on the 
comments received as the type of information requested in these 
responses is generally not available during a typical site inspection. 
The HRS has also been designed so that it can be applied consistently 
to a wide variety of sites. The HRS is not a tool for conducting 
quantitative risk assessment and was designed to be a measure of 
relative risk among sites rather than absolute site-specific risk.
    Question 3: The HRS SsI addition considers source strength in 
delineating ASCs and AOEs, in scoring in likelihood of exposure, in 
assigning waste quantity specifically when estimating hazardous 
constituent quantity and in weighting targets in an ASC. The HRS 
algorithm for all pathways incorporates the consideration of source 
strength in determining an HRS site score. Could EPA further take into 
account source strength in performing an HRS evaluation?
    EPA received multiple comments in response to this question. One 
commenter suggested that EPA assign a higher score when the contaminant 
concentration is high (e.g., when a non-aqueous phase liquid is 
present) to account for source strength. Comments were also received 
that reflected the difficulty of accessing large low concentration 
sources and how to account for that in considering source strength. 
Another commenter remarked that there may be a large ground water plume 
without a discrete source that would cause an increased risk of vapor 
intrusion; and that a large diffuse source is different from having a 
concentrated discrete source. One commenter

[[Page 2774]]

provided a copy of the proposed rule with their suggested edits 
reflecting the evaluation of source strength in assigning HRS specific 
factors.
    The assignment of a degradation factor value (see section 
5.2.1.2.1.2 of the HRS) and the weighting factors for targets in an 
area of subsurface contamination (see Table 5-21 of the HRS) were 
revised to include consideration of source strength; specifically in 
the situation where NAPLs are present.

B. Major Comment Theme Summaries and Responses

Statutory Authority and Rationale for the Proposed HRS Addition
Justification for Revising the HRS
    EPA received comments suggesting that sufficient justification or 
rationale for the need to revise the HRS has not been provided and that 
a revision to the HRS is unnecessary because the 1990 HRS adequately 
evaluates the relative risk posed by a site and identifies those 
priority sites for further investigation.
    The rationale for revising the HRS to add a subsurface intrusion 
component is EPA's statutory authority. Specifically, CERCLA 
105(a)(8)(A), requires EPA to amend the HRS ``to assure to the maximum 
extent feasible, that the HRS accurately assess the relative degree of 
risk to human health and the environment posed by sites and facilities 
subject to review.'' Contamination due to subsurface intrusion is a 
known risk to human health and the ability to evaluate those risks is 
consistent with the CERCLA 105 mandate. The 1990 HRS did not evaluate 
the risk posed by subsurface intrusion when evaluating sites for the 
NPL. As part of the development of this rule, EPA identified high 
priority sites with significant contamination due to SsI that could not 
be evaluated using the 1990 HRS for possible placement on the NPL. With 
the addition of the SsI component to the HRS, sites can now be 
evaluated more comprehensively to consider the relative risk posed by a 
site.
Priority for Drinking Water Sites
    EPA received comments suggesting that the proposed HRS SsI addition 
conflicts with CERCLA's statutory mandate regarding prioritizing 
drinking water sites.
    The revision to the HRS to add a subsurface intrusion component is 
not in conflict with the CERCLA 105 mandate to prioritize drinking 
water sites. The priority given by EPA under CERCLA to sites with a 
high risk of populations exposed to hazardous substances in drinking 
water has not decreased with the addition of a subsurface intrusion 
component to the HRS. In fact, the score for some sites with 
contaminated drinking water supplies may increase because sites with 
contaminated drinking water may also be associated with subsurface 
intrusion contamination and the combination of the ground water 
migration pathway score and the SsI component score may increase the 
overall site score. Furthermore, EPA notes that drinking water is a 
priority identified by CERCLA, but it is not the only priority 
identified in CERCLA 105, which also mandates the prioritization of 
dangers of direct human contact, for which SsI is one example.
    The addition of the SsI component does not change the priority 
given to drinking water sites. It does not change the scoring of 
contaminated drinking water supplies under the HRS, reduce in anyway 
the overall HRS score for any site based on drinking water 
contamination (or any other threat due to exposure to released 
hazardous substances in the HRS), or change the site score of 28.50 
being the HRS score that qualifies sites for placement on the NPL. If a 
site qualifies for placement on the NPL based on its HRS score 
reflecting drinking water contamination prior to the addition of the 
SsI component, it will continue to do so. Adding an evaluation of the 
SsI component can only increase an overall site score. The algorithm 
used to combine pathways scores to obtain an overall site score results 
in an increase in the overall site score with the evaluation of 
additional pathways, components and threats scored. In fact, the SsI 
addition may raise the overall site score at some sites with ground 
water drinking water contamination from below the 28.50 cut-off score 
to above it. This may occur because, as stated above, a site's HRS 
score can increase with the scoring of additional threats. Sites with 
ground water contaminated by volatile substances and used for drinking 
water are also sites at which the ground water contamination may 
volatilize and intrude into overlying regularly occupied structures. 
Thus, a site at which ground water contamination has occurred but does 
not have an HRS score above 28.50 based only on the ground water 
threat, may have an overall HRS site score above 28.50 based on the 
combination of the scores for the contaminated drinking water and SsI 
threats.
    Furthermore, EPA notes that CERCLA 118 refers to CERCLA sections104 
and 108, which address activities that occur pre- or post-NPL-listing, 
and not to the section of CERCLA that addresses site ranking using the 
HRS, which is addressed in CERCLA section 105. CERCLA Section 105 and 
specifically 105(a)(8)(A) requires EPA to prioritize sites based on 
``the population at risk, the hazard potential of hazardous substances 
at such facilities, the potential for contamination of drinking water 
supplies, the potential for direct human contact [and] the potential 
for destruction of sensitive ecosystems.'' Since subsurface intrusion 
contamination is a direct human contact threat, the addition of a 
subsurface intrusion component, which addresses this threat, is 
mandated by CERCLA.
Resource Impacts of the Proposed HRS Addition
Increased Cost and Level of Effort
    EPA received comments suggesting that contrary to EPA's suggestion 
that the HRS SsI addition may not result in more site assessments per 
year and only minimal cost increases, commenters claimed that there 
will be substantial increases in cost and level of effort for states 
and federal agencies, due to the complexity in assessing subsurface 
intrusion sites.
    EPA acknowledges that in some cases the scope of a typical site 
inspection (SI) may need to be expanded to collect the information 
necessary to evaluate the SsI threat present at a site. EPA also 
acknowledges that sites that did not qualify previously for the NPL, 
may now do so. The number of samples and level of effort required to 
evaluate a site using the 1990 HRS pathways or components already 
varies on a site-by-site basis depending on the size and extent of 
contamination at the site. Therefore, it cannot be predicted with 
certainty that there will be an overall increase in cost or level of 
effort for any particular site due to the HRS SsI addition. However, 
the overall budget for performing site assessments per year is not 
expected to change significantly. EPA's budget for site assessment is 
dependent on Congressional appropriation and EPA does not expect the 
rulemaking to impact the appropriation. EPA's budget for site 
assessment has remained relatively constant for the last several years. 
Hence, EPA expects that the allocation of available resources may be 
changed to reflect this rulemaking but will continue to be optimized by 
EPA, its state and tribal partners, and with other federal agencies to 
evaluate priority sites. However, the number of site assessments or NPL 
proposals conducted each year will not significantly increase.

[[Page 2775]]

Potential Limitations With Implementing the HRS SsI Addition
Scope of Site Inspection
    EPA received comments stating that the type and amount of 
information available for collection during a time-limited site 
inspection would be insufficient to properly evaluate a site using the 
HRS SsI addition and would be beyond the scope of site evaluations 
typically conducted at the preliminary assessment or site inspection 
stage.
    During development of the HRS SsI addition EPA considered the type 
of information that could be collected during a time-limited site 
inspection when selecting the factors to include in an evaluation of 
the subsurface intrusion component. The purpose of the site inspection 
(NCP 300.420(c)) is to determine if a release of a hazardous substance 
poses an actual or potential threat to human health or the environment, 
to determine if there is an immediate threat to people or the 
environment, and to collect sufficient data to enable the site to be 
scored using the HRS. EPA also notes that neither the NCP nor the HRS 
requires a certain number of samples be collected during an SI, because 
the number of samples required to evaluate a site varies on a site-by-
site basis and the possible risk pathways being evaluated. However, to 
properly evaluate the subsurface intrusion component, additional 
information may be required beyond that collected during a typical 
current site inspection may be required; this is consistent with the 
need to collect data on the threat posed by a different pathway. In 
these instances, as stated in EPA's Guidance for Performing Site 
Inspections under CERCLA (September 1992), an expanded site inspection 
(ESI) may be required. The objective of the ESI is to collect data that 
was not collected during an initial site inspection. Furthermore, EPA 
found that information required for an SsI evaluation was available 
based on a pilot study which included several candidate NPL sites. The 
pilot study was performed in part to demonstrate the availability of 
the necessary data from screening level investigations. Therefore, EPA 
considers that the information required to properly evaluate the 
subsurface intrusion component can be obtained during the site 
assessment process.
Need for Guidance
    EPA received comments questioning or requesting additional 
information or guidance regarding the type and amount of data to 
collect, data collection methods, and how to apply the subsurface 
intrusion component to a site. Commenters also suggested it was 
difficult to properly evaluate and comment on the proposed HRS SsI 
addition without a thorough understanding of how the SsI component 
would be implemented and that promulgation should be delayed until 
guidance is developed.
    The HRS does not provide prescriptive methods for performing site 
investigations for any HRS pathway evaluation because the methods used 
during the collection and analysis of environmental samples depend on 
site conditions and could not be written to cover all possible 
situations and could also become outdated in the future. Additionally, 
it is outside the scope of the HRS to identify and describe methods for 
conducting a subsurface intrusion screening for HRS purposes. The 
sampling and data collection information in the EPA OSWER VI Guide, 
(particularly in section 6 of the guide) are an appropriate resource 
for gathering data for HRS purposes. For example, Section 6.4 of the 
guide identifies basic principles, methods and procedures for indoor 
air sampling. In addition, states, federal agencies, and private 
contractors have considerable experience in VI investigations and 
collecting VI-related data. Guidance on implementation of the proposed 
SsI addition is not necessary for evaluating the SsI component, which 
is a scoring mechanism not procedures for data collection. Any guidance 
developed will provide details on collecting data to support an HRS SsI 
evaluation. EPA also notes that to delay addressing sites that may pose 
a significant human health risk until all necessary guidance documents 
have been developed would not be consistent with EPA's mandate to 
protect human health. Therefore, EPA does not agree that promulgation 
of the HRS SsI addition needs to be delayed until guidance documents 
related to its implementation have been developed.
Roles of the HRS SsI Addition and the 2015 OSWER VI Guide
    EPA received comments suggesting that the HRS SsI addition is not 
consistent with the VI Guide, published in June 2015 and will create 
confusion when evaluating sites for SsI.
    The VI Guide and HRS SsI rule work in concert to establish national 
consistency in the evaluation of SsI threats. The HRS SsI addition and 
the OSWER VI Guide both address the threat posed by vapor intrusion and 
use the same principles, sampling procedures and concepts to 
characterize the threat posed by vapor intrusion as the sites. However, 
the HRS SsI addition and the OSWER VI Guide serve different purposes 
and support different phases of EPA's site remediation process with 
different data quality requirements and different enabling 
legislations.
    The purpose of the OSWER VI Guide is to guide the investigation and 
assessment of the threat posed by vapor intrusion into structures from 
all sources under all Office of Land and Emergency Management (OLEM, 
formerly OSWER) programs, particularly actions taken under CERCLA and 
RCRA. This guidance is used to support decisions by EPA on whether 
vapor intrusion is posing an unacceptable risk to human health based on 
sufficient site specific data. It contains principles for making such a 
decision, as well as procedures and guidance for collecting the 
information necessary to make these decisions.
    The HRS and the SsI addition is part of the NCP, (the regulations 
implementing CERCLA) required by CERCLA to identify priority sites for 
further investigation based on screening level information (Such sites 
are identified for the public by placing the sites on the NPL, a 
separate rulemaking process). This prioritization is based on the 
possible cumulative relative risk amongst all candidate sites posed by 
releases of hazardous substances to human health and the environment by 
either migration to receptors or by direct contact with the 
contamination, such as by subsurface intrusion. The HRS is only a 
method for assigning a relative score to candidate sites. It is not a 
method for determining site specific risk. The HRS SsI addition is not 
guidance. The HRS SsI addition does not address such subjects as data 
collection and sampling procedures: Many of the procedures and many of 
the guidelines in the OSWER VI Guide are also applicable for HRS 
purposes if they can be implemented as part of a screening level 
assessment.
    Given that the purposes for the two documents are considerably 
different and based on different levels of information, it is not an 
issue that decision criteria are different in the two documents. It is 
certainly possible that, based on an HRS evaluation, EPA may determine 
a site warrants further investigation, and that after further 
investigation is performed EPA may decide no remediation is necessary. 
However until further information is collected during a remedial 
investigation, such an outcome cannot be predicted. Furthermore, such a 
situation is not an indication the results of the HRS evaluation was 
incorrect.

[[Page 2776]]

Application of HRS SsI Component
Inferring Contamination
    EPA received comments suggesting that by inferring contamination 
between sampling locations, the extent of the risk is overstated. The 
commenters considered identifying targets as actually or potentially 
exposed based on inference to inflate the HRS site score. It was also 
suggested that this method conflicts with the other HRS pathways.
    The HRS is not a quantitative risk assessment. Instead, the HRS SsI 
addition score reflects the possible threat posed by subsurface 
intrusion at one site relative to other sites. By inferring 
contamination in an AOE or an ASC between sampling locations, it is not 
assumed that all populations within the two areas are exposed to 
contamination from the subsurface. Inferring contamination also allows 
sites with large populations within the two areas to be ranked higher 
than sites with smaller populations. If the HRS scoring required 
sampling every structure a sufficient number of times to assure that 
all exposed targets were accounted for, the scope of the sampling 
effort would be beyond that of a screening tool and more consistent 
with the scope of a remedial investigation.
    Inference of contamination between sampling locations is also 
assumed in other HRS pathways. The other pathways allow the inference 
of contamination based on the location of samples documenting the 
presence of contamination attributable to the site being investigated. 
For example, in the soil exposure component, inference of contamination 
is done by drawing AOC boundaries based on sample locations and 
inferring that those targets associated with the properties within the 
boundaries are actually exposed.
    In the SsI component, unless site-specific information indicates 
otherwise, when delineating an AOE or an ASC, populations in occupied 
structures within an AOE are inferred to be actually exposed, and, 
populations in occupied structures within an ASC are inferred to likely 
be exposed to contamination.
Purpose of Hazardous Waste Quantity
    Commenters noted that as explained in the TSD for the proposed HRS 
SsI Addition, the hazardous waste quantity factor serves as a surrogate 
for the contaminant dose that populations may be exposed to. Commenters 
asserted that the hazardous waste quantity factor is not adequately 
reflective of this dose to be used as a surrogate.
    The commenters appear to be confusing consideration of waste 
quantity as a surrogate for dose in an HRS evaluation with the 
calculation of a site-specific risk level based on the ratio of waste 
quantity to receptors. EPA is not projecting a specific risk level 
based on the waste quantity alone when it performs an HRS evaluation. 
Other HRS factors such as the population associated with the 
structures, the probability of a release into the occupied structures, 
the possibility of degradation, and the toxicity of the substances are 
also considered,
    The decision to include waste quantity as a surrogate for dose in 
all pathways and components in the HRS algorithm was made when the HRS 
was last revised in 1990 (see Section V.3 of the proposed 1988 HRS, 53 
FR 51692, December 23, 1988; Section III.C of the 1990 HRS, 55 FR 
51542, December 14, 1990). The decision was based on the concept that 
determining an accurate dose that receptors would be exposed to was 
beyond the scope of information available after a site inspection. It 
is not possible to accurately predict the hazardous substance 
concentration that receptors would be exposed to over a representative 
exposure period based on information collected during a site inspection 
due to the variability in exposure levels over time and space. Instead, 
hazardous waste quantity is used as a surrogate for dose in the sense 
that the quantity of the hazardous substances is at least qualitatively 
correlated to the magnitude of the exposure. If there is no waste 
quantity, there will be no exposure; as the waste quantity increases, 
the greater the possibility of exposure to hazardous substances that a 
receptor may come in contact with. EPA agrees this is not a perfect 
correlation, and has built into the HRS four order of magnitude ranges 
for assigning factor values that reflect the imperfection of this 
correlation.
    In addition, the inclusion of hazardous waste quantity in the 
subsurface intrusion component is consistent with its inclusion in all 
the other existing HRS pathway evaluations and is consistent with the 
goal that the scoring of the new component not impact the balance built 
into overall HRS site scoring algorithm among the HRS pathways.
    Furthermore, for determining waste quantity for the SsI component, 
EPA made a specific alteration to how waste quantity is calculated as 
compared to other HRS pathway. EPA decided to only include the amount 
of hazardous substance that actually enters into or that could enter 
into occupied structures, not the total amount in the release to the 
environment, based on the rationale that at least some of the original 
release in the subsurface would vent directly to the atmosphere. 
Therefore only the amount of hazardous substances that has entered into 
occupied structures or the amount located under structures is reflected 
in the estimate. This was achieved by not estimating the waste quantity 
based on the area or the volume of the contaminated media in the 
subsurface, but instead on the volume of the structures, or the basal 
area if the volume cannot be determined.
    Finally, no comments were received that provided a viable 
alternative to the proposed method of estimating hazardous waste 
quantity. Commenters stated the amount of exposure was overestimated 
for large buildings because in general larger buildings have lower air 
exchange rates and suggested that this consideration be built into the 
estimation methods for all structures. However, the commenters did not 
present data to document this generality nor suggest how to determine 
the air exchange rate for all structures if it is not provided by the 
building owner. EPA notes that if air exchange rates are available, the 
present estimation method (which has not changed since proposal) allows 
for a hazardous waste quantity estimate using that information (see, 
HRS section 5.2.1.2.2 Tier B, hazardous wastestream quantity).
    While some commenters suggested procedures for determining a more 
accurate hazardous waste quantity for specific situations they did not 
suggest how the hazardous waste quantity calculated for these 
situations could be relatively ranked against sites where equivalent 
information was not available. When developing a hazardous waste 
quantity factor in 1988, EPA performed studies that showed this level 
of information was not available at all sites, and was not likely to be 
collectible during a limited screening assessment. Therefore, EPA 
considers it inappropriate to incorporate the suggested procedures into 
the HRS.
    In addition, EPA proposed the present hazardous waste quantity 
estimation process as part of the revision of the HRS in 1988. At that 
time EPA requested the Science Advisory Board's (SAB's) assistance on 
the use of concentration data in determining the hazardous waste 
quantity factor as part of the overall SAB peer review of the HRS 
changes. The current method for use of concentration data in 
determining the hazardous waste quantity factor is based on the SAB's 
recommendation.
Establishment of Attribution
    Commenters noted that establishing that indoor air contamination is

[[Page 2777]]

attributable to subsurface intrusion will be very complex to 
demonstrate given all other possible origins of the indoor 
contamination (e.g., outdoor air, consumer products).
    The HRS SsI addition, just as in other HRS pathways and components, 
does not require absolute proof that the significant increase in indoor 
contaminant concentrations is due to subsurface intrusion. It only 
requires at least part of the significant increase be attributable to 
subsurface intrusion. EPA expects to use multiple lines-of-evidence in 
meeting the attribution requirement as discussed in various comments. 
The VI Guide outlines use of multiple lines-of-evidence and provides 
guidance on how to distinguish subsurface intrusion from other sources 
of vapor intrusion. As is done for other HRS pathways and components, 
the HRS standard for establishing attribution is to establish a 
reasoned explanation that is not shown to be incorrect during public 
review of placement of a proposed site on the NPL.
Establishing Observed Exposure
    EPA received comments suggesting that the criteria for establishing 
background for the SsI component is too complex given the variability 
in sampling for SsI and that a significant difference between the 
background level and release concentration is not an adequate measure 
for establishing an observed exposure in a regularly occupied 
structure.
    EPA agrees that establishing a background level for indoor air can 
be difficult. However, it does not mean that the HRS criteria for 
establishing actual exposure should not be used. Methods for 
establishing background levels are too site-specific to be discussed in 
the HRS regulation, which is a scoring methodology. Instead, as 
occurred after the 1990 HRS was promulgated, criteria for establishing 
background was refined based on actual experience gained as sites were 
being scored. EPA expects the same to occur for the HRS SsI component.
    Comparison of background levels and indoor air concentrations are 
used only to establish that the contaminant level in a structure is 
elevated (i.e., significantly different). This is only the first step 
in establishing observed exposure. The second step is to attribute at 
least a part of the significant increase to subsurface intrusion.
    The argument that vapor intrusion rates are too variable to justify 
the use of the same procedure for establishing observed releases or 
exposures as in other parts of the HRS is invalid. Hazardous substance 
concentrations are unpredictably variable temporally and spatially for 
all HRS pathways and SsI variability is no different in that regard. 
For example, in the surface water migration pathway overland flow 
threat, the hazardous substance may only be entering surface water via 
runoff due to rain events. No runoff occurs if it is not raining. The 
amount entering surface water in this situation has been shown to vary 
with the length of time between rains, which impacts the amount of 
material deposited and available for entrainment into the runoff. 
Runoff also varies with the portion of each rain cycle whether the 
sample is collected at the beginning, middle or end of a rain event. At 
the beginning of a rain event all erodible materials are present and 
available. During the middle or during a high intensity period of rain, 
the force of the rain drops can dislodge and entrain hazardous 
substances at greater rates that during low intensity periods. At the 
end of a rain event, it may be that much of the hazardous substances 
have already been washed away. In continuous air releases, the 
contaminant concentration can vary by order of magnitudes with distance 
from the source, with wind direction and wind speed all of which can 
cause differences in concentrations spatially due to the three 
dimensionality of the atmosphere, and cannot be predicted or accounted 
for based on a screening assessment. Even in ground water 
contamination, the contaminant plume's concentration can vary spatially 
depending on the rate of ground water movement from the original spill 
concentrations. It is not possible to account for these factors that 
can drastically impact the contaminant concentration at a sampling 
location, based on screening level information.
    For example, variation in the occurrence of releases is no greater 
in the SsI component than would be expected in point-source air 
releases or spills to surface water.
Degradation
    Commenters suggested changes in how the degradation factor value 
for the subsurface intrusion component is assigned. Other comments 
dealt with conditions associated with assigning different degradation 
factor values based on the depth of biologically active soil and the 
half-lives of individual hazardous substances. In addition, commenters 
suggested moving the consideration of degradation from the waste 
characteristics factor category value calculations to the likelihood of 
exposure factor category value calculations.
    After evaluation of the comments, EPA modified the assignment of 
the degradation factor to simplify the evaluation and to consider the 
presence of non-aqueous phase liquids (NAPLs); other changes suggested 
by commenters were not implemented. Some changes were not made because 
a sufficient rationale was not provided to justify a change. Regarding 
the placement of the degradation factor in the HRS equation, the 
consideration of an individual substance's characteristics in the waste 
characteristics factor category is consistent with other HRS pathways 
and components. Furthermore, whether the degradation factor is put in 
the likelihood of release or waste characteristic factor category, the 
impact of the factor on the score would be similar.
Targets
    EPA received comments on the weightings assigned to targets in both 
the AOE and ASC. Commenters suggested that the weightings reflect the 
strength of the attribution argument that the significant increase in 
indoor air concentrations is due to subsurface intrusion and also 
reflect the concentration of the contaminants in the subsurface.
    After consideration of these comments, EPA has changed the 
weightings of targets in the ASC to reflect the presence of NAPLs 
(i.e., to reflect contaminant concentrations in the subsurface). EPA 
did not incorporate any changes into the weightings of targets based on 
the strength of attribution or concentration of contaminants in the 
subsurface. Regarding the strength of an attribution argument, the HRS 
does not recognize gradations of attribution in any other pathway or 
component and therefore for consistency, will not in this component. 
EPA notes that with the limited sampling that occurs during an SI, it 
is not reasonable to project the concentration of contaminants in the 
subsurface over time or distance.

VI. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive Orders 
can be found at https://www.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 13563: Improving Regulation and Regulatory Review

    This action is a significant regulatory action that was submitted 
to the Office of Management and Budget (OMB) for review. This action 
may raise novel

[[Page 2778]]

legal or policy issues arising out of legal mandates, the President's 
priorities, or the principles set forth in the EO. Any changes made in 
response to OMB recommendations have been documented in the docket.
    EPA prepared an analysis of the potential costs and benefits 
associated with this action. This analysis, Addition of a Subsurface 
Intrusion (SsI) Component to the Hazard Ranking System (HRS): 
Regulatory Impact Analysis is available in the docket for this action.

B. Paperwork Reduction Act (PRA)

    This action does not impose any new information collection burden 
under the PRA. OMB has previously approved the information collection 
activities contained in the existing regulations and has assigned OMB 
control number 2050-0095.
    This regulatory change will only affect how EPA and organizations 
performing work on behalf of EPA (state or tribal partners) conduct 
site assessments and HRS scoring at sites where certain environmental 
conditions exist. This regulatory change will result in data collection 
at these types of sites to allow evaluation under the HRS. EPA expects 
that the total number of site assessments performed and the number of 
sites added to the NPL per year will not increase, but rather expects 
that there will be a realignment and reprioritization of its internal 
resources and state cooperative agreement funding.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. This 
action will not impose any requirements on small entities. This 
regulatory change enables the HRS evaluation to directly consider human 
exposure to hazardous substances that enter building structures through 
subsurface intrusion. This addition to the HRS would not impose direct 
impacts on any other entities. For additional discussion on this 
subject, see section 4.9 of the Regulatory Impact Analysis (see the 
docket for this action).

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in 
UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect 
small governments. The action imposes no enforceable duty on any state, 
local, or tribal governments or the private sector.

E. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have 
substantial direct effects on the states, on the relationship between 
the national government and the states, or on the distribution of power 
and responsibilities among the various levels of government.

F. Executive Order 13175: Consultation and Coordination With Indian 
Tribal Governments

    This action does not have tribal implications, as specified in 
Executive Order 13175. EPA's evaluation of a site using the HRS does 
not impose any costs on a tribe (except those already in a cooperative 
agreement relationship with EPA). Thus, Executive Order 13175 does not 
apply to this action.
    Although Executive Order 13175 does not apply to this action, EPA 
consulted with tribal officials through meetings and correspondence, 
including a letter sent to all federally recognized tribes asking for 
comment on the ``Notice of Opportunity for Public Input'' that was 
published in the Federal Register on January 31, 2011 (76 FR 5370), and 
public listening sessions regarding the decision to proceed with the 
development of this action. All tribal comments indicated support for 
this action.

G. Executive Order 13045: Protection of Children From Environmental 
Health Risks and Safety Risks

    EPA interprets Executive Order 13045 as applying only to those 
regulatory actions that concern environmental health or safety risks 
that EPA has reason to believe may disproportionately affect children, 
per the definition of ``covered regulatory action'' in section 2-202 of 
the Executive Order. This action is not subject to Executive Order 
13045 because it does not concern an environmental health risk or 
safety risk.

H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use

    This action is not a ``significant energy action'' because it is 
not likely to have a significant adverse effect on the supply, 
distribution, or use of energy. The site assessment activities affected 
by this rule are limited in scope and number and rely on existing 
energy distribution systems. Further, we have concluded that this rule 
would not significantly expand the energy demand for site assessments, 
and would not require an entity to conduct any action that would 
require significant energy use, that would significantly affect energy 
supply, distribution, or usage. Thus, Executive Order 13211 does not 
apply to this action.

I. National Technology Transfer and Advancement Act

    This rulemaking does not involve technical standards.

J. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations

    The EPA believes the human health or environmental or environmental 
risk addressed by this action will not have potential 
disproportionately high and adverse human health or environmental 
effects on, low-income or indigenous populations. The results of this 
evaluation are contained in section 4.3 (and all subsections) of the 
Regulatory Impact Analysis for this rulemaking. A copy of the Addition 
of a Subsurface Intrusion (SsI) Component to the Hazard Ranking System 
(HRS): Regulatory Impact Analysis is available in the docket for this 
action.

K. Executive Order 12580--Superfund Implementation

    Executive Order 12580, section 1(d), states that revisions to the 
NCP shall be made in consultation with members of the National Response 
Team (NRT) prior to publication for notice and comment. Revisions shall 
also be made in consultation with the Director of the Federal Emergency 
Management Agency (FEMA) and the Nuclear Regulatory Commission (NRC) to 
avoid inconsistent or duplicative requirements in the emergency 
planning responsibilities of those agencies. Executive Order 12580 
delegates responsibility for revision of the NCP to EPA.
    The agency has complied with Executive Order 12580 to the extent 
that it is related to the addition of a new component to the HRS, 
through consultation with members of the NRT.

L. Congressional Review Act (CRA)

    This action is subject to the CRA, and the EPA will submit a rule 
report to each House of the Congress and to the Comptroller General of 
the United States. This action is not a ``major rule'' as defined by 5 
U.S.C. 804(2).

List of Subjects in 40 CFR Part 300

    Environmental protection, Air pollution control, Chemicals, 
Hazardous substances, Hazardous waste, Intergovernmental relations, 
Natural resources, Oil pollution, Penalties, Reporting and 
recordkeeping

[[Page 2779]]

requirements, Superfund, Water pollution control, Water supply.

    Dated: December 7, 2016.
Gina McCarthy,
Administrator.

    For the reasons set out in the preamble, Title 40, Chapter 1 of the 
Code of Federal Regulations is amended as follows:

PART 300--NATIONAL OIL AND HAZARDOUS SUBSTANCES POLLUTION 
CONTINGENCY PLAN

0
1. The authority citation for part 300 continues to read as follows:

    Authority: 33 U.S.C. 1321(d); 42 U.S.C. 9601-9657; E.O. 13626, 
77 FR 56749, 3 CFR, 2013 Comp., p. 306; E.O. 12777, 56 FR 54757, 3 
CFR, 1991 Comp., p. 351; E.O. 12580, 52 FR 2923, 3 CFR, 1987 Comp., 
p. 193.

0
2. Amend Appendix A to Part 300:
0
a. In section 1.1 by:
0
i. Removing the definition heading ``Ambient Water Quality Criteria 
(AWQC) and adding ``Ambient Water Quality Criteria (AWQC)/National 
Recommended Water Quality Criteria'', in its place; and removing the 
text ``maximum acute or chronic toxicity'' and adding ``maximum acute 
(Criteria Maximum Concentration or CMC) or chronic (Criterion 
Continuous Concentration or CCC) toxicity.'' in its place;
0
ii. Adding in alphabetical order the definition ``Channelized flow'';
0
iii. Revising the definition ``Chronic toxicity'';
0
iv. Adding in alphabetical order the definition ``Crawl space'';
0
v. Revising the definitions ``Distance weight'' and ``Half-life'';
0
vi. Amending the definition ``HRS pathway'' by removing the word 
``soil,'' and adding ``soil exposure and subsurface intrusion,'' in its 
place;
0
vii. Adding in alphabetical order the definitions ``Indoor air'', 
``Inhalation Unit Risk (IUR)'', ``Non-Aqueous Phase Liquid (NAPL)'', 
``Preferential subsurface intrusion pathways'', and ``Reference 
concentration (RfC)'';
0
viii. Revising the definition ``Reference dose (RfD)'';
0
ix. Adding in alphabetical order the definition ``Regularly occupied 
structures'';
0
x. Revising the definition ``Screening concentration'';
0
xi. Adding in alphabetical order the definition ``Shallow ground 
water'';
0
xii. Revising the definition ``Slope factor (also referred to as cancer 
potency factor)'';
0
xiii. Adding in alphabetical order the definitions ``Soil gas'', ``Soil 
porosity''; ``Subslab'', ``Subsurface intrusion'', ``Unit risk'', and 
``Unsaturated zone''; and
0
xiv. Revising the definition ``Weight-of-evidence''.
0
b. Revising section 2.0;
0
c. Revising section 5.0;
0
d. In section 6.0 by revising Table 6-14; and
0
e. In section 7.0 by:
0
i. Revising Table 7-1;
0
ii. Under Table 7-1, the second undesignated paragraph, revising the 
third sentence;
0
iii. Revising sections 7.1, 7.1.1, and 7.1.2; 7.2.1; 7.2.3; 7.2.4; 
7.2.5.1, 7.2.5.1.1 through 7.2.5.1.3; 7.2.5.2; 7.2.5.3; 7.3, 7.3.1, and 
7.3.2; and
0
iv. Adding section 7.3.3.
    The revisions and additions read as follows:

Appendix A to Part 300--Hazard Ranking System

* * * * *

1.1 Definitions

* * * * *
    Channelized flow: Natural geological or manmade features such as 
karst, fractures, lava tubes, and utility conduits (e.g., sewer 
lines), which allow ground water and/or soil gas to move through the 
subsurface environment more easily.
    Chronic toxicity: Measure of toxicological responses that result 
from repeated exposure to a substance over an extended period of 
time (typically 3 months or longer). Such responses may persist 
beyond the exposure or may not appear until much later in time than 
the exposure. HRS measures of chronic toxicity include Reference 
Dose (RfD) and Reference Concentration (RfC) values.
* * * * *
    Crawl space: The enclosed or semi-enclosed area between a 
regularly occupied structure's foundation (e.g., pier and beam 
construction) and the ground surface. Crawl space samples are 
collected to determine the concentration of hazardous substances in 
the air beneath a regularly occupied structure.
* * * * *
    Distance weight: Parameter in the HRS air migration pathway, 
ground water migration pathway, and the soil exposure component of 
the soil exposure and subsurface intrusion pathway that reduces the 
point value assigned to targets as their distance increases from the 
site. [unitless].
* * * * *
    Half-life: Length of time required for an initial concentration 
of a substance to be halved as a result of loss through decay. The 
HRS considers five decay processes for assigning surface water 
persistence: Biodegradation, hydrolysis, photolysis, radioactive 
decay, and volatilization. The HRS considers two decay processes for 
assigning subsurface intrusion degradation: Biodegradation and 
hydrolysis.
* * * * *
    Indoor air: The air present within a structure.
    Inhalation Unit Risk (IUR): The upper-bound excess lifetime 
cancer risk estimated to result from continuous exposure to an agent 
(i.e., hazardous substance) at a concentration of 1[mu]g/m\3\ in 
air.
* * * * *
    Non-Aqueous Phase Liquid (NAPL): Contaminants and substances 
that are water-immiscible liquids composed of constituents with 
varying degrees of water solubility.
* * * * *
    Preferential subsurface intrusion pathways: Subsurface features 
such as animal burrows, cracks in walls, spaces around utility 
lines, or drains through which a hazardous substance moves more 
easily into a regularly occupied structure.
* * * * *
    Reference concentration (RfC): An estimate of a continuous 
inhalation exposure to the human population that is likely to be 
without an appreciable risk of deleterious effects during a 
lifetime.
    Reference dose (RfD): An estimate of a daily oral exposure to 
the human population that is likely to be without an appreciable 
risk of deleterious effects during a lifetime.
    Regularly occupied structures: Structures with enclosed air 
space, where people either reside, attend school or day care, or 
work on a regular basis, or that were previously occupied but 
vacated due to a site-related hazardous substance(s). This also 
includes resource structures (e.g., library, church, tribal 
structure).
* * * * *
    Screening concentration: Media-specific benchmark concentration 
for a hazardous substance that is used in the HRS for comparison 
with the concentration of that hazardous substance in a sample from 
that media. The screening concentration for a specific hazardous 
substance corresponds to its reference concentration for inhalation 
exposures or reference dose for oral exposures, as appropriate, and, 
if the substance is a human carcinogen with either a weight-of-
evidence classification of A, B, or C, or a weight-of-evidence 
classification of carcinogenic to humans, likely to be carcinogenic 
to humans or suggestive evidence of carcinogenic potential, to that 
concentration that corresponds to its 10-6 individual 
lifetime excess cancer risk for inhalation exposures or for oral 
exposures, as appropriate.
    Shallow ground water: The uppermost saturated zone, typically 
unconfined.
* * * * *
    Slope factor (also referred to as cancer potency factor): 
Estimate of the probability of response (for example, cancer) per 
unit intake of a substance over a lifetime. The slope factor is 
typically used to estimate upper-bound probability of an individual 
developing cancer as a result of exposure to a particular level of a 
human carcinogen with either a weight-of-evidence classification of 
A, B, or C, or a weight-of-evidence classification of carcinogenic 
to humans, likely to be carcinogenic to humans or having suggestive 
evidence of carcinogenic potential. [(mg/kg-day)-1 for 
non-radioactive substances and (pCi)-1 for radioactive 
substances].

[[Page 2780]]

    Soil gas: The gaseous elements and compounds in the small spaces 
between particles of soil.
    Soil porosity: The degree to which the total volume of soil is 
permeated with pores or cavities through which fluids (including air 
or gas) can move. It is typically calculated as the ratio of the 
pore spaces within the soil to the overall volume of the soil.
* * * * *
    Subslab: The area immediately beneath a regularly occupied 
structure with a basement foundation or a slab-on-grade foundation. 
Subslab samples are collected to determine the concentration of 
hazardous substances in the soil gas beneath a home or building.
    Subsurface intrusion: The migration of hazardous substances from 
the unsaturated zone and/or ground water into overlying structures.
* * * * *
    Unit risk: The upper-bound excess lifetime cancer risk estimated 
to result from continuous exposure to an agent (i.e., hazardous 
substance) at a concentration of 1 [mu]g/L in water, or 1 [mu]g/m\3\ 
in air.
    Unsaturated zone: The portion of subsurface between the land 
surface and the zone of saturation. It extends from the ground 
surface to the top of the shallowest ground water table (excluding 
localized or perched water).
* * * * *
    Weight-of-evidence: EPA classification system for characterizing 
the evidence supporting the designation of a substance as a human 
carcinogen. The EPA weight-of-evidence, depending on the date EPA 
updated the profile, includes either the groupings:
     Group A: Human carcinogen--sufficient evidence of 
carcinogenicity in humans.
     Group B1: Probable human carcinogen--limited evidence 
of carcinogenicity in humans.
     Group B2: Probable human carcinogen--sufficient 
evidence of carcinogenicity in animals.
     Group C: Possible human carcinogen--limited evidence of 
carcinogenicity in animals.
     Group D: Not classifiable as to human carcinogenicity--
applicable when there is no animal evidence, or when human or animal 
evidence is inadequate.
     Group E: Evidence of noncarcinogenicity for humans.
    Or the descriptors:
     Carcinogenic to humans.
     Likely to be carcinogenic to humans.
     Suggestive evidence of carcinogenic potential.
     Inadequate information to assess carcinogenic 
potential.
     Not likely to be carcinogenic to humans.

2.0 Evaluations Common to Multiple Pathways

    2.1 Overview. The HRS site score (S) is the result of an 
evaluation of four pathways:
     Ground Water Migration (Sgw).
     Surface Water Migration (Ssw).
     Soil Exposure and Subsurface Intrusion 
(Ssessi).
     Air Migration (Sa).
    The ground water and air migration pathways use single threat 
evaluations, while the surface water migration and soil exposure and 
subsurface intrusion pathways use multiple threat evaluations. Three 
threats are evaluated for the surface water migration pathway: 
Drinking water, human food chain, and environmental. These threats 
are evaluated for two separate migration components--overland/flood 
migration and ground water to surface water migration. Two 
components are evaluated for the soil exposure and subsurface 
intrusion pathway: Soil exposure and subsurface intrusion. The soil 
exposure component evaluates two threats: Resident population and 
nearby population, and the subsurface intrusion component is a 
single threat evaluation.
    The HRS is structured to provide a parallel evaluation for each 
of these pathways, components, and threats. This section focuses on 
these parallel evaluations, starting with the calculation of the HRS 
site score and the individual pathway scores.
    2.1.1 Calculation of HRS site score. Scores are first calculated 
for the individual pathways as specified in sections 2 through 7 and 
then are combined for the site using the following root-mean-square 
equation to determine the overall HRS site score, which ranges from 
0 to 100:
[GRAPHIC] [TIFF OMITTED] TR09JA17.062

    2.1.2 Calculation of pathway score. Table 2-1, which is based on 
the air migration pathway, illustrates the basic parameters used to 
calculate a pathway score. As Table 2-1 shows, each pathway 
(component or threat) score is the product of three ``factor 
categories'': Likelihood of release, waste characteristics, and 
targets. (The soil exposure and subsurface intrusion pathway uses 
likelihood of exposure rather than likelihood of release.) Each of 
the three factor categories contains a set of factors that are 
assigned numerical values and combined as specified in sections 2 
through 7. The factor values are rounded to the nearest integer, 
except where otherwise noted.
    2.1.3 Common evaluations. Evaluations common to all four HRS 
pathways include:

 Characterizing sources.
    --Identifying sources (and, for the soil exposure and subsurface 
intrusion pathway, areas of observed contamination, areas of 
observed exposure and/or areas of subsurface contamination [see 
sections 5.1.0 and 5.2.0]).
    --Identifying hazardous substances associated with each source 
(or area of observed contamination, or observed exposure, or 
subsurface contamination).
    --Identifying hazardous substances available to a pathway.

                  Table 2-1--Sample Pathway Scoresheet
------------------------------------------------------------------------
                                                               Value
             Factor category               Maximum value     assigned
------------------------------------------------------------------------
                          Likelihood of Release
------------------------------------------------------------------------
1. Observed Release.....................             550  ..............
2. Potential to Release.................             500  ..............
3. Likelihood of Release (higher of                  550  ..............
 lines 1 and 2).........................
------------------------------------------------------------------------
                          Waste Characteristics
------------------------------------------------------------------------
4. Toxicity/Mobility....................           (\a\)  ..............
5. Hazardous Waste Quantity.............           (\a\)  ..............
6. Waste Characteristics................             100  ..............
------------------------------------------------------------------------
                                 Targets
------------------------------------------------------------------------
7. Nearest Individual...................  ..............  ..............
    7a. Level I.........................              50  ..............
    7b. Level II........................              45  ..............
    7c. Potential Contamination.........              20  ..............
    7d. Nearest Individual (higher of                 50  ..............
     lines 7a, 7b, or 7c)...............
8. Population...........................           (\b\)  ..............
    8a. Level I.........................           (\b\)  ..............
    8b. Level II........................           (\b\)  ..............

[[Page 2781]]

 
    8c. Potential Contamination.........           (\b\)  ..............
    8d. Total Population (lines           ..............  ..............
     8a+8b+8c)..........................
9. Resources............................               5  ..............
10. Sensitive Environments..............           (\b\)  ..............
    10a. Actual Contamination...........           (\b\)  ..............
    10b. Potential Environments.........           (\b\)  ..............
    10c. Sensitive Environments (lines             (\b\)  ..............
     10a+10b)...........................
11. Targets (lines 7d+8d+9+10c).........           (\b\)  ..............
12. Pathway Score is the product of
 Likelihood of Release, Waste
 Characteristics, and Targets, divided
 by 82,500. Pathway scores are limited
 to a maximum of 100 points.............
------------------------------------------------------------------------
\a\ Maximum value applies to waste characteristics category. The product
  of lines 4 and 5 is used in Table 2-7 to derive the value for the
  waste characteristics factor category.
\b\ There is no limit to the human population or sensitive environments
  factor values. However, the pathway score based solely on sensitive
  environments is limited to a maximum of 60 points.

 Scoring likelihood of release (or likelihood of exposure) 
factor category.
    --Scoring observed release (or observed exposure or observed 
contamination).
    --Scoring potential to release when there is no observed 
release.
 Scoring waste characteristics factor category.
    --Evaluating toxicity.
    [ssquf] Combining toxicity with mobility, persistence, 
degradation and/or bioaccumulation (or ecosystem bioaccumulation) 
potential, as appropriate to the pathway (component or threat).
    [ssquf] Evaluating hazardous waste quantity.
    --Combining hazardous waste quantity with the other waste 
characteristics factors.
    [ssquf] Determining waste characteristics factor category value.
 Scoring targets factor category.
    --Determining level of contamination for targets.

    These evaluations are essentially identical for the three 
migration pathways (ground water, surface water, and air). However, 
the evaluations differ in certain respects for the soil exposure and 
subsurface intrusion pathway.
    Section 7 specifies modifications that apply to each pathway 
when evaluating sites containing radioactive substances.
    Section 2 focuses on evaluations common at the pathway, 
component, and threat levels. Note that for the ground water and 
surface water migration pathways, separate scores are calculated for 
each aquifer (see section 3.0) and each watershed (see sections 
4.1.1.3 and 4.2.1.5) when determining the pathway scores for a site. 
Although the evaluations in section 2 do not vary when different 
aquifers or watersheds are scored at a site, the specific factor 
values (for example, observed release, hazardous waste quantity, 
toxicity/mobility) that result from these evaluations can vary by 
aquifer and by watershed at the site. This can occur through 
differences both in the specific sources and targets eligible to be 
evaluated for each aquifer and watershed and in whether observed 
releases can be established for each aquifer and watershed. Such 
differences in scoring at the aquifer and watershed level are 
addressed in sections 3 and 4, not section 2.
    2.2 Characterize sources. Source characterization includes 
identification of the following:
     Sources (and areas of observed contamination, areas of 
observed exposure, or areas of subsurface contamination) at the 
site.
     Hazardous substances associated with these sources (or 
areas of observed contamination, areas of observed exposure, or 
areas of subsurface contamination).
     Pathways potentially threatened by these hazardous 
substances.
    Table 2-2 presents a sample worksheet for source 
characterization.
    2.2.1 Identify sources. For the three migration pathways, 
identify the sources at the site that contain hazardous substances. 
Identify the migration pathway(s) to which each source applies. For 
the soil exposure and subsurface intrusion pathway, identify areas 
of observed contamination, areas of observed exposure, and/or areas 
of subsurface contamination at the site (see sections 5.1.0 and 
5.2.0).

Table 2-2--Sample Source Characterization Worksheet

Source: __
A. Source dimensions and hazardous waste quantity.
Hazardous constituent quantity: __
Hazardous wastestream quantity: __
Volume: __
Area: __
Area of observed contamination: __
Area of observed exposure: __
Area of subsurface contamination: __
B. Hazardous substances associated with the source.

--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                 Available to pathway
                             ---------------------------------------------------------------------------------------------------------------------------
                                         Air                            Surface Water (SW)             Soil Exposure/Subsurface Intrusion (SESSI)
                             --------------------------             ------------------------------------------------------------------------------------
     Hazardous substance                                                                            Soil  exposure             Subsurface Intrusion
                                                           Ground                             ----------------------------------------------------------
                                  Gas      Particulate   Water (GW)   Overland/     GW to SW                                 Area of         Area of
                                                                        flood                    Resident      Nearby       observed        subsurface
                                                                                                                            exposure      contamination
--------------------------------------------------------------------------------------------------------------------------------------------------------
 
--------------------------------------------------------------------------------------------------------------------------------------------------------
 
--------------------------------------------------------------------------------------------------------------------------------------------------------
 
--------------------------------------------------------------------------------------------------------------------------------------------------------
 
--------------------------------------------------------------------------------------------------------------------------------------------------------
 
--------------------------------------------------------------------------------------------------------------------------------------------------------

    2.2.2 Identify hazardous substances associated with a source. 
For each of the three migration pathways, consider those hazardous 
substances documented in a source (for example, by sampling, labels, 
manifests, oral or written statements) to be

[[Page 2782]]

associated with that source when evaluating each pathway. In some 
instances, a hazardous substance can be documented as being present 
at a site (for example, by labels, manifests, oral or written 
statements), but the specific source(s) containing that hazardous 
substance cannot be documented. For the three migration pathways, in 
those instances when the specific source(s) cannot be documented for 
a hazardous substance, consider the hazardous substance to be 
present in each source at the site, except sources for which 
definitive information indicates that the hazardous substance was 
not or could not be present.
    For an area of observed contamination in the soil exposure 
component of the soil exposure and subsurface intrusion pathway, 
consider only those hazardous substances that meet the criteria for 
observed contamination for that area (see section 5.1.0) to be 
associated with that area when evaluating the pathway.
    For an area of observed exposure or area of subsurface 
contamination (see section 5.2.0) in the subsurface intrusion 
component of the soil exposure and subsurface intrusion pathway, 
consider only those hazardous substances that:
     Meet the criteria for observed exposure, or
     Meet the criteria for observed release in an area of 
subsurface contamination and have a vapor pressure greater than or 
equal to one torr or a Henry's constant greater than or equal to 
10-\5\ atm-m\3\/mol, or
     Meet the criteria for an observed release in a 
structure within, or in a sample from below, an area of observed 
exposure and have a vapor pressure greater than or equal to one torr 
or a Henry's constant greater than or equal to 10-\5\ 
atm-m\3\/mol.
    2.2.3 Identify hazardous substances available to a pathway. In 
evaluating each migration pathway, consider the following hazardous 
substances available to migrate from the sources at the site to the 
pathway:

 Ground water migration.
    --Hazardous substances that meet the criteria for an observed 
release (see section 2.3) to ground water.
    --All hazardous substances associated with a source with a 
ground water containment factor value greater than 0 (see section 
3.1.2.1).
 Surface water migration--overland/flood component.
    --Hazardous substances that meet the criteria for an observed 
release to surface water in the watershed being evaluated.
    --All hazardous substances associated with a source with a 
surface water containment factor value greater than 0 for the 
watershed (see sections 4.1.2.1.2.1.1 and 4.1.2.1.2.2.1).
 Surface water migration--ground water to surface water 
component.
    --Hazardous substances that meet the criteria for an observed 
release to ground water.
    --All hazardous substances associated with a source with a 
ground water containment factor value greater than 0 (see sections 
4.2.2.1.2 and 3.1.2.1).
 Air migration.
    --Hazardous substances that meet the criteria for an observed 
release to the atmosphere.
    --All gaseous hazardous substances associated with a source with 
a gas containment factor value greater than 0 (see section 
6.1.2.1.1).
    --All particulate hazardous substances associated with a source 
with a particulate containment factor value greater than 0 (see 
section 6.1.2.2.1).
 For each migration pathway, in those instances when the 
specific source(s) containing the hazardous substance cannot be 
documented, consider that hazardous substance to be available to 
migrate to the pathway when it can be associated (see section 2.2.2) 
with at least one source having a containment factor value greater 
than 0 for that pathway.

    In evaluating the soil exposure and subsurface intrusion 
pathway, consider the following hazardous substances available to 
the pathway:

 Soil exposure component--resident population threat.
    --All hazardous substances that meet the criteria for observed 
contamination at the site (see section 5.1.0).
 Soil exposure component--nearby population threat.
    --All hazardous substances that meet the criteria for observed 
contamination at areas with an attractiveness/accessibility factor 
value greater than 0 (see section 5.1.2.1.1).
 Subsurface intrusion component.
    --All hazardous substances that meet the criteria for observed 
exposure at the site (see section 5.2.0).
    --All hazardous substances with a vapor pressure greater than or 
equal to one torr or a Henry's constant greater than or equal to 
10-\5\ atm-m\3\/mol that meet the criteria for an 
observed release in an area of subsurface contamination (see section 
5.2.0).
    --All hazardous substances that meet the criteria for an 
observed release in a structure within, or in a sample from below, 
an area of observed exposure (see section 5.2.0).

    2.3 Likelihood of release. Likelihood of release is a measure of 
the likelihood that a waste has been or will be released to the 
environment. The likelihood of release factor category is assigned 
the maximum value of 550 for a migration pathway whenever the 
criteria for an observed release are met for that pathway. If the 
criteria for an observed release are met, do not evaluate potential 
to release for that pathway. When the criteria for an observed 
release are not met, evaluate potential to release for that pathway, 
with a maximum value of 500. The evaluation of potential to release 
varies by migration pathway (see sections 3, 4 and 6).
    Establish an observed release either by direct observation of 
the release of a hazardous substance into the media being evaluated 
(for example, surface water) or by chemical analysis of samples 
appropriate to the pathway being evaluated (see sections 3, 4 and 
6). The minimum standard to establish an observed release by 
chemical analysis is analytical evidence of a hazardous substance in 
the media significantly above the background level. Further, some 
portion of the release must be attributable to the site. Use the 
criteria in Table 2-3 as the standard for determining analytical 
significance. (The criteria in Table 2-3 are also used in 
establishing observed contamination for the soil exposure component 
and for establishing areas of observed exposure and areas of 
subsurface contamination in the subsurface intrusion component of 
the soil exposure and subsurface intrusion pathway, see section 
5.1.0 and section 5.2.0). Separate criteria apply to radionuclides 
(see section 7.1.1).

       Table 2-3--Observed Release Criteria for Chemical Analysis
------------------------------------------------------------------------
 
-------------------------------------------------------------------------
Sample Measurement < Sample Quantitation Limit.\a\
No observed release is established.
Sample Measurement >= Sample Quantitation Limit.\a\
An observed release is established as follows:
     If the background concentration is not detected (or is less
     than the detection limit), an observed release is established when
     the sample measurement equals or exceeds the sample quantitation
     limit.\a\
     If the background concentration equals or exceeds the
     detection limit, an observed release is established when the sample
     measurement is 3 times or more above the background concentration.
------------------------------------------------------------------------
\a\ If the sample quantitation limit (SQL) cannot be established,
  determine if there is an observed release as follows:
--If the sample analysis was performed under the EPA Contract Laboratory
  Program, use the EPA contract-required quantitation limit (CRQL) in
  place of the SQL.
--If the sample analysis is not performed under the EPA Contract
  Laboratory Program, use the detection limit (DL) in place of the SQL.

    2.4 Waste characteristics. The waste characteristics factor 
category includes the following factors: Hazardous waste quantity, 
toxicity, and as appropriate to the pathway or threat being 
evaluated, mobility, persistence, degradation, and/or

[[Page 2783]]

bioaccumulation (or ecosystem bioaccumulation) potential.
    2.4.1 Selection of substance potentially posing greatest hazard. 
For all pathways (components and threats), select the hazardous 
substance potentially posing the greatest hazard for the pathway 
(component or threat) and use that substance in evaluating the waste 
characteristics category of the pathway (component or threat). For 
the three migration pathways (and threats), base the selection of 
this hazardous substance on the toxicity factor value for the 
substance, combined with its mobility, persistence, and/or 
bioaccumulation (or ecosystem bioaccumulation) potential factor 
values, as applicable to the migration pathway (or threat). For the 
soil exposure component of the soil exposure and subsurface 
intrusion pathway, base the selection on the toxicity factor alone. 
For the subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway, base the selection on the toxicity 
factor value for the substance, combined with its degradation factor 
value. Evaluation of the toxicity factor is specified in section 
2.4.1.1. Use and evaluation of the mobility, persistence, 
degradation, and/or bioaccumulation (or ecosystem bioaccumulation) 
potential factors vary by pathway (component or threat) and are 
specified under the appropriate pathway (component or threat) 
section. Section 2.4.1.2 identifies the specific factors that are 
combined with toxicity in evaluating each pathway (component or 
threat).
    2.4.1.1 Toxicity factor. Evaluate toxicity for those hazardous 
substances at the site that are available to the pathway being 
scored. For all pathways and threats, except the surface water 
environmental threat, evaluate human toxicity as specified below. 
For the surface water environmental threat, evaluate ecosystem 
toxicity as specified in section 4.1.4.2.1.1.
    Establish human toxicity factor values based on quantitative 
dose-response parameters for the following three types of toxicity:
     Cancer--Use slope factors (also referred to as cancer 
potency factors) combined with weight-of-evidence ratings for 
carcinogenicity for all exposure routes except inhalation. Use 
inhalation unit risk (IUR) for inhalation exposure. If an inhalation 
unit risk or a slope factor is not available for a substance, use 
its ED10 value to estimate a slope factor as follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.063

     Noncancer toxicological responses of chronic exposure--
use reference dose (RfD) or reference concentration (RfC) values as 
applicable.
     Noncancer toxicological responses of acute exposure--
use acute toxicity parameters, such as the LD50.
    Assign human toxicity factor values to a hazardous substance 
using Table 2-4, as follows:
     If RfD/RfC and slope factor/inhalation unit risk values 
are available for the hazardous substance, assign the substance a 
value from Table 2-4 for each. Select the higher of the two values 
assigned and use it as the overall toxicity factor value for the 
hazardous substance.
     If either an RfD/RfC or slope factor/inhalation unit 
risk value is available, but not both, assign the hazardous 
substance an overall toxicity factor value from Table 2-4 based 
solely on the available value (RfD/RfC or slope factor/inhalation 
unit risk).
     If neither an RfD/RfC nor slope factor/inhalation unit 
risk value is available, assign the hazardous substance an overall 
toxicity factor value from Table 2-4 based solely on acute toxicity. 
That is, consider acute toxicity in Table 2-4 only when both RfD/RfC 
and slope factor/IUR values are not available.
     If neither an RfD/RfC, nor slope factor/inhalation unit 
risk, nor acute toxicity value is available, assign the hazardous 
substance an overall toxicity factor value of 0 and use other 
hazardous substances for which information is available in 
evaluating the pathway.

                  Table 2-4--Toxicity Factor Evaluation
------------------------------------------------------------------------
                                                          Assigned value
------------------------------------------------------------------------
                        Chronic Toxicity (Human)
------------------------------------------------------------------------
Reference dose (RfD) (mg/kg-day):
  RfD < 0.0005..........................................          10,000
  0.0005 <= RfD < 0.005.................................           1,000
  0.005 <= RfD < 0.05...................................             100
  0.05 <= RfD < 0.5.....................................              10
  0.5 <= RfD............................................               1
  RfD not available.....................................               0
Reference concentration (RfC) (mg/m\3\):
  RfC < 0.0001..........................................          10,000
  0.0001 <= RfC < 0.006.................................           1,000
  0.006 <= RfC < 0.2....................................             100
  0.2 <= RfC < 2.0......................................              10
  2.0 <= RfC............................................               1
  RfC not available.....................................               0
------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------
                                             Carcinogenicity (human)
-----------------------------------------------------------------------------------------------------------------
                                               B or Likely to be       C or Suggestive evidence
       A or Carcinogenic to humans          carcinogenic to humans     of carcinogenic potential  Assigned value
----------------------------------------------------------------------------------------------------------------
                                 Weight-of-evidencea/Slope factor (mg/kg-day)-1
----------------------------------------------------------------------------------------------------------------
0.5 <= SF \b\...........................  5 <= SF...................  50 <= SF..................          10,000
0.05 <= SF < 0.5........................  0.5 <= SF < 5.............  5 <= SF < 50..............           1,000
SF < 0.05...............................  0.05 <= SF < 0.5..........  0.5 <= SF < 5.............             100
                                          SF < 0.05.................  SF < 0.5..................              10
Slope factor not available..............  Slope factor not available  Slope factor not available               0
----------------------------------------------------------------------------------------------------------------
                              Weight-of-evidence a/Inhalation unit risk ([mu]g/m3)
----------------------------------------------------------------------------------------------------------------
0.00004 <= IUR \c\......................  0.0004 <= IUR.............  0.004 <= IUR..............          10,000
0.00001 <= IUR < 0.00004................  0.0001 <= IUR < 0.0004....  0.001 <= IUR < 0.004......           1,000
IUR < 0.00001...........................  0.00001 <= IUR < 0.0001...  0.0001 <= IUR < 0.001.....             100
                                          < 0.00001.................  IUR < 0.0001..............              10
Inhalation unit risk not available......  Inhalation unit risk not    Inhalation unit risk not                 0
                                           available.                  available.
----------------------------------------------------------------------------------------------------------------
\a\ A, B, and C, as well as Carcinogenic to humans, Likely to be carcinogenic to humans, and Suggestive evidence
  of carcinogenic potential refer to weight-of-evidence categories. Assign substances with a weight-of-evidence
  category of D (inadequate evidence of carcinogenicity) or E (evidence of lack of carcinogenicity), as well as
  inadequate information to assess carcinogenic potential and not likely to be carcinogenic to humans a value of
  0 for carcinogenicity.
\b\ SF = Slope factor.
\c\ IUR = Inhalation Unit Risk.

----------------------------------------------------------------------------------------------------------------
                                             Acute Toxicity (human)
-----------------------------------------------------------------------------------------------------------------
                                   Dermal LD50 (mg/kg)   Dust or mist LC50    Gas or vapor LC50
        Oral LD50 (mg/kg)                                      (mg/l)               (ppm)         Assigned value
----------------------------------------------------------------------------------------------------------------
LD50 < 5.........................  LD50 < 2...........  LC50 < 0.2.........  LC50 < 20..........           1,000

[[Page 2784]]

 
5 <= LD50 < 50...................  2 <= LD50 < 20.....  0.2 <= LC50 < 2....  20 <= LC50 <200....             100
50 <= LD50 < 500.................  20 <= LD50 < 200...  2 <= LC50 <20......  200 <= LC50 <2,000.              10
500 <= LD50......................  200 <= LD50........  20 <= LC50.........  2,000 <= LC50......               1
LD50 not available...............  LD50 not available.  LC50 not available.  LC50 not available.               0
----------------------------------------------------------------------------------------------------------------

    If a toxicity factor value of 0 is assigned to all hazardous 
substances available to a particular pathway (that is, insufficient 
toxicity data are available for evaluating all the substances), use 
a default value of 100 as the overall human toxicity factor value 
for all hazardous substances available to the pathway. For hazardous 
substances having usable toxicity data for multiple exposure routes 
(for example, inhalation and ingestion), consider all exposure 
routes and use the highest assigned value, regardless of exposure 
route, as the toxicity factor value. For HRS purposes, assign both 
asbestos and lead (and its compounds) a human toxicity factor value 
of 10,000.
    Separate criteria apply for assigning factor values for human 
toxicity and ecosystem toxicity for radionuclides (see sections 
7.2.1 and 7.2.2).
    2.4.1.2 Hazardous substance selection. For each hazardous 
substance evaluated for a migration pathway (or threat), combine the 
human toxicity factor value (or ecosystem toxicity factor value) for 
the hazardous substance with a mobility, persistence, and/or 
bioaccumulation (or ecosystem bioaccumulation) potential factor 
value as follows:

 Ground water migration.
    --Determine a combined human toxicity/mobility factor value for 
the hazardous substance (see section 3.2.1).
 Surface water migration--overland/flood migration 
component.
    --Determine a combined human toxicity/persistence factor value 
for the hazardous substance for the drinking water threat (see 
section 4.1.2.2.1).
    --Determine a combined human toxicity/persistence/
bioaccumulation factor value for the hazardous substance for the 
human food chain threat (see section 4.1.3.2.1).
    --Determine a combined ecosystem toxicity/persistence/
bioaccumulation factor value for the hazardous substance for the 
environmental threat (see section 4.1.4.2.1).
 Surface water migration--ground water to surface water 
migration component.
    --Determine a combined human toxicity/mobility/persistence 
factor value for the hazardous substance for the drinking water 
threat (see section 4.2.2.2.1).
    --Determine a combined human toxicity/mobility/persistence/
bioaccumulation factor value for the hazardous substance for the 
human food chain threat (see section 4.2.3.2.1).
    --Determine a combined ecosystem toxicity/mobility/persistence/
bioaccumulation factor value for the hazardous substance for the 
environmental threat (see section 4.2.4.2.1).
 Air migration.
    --Determine a combined human toxicity/mobility factor value for 
the hazardous substance (see section 6.2.1).

    Determine each combined factor value for a hazardous substance 
by multiplying the individual factor values appropriate to the 
pathway (or threat). For each migration pathway (or threat) being 
evaluated, select the hazardous substance with the highest combined 
factor value and use that substance in evaluating the waste 
characteristics factor category of the pathway (or threat).
    For the soil exposure and subsurface intrusion pathway, 
determine toxicity and toxicity/degradation factor values as 
follows:

 Soil exposure and subsurface intrusion--soil exposure 
component.
    --Select the hazardous substance with the highest human toxicity 
factor value from among the substances that meet the criteria for 
observed contamination for the threat evaluated and use that 
substance in evaluating the waste characteristics factor category 
(see section 5.1.1.2.1).
 Soil exposure and subsurface intrusion--subsurface 
intrusion component.
    --Determine a combined human toxicity/degradation factor value 
for each hazardous substance being evaluated that:
    [ssquf] Meets the criteria for observed exposure, or
    [ssquf] Meets the criteria for observed release in an area of 
subsurface contamination and has a vapor pressure greater than or 
equal to one torr or a Henry's constant greater than or equal to 
10-5 atm-m\3\/mol, or
    [ssquf] Meets the criteria for an observed release in a 
structure within, or in a sample from below, an area of observed 
exposure and has a vapor pressure greater than or equal to one torr 
or a Henry's constant greater than or equal to 10-5 atm-
m\3\/mol.
    --Select the hazardous substance with the highest combined 
factor value and use that substance in evaluating the waste 
characteristics factor category (see sections 5.2.1.2.1 and 
5.2.1.2).

    2.4.2 Hazardous waste quantity. Evaluate the hazardous waste 
quantity factor by first assigning each source (or area of observed 
contamination, area of observed exposure, or area of subsurface 
contamination) a source hazardous waste quantity value as specified 
below. Sum these values to obtain the hazardous waste quantity 
factor value for the pathway being evaluated.
    In evaluating the hazardous waste quantity factor for the three 
migration pathways, allocate hazardous substances and hazardous 
wastestreams to specific sources in the manner specified in section 
2.2.2, except: Consider hazardous substances and hazardous 
wastestreams that cannot be allocated to any specific source to 
constitute a separate ``unallocated source'' for purposes of 
evaluating only this factor for the three migration pathways. Do 
not, however, include a hazardous substance or hazardous wastestream 
in the unallocated source for a migration pathway if there is 
definitive information indicating that the substance or wastestream 
could only have been placed in sources with a containment factor 
value of 0 for that migration pathway.
    In evaluating the hazardous waste quantity factor for the soil 
exposure component of the soil exposure and subsurface intrusion 
pathway, allocate to each area of observed contamination only those 
hazardous substances that meet the criteria for observed 
contamination for that area of observed contamination and only those 
hazardous wastestreams that contain hazardous substances that meet 
the criteria for observed contamination for that area of observed 
contamination. Do not consider other hazardous substances or 
hazardous wastestreams at the site in evaluating this factor for the 
soil exposure component of the soil exposure and subsurface 
intrusion pathway.
    In evaluating the hazardous waste quantity factor for the 
subsurface intrusion component of the soil exposure and subsurface 
intrusion pathway, allocate to each area of observed exposure or 
area of subsurface contamination only those hazardous substances and 
hazardous wastestreams that contain hazardous substances that:

 Meet the criteria for observed exposure, or
 Meet the criteria for observed release in an area of 
subsurface contamination and have a vapor pressure greater than or 
equal to one torr or a Henry's constant greater than or equal to 
10-5 atm-m\3\/mol, or
 Meet the criteria for an observed release in a structure 
within, or in a sample from below, an area of observed exposure and 
have a vapor pressure greater than or equal to one torr or a Henry's 
constant greater than or equal to 10-5 atm-m\3\/mol.

    Do not consider other hazardous substances or hazardous 
wastestreams at the site in evaluating this factor for the 
subsurface intrusion component of the soil exposure and subsurface 
intrusion pathway. When determining the hazardous waste quantity for 
multi-subunit structures, use the procedures identified in section 
5.2.1.2.2.
    2.4.2.1 Source hazardous waste quantity. For each of the three 
migration pathways, assign a source hazardous waste quantity

[[Page 2785]]

value to each source (including the unallocated source) having a 
containment factor value greater than 0 for the pathway being 
evaluated. Consider the unallocated source to have a containment 
factor value greater than 0 for each migration pathway.
    For the soil exposure component of the soil exposure and 
subsurface intrusion pathway, assign a source hazardous waste 
quantity value to each area of observed contamination, as applicable 
to the threat being evaluated.
    For the subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway, assign a source hazardous waste 
quantity value to each regularly occupied structure within an area 
of observed exposure or an area of subsurface contamination that has 
a structure containment factor value greater than 0. If sufficient 
data is available and state of the science shows there is no 
unacceptable risk due to subsurface intrusion into a regularly 
occupied structure located within an area of subsurface 
contamination, that structure can be excluded from the area of 
subsurface contamination.
    For determining all hazardous waste quantity calculations except 
for an unallocated source or an area of subsurface contamination, 
evaluate using the following four measures in the following 
hierarchy:
     Hazardous constituent quantity.
     Hazardous wastestream quantity.
     Volume.
     Area.
    For the unallocated source, use only the first two measures. For 
an area of subsurface contamination, evaluate non-radioactive 
hazardous substances using only the last two measures and evaluate 
radioactive hazardous substances using hazardous wastestream 
quantity only. See also section 7.0 regarding the evaluation of 
radioactive substances.
    Separate criteria apply for assigning a source hazardous waste 
quantity value for radionuclides (see section 7.2.5).
    2.4.2.1.1 Hazardous constituent quantity. Evaluate hazardous 
constituent quantity for the source (or area of observed 
contamination) based solely on the mass of CERCLA hazardous 
substances (as defined in CERCLA section 101(14), as amended) 
allocated to the source (or area of observed contamination), except:
     For a hazardous waste listed pursuant to section 3001 
of the Solid Waste Disposal Act, as amended by the Resource 
Conservation and Recovery Act of 1976 (RCRA), 42 U.S.C. 6901 et 
seq., determine its mass for the evaluation of this measure as 
follows:

--If the hazardous waste is listed solely for Hazard Code T (toxic 
waste), include only the mass of constituents in the hazardous waste 
that are CERCLA hazardous substances and not the mass of the entire 
hazardous waste.
--If the hazardous waste is listed for any other Hazard Code 
(including T plus any other Hazard Code), include the mass of the 
entire hazardous waste.

     For a RCRA hazardous waste that exhibits the 
characteristics identified under section 3001 of RCRA, as amended, 
determine its mass for the evaluation of this measure as follows:

--If the hazardous waste exhibits only the characteristic of 
toxicity (or only the characteristic of EP toxicity), include only 
the mass of constituents in the hazardous waste that are CERCLA 
hazardous substances and not the mass of the entire hazardous waste.
--If the hazardous waste exhibits any other characteristic 
identified under section 3001 (including any other characteristic 
plus the characteristic of toxicity [or the characteristic of EP 
toxicity]), include the mass of the entire hazardous waste.

    Based on this mass, designated as C, assign a value for 
hazardous constituent quantity as follows:
     For the migration pathways, assign the source a value 
for hazardous constituent quantity using the Tier A equation of 
Table 2-5.
     For the soil exposure and subsurface intrusion 
pathway--soil exposure component, assign the area of observed 
contamination a value using the Tier A equation of Table 5-2 
(section 5.1.1.2.2).
     For the soil exposure and subsurface intrusion 
pathway--subsurface intrusion component, assign the area of observed 
exposure a value using the Tier A equation of Table 5-19 (section 
5.2.1.2.2).
    If the hazardous constituent quantity for the source (or area of 
observed contamination or area of observed exposure) is adequately 
determined (that is, the total mass of all CERCLA hazardous 
substances in the source and releases from the source [or in the 
area of observed contamination or area of observed exposure] is 
known or is estimated with reasonable confidence), do not evaluate 
the other three measures discussed below. Instead assign these other 
three measures a value of 0 for the source (or area of observed 
contamination or area of observed exposure) and proceed to section 
2.4.2.1.5.
    If the hazardous constituent quantity is not adequately 
determined, assign the source (or area of observed contamination or 
area of observed exposure) a value for hazardous constituent 
quantity based on the available data and proceed to section 
2.4.2.1.2.

        Table 2-5--Hazardous Waste Quantity Evaluation Equations
------------------------------------------------------------------------
                                                          Equation for
      Tier              Measure             Units       assigning value
                                                              \a\
------------------------------------------------------------------------
A..............  Hazardous constituent  lb...........  C
                  quantity (C).
B \b\..........  Hazardous wastestream  lb...........  W/5,000
                  quantity (W).
C \b\..........  Volume (V)...........                 .................
                 Landfill.............  yd\3\........  V/2,500
                 Surface impoundment..  yd\3\........  V/2.5
                 Surface impoundment    yd\3\........  V/2.5
                  (buried/backfilled).
                 Drums \c\............  gallon.......  V/2.5
                 Tanks and containers   yd\3\........  V/2.5
                  other than drums.
                 Contaminated soil....  yd\3\........  V/2,500
                 Pile.................  yd\3\........  V/2.5
                 Other................  yd\3\........  V/2.5
D \b\..........  Area (A).............
                 Landfill.............  ft\2\........  A/3,400
                 Surface impoundment..  ft\2\........  A/13
                 Surface impoundment    ft\2\........  A/13
                  (buried/backfilled).
                 Land treatment.......  ft\2\........  A/270
                 Pile \d\.............  ft\2\........  A/13
                 Contaminated soil....  ft\2\........  A/34,000
------------------------------------------------------------------------
\a\ Do not round to nearest integer.
\b\ Convert volume to mass when necessary: 1 ton = 2,000 pounds = 1
  cubic yard = 4 drums = 200 gallons.
\c\ If actual volume of drums is unavailable, assume 1 drum = 50
  gallons.
\d\ Use land surface area under pile, not surface area of pile.

    2.4.2.1.2 Hazardous wastestream quantity. Evaluate hazardous 
wastestream quantity for the source (or area of observed 
contamination or area of observed exposure) based on the mass of 
hazardous wastestreams plus the mass of any additional CERCLA 
pollutants and contaminants (as defined in CERCLA section 101[33], 
as amended) that are allocated to the source (or area of

[[Page 2786]]

observed contamination or area of observed exposure). For a 
wastestream that consists solely of a hazardous waste listed 
pursuant to section 3001 of RCRA, as amended or that consists solely 
of a RCRA hazardous waste that exhibits the characteristics 
identified under section 3001 of RCRA, as amended, include the mass 
of that entire hazardous waste in the evaluation of this measure.
    Based on this mass, designated as W, assign a value for 
hazardous wastestream quantity as follows:
     For the migration pathways, assign the source a value 
for hazardous wastestream quantity using the Tier B equation of 
Table 2-5.
     For the soil exposure and subsurface intrusion 
pathway--soil exposure component, assign the area of observed 
contamination a value using the Tier B equation of Table 5-2 
(section 5.1.1.2.2).
     For the soil exposure and subsurface intrusion 
pathway--subsurface intrusion component, assign the area of observed 
exposure a value using the Tier B equation of Table 5-19 (section 
5.2.1.2.2).
    Do not evaluate the volume and area measures described below if 
the source is the unallocated source or if the following condition 
applies:
     The hazardous wastestream quantity for the source (or 
area of observed contamination or area of observed exposure) is 
adequately determined--that is, total mass of all hazardous 
wastestreams and CERCLA pollutants and contaminants for the source 
and releases from the source (or for the area of observed 
contamination) is known or is estimated with reasonable confidence.
    If the source is the unallocated source or if this condition 
applies, assign the volume and area measures a value of 0 for the 
source (or area of observed contamination) and proceed to section 
2.4.2.1.5. Otherwise, assign the source (or area of observed 
contamination) a value for hazardous wastestream quantity based on 
the available data and proceed to section 2.4.2.1.3.
    2.4.2.1.3 Volume. Evaluate the volume measure using the volume 
of the source (or the volume of the area of observed contamination, 
area of observed exposure, or area of subsurface contamination). For 
the soil exposure and subsurface intrusion pathway, restrict the use 
of the volume measure to those areas of observed contamination, 
areas of observed exposure, or areas of subsurface contamination as 
specified in sections 5.1.1.2.2 and 5.2.1.2.2.
    Based on the volume, designated as V, assign a value to the 
volume measure as follows:
     For the migration pathways, assign the source a value 
for volume using the appropriate Tier C equation of Table 2-5.
     For the soil exposure and subsurface intrusion 
pathway--soil exposure component, assign the area of observed 
contamination a value for volume using the appropriate Tier C 
equation of Table 5-2 (section 5.1.1.2.2).
     For the soil exposure and subsurface intrusion 
pathway--subsurface intrusion component, assign the value based on 
the volume of the regularly occupied structures within the area of 
observed exposure or area of subsurface contamination using the Tier 
C equation of Table 5-19 (section 5.2.1.2.2).
    If the volume of the source (or volume of the area of observed 
contamination, area of observed exposure, or area of subsurface 
contamination, if applicable) can be determined, do not evaluate the 
area measure. Instead, assign the area measure a value of 0 and 
proceed to section 2.4.2.1.5. If the volume cannot be determined (or 
is not applicable for the soil exposure and subsurface intrusion 
pathway), assign the source (or area of observed contamination, area 
of observed exposure, or area of subsurface contamination) a value 
of 0 for the volume measure and proceed to section 2.4.2.1.4.
    2.4.2.1.4 Area. Evaluate the area measure using the area of the 
source (or the area of the area of observed contamination, area of 
observed exposure, or area of subsurface contamination). Based on 
this area, designated as A, assign a value to the area measure as 
follows:
     For the migration pathways, assign the source a value 
for area using the appropriate Tier D equation of Table 2-5.
     For the soil exposure and subsurface intrusion 
pathway--soil exposure component, assign the area of observed 
contamination a value for area using the appropriate Tier D equation 
of Table 5-2 (section 5.1.1.2.2).
     For the soil exposure and subsurface intrusion 
pathway--subsurface intrusion component, assign a value based on the 
area of regularly occupied structures within the area of observed 
exposure or area of subsurface contamination using the Tier D 
equation of Table 5-19 (section 5.2.1.2.2).
    2.4.2.1.5 Calculation of source hazardous waste quantity value. 
Select the highest of the values assigned to the source (or areas of 
observed contamination, areas of observed exposure, or areas of 
subsurface contamination) for the hazardous constituent quantity, 
hazardous wastestream quantity, volume, and area measures. Assign 
this value as the source hazardous waste quantity value. Do not 
round to the nearest integer.
    2.4.2.2 Calculation of hazardous waste quantity factor value. 
Sum the source hazardous waste quantity values assigned to all 
sources (including the unallocated source) or areas of observed 
contamination, areas of observed exposure, or areas of subsurface 
contamination for the pathway being evaluated and round this sum to 
the nearest integer, except: If the sum is greater than 0, but less 
than 1, round it to 1. Based on this value, select a hazardous waste 
quantity factor value for the pathway from Table 2-6.

            Table 2-6--Hazardous Waste Quantity Factor Values
------------------------------------------------------------------------
             Hazardous waste quantity value               Assigned value
------------------------------------------------------------------------
0.......................................................               0
1 \a\ to 100............................................           \b\ 1
Greater than 100 to 10,000..............................             100
Greater than 10,000 to 1,000,000........................          10,000
Greater than 1,000,000..................................       1,000,000
------------------------------------------------------------------------
\a\ If the hazardous waste quantity value is greater than 0, but less
  than 1, round it to 1 as specified in text.
\b\ For the pathway, if hazardous constituent quantity is not adequately
  determined, assign a value as specified in the text; do not assign the
  value of 1.

    For a migration pathway, if the hazardous constituent quantity 
is adequately determined (see section 2.4.2.1.1) for all sources (or 
all portions of sources and releases remaining after a removal 
action), assign the value from Table 2-6 as the hazardous waste 
quantity factor value for the pathway. If the hazardous constituent 
quantity is not adequately determined for one or more sources (or 
one or more portions of sources or releases remaining after a 
removal action) assign a factor value as follows:
     If any target for that migration pathway is subject to 
Level I or Level II concentrations (see section 2.5), assign either 
the value from Table 2-6 or a value of 100, whichever is greater, as 
the hazardous waste quantity factor value for that pathway.
     If none of the targets for that pathway is subject to 
Level I or Level II concentrations, assign a factor value as 
follows:

--If there has been no removal action, assign either the value from 
Table 2-6 or a value of 10, whichever is greater, as the hazardous 
waste quantity factor value for that pathway.
--If there has been a removal action:
    [ssquf] Determine values from Table 2-6 with and without 
consideration of the removal action.
    [ssquf] If the value that would be assigned from Table 2-6 
without consideration of the removal action would be 100 or greater, 
assign either the value from Table 2-6 with consideration of the 
removal action or a value of 100, whichever is greater, as the 
hazardous waste quantity factor value for the pathway.
    [ssquf] If the value that would be assigned from Table 2-6 
without consideration of the removal action would be less than 100, 
assign a value of 10 as the hazardous waste quantity factor value 
for the pathway.

    For the soil exposure component of the soil exposure and 
subsurface intrusion pathway, if the hazardous constituent quantity 
is adequately determined for all areas of observed contamination, 
assign the value from Table 2-6 as the hazardous waste quantity 
factor value. If the hazardous constituent quantity is not 
adequately determined for one or more areas of observed 
contamination, assign either the value from Table 2-6 or a value of 
10, whichever is greater, as the hazardous waste quantity factor 
value.
    For the subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway, if the hazardous constituent

[[Page 2787]]

quantity is adequately determined for all areas of observed 
exposure, assign the value from Table 2-6 as the hazardous waste 
quantity factor value. If the hazardous constituent quantity is not 
adequately determined for one or more areas of observed exposure, 
assign either the value from Table 2-6 or assign a factor value as 
follows:
     If any target for the subsurface intrusion component is 
subject to Level I or Level II concentrations (see section 2.5), 
assign either the value from Table 2-6 or a value of 100, whichever 
is greater, as the hazardous waste quantity factor value for this 
component.
     If none of the targets for the subsurface intrusion 
component is subject to Level I or Level II concentrations and if 
there has been a removal or other temporary response action that 
does not permanently interrupt target exposure form subsurface 
intrusion, assign a factor value as follows:

--Determine the values from Table 2-6 with and without consideration 
of the removal or other temporary response action.
--If the value that would be assigned from Table 2-6 without 
consideration of the removal or other temporary response action 
would be 100 or greater, assign either the value from Table 2-6 with 
consideration of the removal action or a value of 100, whichever is 
greater, as the hazardous waste quantity factor value for the 
component.
--If the value that would be assigned from Table 2-6 without 
consideration of the removal or other temporary response action 
would be less than 100, assign a value of 10 as the hazardous waste 
quantity factor value for the component.

     Otherwise, if none of the targets for the subsurface 
intrusion component is subject to Level I or Level II concentrations 
and there has not been a removal action, assign a value from Table 
2-6 or a value of 10, whichever is greater.
    2.4.3 Waste characteristics factor category value. Determine the 
waste characteristics factor category value as specified in section 
2.4.3.1 for all pathways and threats, except the surface water-human 
food chain threat and the surface water-environmental threat. 
Determine the waste characteristics factor category value for these 
latter two threats as specified in section 2.4.3.2.
    2.4.3.1 Factor category value. For the pathway (component or 
threat) being evaluated, multiply the toxicity or combined factor 
value, as appropriate, from section 2.4.1.2 and the hazardous waste 
quantity factor value from section 2.4.2.2, subject to a maximum 
product of 1x10\8\. Based on this waste characteristics product, 
assign a waste characteristics factor category value to the pathway 
(component or threat) from Table 2-7.

         Table 2-7--Waste Characteristics Factor Category Values
------------------------------------------------------------------------
              Waste characteristics product               Assigned value
------------------------------------------------------------------------
0.......................................................               0
Greater than 0 to less than 10..........................               1
10 to less than 1x10\2\.................................               2
1x10\2\ to less than 1x10\3\............................               3
1x10\3\ to less than 1x10\4\............................               6
1x10\4\ to less than 1x10\5\............................              10
1x10\5\ to less than 1x10\6\............................              18
1x10\6\ to less than 1x10\7\............................              32
1x10\7\ to less than 1x10\8\............................              56
1x10\8\ to less than 1x10\9\............................             100
1x10\9\ to less than 1x10\10\...........................             180
1x10\10\ to less than 1x10\11\..........................             320
1x10\11\ to less than 1x10\12\..........................             560
1x10\12\................................................           1,000
------------------------------------------------------------------------

    2.4.3.2 Factor category value, considering bioaccumulation 
potential. For the surface water-human food chain threat and the 
surface water-environmental threat, multiply the toxicity or 
combined factor value, as appropriate, from section 2.4.1.2 and the 
hazardous waste quantity factor value from section 2.4.2.2, subject 
to:
     A maximum product of 1x10\12\, and
     A maximum product exclusive of the bioaccumulation (or 
ecosystem bioaccumulation) potential factor of 1x10\8\.
    Based on the total waste characteristics product, assign a waste 
characteristics factor category value to these threats from Table 2-
7.
    2.5 Targets. The types of targets evaluated include the 
following:
     Individual (factor name varies by pathway, component, 
and threat).
     Human population.
     Resources (these vary by pathway, component, and 
threat).
     Sensitive environments (included for the surface water 
migration pathway, air migration pathway, and soil exposure 
component of the soil exposure and subsurface intrusion pathway).
    The factor values that may be assigned to each type of target 
have the same range for each pathway for which that type of target 
is evaluated. The factor value for most types of targets depends on 
whether the target is subject to actual or potential contamination 
for the pathway and whether the actual contamination is Level I or 
Level II:
     Actual contamination: Target is associated either with 
a sampling location that meets the criteria for an observed release 
(or observed contamination or observed exposure) for the pathway or 
with an observed release based on direct observation for the pathway 
(additional criteria apply for establishing actual contamination for 
the human food chain threat in the surface water migration pathway, 
see sections 4.1.3.3 and 4.2.3.3). Sections 3 through 6 specify how 
to determine the targets associated with a sampling location or with 
an observed release based on direct observation. Determine whether 
the actual contamination is Level I or Level II as follows:

--Level I:
    [ssquf] Media-specific concentrations for the target meet the 
criteria for an observed release (or observed contamination or 
observed exposure) for the pathway and are at or above media-
specific benchmark values. These benchmark values (see section 
2.5.2) include both screening concentrations and concentrations 
specified in regulatory limits (such as Maximum Contaminant Level 
(MCL) values), or
    [ssquf] For the human food chain threat in the surface water 
migration pathway, concentrations in tissue samples from aquatic 
human food chain organisms are at or above benchmark values. Such 
tissue samples may be used in addition to media-specific 
concentrations only as specified in sections 4.1.3.3 and 4.2.3.3.
--Level II:
    [ssquf] Media-specific concentrations for the target meet the 
criteria for an observed release (or observed contamination or 
observed exposure) for the pathway, but are less than media-specific 
benchmarks. If none of the hazardous substances eligible to be 
evaluated for the sampling location has an applicable benchmark, 
assign Level II to the actual contamination at the sampling 
location, or
    [ssquf] For observed releases or observed exposures based on 
direct observation, assign Level II to targets as specified in 
sections 3, 4, 5, and 6, or
    [ssquf] For the human food chain threat in the surface water 
migration pathway, concentrations in tissue samples from aquatic 
human food chain organisms, when applicable, are below benchmark 
values.
--If a target is subject to both Level I and Level II concentrations 
for a pathway (component or threat), evaluate the target using Level 
I concentrations for that pathway (component or threat).

     Potential contamination: Target is subject to a 
potential release (that is, target is not associated with actual 
contamination for that pathway or threat).
    Assign a factor value for individual risk as follows (select the 
highest value that applies to the pathway, component or threat):
     50 points if any individual is exposed to Level I 
concentrations.
     45 points if any individual is exposed to Level II 
concentrations.
     Maximum of 20 points if any individual is subject to 
potential contamination. The value assigned is 20 unless reduced by 
a distance or dilution weight appropriate to the pathway. Assign 
factor values for population and sensitive environments as follows:
     Sum Level I targets and multiply by 10. (Level I is not 
used for sensitive environments in the soil exposure component of 
the soil exposure and subsurface intrusion and air migration 
pathways.)
     Sum Level II targets.
     Multiply potential targets in all but the soil exposure 
and subsurface intrusion pathway by distance or dilution weights 
appropriate to the pathway, sum, and divide by 10. Distance or 
dilution weighting accounts for diminishing exposure with increasing 
distance or dilution within the different pathways. For targets 
within an area

[[Page 2788]]

of subsurface contamination in the subsurface intrusion component of 
the soil exposure and subsurface intrusion pathway, multiply by a 
weighting factor as directed in section 5.2.1.3.2.3.
     Sum the values for the three levels.
    In addition, resource value points are assigned within all 
pathways for welfare-related impacts (for example, impacts to 
agricultural land), but do not depend on whether there is actual or 
potential contamination.
    2.5.1 Determination of level of actual contamination at a 
sampling location. Determine whether Level I concentrations or Level 
II concentrations apply at a sampling location (and thus to the 
associated targets) as follows:
     Select the benchmarks applicable to the pathway 
(component or threat) being evaluated.
     Compare the concentrations of hazardous substances in 
the sample (or comparable samples) to their benchmark concentrations 
for the pathway (component or threat), as specified in section 
2.5.2.
     Determine which level applies based on this comparison.
     If none of the hazardous substances eligible to be 
evaluated for the sampling location has an applicable benchmark, 
assign Level II to the actual contamination at that sampling 
location for the pathway (component or threat).
    In making the comparison, consider only those samples, and only 
those hazardous substances in the sample, that meet the criteria for 
an observed release (or observed contamination or observed exposure) 
for the pathway, except: Tissue samples from aquatic human food 
chain organisms may also be used as specified in sections 4.1.3.3 
and 4.2.3.3 of the surface water-human food chain threat. If any 
hazardous substance is present in more than one comparable sample 
for the sampling location, use the highest concentration of that 
hazardous substance from any of the comparable samples in making the 
comparisons.
    Treat sets of samples that are not comparable separately and 
make a separate comparison for each such set.
    2.5.2 Comparison to benchmarks. Use the following media-specific 
benchmarks for making the comparisons for the indicated pathway (or 
threat):
     Maximum Contaminant Level Goals (MCLGs)--ground water 
migration pathway and drinking water threat in surface water 
migration pathway. Use only MCLG values greater than 0.
     Maximum Contaminant Levels (MCLs)--ground water 
migration pathway and drinking water threat in surface water 
migration pathway.
     Food and Drug Administration Action Level (FDAAL) for 
fish or shellfish--human food chain threat in surface water 
migration pathway.
     EPA Ambient Water Quality Criteria (AWQC/National 
Recommended Water Quality Criteria) for protection of aquatic life--
environmental threat in surface water migration pathway.
     EPA Ambient Aquatic Life Advisory Concentrations 
(AALAC)--environmental threat in surface water migration pathway.
     National Ambient Air Quality Standards (NAAQS)--air 
migration pathway.
     National Emission Standards for Hazardous Air 
Pollutants (NESHAPs)--air migration pathway. Use only those NESHAPs 
promulgated in ambient concentration units.
     Screening concentration for cancer corresponding to 
that concentration that corresponds to the 10-6 
individual cancer risk for inhalation exposures (air migration 
pathway or subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway) or for oral exposures (ground water 
migration pathway; drinking water and human food chain threats in 
surface water migration pathway; and soil exposure and subsurface 
intrusion pathway).
     Screening concentration for noncancer toxicological 
responses corresponding to the RfC for inhalation exposures (air 
migration pathway and subsurface intrusion component of the soil 
exposure and subsurface intrusion pathway) or RfD for oral exposures 
(ground water migration pathway; drinking water and human food chain 
threats in surface water migration pathway; and soil exposure and 
subsurface intrusion pathway).
    Select the benchmark(s) applicable to the pathway (component or 
threat) being evaluated as specified in sections 3 through 6. 
Compare the concentration of each hazardous substance from the 
sampling location to its benchmark concentration(s) for that pathway 
(component or threat). Use only those samples and only those 
hazardous substances in the sample that meet the criteria for an 
observed release (or observed contamination or observed exposure) 
for the pathway, except: Tissue samples from aquatic human food 
chain organisms may be used as specified in sections 4.1.3.3 and 
4.2.3.3. If the concentration of any applicable hazardous substance 
from any sample equals or exceeds its benchmark concentration, 
consider the sampling location to be subject to Level I 
concentrations for that pathway (or threat). If more than one 
benchmark applies to the hazardous substance, assign Level I if the 
concentration of the hazardous substance equals or exceeds the 
lowest applicable benchmark concentration.
    If no hazardous substance individually equals or exceeds its 
benchmark concentration, but more than one hazardous substance 
either meets the criteria for an observed release (or observed 
contamination or observed exposure) for the sample (or comparable 
samples) or is eligible to be evaluated for a tissue sample (see 
sections 4.1.3.3 and 4.2.3.3), calculate the indices I and J 
specified below based on these hazardous substances.
    For those hazardous substances that are carcinogens (that is, 
those having either a carcinogen weight-of-evidence classification 
of A, B, or C or a weight-of-evidence classification of carcinogenic 
to humans, likely to be carcinogenic to humans, or suggestive 
evidence of carcinogenic potential), calculate an index I for the 
sample location as follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.064

Where:

Ci = Concentration of hazardous substance i in sample (or 
highest concentration of hazardous substance i from among comparable 
samples).
SCi = Screening concentration for cancer corresponding to 
that concentration that corresponds to its 10-6 
individual cancer risk for applicable exposure (inhalation or oral) 
for hazardous substance i.
n = Number of applicable hazardous substances in sample (or 
comparable samples) that are carcinogens and for which an 
SCi is available.

    For those hazardous substances for which an RfD or RfC is 
available, calculate an index J for the sample location as follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.065

Where:

Cj = Concentration of hazardous substance j in sample (or 
highest concentration of hazardous substance j from among comparable 
samples).
CRj = Screening concentration for noncancer toxicological 
responses corresponding to RfD or RfC for applicable exposure 
(inhalation or oral) for hazardous substance j.
m = Number of applicable hazardous substances in sample (or 
comparable samples) for which a CRj is available.
    If either I or J equals or exceeds 1, consider the sampling 
location to be subject to Level I concentrations for that pathway 
(component or threat). If both I and J are less than 1, consider the 
sampling location to be subject to Level II concentrations for that 
pathway (component or threat). If, for the sampling location, there 
are sets of samples that are not comparable, calculate I and J 
separately for each such set, and use the highest calculated values 
of I and J to assign Level I and Level II.
    See sections 7.3.1 and 7.3.2 for criteria for determining the 
level of contamination for radioactive substances.
* * * * *

5.0 Soil Exposure and Subsurface Intrusion Pathway

    5.0.1 Exposure components. Evaluate the soil exposure and 
subsurface intrusion pathway based on two exposure components:
     Soil exposure component (see section 5.1).
     Subsurface intrusion component (see section 5.2).
    Score one or both components considering their relative 
importance. If only one component is scored, assign its score as the 
soil exposure and subsurface intrusion pathway score. If both 
components are scored, sum the two scores and assign it as the soil 
exposure and subsurface intrusion pathway score, subject to a 
maximum of 100.

[[Page 2789]]

[GRAPHIC] [TIFF OMITTED] TR09JA17.066

    5.1 Soil exposure component. Evaluate the soil exposure 
component based on two threats: Resident population threat and 
nearby population threat. Evaluate both threats based on three 
factor categories: Likelihood of exposure, waste characteristics,

[[Page 2790]]

and targets. Figure 5-1 indicates the factors included within each 
factor category for each type of threat.
    Determine the soil exposure component score (Sse) in 
terms of the factor category values as follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.067

Where:

LEi = Likelihood of exposure factor category value for 
threat i (that is, resident population threat or nearby population 
threat).
WCi = Waste characteristics factor category value for 
threat i.
Ti = Targets factor category value for threat i.
SF = Scaling factor.

    Table 5-1 outlines the specific calculation procedure.

              Table 5-1--Soil Exposure Component Scoresheet
------------------------------------------------------------------------
      Factor categories and factors        Maximum value  Value assigned
------------------------------------------------------------------------
                       Resident Population Threat
------------------------------------------------------------------------
Likelihood of Exposure:
    1. Likelihood of Exposure...........             550
Waste Characteristics:
    2. Toxicity.........................           (\a\)
    3. Hazardous Waste Quantity.........           (\a\)
    4. Waste Characteristics............             100
Targets:
    5. Resident Individual..............              50
    6. Resident Population:.............
        6a. Level I Concentrations......           (\b\)
        6b. Level II Concentrations.....           (\b\)
        6c. Resident Population (lines             (\b\)
         6a + 6b).......................
    7. Workers..........................              15
    8. Resources........................               5
    9. Terrestrial Sensitive                       (\c\)
     Environments.......................
    10. Targets (lines 5 + 6c + 7 + 8 +            (\b\)
     9).................................
Resident Population Threat Score:
    11. Resident Population Threat                 (\b\)
     (lines 1 x 4 x 10).................
------------------------------------------------------------------------
                        Nearby Population Threat
------------------------------------------------------------------------
Likelihood of Exposure:
    12. Attractiveness/Accessibility....             100
    13. Area of Contamination...........             100
    14. Likelihood of Exposure..........             500
Waste Characteristics:
    15. Toxicity........................           (\a\)
    16. Hazardous Waste Quantity........           (\a\)
    17. Waste Characteristics...........             100
Targets:
    18. Nearby Individual...............               1
    19. Population Within 1 Mile........           (\b\)
    20. Targets (lines 18 + 19).........           (\b\)
Nearby Population Threat Score:
    21. Nearby Population Threat (lines            (\b\)
     14 x 17 x 20)......................
Soil Exposure Component Score:
    22. Soil Exposure Component Score                100
     \d\ (Sse), (lines [11 + 21]/82,500,
     subject to a maximum of 100).......
------------------------------------------------------------------------
\a\ Maximum value applies to waste characteristics category.
\b\ Maximum value not applicable.
\c\ No specific maximum value applies to factor. However, pathway score
  based solely on terrestrial sensitive environments is limited to
  maximum of 60.
\d\ Do not round to nearest integer.

    5.1.0 General considerations. Evaluate the soil exposure 
component based on areas of observed contamination:
     Consider observed contamination to be present at 
sampling locations where analytic evidence indicates that:

--A hazardous substance attributable to the site is present at a 
concentration significantly above background levels for the site 
(see Table 2-3 in section 2.3 for the criteria for determining 
analytical significance), and
--This hazardous substance, if not present at the surface, is 
covered by 2 feet or less of cover material (for example, soil).

     Establish areas of observed contamination based on 
sampling locations at which there is observed contamination as 
follows:

--For all sources except contaminated soil, if observed 
contamination from the site is present at any sampling location 
within the source, consider that entire source to be an area of 
observed contamination.
--For contaminated soil, consider both the sampling location(s) with 
observed contamination from the site and the area lying between such 
locations to be an area of observed contamination, unless available 
information indicates otherwise.

     If an area of observed contamination (or portion of 
such an area) is covered by a permanent, or otherwise maintained, 
essentially impenetrable material (for example, asphalt) that is not 
more than 2 feet thick, exclude that area (or portion of the area) 
in evaluating the soil exposure component.
     For an area of observed contamination, consider only 
those hazardous substances that meet the criteria for observed 
contamination for that area to be associated with that area in 
evaluating the soil exposure component (see section 2.2.2).

[[Page 2791]]

    If there is observed contamination, assign scores for the 
resident population threat and the nearby population threat, as 
specified in sections 5.1.1 and 5.1.2. If there is no observed 
contamination, assign the soil exposure component of the soil 
exposure and subsurface intrusion pathway a score of 0.
    5.1.1 Resident population threat. Evaluate the resident 
population threat only if there is an area of observed contamination 
in one or more of the following locations:
     Within the property boundary of a residence, school, or 
day care center and within 200 feet of the respective residence, 
school, or day care center, or
     Within a workplace property boundary and within 200 
feet of a workplace area, or
     Within the boundaries of a resource specified in 
section 5.1.1.3.4, or
     Within the boundaries of a terrestrial sensitive 
environment specified in section 5.1.1.3.5.
    If not, assign the resident population threat a value of 0, 
enter this value in Table 5-1, and proceed to the nearby population 
threat (section 5.1.2).
    5.1.1.1 Likelihood of exposure. Assign a value of 550 to the 
likelihood of exposure factor category for the resident population 
threat if there is an area of observed contamination in one or more 
locations listed in section 5.1.1. Enter this value in Table 5-1.
    5.1.1.2 Waste characteristics. Evaluate waste characteristics 
based on two factors: toxicity and hazardous waste quantity. 
Evaluate only those hazardous substances that meet the criteria for 
observed contamination at the site (see section 5.1.0).
    5.1.1.2.1 Toxicity. Assign a toxicity factor value to each 
hazardous substance as specified in section 2.4.1.1. Use the 
hazardous substance with the highest toxicity factor value to assign 
the value to the toxicity factor for the resident population threat. 
Enter this value in Table 5-1.
    5.1.1.2.2 Hazardous waste quantity. Assign a hazardous waste 
quantity factor value as specified in section 2.4.2. In estimating 
the hazardous waste quantity, use Table 5-2 and:
     Consider only the first 2 feet of depth of an area of 
observed contamination, except as specified for the volume measure.
     Use the volume measure (see section 2.4.2.1.3) only for 
those types of areas of observed contamination listed in Tier C of 
Table 5-2. In evaluating the volume measure for these listed areas 
of observed contamination, use the full volume, not just the volume 
within the top 2 feet.
     Use the area measure (see section 2.4.2.1.4), not the 
volume measure, for all other types of areas of observed 
contamination, even if their volume is known.
    Enter the value assigned in Table 5-1.

              Table 5-2--Hazardous Waste Quantity Evaluation Equations for Soil Exposure Component
----------------------------------------------------------------------------------------------------------------
                                                                                         Equation for assigning
               Tier                           Measure                   Units                  value \a\
----------------------------------------------------------------------------------------------------------------
A.................................  Hazardous Constituent       lb...................  C.
                                     Quantity (C).
 B\b\.............................  Hazardous Wastestream       lb...................  W/5,000.
                                     Quantity (W).
C \b\.............................  Volume (V)................
                                    Surface Impoundment \c\...  yd\3\................  V/2.5.
                                    Drums \d\.................  gallon...............  V/500.
                                    Tanks and Containers Other  yd \3\...............  V/2.5.
                                     Than Drums.
D \b\.............................  Area (A)..................
                                    Landfill..................  ft \2\...............  A/34,000.
                                    Surface Impoundment.......  ft \2\...............  A/13.
                                    Surface Impoundment         ft \2\...............  A/13.
                                     (Buried/backfilled).
                                    Land treatment............  ft \2\...............  A/270.
                                    Pile \e\..................  ft \2\...............  A/34.
                                    Contaminated Soil.........  ft \2\...............  A/34,000.
----------------------------------------------------------------------------------------------------------------
\a\ Do not round nearest integer.
\b\ Convert volume to mass when necessary: 1 ton = 2,000 pounds = 1 cubic yard = 4 drums = 200 gallons.
\c\ Use volume measure only for surface impoundments containing hazardous substances present as liquids. Use
  area measures in Tier D for dry surface impoundments and for buried/backfilled surface impoundments.
\d\ If actual volume of drums is unavailable, assume 1 drum = 50 gallons.
\e\ Use land surface area under pile, not surface area of pile.

    5.1.1.2.3 Calculation of waste characteristics factor category 
value. Multiply the toxicity and hazardous waste quantity factor 
values, subject to a maximum product of 1 x 10 \8\. Based on this 
product, assign a value from Table 2-7 (section 2.4.3.1) to the 
waste characteristics factor category. Enter this value in Table 5-
1.
    5.1.1.3 Targets. Evaluate the targets factor category for the 
resident population threat based on five factors: Resident 
individual, resident population, workers, resources, and terrestrial 
sensitive environments.
    In evaluating the targets factor category for the resident 
population threat, count only the following as targets:
     Resident individual--a person living or attending 
school or day care on a property with an area of observed 
contamination and whose residence, school, or day care center, 
respectively, is on or within 200 feet of the area of observed 
contamination.
     Worker--a person working on a property with an area of 
observed contamination and whose workplace area is on or within 200 
feet of the area of observed contamination.
     Resources located on an area of observed contamination, 
as specified in section 5.1.1.
     Terrestrial sensitive environments located on an area 
of observed contamination, as specified in section 5.1.1.
    5.1.1.3.1 Resident individual. Evaluate this factor based on 
whether there is a resident individual, as specified in section 
5.1.1.3, who is subject to Level I or Level II concentrations.
    First, determine those areas of observed contamination subject 
to Level I concentrations and those subject to Level II 
concentrations as specified in sections 2.5.1 and 2.5.2. Use the 
health-based benchmarks from Table 5-3 in determining the level of 
contamination. Then assign a value to the resident individual factor 
as follows:
     Assign a value of 50 if there is at least one resident 
individual for one or more areas subject to Level I concentrations.
     Assign a value of 45 if there is no such resident 
individuals, but there is at least one resident individual for one 
or more areas subject to Level II concentrations.
     Assign a value of 0 if there is no resident individual.

Enter the value assigned in Table 5-1.
    5.1.1.3.2 Resident population. Evaluate resident population 
based on two factors: Level I concentrations and Level II 
concentrations. Determine which factor applies as specified in 
sections 2.5.1 and 2.5.2, using the health-based benchmarks from 
Table 5-3. Evaluate populations subject to Level I concentrations as 
specified in section 5.1.1.3.2.1 and populations subject to Level II 
concentrations as specified in section 5.1.1.3.2.2.

[[Page 2792]]

  Table 5-3--Health-Based Benchmarks for Hazardous Substances in Soils
------------------------------------------------------------------------
 
-------------------------------------------------------------------------
Screening concentration for cancer corresponding to that concentration
 that corresponds to the 10 -6 individual cancer risk for oral
 exposures.
Screening concentration for noncancer toxicological responses
 corresponding to the Reference Dose (RfD) for oral exposures.
------------------------------------------------------------------------

    Count only those persons meeting the criteria for resident 
individual as specified in section 5.1.1.3. In estimating the number 
of people living on property with an area of observed contamination, 
when the estimate is based on the number of residences, multiply 
each residence by the average number of persons per residence for 
the county in which the residence is located.
    5.1.1.3.2.1 Level I concentrations. Sum the number of resident 
individuals subject to Level I concentrations and multiply this sum 
by 10. Assign the resulting product as the value for this factor. 
Enter this value in Table 5-1.
    5.1.1.3.2.2 Level II concentrations. Sum the number of resident 
individuals subject to Level II concentrations. Do not include those 
people already counted under the Level I concentrations factor. 
Assign this sum as the value for this factor. Enter this value in 
Table 5-1.
    5.1.1.3.2.3 Calculation of resident population factor value. Sum 
the factor values for Level I concentrations and Level II 
concentrations. Assign this sum as the resident population factor 
value. Enter this value in Table 5-1.
    5.1.1.3.3 Workers. Evaluate this factor based on the number of 
workers that meet the section 5.1.1.3 criteria. Assign a value for 
these workers using Table 5-4. Enter this value in Table 5-1.

                  Table 5-4--Factor Values for Workers
------------------------------------------------------------------------
                                                               Assigned
                     Number of workers                          value
------------------------------------------------------------------------
0..........................................................            0
1 to 100...................................................            5
101 to 1,000...............................................           10
Greater than 1,000.........................................           15
------------------------------------------------------------------------

    5.1.1.3.4 Resources. Evaluate the resources factor as follows:
     Assign a value of 5 to the resources factor if one or 
more of the following is present on an area of observed 
contamination at the site:
--Commercial agriculture.
--Commercial silviculture.
--Commercial livestock production or commercial livestock grazing.

     Assign a value of 0 if none of the above are present.
    Enter the value assigned in Table 5-1.
    5.1.1.3.5 Terrestrial sensitive environments. Assign value(s) 
from Table 5-5 to each terrestrial sensitive environment that meets 
the eligibility criteria of section 5.1.1.3.
    Calculate a value (ES) for terrestrial sensitive environments as 
follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.068

Where:

Si = Value(s) assigned from Table 5-5 to terrestrial 
sensitive environment i.
n = Number of terrestrial sensitive environments meeting section 
5.1.1.3 criteria.

    Because the pathway score based solely on terrestrial sensitive 
environments is limited to a maximum of 60, determine the value for 
the terrestrial sensitive environments factor as follows:

       Table 5-5--Terrestrial Sensitive Environments Rating Values
------------------------------------------------------------------------
                                                             Assigned
           Terrestrial sensitive environments                  value
------------------------------------------------------------------------
Terrestrial critical habitat \a\ for Federal designated              100
 endangered or threatened species.......................
    National Park
    Designated Federal Wilderness Area..................
    National Monument...................................
Terrestrial habitat known to be used by Federal                       75
 designated or proposed threatened or endangered species
    National Preserve (terrestrial)
     National or State Terrestrial Wildlife Refuge......
    Federal land designated for protection of natural
     ecosystems.........................................
     Administratively proposed Federal Wilderness Area..
    Terrestrial areas utilized for breeding by large or
     dense aggregations of animals \b\..................
Terrestrial habitat known to be used by State designated              50
 endangered or threatened species.......................
    Terrestrial habitat known to be used by species
     under review as to its Federal designated
     endangered or threatened status
State lands designated for wildlife or game management..              25
    State designated Natural Areas
    Particular areas, relatively small in size,
     important to maintenance of unique biotic
     communities........................................
------------------------------------------------------------------------
\a\ Critical habitat as defined in 50 CFR 424.02.
\b\ Limit to vertebrate species.

     Multiply the values assigned to the resident population 
threat for likelihood of exposure (LE), waste characteristics (WC), 
and ES. Divide the product by 82,500.
--If the result is 60 or less, assign the value ES as the 
terrestrial sensitive environments factor value.
--If the result exceeds 60, calculate a value EC as follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.069

    Assign the value EC as the terrestrial sensitive environments 
factor value. Do not round this value to the nearest integer.
    Enter the value assigned for the terrestrial sensitive 
environments factor in Table 5-1.
    5.1.1.3.6 Calculation of resident population targets factor 
category value. Sum the values for the resident individual, resident 
population, workers, resources, and terrestrial sensitive 
environments factors. Do not round to the nearest integer. Assign 
this sum as the targets factor category value for the resident 
population threat. Enter this value in Table 5-1.
    5.1.1.4 Calculation of resident population threat score. 
Multiply the values for likelihood of exposure, waste 
characteristics, and targets for the resident population threat, and 
round the product to the nearest integer. Assign this product as the 
resident population threat score. Enter this score in Table 5-1.
    5.1.2 Nearby population threat. Include in the nearby population 
only those individuals who live or attend school within a 1-mile 
travel distance of an area of observed contamination at the site and 
who do not meet the criteria for resident individual as specified in 
section 5.1.1.3.
    Do not consider areas of observed contamination that have an 
attractiveness/accessibility factor value of 0 (see section

[[Page 2793]]

5.1.2.1.1) in evaluating the nearby population threat.
    5.1.2.1 Likelihood of exposure. Evaluate two factors for the 
likelihood of exposure factor category for the nearby population 
threat: attractiveness/accessibility and area of contamination.
    5.1.2.1.1 Attractiveness/accessibility. Assign a value for 
attractiveness/accessibility from Table 5-6 to each area of observed 
contamination, excluding any land used for residences. Select the 
highest value assigned to the areas evaluated and use it as the 
value for the attractiveness/accessibility factor. Enter this value 
in Table 5-1.
    5.1.2.1.2 Area of contamination. Evaluate area of contamination 
based on the total area of the areas of observed contamination at 
the site. Count only the area(s) that meet the criteria in section 
5.1.0 and that receive an attractiveness/accessibility value greater 
than 0. Assign a value to this factor from Table 5-7. Enter this 
value in Table 5-1.

             Table 5-6--Attractiveness/Accessibility Values
------------------------------------------------------------------------
             Area of observed contamination               Assigned value
------------------------------------------------------------------------
Designated recreational area............................             100
Regularly used for public recreation (for example,                    75
 fishing, hiking, softball).............................
Accessible and unique recreational area (for example,                 75
 vacant lots in urban area).............................
Moderately accessible (may have some access                           50
 improvements, for example, gravel road), with some
 public recreation use..................................
Slightly accessible (for example, extremely rural area                25
 with no road improvement), with some public recreation
 use....................................................
Accessible, with no public recreation use...............              10
Surrounded by maintained fence or combination of                       5
 maintained fence and natural barriers..................
Physically inaccessible to public, with no evidence of                 0
 public recreation use..................................
------------------------------------------------------------------------

             Table 5-7--Area of Contamination Factor Values
------------------------------------------------------------------------
    Total area of the areas of observed contamination
                      (square feet)                       Assigned value
------------------------------------------------------------------------
Less than or equal to 5,000.............................               5
Greater than 5,000 to 125,000...........................              20
Greater than 125,000 to 250,000.........................              40
Greater than 250,000 to 375,000.........................              60
Greater than 375,000 to 500,000.........................              80
Greater than 500,000....................................             100
------------------------------------------------------------------------

    5.1.2.1.3 Likelihood of exposure factor category value. Assign a 
value from Table 5-8 to the likelihood of exposure factor category, 
based on the values assigned to the attractiveness/accessibility and 
area of contamination factors. Enter this value in Table 5-1.

                                            Table 5-8--Nearby Population Likelihood of Exposure Factor Values
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                             Attractiveness/accessibility factor value
   Area of contamination factor value    ---------------------------------------------------------------------------------------------------------------
                                                100             75              50              25              10               5               0
--------------------------------------------------------------------------------------------------------------------------------------------------------
100.....................................             500             500             375             250             125              50               0
80......................................             500             375             250             125              50              25               0
60......................................             375             250             125              50              25               5               0
40......................................             250             125              50              25               5               5               0
20......................................             125              50              25               5               5               5               0
5.......................................              50              25               5               5               5               5               0
--------------------------------------------------------------------------------------------------------------------------------------------------------

    5.1.2.2 Waste characteristics. Evaluate waste characteristics 
based on two factors: toxicity and hazardous waste quantity. 
Evaluate only those hazardous substances that meet the criteria for 
observed contamination (see section 5.1.0) at areas that can be 
assigned an attractiveness/accessibility factor value greater than 
0.
    5.1.2.2.1 Toxicity. Assign a toxicity factor value as specified 
in section 2.4.1.1 to each hazardous substance meeting the criteria 
in section 5.1.2.2. Use the hazardous substance with the highest 
toxicity factor value to assign the value to the toxicity factor for 
the nearby population threat. Enter this value in Table 5-1.
    5.1.2.2.2 Hazardous waste quantity. Assign a value to the 
hazardous waste quantity factor as specified in section 5.1.1.2.2, 
except: consider only those areas of observed contamination that can 
be assigned an attractiveness/accessibility factor value greater 
than 0. Enter the value assigned in Table 5-1.
    5.1.2.2.3 Calculation of waste characteristics factor category 
value. Multiply the toxicity and hazardous waste quantity factor 
values, subject to a maximum product of 1 x 10\8\. Based on this 
product, assign a value from Table 2-7 (section 2.4.3.1) to the 
waste characteristics factor category. Enter this value in Table 5-
1.
    5.1.2.3 Targets. Evaluate the targets factory category for the 
nearby population threat based on two factors: nearby individual and 
population within a 1-mile travel distance from the site.
    5.1.2.3.1 Nearby individual. If one or more persons meet the 
section 5.1.1.3 criteria for a resident individual, assign this 
factor a value of 0. Enter this value in Table 5-1.
    If no person meets the criteria for a resident individual, 
determine the shortest travel distance from the site to any 
residence or school. In determining the travel distance, measure the 
shortest overland distance an individual would travel from a 
residence or school to the nearest area of observed contamination 
for the site with an attractiveness/accessibility factor value 
greater than 0. If there are no natural barriers to travel, measure 
the travel distance as the shortest straight-line distance from the 
residence or school to the area of observed contamination. If 
natural barriers exist (for example, a river), measure the travel 
distance as the shortest straight-line distance from the residence 
or school to the nearest crossing point and from there as the 
shortest straight-

[[Page 2794]]

line distance to the area of observed contamination. Based on the 
shortest travel distance, assign a value from Table 5-9 to the 
nearest individual factor. Enter this value in Table 5-1.

               Table 5-9--Nearby Individual Factor Values
------------------------------------------------------------------------
                                                                Assigned
        Travel distance for nearby individual (miles)            value
------------------------------------------------------------------------
Greater than 0 to \1/4\......................................      \a\ 1
Greater than \1/4\ to 1......................................          0
------------------------------------------------------------------------
\a\ Assign a value of 0 if one or more persons meet the section 5.1.1.3
  criteria for resident individual.

    5.1.2.3.2 Population within 1 mile. Determine the population 
within each travel distance category of Table 5-10. Count residents 
and students who attend school within this travel distance. Do not 
include those people already counted in the resident population 
threat. Determine travel distances as specified in section 
5.1.2.3.1.
    In estimating residential population, when the estimate is based 
on the number of residences, multiply each residence by the average 
number of persons per residence for the county in which the 
residence is located.
    Based on the number of people included within a travel distance 
category, assign a distance-weighted population value for that 
travel distance from Table 5-10.
    Calculate the value for the population within 1 mile factor (PN) 
as follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.070

Where:

Wi=Distance-weighted population value from Table 5-10 for 
travel distance category i.

    If PN is less than 1, do not round it to the nearest integer; if 
PN is 1 or more, round to the nearest integer. Enter this value in 
Table 5-1.
    5.1.2.3.3 Calculation of nearby population targets factor 
category value. Sum the values for the nearby individual factor and 
the population within 1 mile factor. Do not round this sum to the 
nearest integer. Assign this sum as the targets factor category 
value for the nearby population threat. Enter this value in Table 5-
1.

                                     Table 5-10--Distance Weighted Population Values for Nearby Population Threat a
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                          Number of people within the travel distance category
                                               ---------------------------------------------------------------------------------------------------------
       Travel distance category (miles)                                                          1,001   3,001    10,001   30,001    100,001
                                                   0     1 to    11 to   31 to  101 to  301 to    to       to       to       to        to     300,001 to
                                                          10      30      100     300    1,000   3,000   10,000   30,000   100,000   300,000   1,000,000
--------------------------------------------------------------------------------------------------------------------------------------------------------
Greater than 0 to \1/4\.......................       0     0.1     0.4     1.0       4      13      41      130      408     1,303     4,081      13,034
Greater than \1/4\ to \1/2\...................       0    0.05     0.2     0.7       2       7      20       65      204       652     2,041       6,517
Greater than \1/2\ to 1.......................       0    0.02     0.1     0.3       1       3      10       33      102       326     1,020       3,258
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ Round the number of people present within a travel distance category to nearest integer. Do not round the assigned distance-weighted population
  value to nearest integer.

    5.1.2.4 Calculation of nearby population threat score. Multiply 
the values for likelihood of exposure, waste characteristics, and 
targets for the nearby population threat, and round the product to 
the nearest integer. Assign this product as the nearby population 
threat score. Enter this score in Table 5-1.
    5.1.3 Calculation of soil exposure component score. Sum the 
resident population threat score and the nearby population threat 
score, and divide the sum by 82,500. Assign the resulting value, 
subject to a maximum of 100, as the soil exposure component score 
(Sse). Enter this score in Table 5-1.
    5.2 Subsurface intrusion component. Evaluate the subsurface 
intrusion component based on three factor categories: likelihood of 
exposure, waste characteristics, and targets. Figure 5-1 indicates 
the factors included within each factor category for the subsurface 
intrusion component.
    Determine the component score (Sssi) in terms of the 
factor category values as follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.071

Where:

LE=Likelihood of exposure factor category value.
WC=Waste characteristics factor category value.
T=Targets factor category value.
SF=Scaling factor.

    Table 5-11 outlines the specific calculation procedure.

          Table 5-11--Subsurface Intrusion Component Scoresheet
------------------------------------------------------------------------
      Factor categories and factors        Maximum value  Value assigned
------------------------------------------------------------------------
Subsurface Intrusion Component:
Likelihood of Exposure:
    1. Observed Exposure................             550
    2. Potential for Exposure...........
        2a. Structure Containment.......              10
        2b. Depth to contamination......              10
        2c. Vertical Migration..........              15
        2d. Vapor Migration Potential...              25
    3. Potential for Exposure (lines 2a              500
     * (2b + 2c + 2d), subject to a
     maximum of 500)....................
    4. Likelihood of Exposure (higher of             550
     lines 1 or 3)......................
Waste Characteristics:
    5. Toxicity/Degradation.............           (\a\)
    6. Hazardous Waste Quantity.........           (\a\)
    7. Waste Characteristics (subject to             100
     a maximum of 100)..................
Targets:
    8. Exposed Individual...............              50
    9. Population:......................
        9a. Level I Concentrations......           (\b\)
        9b. Level II Concentrations.....           (\b\)
        9c. Population within an Area of           (\b\)
         Subsurface Contamination.......
        9d. Total Population (lines 9a +           (\b\)
         9b + 9c).......................

[[Page 2795]]

 
    10. Resources.......................               5
    11. Targets (lines 8 + 9d + 10).....           (\b\)
Subsurface Intrusion Component Score:
    12. Subsurface Intrusion Component               100
     (lines 4 x 7 x 11)/82,500 \c\
     (subject to a maximum of 100)......
Soil Exposure and Subsurface Intrusion
 Pathway Score:
    13. Soil Exposure Component +                    100
     Subsurface Intrusion Component
     (subject to a maximum of 100)......
------------------------------------------------------------------------
\a\ Maximum value applies to waste characteristics category.
\b\ Maximum value not applicable.
\c\ Do not round to the nearest integer.

    5.2.0 General considerations. The subsurface intrusion component 
evaluates the threats from hazardous substances that have or could 
intrude into regularly occupied structures from the subsurface. 
Evaluate the subsurface intrusion component based on the actual or 
potential intrusion of hazardous substances into all regularly 
occupied structures that have structure containment values greater 
than zero and meet the criteria identified in the section below as 
being either in an area of observed exposure or in an area of 
subsurface contamination. These structures may or may not have 
subunits. Subunits are partitioned areas within a structure with 
separate heating, ventilating, and air conditioning (HVAC) systems 
or distinctly different air exchange rates. Subunits include 
regularly occupied partitioned tenant spaces such as office suites, 
apartments, condos, common or shared areas, and portions of 
residential, commercial or industrial structures with separate 
heating, ventilating, and air conditioning (HVAC) systems.
    In evaluating the subsurface intrusion component, consider the 
following:
     Area(s) of observed exposure: An area of observed 
exposure is delineated by regularly occupied structures with 
documented contamination meeting observed exposure criteria; an area 
of observed exposure includes regularly occupied structures with 
samples meeting observed exposure criteria or inferred to be within 
an area of observed exposure based on samples meeting observed 
exposure criteria (see section 5.2.1.1.1 Observed exposure). 
Establish areas of observed exposure as follows:

--For regularly occupied structures that have no subunits, consider 
both the regularly occupied structures containing sampling 
location(s) meeting observed exposure criteria for the site and the 
regularly occupied structure(s) in the area lying between such 
locations to be an area of observed exposure (i.e., inferred to be 
in an area of observed exposure), unless available information 
indicates otherwise.
--In multi-story, multi-subunit, regularly occupied structures, 
consider all subunits on a level with sampling locations meeting 
observed exposure criteria from the site and all levels below, if 
any, to be within an area of observed exposure, unless available 
information indicates otherwise.
--In multi-tenant structures, that do not have a documented observed 
exposure, but are located in an area lying between locations where 
observed exposures have been documented, consider only those 
regularly occupied subunits, if any, on the lowest level of the 
structure, to be within an area of observed exposure (i.e., inferred 
to be in an area of observed exposure, unless available information 
indicates otherwise.

     Area(s) of subsurface contamination: An area of 
subsurface contamination is delineated by sampling locations meeting 
observed release criteria for subsurface intrusion, excluding areas 
of observed exposure (see Table 2-3 in section 2.3). The area within 
an area of subsurface contamination includes potentially exposed 
populations. If the significant increase in hazardous substance 
levels cannot be attributed at least in part to the site, and cannot 
be attributed to other sites, attribution can be established based 
on the presence of hazardous substances in the area of subsurface 
contamination. Establish areas of subsurface contamination as 
follows:

--Exclude those areas that contain structures meeting the criteria 
defined as an area of observed exposure.
--Consider both the sampling location(s) with subsurface 
contamination meeting observed release criteria from the site and 
the area lying between such locations to be an area of subsurface 
contamination (i.e., inferred to be in an area of subsurface 
contamination). If sufficient data is available and state of the 
science shows there is no unacceptable risk due to subsurface 
intrusion into a regularly occupied structure located within an area 
of subsurface contamination, that structure can be excluded from the 
area of subsurface contamination.
Evaluate an area of subsurface contamination based on hazardous 
substances that:
    [ssquf] Meet the criteria for observed exposure of a chemical 
that has a vapor pressure greater than or equal to one torr or a 
Henry's constant greater than or equal to 10-\5\ atm-
m\3\/mol, or
    [ssquf] Meet the criteria for observed release in an area of 
subsurface contamination and have a vapor pressure greater than or 
equal to one torr or a Henry's constant greater than or equal to 
10-\5\ atm-m\3\/mol, or
    [ssquf] Meet the criteria for an observed release in a structure 
within, or in a sample from below, an area of observed exposure and 
have a vapor pressure greater than or equal to one torr or a Henry's 
constant greater than or equal to 10-\5\ atm-m\3\/mol.
    --Evaluate all structures with no subunits that have containment 
factor values greater than zero, and not documented to meet observed 
exposure criteria to be in an area of subsurface contamination if 
they are lying between locations of subsurface intrusion samples 
meeting observed release criteria.
    --Evaluate multi-subunit structures as follows:
    [ssquf] If an observed exposure has been documented based on a 
gaseous indoor air sample, consider all regularly occupied 
subunit(s), if any, on the level immediately above the level where 
an observed exposure has been documented (or has been inferred to be 
within an area of observed exposure), to be within an area of 
subsurface contamination. If sufficient data is available and state 
of the science shows there is no unacceptable risk due to subsurface 
intrusion on the level immediately above the level where an observed 
exposure has been documented (or has been inferred to be within an 
area of observed exposure) that level can be excluded from the area 
of subsurface contamination.
    [ssquf] If observed release criteria have been met based on a 
gaseous indoor air sample collected from a level not regularly 
occupied, consider all regularly occupied subunit(s), if any, on the 
level immediately above the level where the observed release 
criteria has been documented, to be within an area of subsurface 
contamination. If sufficient data is available and state of the 
science shows there is no unacceptable risk due to subsurface 
intrusion on the level immediately above the level where the 
observed release criteria has been documented that level can be 
excluded from the area of subsurface contamination.
    [ssquf] If any regularly occupied multi-subunit structure is 
inferred to be in an area of subsurface contamination, consider only 
those regularly occupied subunit(s), if any, on the lowest level, to 
be within an area of subsurface contamination. If sufficient data is 
available and state of the science shows there is no unacceptable 
risk due to subsurface intrusion on the lowest level, that structure 
can be excluded from the area of subsurface contamination.

    See Section 7.0 for establishing an area of subsurface 
contamination based on the

[[Page 2796]]

presence of radioactive hazardous substances.
    If there is no area of observed exposure and no area of 
subsurface contamination, assign a score of 0 for the subsurface 
intrusion component.
    5.2.1 Subsurface intrusion component. Evaluate this component 
only if there is an area of observed exposure or area of subsurface 
contamination:
     Within or underlying a residence, school, day care 
center, workplace, or
     Within or underlying a resource specified in section 
5.2.1.3.3.
    5.2.1.1 Likelihood of exposure. Assign a value of 550 to the 
likelihood of exposure factor category for the subsurface intrusion 
component if there is an area of observed exposure in one or more 
locations listed in section 5.2.1. Enter this value in Table 5-11.
    5.2.1.1.1 Observed exposure. Establish observed exposure in a 
regularly occupied structure by demonstrating that a hazardous 
substance has been released into a regularly occupied structure via 
the subsurface. Base this demonstration on either of the following 
criteria:
     Direct observation:

--A solid, liquid, or gaseous material that contains one or more 
hazardous substances attributable to the site has been observed 
entering a regularly occupied structure through migration via the 
subsurface or is known to have entered a regularly occupied 
structure via the subsurface, or
--When evidence supports the inference of subsurface intrusion of a 
material that contains one or more hazardous substances associated 
with the site into a regularly occupied structure, demonstrated 
adverse effects associated with that release may be used to 
establish observed exposure.

     Chemical analysis:

--Analysis of indoor samples indicates that the concentration of 
hazardous substance(s) is significantly above the background 
concentration for the site for that type of sample (see section 
2.3).
--Some portion of the significant increase above background must be 
attributable to the site to establish the observed exposure. 
Documentation of this attribution should account for possible 
concentrations of the hazardous substance(s) in outdoor air or from 
materials found in the regularly occupied structure, and should 
provide a rationale for the increase being from subsurface 
intrusion.

    If observed exposure can be established in a regularly occupied 
structure, assign an observed exposure factor value of 550, enter 
this value in Table 5-11, and proceed to section 5.2.1.1.3. If no 
observed exposure can be established, assign an observed exposure 
factor value of 0, enter this value in Table 5-11, and proceed to 
section 5.2.1.1.2.
    5.2.1.1.2 Potential for exposure. Evaluate potential for 
exposure only if an observed exposure cannot be established, but an 
area of subsurface contamination has been delineated. Evaluate 
potential for exposure based only on the presence of hazardous 
substances with a vapor pressure greater than or equal to one torr 
or a Henry's constant greater than or equal to 10-\5\ 
atm-m\3\/mol. Evaluate potential for exposure for each area of 
subsurface contamination based on four factors: Structure 
containment (see section 5.2.1.1.2.1), depth to contamination (see 
section 5.2.1.1.2.2), vertical migration (see section 5.2.1.1.2.3) 
and vapor migration potential (see section 5.2.1.1.2.4). For each 
area of subsurface contamination, assign the highest value for each 
factor. If information is insufficient to calculate any single 
factor value used to calculate the potential for exposure factor 
values at an identified area of subsurface contamination, 
information collected for another area of subsurface contamination 
at the site may be used when evaluating potential for exposure. 
Calculate the potential for exposure value for the site as specified 
in section 5.2.1.1.2.5.
    5.2.1.1.2.1 Structure containment. Calculate containment for 
eligible hazardous substances within this component as directed in 
Table 5-12 and enter this value into Table 5-11. Assign each 
regularly occupied structure within an area of subsurface 
contamination the highest appropriate structure containment value 
from Table 5-12 and use the regularly occupied structure at the site 
with the highest structure containment value in performing the 
potential for exposure calculation. For all regularly occupied 
structures with unknown containment features assign a structure 
containment value of greater than zero for the purposes of 
evaluating targets (see section 5.2.1.3).

                    Table 5-12--Structure Containment
------------------------------------------------------------------------
                                Evidence of structure
            No.                      containment          Assigned value
------------------------------------------------------------------------
1.........................  Regularly occupied structure              10
                             with evidence of subsurface
                             intrusion, including
                             documented observed
                             exposure or sampling of bio
                             or inert gases, such as
                             methane and radon.
2.........................  Regularly occupied structure              10
                             with open preferential
                             subsurface intrusion
                             pathways (e.g., sumps,
                             foundation cracks, unsealed
                             utility lines).
3.........................  Regularly occupied structure               7
                             with an engineered vapor
                             migration barrier system
                             that does not address all
                             preferential subsurface
                             intrusion pathways.
4.........................  Regularly occupied structure               6
                             with an engineered passive
                             vapor mitigation system
                             without documented
                             institutional controls
                             (e.g., deed restrictions)
                             or evidence of regular
                             maintenance and inspection.
5.........................  Regularly occupied structure               4
                             with no visible open
                             preferential subsurface
                             intrusion pathways from the
                             subsurface (e.g., sumps,
                             foundation cracks, unsealed
                             utility lines).
6.........................  Regularly occupied structure               3
                             with an engineered passive
                             vapor mitigation system
                             (e.g., passive venting)
                             with documented
                             institutional controls
                             (e.g., deed restrictions)
                             or evidence of regular
                             maintenance and inspection.
7.........................  Regularly occupied structure               2
                             with an engineered, active
                             vapor mitigation system
                             (e.g., active venting)
                             without documented
                             institutional controls
                             (e.g., deed restrictions)
                             and funding in place for on-
                             going operation, inspection
                             and maintenance.
8.........................  Regularly occupied structure               1
                             with a permanent
                             engineered, active vapor
                             mitigation system (e.g.,
                             active venting) with
                             documented institutional
                             controls (e.g., deed
                             restrictions) and funding
                             in place for on-going
                             operation, inspection and
                             maintenance.
9.........................  Regularly occupied structure               0
                             with a foundation raised
                             greater than 6 feet above
                             ground surface (e.g.,
                             structure on stilts) or
                             structure that has been
                             built, and maintained, in a
                             manner to prevent
                             subsurface intrusion.
------------------------------------------------------------------------

    5.2.1.1.2.2 Depth to contamination. Assign each area of 
subsurface contamination a depth to contamination based on the least 
depth to either contaminated crawl space or subsurface media 
underlying a regularly occupied structure. Measure this depth to 
contamination based on the distance between the lowest point of a 
regularly occupied structure to the highest known point of hazardous 
substances eligible to be evaluated. Use any regularly occupied 
structure within an area of subsurface contamination with a 
structure containment factor value greater than zero. Subtract from 
the depth to contamination the thickness of any subsurface layer 
composed of features that would allow channelized flow (e.g., karst, 
lava tubes, open fractures, as well as manmade preferential pathways 
such as utility conduits or drainage systems).
    Based on this calculated depth, assign a factor value from Table 
5-13. If the necessary information is available at multiple 
locations, calculate the depth to contamination at each location. 
Use the location having the least depth to contamination to assign 
the factor value. Enter this value in Table 5-11.\\

[[Page 2797]]

                   Table 5-13--Depth to Contamination
------------------------------------------------------------------------
                                                             Depth to
                     Depth range 1 2                       contamination
                                                          assigned value
------------------------------------------------------------------------
0 to <10 ft (Including subslab and semi-enclosed or                   10
 enclosed crawl space contamination)....................
>10 to 20 ft............................................               8
>20 to 50 ft............................................               6
>50 to 100 ft...........................................               4
>100 to 150 ft..........................................               2
>150 ft.................................................               0
------------------------------------------------------------------------
\1\ If any part of the subsurface profile has channelized flow features,
  assign that portion of the subsurface profile a depth of 0.
\2\ Measure elevation below any regularly occupied structure within an
  area of subsurface contamination at a site. Select the regularly
  occupied structure with the least depth to contamination below a
  structure.

    5.2.1.1.2.3 Vertical migration. Evaluate the vertical migration 
factor for each area of subsurface contamination based on the 
geologic materials in the interval between the lowest point of a 
regularly occupied structure and the highest known point of 
hazardous substances in the subsurface. Use any regularly occupied 
structure either within an area of subsurface contamination or 
overlying subsurface soil gas or ground water contamination. Assign 
a value to the vertical migration factor as follows:
     If the depth to contamination (see section 5.2.1.1.2.2) 
is 10 feet or less, assign a value of 15.
     If the depth to contamination is greater than 10 feet, 
do not consider layers or portions of layers within the first 10 
feet of the depth to contamination (as assigned in section 
5.2.1.1.2.2).
     If, for the interval between the lowest point of a 
regularly occupied structure and the highest point of hazardous 
substances in the subsurface, all layers that underlie a portion of 
a regularly occupied structure at the site are karst or otherwise 
allow channelized flow, assign a value of 15.
     Otherwise:

--Select the lowest effective porosity/permeability layer(s) from 
within the interval identified above. Consider only layers at least 
1 foot thick.--Assign a value for individual layers from Table 5-14 
using the hydraulic conductivity of the layer, if available. If the 
hydraulic conductivity is not available, assign a value based on the 
type of material in the selected layer.
--If more than one layer has the same assigned porosity/permeability 
value, include all such layers and sum their thicknesses. Assign a 
thickness of 0 feet to a layer with channelized flow features found 
within any area of subsurface contamination at the site.
--Assign a value from Table 5-15 to the vertical migration factor, 
based on the thickness and assigned porosity/permeability value of 
the lowest effective porosity/permeability layer(s).

    Determine vertical migration only at locations within an area of 
subsurface contamination at the site. If the necessary subsurface 
geologic information is available at multiple locations, evaluate 
the vertical migration factor at each location. Use the location 
having the highest vertical migration factor value to assign the 
factor value. Enter this value in Table 5-11.

    Table 5-14--Effective Porosity/Permeability of Geologic Materials
------------------------------------------------------------------------
                                                             Assigned
                                          Hydraulic          porosity/
         Type of material             conductivity (cm/    permeability
                                            sec)               value
------------------------------------------------------------------------
Gravel; clean sand; highly          Greater than or                    1
 permeable fractured igneous and     equal to 1 x 10-\3\.
 metamorphic rocks; permeable
 basalt; karst limestones and
 dolomites.
Sand; sandy clays; sandy loams;     Less than 1 x 10-\3\               2
 loamy sands; sandy silts;
 sediments that are predominantly
 sand; highly permeable till
 (coarse-grained, unconsolidated
 or compact and highly fractured);
 peat; moderately permeable
 limestones and dolomites (no
 karst); moderately permeable
 sandstone; moderately permeable
 fractured igneous and metamorphic
 rocks.
Silt; loams; silty loams; loesses;  Less than 1 x 10-\5\               3
 silty clays; sediments that are
 predominantly silts; moderately
 permeable till (fine-grained,
 unconsolidated till, or compact
 till with some fractures); low
 permeability limestones and
 dolomites (no karst); low
 permeability sandstone; low
 permeability fractured igneous
 and metamorphic rocks.
Clay; low permeability till         Less than 1 x 10-\7\               4
 (compact unfractured till);
 shale; unfractured metamorphic
 and igneous rocks.
------------------------------------------------------------------------

                                                     Table 5-15--Vertical Migration Factor Values a
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                 Thickness of lowest porosity layer(s) \b\ (feet)
                                                         -----------------------------------------------------------------------------------------------
          Assigned porosity/permeability value                            Greater than 5   Greater than    Greater than    Greater than    Greater than
                                                              0 to 5           to 10         10 to 20        20 to 50        50 to 100      100 to 150
--------------------------------------------------------------------------------------------------------------------------------------------------------
1.......................................................              15              15              14              11               8               6
2.......................................................              15              14              12               9               6               4
3.......................................................              15              13              10               7               5               2
4.......................................................              15              12               9               6               3               1
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ If depth to contamination is 10 feet or less or if, for the interval being evaluated, all layers that underlie a portion of the structure at the
  site are karst or have other channelized flow features, assign a value of 15.
\b\ Consider only layers at least 1 foot thick.

    5.2.1.1.2.4 Vapor migration potential. Evaluate this factor for 
each area of subsurface contamination as follows:
     If the depth to contamination (see section 5.2.1.1.2.2) 
is 10 feet or less, assign a value of 25.
     Assign a value for vapor migration potential to each of 
the gaseous hazardous substances associated with the area of 
subsurface contamination (see section 2.2.2) as follows:
--Assign values from Table 5-16 for both vapor pressure and Henry's 
constant to each hazardous substance. If Henry's constant cannot be 
determined for a hazardous substance, assign that hazardous 
substance a value of 2 for the Henry's constant component.
--Sum the two values assigned to each hazardous substance.
--Based on this sum, assign each hazardous substance a value from 
Table 5-17 for vapor migration potential.
     Assign a value for vapor migration potential to each 
area of subsurface contamination as follows:
--Select the hazardous substance associated with the area of 
subsurface contamination

[[Page 2798]]

with the highest vapor migration potential value and assign this 
value as the vapor migration potential factor value for the area of 
subsurface contamination.
    Enter this value in Table 5-11.

       Table 5-16--Values for Vapor Pressure and Henry's Constant
------------------------------------------------------------------------
                  Vapor pressure (torr)                   Assigned value
------------------------------------------------------------------------
Greater than 10.........................................               3
1 to 10.................................................               2
Less than 1.............................................               0
------------------------------------------------------------------------
Henry's constant                                                Assigned
(atm-m\3\/mol)                                                     value
------------------------------------------------------------------------
Greater than 10-\3\.....................................               3
Greater than 10\4\ to 10-\3\............................               2
10-\5\ to 10-\4\........................................               1
Less than 10-\5\........................................               0
------------------------------------------------------------------------

   Table 5-17--Vapor Migration Potential Factor Values for a Hazardous
                                Substance
------------------------------------------------------------------------
  Sum of values for vapor pressure and Henry's constant   Assigned value
------------------------------------------------------------------------
0.......................................................               0
1 or 2..................................................               5
3 or 4..................................................              15
5 or 6..................................................              25
------------------------------------------------------------------------

    5.2.1.1.2.5 Calculation of potential for exposure factor value. 
For each identified area of subsurface contamination, sum the factor 
values for depth to contamination, vertical migration, and vapor 
migration potential, and multiply this sum by the factor value for 
structure containment. Select the highest product for any area of 
subsurface contamination and assign this value as the potential for 
exposure factor value for the component. Enter this value in Table 
5-11.
    5.2.1.1.3 Calculation of likelihood of exposure factor category 
value. If observed exposure is established for the site, assign the 
observed exposure factor value of 550 as the likelihood of exposure 
factor category value for the site. Otherwise, assign the potential 
for exposure factor value for the component as the likelihood of 
exposure value. Enter the value assigned in Table 5-11.
    5.2.1.2 Waste characteristics. Evaluate waste characteristics 
based on two factors: toxicity/degradation and hazardous waste 
quantity.
    5.2.1.2.1 Toxicity/degradation. For each hazardous substance, 
assign a toxicity factor value, a degradation factor value and a 
combined toxicity/degradation factor value as specified in sections 
2.2.3, 2.4.1.2 and 5.2.1.2.1.1 through 5.2.1.2.1.3.
    5.2.1.2.1.1 Toxicity. Assign a toxicity factor value to each 
hazardous substance as specified in sections 2.2.2 and 2.4.1.1.
    5.2.1.2.1.2 Degradation. Assign a degradation factor value to 
each hazardous substance as follows:
     For any hazardous substance that meets the criteria for 
an observed exposure, or if a NAPL is present in the subsurface 
below an area of observed exposure or area of subsurface 
contamination at a depth less than or equal to 30 feet, assign that 
substance a degradation factor value of 1.
     For all other situations, assign a degradation factor 
value using Table 5-18. Assign the depth to contamination as 
directed in section 5.2.1.1.2.2, except if evidence indicates that 
biologically active soil is not present throughout the depth beneath 
any regularly occupied structure. In this situation, subtract any 
thickness of non-biologically active soil from the estimated depth 
to contamination.

                                   Table 5-18--Degradation Factor Value Table
----------------------------------------------------------------------------------------------------------------
                                                                                Half-life
                                                        --------------------------------------------------------
           Depth to contamination (feet) \a\                                   >30 days and
                                                             >100 Days          <=100 days         <=30 days
----------------------------------------------------------------------------------------------------------------
<10....................................................                  1                  1                  1
10 to <=30.............................................                  1                  1                0.1
>30....................................................                  1                0.5                0.1
----------------------------------------------------------------------------------------------------------------
\a\ When determining the depth to contamination do not include layers of non-biologically-active soil, nor
  subsurface intervals with channelized flow (e.g., karst, lava tubes, open fractures, and manmade preferential
  pathways as directed in section 5.2.1.1.2.2).

Calculate the half-life for each hazardous substance that meets 
subsurface intrusion observed release criteria as follows:
    The half-life of a substance in the subsurface is defined for 
HRS purposes as the time required to reduce the initial 
concentration of the substance in the subsurface by one-half as a 
result of the combined decay processes of two components: 
Biodegradation and hydrolysis.
    Estimate the half-life (t1/2) of a hazardous 
substance as follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.072

Where:

h=Hydrolysis half-life.
b=Biodegradation half-life.

    If either of these component half-lives cannot be estimated for 
the hazardous substance from available data, delete that component 
half-life from the above equation.
    If no half-life information is available for a hazardous 
substance and the substance is not already assigned a value of 1, 
unless information indicates otherwise, assign a value of 1.
    5.2.1.2.1.3 Calculation of toxicity/degradation factor value. 
Assign each substance a toxicity/degradation value by multiplying 
the toxicity factor value by the degradation factor value. Use the 
hazardous substance with the highest combined toxicity/degradation 
value to assign the factor value to the toxicity/degradation factor 
for the subsurface intrusion threat. Enter this value in Table 5-11.
    5.2.1.2.2 Hazardous waste quantity. Assign a hazardous waste 
quantity factor value as specified in section 2.4.2. Consider only 
those regularly occupied structures or subunits with a non-zero 
structure containment value. Also include all regularly occupied 
structures or subunits that have had mitigation systems installed as 
part of a removal or other temporary response action. If sufficient 
structure-specific concentration data is available and state of the 
science shows there is no unacceptable risk of exposure to 
populations in a regularly occupied structure or subunit in an area 
of subsurface contamination, that structure or subunit is not 
included in the hazardous waste quantity evaluation. In estimating 
the hazardous waste quantity, use Tables 2-5 and 5-19 and:
     For Tier A, hazardous constituent quantity, use the 
mass of constituents found in the regularly occupied structure(s) 
where the observed exposure has been identified.
--For multi-subunit structures, when calculating Tier A, use the 
mass of constituents found in the regularly occupied subunit 
space(s) where the observed exposure has been identified.
     For Tier B, hazardous wastestream quantity, use the 
flow-through volume of the regularly occupied structures where the 
observed exposure has been identified.

--For multi-subunit structures, when calculating Tier B, use the 
flow-through volume of the regularly occupied subunit spaces where 
the observed exposure has been identified.

     For Tier C, volume, use the volume divisor listed in 
Tier C of Table 5-19. Volume is calculated for those regularly 
occupied structures located within areas of observed exposure with 
observed or inferred

[[Page 2799]]

intrusion and within areas of subsurface contamination.

--In evaluating the volume measure for these listed areas of 
observed exposure and areas of subsurface contamination based on a 
gaseous/vapor intrusion or the potential for gaseous/vapor 
intrusion, consider the following:
    [ssquf] Calculate the volume of each regularly occupied 
structure based on actual data. If unknown, use a ceiling height of 
8 feet.
    [ssquf] For multi-subunit structures, when calculating Tier C, 
calculate volume for those subunit spaces with observed or inferred 
exposure and all other regularly occupied subunit spaces on that 
level, unless available information indicates otherwise. If the 
structure has multiple stories, also include the volume of all 
regularly occupied subunit spaces below the floor with an observed 
exposure and one story above, unless evidence indicates otherwise.
    [ssquf] For multi-subunit structures within an area of 
subsurface contamination and no observed or inferred exposure, 
consider only the volume of the regularly occupied subunit spaces on 
the lowest story, unless available information indicates otherwise.

     For Tier D, area, if volume is unknown, use the area 
divisor listed in Tier D of Table 5-19 for those regularly occupied 
structures within areas of observed exposure with observed or 
inferred intrusion and within areas of subsurface contamination.

--In evaluating the area measure for these listed areas of observed 
exposure and areas of subsurface contamination, calculate the area 
of each regularly occupied structure (including multi-subunit 
structures) or subunit based on actual footprint area data.
    [ssquf] If the actual footprint area of the structure(s) is 
unknown, use an area of 1,740 square feet for each structure (or 
subunit space).
    [ssquf] For multi-subunit structures, when calculating Tier D, 
calculate area for those subunit spaces with observed or inferred 
exposure and all other regularly occupied subunit spaces on that 
level, unless available information indicates otherwise. If the 
structure has multiple stories, also include the area of all 
regularly occupied subunit spaces below the floor with an observed 
exposure and one story above, unless evidence indicates otherwise.
    [ssquf] For multi-subunit structures within an area of 
subsurface contamination and no observed or inferred exposure, 
consider only the area of the regularly occupied subunit spaces on 
the lowest story, unless available information indicates otherwise.

          Table 5-19--Hazardous Waste Quantity Evaluation Equations for Subsurface Intrusion Component
----------------------------------------------------------------------------------------------------------------
                                                                                        Equation for assigning
           Tier                        Measure                       Units                     value \a\
----------------------------------------------------------------------------------------------------------------
A.........................  Hazardous Constituent         Lb........................  C
                             Quantity (C).
B\b\......................  Hazardous Wastestream         Lb........................  W/5,000
                             Quantity (W).
C\b,c\....................  Volume (V)..................
                            Regularly occupied            yd\3\.....................  V/2.5
                             structure(s) in areas of
                             observed exposure or
                             subsurface contamination.
D\b,d\....................  Area (A)....................
                            Regularly occupied            ft\2\.....................  A/13
                             structure(s) in areas of
                             observed exposure or
                             subsurface contamination.
----------------------------------------------------------------------------------------------------------------
\a\ Do not round to the nearest integer.
\b\ Convert volume to mass when necessary: 1 ton=2,000 pounds=1 cubic yard=4 drums=200 gallons.
\c\ Calculate volume of each regularly occupied structure or subunit space in areas of observed exposure and
  areas of subsurface contamination--Assume 8-foot ceiling height unless actual value is known.
\d\ Calculate area of the footprint of each regularly occupied structure in areas of observed exposure and areas
  of subsurface contamination. If the footprint area of a regularly occupied structure is unknown, use 1,740
  square feet as the footprint area of the structure or subunit space.

    For the subsurface intrusion component, if the hazardous 
constituent quantity is adequately determined for all areas of 
observed exposure, assign the value from Table 2-6 as the hazardous 
waste quantity factor value. If the hazardous constituent quantity 
is not adequately determined for one or more areas of observed 
exposure or if one or more areas of subsurface contamination are 
present, assign either the value from Table 2-6 or assign a factor 
value as follows:
     If any target for the subsurface intrusion component is 
subject to Level I or Level II concentrations (see section 2.5), 
assign either the value from Table 2-6 or a value of 100, whichever 
is greater, as the hazardous waste quantity factor value for this 
component.
     If none of the targets for the subsurface intrusion 
component is subject to Level I or Level II concentrations and if 
there has been a removal action that does not permanently interrupt 
target exposure from subsurface intrusion, and if an area of 
subsurface contamination exists, assign a factor value as follows:

--Determine the values from Table 2-6 with and without consideration 
of the removal action.
--If the value that would be assigned from Table 2-6 without 
consideration of the removal action would be 100 or greater, assign 
either the value from Table 2-6 with consideration of the removal 
action or a value of 100, whichever is greater, as the hazardous 
waste quantity factor value for the component.
--If the value that would be assigned from Table 2-6 without 
consideration of the removal action would be less than 100, assign a 
value of 10 as the hazardous waste quantity factor value for the 
component.

     Otherwise, if none of the targets for the subsurface 
intrusion component is subject to Level I or Level II concentrations 
and there has not been a removal action, assign a value from Table 
2-6 or a value of 10, whichever is greater.
    Enter the value assigned in Table 5-11.
    5.2.1.2.3 Calculation of waste characteristics factor category 
value. Multiply the toxicity/degradation and hazardous waste 
quantity factor values, subject to a maximum product of 1 x 10\8\. 
Based on this product, assign a value from Table 2-7 (section 
2.4.3.1) to the waste characteristics factor category. Enter this 
value in Table 5-11.
    5.2.1.3 Targets. Evaluate the targets factor category for the 
subsurface intrusion threat based on three factors: Exposed 
individual, population, and resources in regularly occupied 
structures with structure containment factors greater than 0. 
Evaluate only those targets within areas of observed exposure and 
areas of subsurface contamination (see section 5.2.0).
    In evaluating the targets factor category for the subsurface 
intrusion threat, count only the following as targets:
     Exposed individual--a person living, attending school 
or day care, or working in a regularly occupied structure with 
observed exposure or in a structure within an area of observed 
exposure or within an area of subsurface contamination.
     Population--exposed individuals in a regularly occupied 
structure within an area of observed exposure or within an area of 
subsurface contamination.
     Resources--located within an area of observed exposure 
or within an area of subsurface contamination as specified in 
section 5.2.1.3.3.
    If a formerly occupied structure has been vacated due to 
subsurface intrusion attributable to the site, count the initial 
targets as if they were still residing in the structure. In 
addition, if a removal or temporary response action has occurred 
that has not completely mitigated the release, count the initial 
targets as if the removal or temporary response action has not 
permanently interrupted target exposure from subsurface intrusion. 
Evaluate those targets based on conditions at the time of removal of 
temporary response action.

[[Page 2800]]

    For populations residing in or working in a multi-subunit 
structure with multiple stories in an area of observed exposure or 
area of subsurface contamination, count these targets as follows:
     If there is no observed exposure within the structure, 
include in the evaluation only those targets, if any, in the lowest 
occupied level. If sufficient structure-specific concentration data 
is available and state of the science shows there is no unacceptable 
risk of exposure to targets in the lowest level, those targets are 
not included in the evaluation.
     If there is an observed exposure in any level, include 
in the evaluation those targets in that level, the level above and 
all levels below. (The weighting of these targets is specified in 
Section 5.2.1.3.2.) If sufficient structure-specific concentration 
data is available and state of the science shows there is no 
unacceptable risk of exposure to targets in the level above where 
the observed exposure has been documented, those targets are not 
included in the evaluation.
    5.2.1.3.1 Exposed individual. Evaluate this factor based on 
whether there is an exposed individual, as specified in sections 
2.5.1, 2.5.2 and 5.2.1.3, who is subject to Level I or Level II 
concentrations.
    First, determine those regularly occupied structures or 
partitioned subunit(s) within structures in an area of observed 
exposure subject to Level I concentrations and those subject to 
Level II concentrations as specified as follows (see section 5.2.0):
     Level I Concentrations: For contamination resulting 
from subsurface intrusion, compare the hazardous substance 
concentrations in any sample meeting the observed exposure by 
chemical analysis criteria to the appropriate benchmark. Use the 
health-based benchmarks from Table 5-20 to determine the level of 
contamination.

--If the sample is from a structure with no subunits and the 
concentration equals or exceeds the appropriate benchmark, assign 
Level I concentrations to the entire structure.
--If the sample is from a subunit within a structure and the 
concentration from that subunit equals or exceeds the appropriate 
benchmark, assign Level I concentrations to that subunit.

     Level II Concentrations: Structures, or subunits within 
structures, with one or more samples that meet observed exposure by 
chemical analysis criteria but do not equal or exceed the 
appropriate benchmark; structures, or subunits, that have an 
observed exposure by direct observation; and structures inferred to 
be in an area of observed exposure based on samples meeting observed 
exposure, are assigned Level II concentrations.

--For all regularly occupied structures, or subunits in such 
structures, in an area of observed exposure that are not assigned 
Level I concentrations, assign Level II concentrations.

    Then assign a value to the exposed individual factor as follows:
     Assign a value of 50 if there is at least one exposed 
individual in one or more regularly occupied structures subject to 
Level I concentrations.
     Assign a value of 45 if there are no Level I exposed 
individuals, but there is at least one exposed individual in one or 
more regularly occupied structures subject to Level II 
concentrations.
     Assign a value of 20 if there is no Level I or Level II 
exposed individual but there is at least one individual in a 
regularly occupied structure within an area of subsurface 
contamination. Enter the value assigned in Table 5-11.
    5.2.1.3.2 Population. Evaluate population based on three 
factors: Level I concentrations, Level II concentrations, and 
population within an area of subsurface contamination. Determine 
which factors apply as specified in section 5.2.1.3.1, using the 
health-based benchmarks from Table 5-20. Evaluate populations 
subject to Level I and Level II concentrations as specified in 
section 2.5.

   Table 5-20--Health-Based Benchmarks for Hazardous Substances in the
                     Subsurface Intrusion Component
------------------------------------------------------------------------
 
-------------------------------------------------------------------------
Screening concentration for cancer corresponding to that concentration
 that corresponds to the 10-6 individual cancer risk using the
 inhalation unit risk. For oral exposures use the oral cancer slope
 factor.
Screening concentration for noncancer toxicological responses
 corresponding to the reference dose (RfD) for oral exposure and the
 reference concentration (RfC) for inhalation exposures.
------------------------------------------------------------------------

    Count only those persons meeting the criteria for population as 
specified in section 5.2.1.3. In estimating the number of 
individuals in structures in an area of observed exposure or area of 
subsurface contamination if the actual number of residents is not 
known, multiply each residence by the average number of persons per 
residence for the county in which the residence is located.
    5.2.1.3.2.1 Level I concentrations. Assign the population 
subject to Level I concentrations as follows:
    1. Identify all exposed individuals regularly present in an 
eligible structure with a structure containment value greater than 
zero, or if the structure has subunits, identify those regularly 
present in each subunit, located in an area of observed exposure 
subject to Level I concentrations as described in sections 5.2.0 and 
5.2.1.3.1. Identify only once per structure those exposed 
individuals that are using more than one eligible subunit of the 
same structure (e.g., using a common or shared area and other parts 
of the same structure).
    2. For each structure or subunit count the number of individuals 
residing in or attending school or day care in the structure or 
subunit.
    3. Count the number of full-time and part-time workers in the 
structure or subunit(s) subject to Level I concentrations. If 
information is unavailable to classify a worker as full- or part-
time, evaluate that worker as being full-time. Divide the number of 
full-time workers by 3 and the number of part-time workers by 6, and 
then sum these products with the number of other individuals for 
each structure or subunit.
    4. Sum this combined value for all structures, or subunits, 
within areas of observed exposure and multiply this sum by 10.
    Assign the resulting product as the combined population factor 
value subject to Level I concentrations for the site. Enter this 
value in line 9a of Table 5-11.
    5.2.1.3.2.2 Level II concentrations. Assign the population 
subject to Level II concentrations as follows:
    1. Identify all exposed individuals regularly present in an 
eligible structure with a structure containment value greater than 
zero, or if the structure has subunits, identify those regularly 
present in each subunit, located in an area of observed exposure 
subject to Level II concentrations as described in sections 5.2.0 
and 5.2.1.3.1. Identify only once per structure those exposed 
individuals that are using more than one eligible subunit of the 
same structure (e.g., using a common or shared area and other parts 
of the same structure).
    2. Do not include exposed individuals already counted under the 
Level I concentrations factor.
    3. For each structure or subunit(s), count the number of 
individuals residing in or attending school or day care in the 
structure, or subunit, subject to Level II concentrations.
    4. Count the number of full-time and part-time workers in the 
structure or subunit(s) subject to Level II concentrations. If 
information is unavailable to classify a worker as full- or part-
time, evaluate that worker as being full-time. Divide the number of 
full-time workers by 3 and the number of part-time workers by 6, and 
then sum these products with the number of other individuals for 
each structure or subunit.
    5. Sum the combined population value for all structures within 
the areas of observed exposure for the site.
    Assign this sum as the combined population factor value subject 
to Level II concentrations for this site. Enter this value in line 
9b of Table 5-11.
    5.2.1.3.2.3 Population within area(s) of subsurface 
contamination. Assign the population in area(s) of subsurface 
contamination factor value as follows. If sufficient structure-
specific concentration data is available and state of the science 
shows there is no unacceptable risk of exposure to populations in a 
regularly occupied structure in an area of subsurface contamination, 
those populations are not included in the evaluation. (see sections 
5.2.0 and 5.2.1.3.1):
    1. Identify the regularly occupied structures with a structure 
containment value greater than zero and the eligible population 
associated with the structures or portions of structures in each 
area of subsurface contamination:
     For each regularly occupied structure or portion of a 
structure in an area of subsurface contamination, sum the number of 
all individuals residing in or attending school or day care, in the 
structure or portion of the structure in the area of subsurface 
contamination.
     Count the number of full-time and part-time workers 
regularly present in each

[[Page 2801]]

structure or portion of a structure in an area of subsurface 
contamination. If information is unavailable to classify a worker as 
full- or part-time, evaluate that worker as being full-time. Divide 
the number of full-time workers by 3 and the number of part-time 
workers by 6. Sum these products with the number of individuals 
residing in or attending school or day care in the structure.
     Use this sum as the population for the structure.
    2. Estimate the depth or distance to contamination at each 
regularly occupied structure within an area of subsurface 
contamination based on available sampling data, and categorize each 
eligible structure based on the depth or distance to contamination 
and sample media as presented in Table 5-21. Weight the population 
in each structure using the appropriate weighting factors in Table 
5-21. If samples from multiple media are available, use the sample 
that results in the highest weighting factor.
    3. Sum the weighted population in all structures within the 
area(s) of subsurface contamination and assign this sum as the 
population within an area of subsurface contamination factor value. 
Enter this value in line 9c of Table 5-11.

  Table 5-21--Weighting Factor Values for Populations Within an Area of
                        Subsurface Contamination
------------------------------------------------------------------------
                                                            Population
  Eligible populations \a\ in structures \b\ within an       weighting
            area of subsurface contamination                  factor
------------------------------------------------------------------------
            Samples From Within Structures or in Crawl Spaces
------------------------------------------------------------------------
1. Population in a structure with levels of                          0.9
 contamination in a semi-enclosed or enclosed crawl
 space sample meeting observed release criteria or
Population in a subunit of a multi-story structure
 within an area of subsurface contamination located
 directly above a level in an area of observed exposure
 or a gaseous indoor air sample meeting observed release
 criteria or
Population within a structure where a mitigation system
 has been installed as part of a removal or other
 temporary response action.
2. Population in a structure where levels of                         0.8
 contaminants meeting observed release criteria are
 inferred based on semi-enclosed or enclosed crawl space
 samples in surrounding structures, and a NAPL is
 present in those samples...............................
3. Population in a structure where levels of                         0.4
 contaminants meeting observed release criteria are
 inferred based on semi-enclosed or enclosed crawl space
 samples in surrounding structures, but no NAPL is
 present................................................
------------------------------------------------------------------------
 Subsurface Samples From Less Than or Equal to 5 Feet From a Foundation
------------------------------------------------------------------------
4. Population in a structure where levels of                         0.8
 contaminants meeting observed release criteria are
 found or inferred based on any sampling media at or
 within five feet horizontally or vertically of the
 structure foundation, and a NAPL is present within that
 depth..................................................
5. Population in a structure where levels of                         0.4
 contaminants meeting observed release criteria are
 found or inferred based on any sampling media at or
 within five feet horizontally or vertically of the
 structure foundation, but no NAPL is present within
 that depth.............................................
------------------------------------------------------------------------
Subsurface Samples From Greater Than 5 Feet But Less Than or Equal to 30
                               Feet Depth
------------------------------------------------------------------------
6. Population in a structure where levels of                         0.4
 contaminants meeting observed release criteria are
 found or inferred based on any underlying non-ground
 water subsurface sample at a depth greater than 5 feet
 but less than or equal to 30 feet from a structure
 foundation and a NAPL is present within that depth.....
7. Population in a structure where levels of                         0.2
 contaminants meeting observed release criteria are
 found or inferred based on any underlying non-ground
 water subsurface sample at a depth greater than 5 feet
 but less than or equal to 30 feet, but no NAPL is
 present within that depth..............................
8. Population in a structure where levels of                         0.2
 contaminants meeting observed release criteria are
 found or inferred based on underlying ground water
 samples greater than 5 feet from the structure
 foundation but less than or equal to 30 feet, and a
 NAPL is present in those samples.......................
9. Population in a structure where levels of                         0.1
 contaminants meeting observed release criteria are
 found or inferred based on underlying ground water
 samples greater than 5 feet from the structure
 foundation but less than or equal to 30 feet, but no
 NAPL is present in those samples.......................
------------------------------------------------------------------------
           Subsurface Samples From Greater Than 30 Feet Depth
------------------------------------------------------------------------
10. Population in a structure where levels of                        0.1
 contaminants meeting observed release criteria are
 found or inferred based on any underlying sample at
 depths greater than 30 feet............................
------------------------------------------------------------------------
\a\ Eligible populations include residents (including individuals living
  in, or attending school or day care in the structure), and workers in
  regularly occupied structures (see HRS Section 5.2.1.3).
\b\ Eligible structures may include single- or multi-tenant structures
  where eligible populations reside, attend school or day care, or work.
  These structures may also be mixed use structures.

    5.2.1.3.2.4 Calculation of population factor value. Sum the 
factor values for Level I concentrations, Level II concentrations, 
and population within the area(s) of subsurface contamination. 
Assign this sum as the population factor value. Enter this value in 
line 9d of Table 5-11.
    5.2.1.3.3 Resources. Evaluate the resources factor as follows:
     Assign a value of 5 if a resource structure (e.g., 
library, church, tribal facility) is present and regularly occupied 
within either an area of observed exposure or area of subsurface 
contamination.
     Assign a value of 0 if there is no resource structure 
within an area of observed exposure or area of subsurface 
contamination.
    Enter the value assigned in Table 5-11.
    5.2.1.3.4 Calculation of targets factor category value. Sum the 
values for the exposed individual, population, and resources 
factors. Do not round to the nearest integer. Assign this sum as the 
targets factor category value for the subsurface intrusion 
component. Enter this value in Table 5-11.
    5.2.2 Calculation of subsurface intrusion component score. 
Multiply the factor category values for likelihood of exposure, 
waste characteristics, and targets and round the product to the 
nearest integer. Divide the product by 82,500. Assign the resulting 
value, subject to a maximum of 100, as the

[[Page 2802]]

subsurface intrusion component score and enter this score in Table 
5-11.
    5.3 Calculation of the soil exposure and subsurface intrusion 
pathway score. Sum the soil exposure component score and subsurface 
intrusion component score. Assign the resulting value, subject to a 
maximum of 100, as the soil exposure and subsurface intrusion 
pathway score (Ssessi). Enter this score in Table 5-11.

6.0 Air Migration Pathway

* * * * *

   Table 6-14--Health-based Benchmarks for Hazardous Substances in Air
------------------------------------------------------------------------
 
-------------------------------------------------------------------------
 Concentration corresponding to National Ambient Air Quality
 Standard (NAAQS).
 Concentration corresponding to National Emission Standards for
 Hazardous Air Pollutants (NESHAPs).
 Screening concentration for cancer corresponding to that
 concentration that corresponds to the 10-\6\ individual cancer risk for
 inhalation exposures.
 Screening concentration for noncancer toxicological responses
 corresponding to the Reference Concentration (RfC) for inhalation
 exposures.
------------------------------------------------------------------------

* * * * *

7.0 Sites Containing Radioactive Substances

* * * * *

                                                                 Table 7-1--HRS Factors Evaluated Differently for Radionuclides
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                                             Subsurface
                                                    Surface water                       Soil exposure                         intrusion
     Ground water pathway          Status \a\          pathway         Status \a\       component of       Status \a\       component of       Status \a\        Air pathway       Status \a\
                                                                                        SESSI pathway                       SESSI pathway
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Likelihood of Release.........                    Likelihood of                       Likelihood of                       Likelihood of                       Likelihood of
                                                   Release.                            Exposure.                           Exposure.                           Release.
Observed Release..............  Yes.............  Observed Release  Yes.............  Observed          Yes.............  Observed          Yes.............  Observed Release  Yes.
                                                                                       Contamination.                      Exposure.
Potential to Release..........  No..............  Potential to      No..............  Attractiveness/   No..............  Potential for     Yes.............  Gas Potential to  No.
                                                   Release.                            Accessibility                       Exposure.                           Release.
                                                                                       to Nearby
                                                                                       Residents.
Containment...................  No..............  Overland Flow     No..............  Area of           No..............  Structure         No..............  Gas Containment.  No.
                                                   Containment.                        Contamination.                      Containment.
Net Precipitation.............  No..............  Runoff..........  No..............  Area of Observed  No..............  Depth to          Yes.............  Gas Source Type.  No.
                                                                                       Exposure.                           Contamination.
Depth to Aquifer..............  No..............  Distance to       No..............  Area of           No..............  Vertical          No..............  Gas Migration     No.
                                                   Surface water.                      Subsurface                          migration.                          Potential.
                                                                                       Contamination.
Travel Time...................  No..............  Flood Frequency.  No..............                                      Vapor Migration   No..............  Particulate       No.
                                                                                                                           Potential.                          Potential to
                                                                                                                                                               Release.
                                                  Flood             No..............                                                                          Particulate       No.
                                                   Containment.                                                                                                Containment.
                                                                                                                                                              Particulate       No.
                                                                                                                                                               Source Type.
                                                                                                                                                              Particulate       No.
                                                                                                                                                               Migration
                                                                                                                                                               Potential.
Waste Characteristics.........                    Waste                               Waste                               Waste                               Waste
                                                   Characteristics.                    Characteristics.                    Characteristics.                    Characteristics.
Toxicity......................  Yes.............  Toxicity/         Yes/Yes.........  Toxicity........  Yes.............  Toxicity/         Yes/Yes.........  Toxicity........  Yes.
                                                   Ecotoxicity.                                                            Degradation.
Mobility......................  No..............  Persistence/....  Yes/No..........  Hazardous Waste   Yes.............  Hazardous Waste   Yes.............  Mobility........  No.
                                                  Mobility........                     Quantity.                           Quantity.
Hazardous Waste Quantity......  Yes.............  Bioaccumu-lation  No..............                                                                          Hazardous Waste   Yes.
                                                   Potential.                                                                                                  Quantity.
                                                  Hazardous Waste   Yes.............
                                                   Quantity.
Targets.......................                    Targets.........                    Targets.........                    Targets.........                    Targets.........
Nearest Well..................  Yes.\b\.........  Nearest Intake..  Yes.\b\.........  Resident          Yes.\b\.........  Exposed.........  Yes.\b\.........  Nearest           Yes.\b\
                                                                                       Individual.                        Individual......                     Individual.

[[Page 2803]]

 
Population....................  Yes.\b\.........  Drinking Water    Yes.\b\.........  Resident          Yes.\b\.........  Population......  Yes.\b\.........  Population......  Yes.\b\
                                                   Population.                         Population.
Resources.....................  No..............  Resources.......  No..............  Workers.........  No..............  Resources.......  No..............  Resources.......  No
Wellhead Protection Area......  No..............  Sensitive         Yes.\b\.........  Resources.......  No..............                                      Sensitive         No
                                                   Environments.                                                                                               Environments.
                                                  Human Food Chain  Yes.\b\.........  Terrestrial       No..............
                                                   Individual.                         Sensitive
                                                                                       Environments.
                                                  Human Food Chain  Yes.\b\.........  Nearby            No..............
                                                   Population.                         Individual.
                                                                                      Population        No..............
                                                                                       Within 1 Mile.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ Factors evaluated differently are denoted by ``yes''; factors not evaluated differently are denoted by ``no''.
\b\ Difference is in the determination of Level I and Level II concentrations.

* * * * *
    * * * These differences apply largely to the soil exposure and 
subsurface intrusion pathway and to sites containing mixed 
radioactive and other hazardous substances. * * *
    7.1 Likelihood of release/likelihood of exposure. Evaluate 
likelihood of release for the three migration pathways and 
likelihood of exposure for the soil exposure and subsurface 
intrusion pathway as specified in sections 2 through 6, except: 
establish an observed release, observed contamination, and/or 
observed exposure as specified in section 7.1.1. When an observed 
release or exposure cannot be established for a migration pathway or 
the subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway, evaluate potential to release as 
specified in section 7.1.2. When observed contamination cannot be 
established, do not evaluate the soil exposure component of the soil 
exposure and subsurface intrusion pathway.
    7.1.1 Observed release/observed contamination/observed exposure. 
For radioactive substances, establish an observed release for each 
migration pathway by demonstrating that the site has released a 
radioactive substance to the pathway (or watershed or aquifer, as 
appropriate); establish observed contamination or observed exposure 
for the soil exposure and subsurface intrusion pathway as indicated 
below. Base these demonstrations on one or more of the following, as 
appropriate to the pathway being evaluated:
     Direct observation:

--For each migration pathway, a material that contains one or more 
radionuclides has been seen entering the atmosphere, surface water, 
or ground water, as appropriate, or is known to have entered ground 
water or surface water through direct deposition, or
--For the surface water migration pathway, a source area containing 
radioactive substances has been flooded at a time that radioactive 
substances were present and one or more radioactive substances were 
in contact with the flood waters.
--For the subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway, a material that contains one or more 
radionuclides has been observed entering a regularly occupied 
structure via the subsurface or is known to have entered a regularly 
occupied structure via the subsurface. Also, when evidence supports 
the inference of subsurface intrusion of a material that contains 
one or more radionuclides by the site into a regularly occupied 
structure, demonstrated adverse effects associated with that release 
may also be used to establish observed exposure by direct 
observation.

     Analysis of radionuclide concentrations in samples 
appropriate to the pathway (that is, ground water, soil, air, indoor 
air, soil gas, surface water, benthic, or sediment samples):

--For radionuclides that occur naturally and for radionuclides that 
are ubiquitous in the environment:
    [ssquf] Measured concentration (in units of activity, for 
example, pCi per kilogram [pCi/kg], pCi per liter [pCi/L], pCi per 
cubic meter [pCi/m3]) of a given radionuclide in the sample are at a 
level that:
    [cir] Equals or exceeds a value 2 standard deviations above the 
mean site-specific background concentration for that radionuclide in 
that type of sample, or
    [cir] Exceeds the upper-limit value of the range of regional 
background concentration values for that specific radionuclide in 
that type of sample.
    [ssquf] Some portion of the increase must be attributable to the 
site to establish the observed release (or observed contamination or 
observed exposure), and
    [ssquf] For the soil exposure component of the soil exposure and 
subsurface intrusion pathway only, the radionuclide must also be 
present at the surface or covered by 2 feet or less of cover 
material (for example, soil) to establish observed contamination.
--For man-made radionuclides without ubiquitous background 
concentrations in the environment:
    [ssquf] Measured concentration (in units of activity) of a given 
radionuclide in a sample equals or exceeds the sample quantitation 
limit for that specific radionuclide in that type of media and is 
attributable to the site.
    [ssquf] However, if the radionuclide concentration equals or 
exceeds its sample quantitation limit, but its release can also be 
attributed to one or more neighboring sites, then the measured 
concentration of that radionuclide must also equal or exceed a value 
either 2 standard deviations above the mean concentration of that 
radionuclide contributed by those neighboring sites or 3 times its 
background concentration, whichever is lower.
    [ssquf] If the sample quantitation limit cannot be established:
    [cir] If the sample analysis was performed under the EPA 
Contract Laboratory Program, use the EPA contract-required 
quantitation limit (CRQL) in place of the sample quantitation limit 
in establishing an observed release (or observed contamination or 
observed exposure).
    [cir] If the sample analysis is not performed under the EPA 
Contract Laboratory Program, use the detection limit in place of the 
sample quantitation limit.
    [ssquf] For the soil exposure component of the soil exposure and 
subsurface intrusion pathway only, the radionuclide must also be 
present at the surface or covered by 2 feet or less of cover 
material (for example, soil) to establish observed contamination.
     Gamma radiation measurements (applies only to observed 
contamination or observed exposure in the soil exposure and 
subsurface intrusion pathway):

[[Page 2804]]

--The gamma radiation exposure rate, as measured in microroentgens 
per hour ([mu]R/hr) using a survey instrument held 1 meter above the 
ground surface or floor or walls of a structure (or 1 meter away 
from an aboveground source for the soil exposure component), equals 
or exceeds 2 times the site-specific background gamma radiation 
exposure rate.
--Some portion of the increase must be attributable to the site to 
establish observed contamination or observed exposure. The gamma-
emitting radionuclides do not have to be within 2 feet of the 
surface of the source.

    For the three migration pathways and for the subsurface 
intrusion component of the soil exposure and subsurface intrusion 
pathway, if an observed release or observed exposure can be 
established for the pathway (or component, threat, aquifer, or 
watershed, as appropriate), assign the pathway (or component, 
threat, aquifer, or watershed) an observed release or observed 
exposure factor value of 550 and proceed to section 7.2. If an 
observed release or observed exposure cannot be established, assign 
an observed release or observed exposure factor value of 0 and 
proceed to section 7.1.2.
    For the soil exposure component of the soil exposure and 
subsurface intrusion pathway, if observed contamination can be 
established, assign the likelihood of exposure factor for resident 
population a value of 550 if there is an area of observed 
contamination in one or more locations listed in section 5.1.1; 
evaluate the likelihood of exposure factor for nearby population as 
specified in section 5.1.2.1; and proceed to section 7.2. If 
observed contamination cannot be established, do not evaluate the 
soil exposure component of the soil exposure and subsurface 
intrusion pathway.
    At sites containing mixed radioactive and other hazardous 
substances, evaluate observed release (or component, observed 
contamination or observed exposure) separately for radionuclides as 
described in this section and for other hazardous substances as 
described in sections 2 through 6.
    For the three migration pathways and the subsurface intrusion 
component of the soil exposure and subsurface intrusion pathway, if 
an observed release or observed exposure can be established based on 
either radionuclides or other hazardous substances, or both, assign 
the pathway (or threat, aquifer, or watershed) an observed release 
or observed exposure factor value of 550 and proceed to section 7.2. 
If an observed release or observed exposure cannot be established 
based on either radionuclides or other hazardous substances, assign 
an observed release or observed exposure factor value of 0 and 
proceed to section 7.1.2.
    For the soil exposure component of the soil exposure and 
subsurface intrusion pathway, if observed contamination can be 
established based on either radionuclides or other hazardous 
substances, or both, assign the likelihood of exposure factor for 
resident population a value of 550 if there is an area of observed 
contamination in one or more locations listed in section 5.1.1; 
evaluate the likelihood of exposure factor for nearby population as 
specified in section 5.1.2.1; and proceed to section 7.2. If 
observed contamination cannot be established based on either 
radionuclides or other hazardous substances, do not evaluate the 
soil exposure component of the soil exposure and subsurface 
intrusion pathway.
    7.1.2 Potential to release/potential for exposure. For the three 
migration pathways and the subsurface intrusion component of the 
soil exposure and subsurface intrusion pathway, evaluate potential 
to release or potential for exposure for sites containing 
radionuclides in the same manner as specified for sites containing 
other hazardous substances. Base the evaluation on the physical and 
chemical properties of the radionuclides, not on their level of 
radioactivity. For the subsurface intrusion component of the soil 
exposure and subsurface intrusion pathway, if the potential for 
exposure is based on the presence of gamma emitting radioactive 
substances, assign a potential for exposure factor value of 500 only 
if the contamination is found within 2 feet beneath a regularly 
occupied structure, otherwise assign a potential for exposure factor 
value of 0.
    For sites containing mixed radioactive and other hazardous 
substances, evaluate potential to release or potential for exposure 
considering radionuclides and other hazardous substances together. 
Evaluate potential to release for each migration pathway and the 
potential for exposure for the subsurface intrusion component of the 
soil exposure and subsurface intrusion pathway as specified in 
sections 3 through 6, as appropriate.
* * * * *
    7.2.1 Human Toxicity. For radioactive substances, evaluate the 
human toxicity factor as specified below, not as specified in 
section 2.4.1.1.
    Assign human toxicity factor values to those radionuclides 
available to the pathway based on quantitative dose-response 
parameters for cancer risks as follows:
     Evaluate radionuclides only on the basis of 
carcinogenicity and assign all radionuclides to weight-of-evidence 
category A, or weight-of-evidence category ``Carcinogenic to 
Humans''.
     Assign a human toxicity factor value from Table 7-2 to 
each radionuclide based on its slope factor (also referred to as a 
cancer potency factor).

--For each radionuclide, use the higher of the slope factors for 
inhalation and ingestion to assign the factor value.
--If only one slope factor is available for the radionuclide use it 
to assign the toxicity factor value.
--If no slope factor is available for the radionuclide, assign that 
radionuclide a toxicity factor value of 0 and use other 
radionuclides for which a slope factor is available to evaluate the 
pathway.

     If all radionuclides available to a particular pathway 
are assigned a human toxicity factor value of 0 (that is, no slope 
factor is available for all the radionuclides), use a default human 
toxicity factor value of 1,000 as the human toxicity factor value 
for all radionuclides available to the pathway.
    At sites containing mixed radioactive and other hazardous 
substances, evaluate the toxicity factor separately for the 
radioactive and other hazardous substances and assign each a 
separate toxicity factor value. This applies regardless of whether 
the radioactive and other hazardous substances are physically 
separated, combined chemically, or simply mixed together. Assign 
toxicity factor values to the radionuclides as specified above and 
to the other hazardous substances as specified in section 2.4.1.1.
    At sites containing mixed radioactive and other hazardous 
substances, if all radionuclides available to a particular pathway 
are assigned a human toxicity factor value of 0, use a default human 
toxicity factor value of 1,000 for all those radionuclides even if 
nonradioactive hazardous substances available to the pathway are 
assigned human toxicity factor values greater than 0. Similarly, if 
all nonradioactive hazardous substances available to the pathway are 
assigned a human toxicity factor value of 0, use a default human 
toxicity factor value of 100 for all these nonradioactive hazardous 
substances even if radionuclides available to the pathway are 
assigned human toxicity factor values greater than 0.
* * * * *
    7.2.3 Persistence/Degradation. In determining the surface water 
persistence factor for radionuclides, evaluate this factor based 
solely on half-life; do not include sorption to sediments in the 
evaluation as is done for nonradioactive hazardous substances. 
Assign a persistence factor value from Table 4-10 (section 
4.1.2.2.1.2) to each radionuclide based on half-life (t 
1/2) calculated as follows:
[GRAPHIC] [TIFF OMITTED] TR09JA17.073

Where:

r = Radioactive half-life.
V = Volatilization half-life.

    If the volatilization half-life cannot be estimated for a 
radionuclide from available data, delete it from the equation. 
Select the portion of Table 4-10 to use in assigning the persistence 
factor value as specified in section 4.1.2.2.1.2.
    At sites containing mixed radioactive and other hazardous 
substances, evaluate the persistence factor separately for each 
radionuclide and for each nonradioactive hazardous substance, even 
if the available data indicate that they are combined chemically. 
Assign a persistence factor value to each radionuclide as specified 
in this section and to each nonradioactive hazardous substance as 
specified in section 4.1.2.2.1.2. When combined chemically, assign a 
single persistence factor value based on the higher of the two 
values assigned (individually) to the radioactive and nonradioactive 
components.
    In determining the subsurface intrusion degradation factor for 
radionuclides, when evaluating this factor based solely on half-
life, assign a degradation factor value from section 5.2.1.2.1.2 to 
each radionuclide based on half-life (t1/2) calculated as 
follows:

[[Page 2805]]

[GRAPHIC] [TIFF OMITTED] TR09JA17.074

Where:

r=Radioactive half-life.

    If no radioactive half-life information is available for a 
radionuclide and the substance is not already assigned a value of 1, 
unless information indicates otherwise, assign a value of 1.
    At sites containing mixed radioactive and other hazardous 
substances, evaluate the degradation factor separately for each 
radionuclide and for each nonradioactive hazardous substance, even 
if the available data indicate that they are combined chemically. 
Assign a degradation factor value to each radionuclide as specified 
in this section and to each nonradioactive hazardous substance as 
specified in section 5.2.1.2.1.2. If no radioactive half-life 
information is available for a radionuclide and the substance is not 
already assigned a value of 1, unless information indicates 
otherwise, assign a value of 1. Similarly, if no half-life 
information is available for a nonradioactive substance, and the 
substance is not already assigned a value of 1, unless information 
indicates otherwise, assign a value of 1. When combined chemically, 
assign a single persistence or degradation factor value based on the 
higher of the two values assigned (individually) to the radioactive 
and nonradioactive components.
    7.2.4  Selection of substance potentially posing greatest 
hazard. For the subsurface intrusion component of the soil exposure 
and subsurface intrusion pathway and each migration pathway (or 
threat, aquifer, or watershed, as appropriate), select the 
radioactive substance or nonradioactive hazardous substance that 
potentially poses the greatest hazard based on its toxicity factor 
value, combined with the applicable mobility, persistence, 
degradation and/or bioaccumulation (or ecosystem bioaccumulation) 
potential factor values. Combine these factor values as specified in 
sections 2 through 6. For the soil exposure component of the soil 
exposure and subsurface intrusion pathway, base the selection on the 
toxicity factor alone (see sections 2 and 5).
* * * * *
    7.2.5.1 Source hazardous waste quantity for radionuclides. For 
each migration pathway, assign a source hazardous waste quantity 
value to each source having a containment factor value greater than 
0 for the pathway being evaluated. For the soil exposure component 
of the soil exposure and subsurface intrusion pathway, assign a 
source hazardous waste quantity value to each area of observed 
contamination, as applicable to the threat being evaluated. For the 
subsurface intrusion component, assign a source hazardous waste 
quantity value to each regularly occupied structure located within 
areas of observed exposure or areas of subsurface contamination. 
Allocate hazardous substances and hazardous wastestreams to specific 
sources (or areas of observed contamination, areas of observed 
exposure or areas of subsurface contamination) as specified in 
sections 2.4.2 and 5.2.0.
    7.2.5.1.1 Radionuclide constituent quantity (Tier A). Evaluate 
radionuclide constituent quantity for each source (or area of 
observed contamination or area of observed exposure) based on the 
activity content of the radionuclides allocated to the source (or 
area of observed contamination or area of observed exposure) as 
follows:
     Estimate the net activity content (in curies) for the 
source (or area of observed contamination or area of observed 
exposure) based on:
--Manifests, or
--Either of the following equations, as applicable:
[GRAPHIC] [TIFF OMITTED] TR09JA17.075

Where:

    N=Estimated net activity content (in curies) for the source (or 
area of observed contamination or area of observed exposure).
    V=Total volume of material (in cubic yards) in a source (or area 
of observed contamination or area of observed exposure) containing 
radionuclides.
    ACi=Activity concentration above the respective 
background concentration (in pCi/g) for each radionuclide i 
allocated to the source (or area of observed contamination or area 
of observed exposure).
    n=Number of radionuclides allocated to the source (or area of 
observed contamination or area of observed exposure) above the 
respective background concentrations.
or,
[GRAPHIC] [TIFF OMITTED] TR09JA17.076

Where:

N=Estimated net activity content (in curies) for the source (or area 
of observed contamination or area of observed exposure).
V=Total volume of material (in gallons) in a source (or area of 
observed contamination or area of observed exposure) containing 
radionuclides.
ACi=Activity concentration above the respective 
background concentration (in pCi/1) for each radionuclide i 
allocated to the source (or area of observed contamination or area 
of observed exposure).
n=Number of radionuclides allocated to the source (or area of 
observed contamination or area of observed exposure) above the 
respective background concentrations.

--Estimate volume for the source (or volume for the area of observed 
contamination or area of observed exposure) based on records or 
measurements.
--For the soil exposure component of the soil exposure and 
subsurface intrusion pathway, in estimating the volume for areas of 
observed contamination, do not include more than the first 2 feet of 
depth, except: for those types of areas of observed contamination 
listed in Tier C of Table 5-2 (section 5.1.1.2.2), include the 
entire depth, not just that within 2 feet of the surface.
--For the subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway, in estimating the volume for areas of 
observed exposure, only use the volume of air in the regularly 
occupied structures where observed exposure has been documented.
     Convert from curies of radionuclides to equivalent 
pounds of nonradioactive hazardous substances by multiplying the 
activity estimate for the source (or area of observed contamination 
or area of observed exposure) by 1,000.
     Assign this resulting product as the radionuclide 
constituent quantity value for the source (or area of observed 
contamination or area of observed exposure).
    If the radionuclide constituent quantity for the source (or area 
of observed contamination or area of observed exposure) is 
adequately determined (that is, the total activity of all 
radionuclides in the source and releases from the source [or in the 
area of observed contamination or area of observed exposure] is 
known or is estimated with reasonable confidence), do not evaluate 
the radionuclide wastestream quantity measure in section 7.2.5.1.2. 
Instead, assign radionuclide wastestream quantity a value of 0 and 
proceed to section 7.2.5.1.3. If the radionuclide constituent 
quantity is not adequately determined, assign the source (or area of 
observed contamination or area of observed exposure) a value for 
radionuclide constituent quantity based on the available data and 
proceed to section 7.2.5.1.2.
    7.2.5.1.2 Radionuclide wastestream quantity (Tier B). Evaluate 
radionuclide wastestream quantity for the source (or area of 
observed contamination, area of observed exposure, or area of 
subsurface contamination) based on the activity content of 
radionuclide wastestreams allocated to the source (or area of 
observed contamination, area of observed exposure, or area of 
subsurface contamination) as follows:
     Estimate the total volume (in cubic yards or in 
gallons) of wastestreams containing radionuclides allocated to the 
source (or area of observed contamination, area of observed 
exposure, or area of subsurface contamination).
     Divide the volume in cubic yards by 0.55 (or the volume 
in gallons by 110) to convert to the activity content expressed in 
terms of equivalent pounds of nonradioactive hazardous substances.
     Assign the resulting value as the radionuclide 
wastestream quantity value for the source (or area of observed 
contamination, area of observed exposure, or area of subsurface 
contamination).
     For the subsurface intrusion component of the soil 
exposure and subsurface intrusion pathway, estimate the total 
wastestream volume for all regularly occupied structures that have a 
containment value >0 and that are located within areas of observed 
exposure with observed or inferred intrusion, and

[[Page 2806]]

within areas of subsurface contamination. Calculate the volume of 
each regularly occupied structure based on actual data. If unknown, 
use a ceiling height of 8 feet.
    7.2.5.1.3 Calculation of source hazardous waste quantity value 
for radionuclides. Select the higher of the values assigned to the 
source (or area of observed contamination, area of observed 
exposure, and/or area of subsurface contamination) for radionuclide 
constituent quantity and radionuclide wastestream quantity. Assign 
this value as the source hazardous waste quantity value for the 
source (or area of observed contamination, area of observed 
exposure, or area of subsurface contamination). Do not round to the 
nearest integer.
    7.2.5.2 Calculation of hazardous waste quantity factor value for 
radionuclides. Sum the source hazardous waste quantity values 
assigned to all sources (or areas of observed contamination, areas 
of observed exposure, or areas of subsurface contamination) for the 
pathway being evaluated and round this sum to the nearest integer, 
except: if the sum is greater than 0, but less than 1, round it to 
1. Based on this value, select a hazardous waste quantity factor 
value for this pathway from Table 2-6 (section 2.4.2.2).
    For a migration pathway, if the radionuclide constituent 
quantity is adequately determined (see section 7.2.5.1.1) for all 
sources (or all portions of sources and releases remaining after a 
removal action), assign the value from Table 2-6 as the hazardous 
waste quantity factor value for the pathway. If the radionuclide 
constituent quantity is not adequately determined for one or more 
sources (or one or more portions of sources or releases remaining 
after a removal action), assign a factor value as follows:
     If any target for that migration pathway is subject to 
Level I or Level II concentrations (see section 7.3), assign either 
the value from Table 2-6 or a value of 100, whichever is greater, as 
the hazardous waste quantity factor value for that pathway.
     If none of the targets for that pathway is subject to 
Level I or Level II concentrations, assign a factor value as 
follows:
--If there has been no removal action, assign either the value from 
Table 2-6 or a value of 10, whichever is greater, as the hazardous 
waste quantity factor value for that pathway.
--If there has been a removal action:
    [ssquf] Determine values from Table 2-6 with and without 
consideration of the removal action.
    [ssquf] If the value that would be assigned from Table 2-6 
without consideration of the removal action would be 100 or greater, 
assign either the value from Table 2-6 with consideration of the 
removal action or a value of 100, whichever is greater, as the 
hazardous waste quantity factor value for the pathway.
    [ssquf] If the value that would be assigned from Table 2-6 
without consideration of the removal action would be less than 100, 
assign a value of 10 as the hazardous waste quantity factor value 
for the pathway.

    For the soil exposure component of the soil exposure and 
subsurface intrusion pathway, if the radionuclide constituent 
quantity is adequately determined for all areas of observed 
contamination, assign the value from Table 2-6 as the hazardous 
waste quantity factor value. If the radionuclide constituent 
quantity is not adequately determined for one or more areas of 
observed contamination, assign either the value from Table 2-6 or a 
value of 10, whichever is greater, as the hazardous waste quantity 
factor value.
    For the subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway, if the radionuclide constituent 
quantity is adequately determined for all areas of observed 
exposure, assign the value from Table 2-6 as the hazardous waste 
quantity factor value. If the radionuclide constituent quantity is 
not adequately determined for one or more areas of observed 
exposure, assign either the value from Table 2-6 or a value of 10, 
whichever is greater, as the hazardous waste quantity factor value.
    7.2.5.3 Calculation of hazardous waste quantity factor value for 
sites containing mixed radioactive and other hazardous substances. 
For each source (or area of observed contamination, area of observed 
exposure, or area of subsurface contamination) containing mixed 
radioactive and other hazardous substances, calculate two source 
hazardous waste quantity values--one based on radionuclides as 
specified in sections 7.2.5.1 through 7.2.5.1.3 and the other based 
on the nonradioactive hazardous substances as specified in sections 
2.4.2.1 through 2.4.2.1.5, and sections 5.1.1.2.2, 5.1.2.2.2 and 
5.2.1.2.2 (that is, determine each value as if the other type of 
substance was not present). Sum the two values to determine a 
combined source hazardous waste quantity value for the source (or 
area of observed contamination, area of observed exposure, or area 
of subsurface contamination). Do not round this value to the nearest 
integer.
    Use this combined source hazardous waste quantity value to 
calculate the hazardous waste quantity factor value for the pathway 
as specified in section 2.4.2.2, except: if either the hazardous 
constituent quantity or the radionuclide constituent quantity, or 
both, are not adequately determined for one or more sources (or one 
or more portions of sources or releases remaining after a removal 
action) or for one or more areas of observed contamination or areas 
of observed exposure, as applicable, assign the value from Table 2-6 
or the default value applicable for the pathway, whichever is 
greater, as the hazardous waste quantity factor value for the 
pathway.
    7.3 Targets. For radioactive substances, evaluate the targets 
factor category as specified in section 2.5 and sections 3 through 
6, except: Establish Level I and Level II concentrations at sampling 
locations as specified in sections 7.3.1 and 7.3.2 and establish 
weighting factors for populations associated with an area of 
subsurface contamination in the subsurface intrusion component of 
the soil exposure and subsurface intrusion pathway as specified in 
section 7.3.3.
    For all pathways (components and threats), use the same target 
distance limits for sites containing radioactive substances as is 
specified in sections 3 through 6 for sites containing 
nonradioactive hazardous substances. At sites containing mixed 
radioactive and other hazardous substances, include all sources (or 
areas of observed contamination, areas of observed exposure, or 
areas of subsurface contamination) at the site in identifying the 
applicable targets for the pathway.
    7.3.1  Level of contamination at a sampling location. Determine 
whether Level I or Level II concentrations apply at a sampling 
location (and thus to the associated targets) as follows:
     Select the benchmarks from section 7.3.2 applicable to 
the pathway (or component or threat) being evaluated.
     Compare the concentrations of radionuclides in the 
sample (or comparable samples) to their benchmark concentrations for 
the pathway (or component or threat) as specified in section 7.3.2. 
Treat comparable samples as specified in section 2.5.1.
     Determine which level applies based on this comparison.
     If none of the radionuclides eligible to be evaluated 
for the sampling location have an applicable benchmark, assign Level 
II to the actual contamination at that sampling location for the 
pathway (or component or threat).
     In making the comparison, consider only those samples, 
and only those radionuclides in the sample, that meet the criteria 
for an observed release (or observed contamination or observed 
exposure) for the pathway, except: Tissue samples from aquatic human 
food chain organisms may also be used for the human food chain 
threat of the surface water pathway as specified in sections 4.1.3.3 
and 4.2.3.3.
    7.3.2  Comparison to benchmarks. Use the following media 
specific benchmarks (expressed in activity units, for example, pCi/l 
for water, pCi/kg for soil and for aquatic human food chain 
organisms, and pCi/m3 for air) for making the comparisons for the 
indicated pathway (or threat):
     Maximum Contaminant Levels (MCLs)--ground water 
migration pathway and drinking water threat in surface water 
migration pathway.
     Uranium Mill Tailings Radiation Control Act (UMTRCA) 
standards--soil exposure component of the soil exposure and 
subsurface intrusion pathway only.
     Screening concentration for cancer corresponding to 
that concentration that corresponds to the 10-6 
individual cancer risk for inhalation exposures (air migration 
pathway and subsurface intrusion component of the soil exposure and 
subsurface intrusion pathway) or for oral exposures (ground water 
migration pathway; drinking water or human food chain threats in 
surface water migration pathway; and soil exposure and subsurface 
intrusion pathway).

--For the soil exposure component of the soil exposure and 
subsurface intrusion pathway, include two screening concentrations 
for cancer--one for ingestion of surface materials and one for 
external radiation exposures from gamma-emitting radionuclides in 
surface materials.
    Select the benchmark(s) applicable to the pathway (component or 
threat) being

[[Page 2807]]

evaluated. Compare the concentration of each radionuclide from the 
sampling location to its benchmark concentration(s) for that pathway 
(component or threat). Use only those samples and only those 
radionuclides in the sample that meet the criteria for an observed 
release (or observed contamination or observed exposure) for the 
pathway, except: Tissue samples from aquatic human food chain 
organisms may be used as specified in sections 4.1.3.3 and 4.2.3.3. 
If the concentration of any applicable radionuclide from any sample 
equals or exceeds its benchmark concentration, consider the sampling 
location to be subject to Level I concentrations for that pathway 
(component or threat). If more than one benchmark applies to the 
radionuclide, assign Level I if the radionuclide concentration 
equals or exceeds the lowest applicable benchmark concentration. In 
addition, for the soil exposure and subsurface intrusion pathway, 
assign Level I concentrations at the sampling location if measured 
gamma radiation exposure rates equal or exceed 2 times the 
background level (see section 7.1.1).
    If no radionuclide individually equals or exceeds its benchmark 
concentration, but more than one radionuclide either meets the 
criteria for an observed release (or observed contamination or 
observed exposure) for the sample or is eligible to be evaluated for 
a tissue sample (see sections 4.1.3.3 and 4.2.3.3), calculate a 
value for index I for these radionuclides as specified in section 
2.5.2. If I equals or exceeds 1, assign Level I to the sampling 
location. If I is less than 1, assign Level II.
    At sites containing mixed radioactive and other hazardous 
substances, establish the level of contamination for each sampling 
location considering radioactive substances and nonradioactive 
hazardous substances separately. Compare the concentration of each 
radionuclide and each nonradioactive hazardous substance from the 
sampling location to its respective benchmark concentration(s). Use 
only those samples and only those substances in the sample that meet 
the criteria for an observed release (or observed contamination or 
observed exposure) for the pathway except: Tissue samples from 
aquatic human food chain organisms may be used as specified in 
sections 4.1.3.3 and 4.2.3.3. If the concentration of one or more 
applicable radionuclides or other hazardous substances from any 
sample equals or exceeds its benchmark concentration, consider the 
sampling location to be subject to Level I concentrations. If more 
than one benchmark applies to a radionuclide or other hazardous 
substance, assign Level I if the concentration of the radionuclide 
or other hazardous substance equals or exceeds its lowest applicable 
benchmark concentration.
    If no radionuclide or other hazardous substance individually 
exceed a benchmark concentration, but more than one radionuclide or 
other hazardous substance either meets the criteria for an observed 
release (or observed contamination or observed exposure) for the 
sample or is eligible to be evaluated for a tissue sample, calculate 
an index I for both types of substances as specified in section 
2.5.2. Sum the index I values for the two types of substances. If 
the value, individually or combined, equals or exceeds 1, assign 
Level I to the sample location. If it is less than 1, calculate an 
index J for the nonradioactive hazardous substances as specified in 
section 2.5.2. If J equals or exceeds 1, assign Level I to the 
sampling location. If J is less than 1, assign Level II.
    7.3.3 Weighting of targets within an area of subsurface 
contamination. For the subsurface intrusion component of the soil 
exposure and subsurface intrusion pathway, assign a weighting factor 
as specified in section 5.2.1.3.2.3 except when a structure in an 
area of subsurface contamination is delineated or inferred to be 
delineated by gamma radiation exposure rates meeting observed 
release criteria with a depth to contamination of 2 feet or less. 
For those populations residing, working, or attending school or day 
care in a structure delineated or inferred to be delineated by gamma 
radiation exposure rates meeting observed release criteria with a 
depth to contamination of 2 feet or less, assign a weighting factor 
of 0.9.

[FR Doc. 2016-30640 Filed 1-6-17; 8:45 am]
 BILLING CODE 6560-50-P