1 

Technical Support Document for U.S. EPA’s Proposed Rule for Addition of 

a Subsurface Intrusion Component to the Hazard Ranking System

January 14, 2016 
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Table of Contents

1.0 Introduction and Rationale for the Addition of a Subsurface Intrusion Component to the HRS ........9

1.1 Introduction ...............................................................................................................................................9

1.2 What is Subsurface Intrusion? ...................................................................................................................9

1.3 Rationale for the Addition of Subsurface Intrusion to the Hazard Ranking System............................... 10

a. Regulatory Rationale (CERCLA/SARA) ..................................................................................................... 10

b. Identification and Classification of Sites with Potential Subsurface Intrusion Contamination............... 11

1.4 Why Should Subsurface Intrusion Be Considered When Identifying Sites for Placement on the NPL? . 14

2.0 Evaluation Options .....................................................................................................................................15

2.1 Methods for a Site to be Placed on the NPL ........................................................................................... 15

a. Current Methods..................................................................................................................................... 15

b. Alternative Methods Considered for Addressing Subsurface Intrusion Sites......................................... 17

c. Proposed Method and Rationale for Expanding the 1990 HRS to Include Subsurface Intrusion ........... 21

2.2 Methods for Addressing Subsurface Intrusion in the 1990 HRS............................................................. 22

a. Current Structure of the HRS (i.e., pathway level description)............................................................... 22

b. Alternative Methods Considered for Incorporating Subsurface Intrusion into the 1990 HRS................ 24

c. Proposed Method and Rationale for Including Subsurface Intrusion in the Soil Exposure Pathway ..... 28

3.0 Addition of New Component to a Renamed Soil Exposure Pathway ....................................................29

3.1 1990 Structure of the Soil Exposure Pathway (i.e., factor category level description) .......................... 29

3.2 Alternative Methods Considered for Including Subsurface Intrusion in the Soil Exposure Pathway..... 29

3.3 Proposed Method and Rationale for Adding a Single Subsurface Intrusion Component....................... 30

3.4 External Peer Review of the Proposed Subsurface Intrusion Component ............................................. 32

4.0 Component Structure: Likelihood of Exposure.......................................................................................39

4.1 Basic Concept and Structure of the Likelihood of Exposure Factor Category ........................................ 39

4.2 Observed Exposure ................................................................................................................................. 40

a. Evidence by Direct Observation (Observed Exposure by Direct Observation) ....................................... 40

b. Evidence by Comparison to Background (Observed Exposure by Chemical Analysis) ........................... 43

c. Special Considerations for Radioactive Hazardous Substances.............................................................. 46

d. Other Option Considered for Establishing Observed Exposure – Use of Subslab and/or Crawl Space

Sampling to Document Observed Exposure.................................................................................................... 47

4.3 Potential for Exposure............................................................................................................................. 47

a. Rationale for Including Potential for Exposure Risks in the Subsurface Intrusion Addition ................... 47

b. Overview of Potential for Exposure Structure ........................................................................................ 48

c. Selection and Weighting of Structure Containment Features ................................................................ 50

d. Selection and Weighting of Key Route Parameters (Route Characteristics)........................................... 53

5.0 Component Structure: Waste Characteristics .........................................................................................66

5.1 Inclusion of a Toxicity/Degradation Value .............................................................................................. 66

a. Toxicity .................................................................................................................................................... 66

b. Degradation............................................................................................................................................. 67

c. Establishing a Combined Toxicity/Degradation Factor Value................................................................. 70

5.2 Other Option Considered – Inclusion of a Mobility Factor..................................................................... 70
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5.3 Determination of “Exposure” Waste Quantity ....................................................................................... 70

a. Summary of Tiered Approach ................................................................................................................. 71

b. Assigning a Hazardous Waste Quantity Value ........................................................................................ 77

c. Component Hazardous Waste Quantity Minimum Values..................................................................... 77

d. Special Considerations for Radioactive Hazardous Substances.............................................................. 78

e. Other Options Considered – Hazardous Waste Quantity ....................................................................... 79

5.4 Considerations in Evaluating Multi-Subunit Structures.......................................................................... 79

5.5 Determination of Waste Characteristics Factor Category Value ............................................................ 80

5.6 Method to Determine the Waste Characteristics Factor Value.............................................................. 80

6.0 Component Structure: Targets .................................................................................................................81

6.1 Delineation of Target Areas .................................................................................................................... 81

a. Approach within the 1990 HRS ............................................................................................................... 81

b. Proposed Option ..................................................................................................................................... 82

c. Other Options Considered ...................................................................................................................... 87

d. Rationale for the Selected Option........................................................................................................... 91

6.2 Selection of Eligible Targets.................................................................................................................... 91

a. Approach within the 1990 HRS ............................................................................................................... 92

6.3 Proposed Option ..................................................................................................................................... 94

a. Assigning Level of Exposure to Structures.............................................................................................. 94

b. Populations (i.e., Individuals Living, Working, or Attending School or Daycare).................................. 100

c. Resources.............................................................................................................................................. 110

d. Special Consideration for Radioactive Hazardous Substances when Identifying and Evaluating the

Targets Factor Category ................................................................................................................................ 110

7.0 Overview of Scoring Methodology for New Soil Exposure and Subsurface Intrusion Pathway ......111

8.0 Testing of the Proposed HRS Addition...................................................................................................114

8.1 Development of Example Site Scoring Scenarios.................................................................................. 114

a. Example Site Scenarios – 1st Phase....................................................................................................... 114

b. Example Site Scenarios – 2nd Phase ..................................................................................................... 114

c. Example Site Scoring Scenarios – 3rd Phase ......................................................................................... 117

8.2 Tier 1 Sites (Test Sites) with Existing Vapor Intrusion Data .................................................................. 127

8.3 Planned Future Efforts.......................................................................................................................... 130

9.0 Terminology Updates Affecting Specific Sections of the HRS (Sections 1, 2, 5 & 6) .........................132

10.0 Resources ...................................................................................................................................................138

10.1 EPA Resources....................................................................................................................................... 138

10.2 Other Federal Agency Resources.......................................................................................................... 143

10.3 Other Published References.................................................................................................................. 143
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List of Figures

Figure 2-1. Superfund Site Assessment Process ......................................................................................................16

Figure 2-2. Structure of the 1990 HRS .....................................................................................................................23

Figure 3-1. Structure of the HRS with the Proposed Addition .................................................................................31

Figure 4-1. Observed Exposure by Direct Observation Example 1 .........................................................................41

Figure 4-2. Observed Exposure by Direct Observation Example 2 .........................................................................42

Figure 4-3. Observed Exposure by Direct Observation Example 3 .........................................................................43

Figure 4-4. Observed Exposure by Chemical Analysis ............................................................................................46

Figure 4-5. Potential for Exposure: Overview ..........................................................................................................50

Figure 4-6. Potential for Exposure: Structure Containment Factor ..........................................................................52

Figure 4-7. Depth to Contamination Attenuation Factor Trends ..............................................................................54

Figure 4-8. Potential for Exposure: Depth to Contamination Factor (Ground Water Plume) ..................................57

Figure 4-9. Potential for Exposure: Depth to Contamination Factor (Soil Gas Plume) ...........................................58

Figure 4-10. Potential for Exposure: Depth to Contamination Factor (Subslab Contamination) .............................59

Figure 4-11. Potential for Exposure: Vertical Migration Factor (Ground Water Plume) .........................................61

Figure 4-12. Potential for Exposure: Vertical Migration Factor (Soil Gas Plume) ..................................................62

Figure 4-13. Potential for Exposure: Vapor Migration Potential Factor (Contaminated Ground Water Plume) .....63

Figure 5-1. Hazardous Waste Quantity: Tier A ........................................................................................................73

Figure 5-2. Hazardous Waste Quantity: Tier B ........................................................................................................74

Figure 5-3. Hazardous Waste Quantity: Tier C ........................................................................................................75

Figure 5-4. Hazardous Waste Quantity: Tier C ........................................................................................................76

Figure 5-5. Hazardous Waste Quantity: Tier D ........................................................................................................77

Figure 6-1. Proposed Method for Delineating Target Areas in the Subsurface Intrusion Component ....................83

Figure 6-2. Inference of Ground Water Intrusion .....................................................................................................85

Figure 6-3. Consideration of Only Actually Exposed Targets without Inference of Contamination .......................87

Figure 6-4. Consideration of Only Actually Exposed Targets and Those with Inference of Contamination ...........89

Figure 6-5. Consideration of Actually Exposed and All Potentially Exposed Targets ............................................90

Figure 6-6. Potential Scenarios for Evaluating Target Weighting in Multi-Subunit Structures ...............................97

Figure 6-7. Inferring Level of Exposure in Occupied Multi-Subunit Buildings within an Area of Observed 

Exposure ...................................................................................................................................................................97

Figure 6-8. Inferring Level of Exposure in Occupied Multi-Subunit Buildings within an Area of Subsurface 

Contamination ..........................................................................................................................................................98

Figure 6-9. Surficial Ground Water Intrusion into an Occupied Multi-Subunit Building .......................................98

Figure 6-10. Example of “Information Indicating Otherwise” - Structure Containment Factor Value of Zero .......99

Figure 6-11. Example Calculation of Actually Exposed Targets ...........................................................................103

Figure 6-12. Example Calculation of Actually Exposed Targets Including Inference of Contamination ..............104

Figure 6-13. Example Calculation of Actually Exposed Targets Excluding Areas Where Information Indicates 

Otherwise ................................................................................................................................................................105

Figure 6-14. Example Weightings of Targets for Structures without Subunits within an Area of Subsurface 

Contamination ........................................................................................................................................................107

Figure 6-15. Example Weightings of Targets for a Multi-story Multi-subunit Structure within an Area of 

Subsurface Contamination ......................................................................................................................................108

Figure 6-16. Example Calculation of Residential Populations within an Area of Subsurface Contamination ......109

Figure 8-1. Example Site Scoring Scenario No. 1 ..................................................................................................119
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Figure 8-2. Example Site Scoring Scenario No. 2 ..................................................................................................122

Figure 8-3. Example Site Scoring Scenario No. 3 ..................................................................................................125

Figure 8-4. Vapor Intrusion Pilot Study Site Distribution ......................................................................................131
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List of Tables

Table 4-1. Excerpt of Subsurface Intrusion Scoresheet (Table 7-1 of this document) .............................................49

Table 4-2. Structure Containment (Table 5-12 in Proposed HRS Addition)............................................................51

Table 4-3. Effect of Depth to Contamination ...........................................................................................................54

Table 4-4. Effect of Soil Type (i.e., Porosity/Permeability) .....................................................................................55

Table 4-5. Effect of Contaminant (i.e., Volatility) ...................................................................................................55

Table 4-6. Effective Porosity/Permeability of Geologic Materials (Table 5-14 in the Proposed HRS Addition)....60

Table 4-7. Effect of Ground Water Temperature .....................................................................................................64

Table 4-8. Example Score Sheet - Potential for Exposure Factor Value Score........................................................65

Table 6-1. Weighting Factor Values for Populations within an Area of Subsurface Contamination .....................106

Table 7-1. Subsurface Intrusion Component Scoresheet (Table 5-11 in Proposed HRS Addition) .......................112

Table 8-1. Scoresheet for Example Site Scoring Scenario No. 1 ...........................................................................120

Table 8-2. Scoresheet for Example Site Scoring Scenario No. 2 ...........................................................................123

Table 8-3. Scoresheet for Example Site Scoring Scenario No. 3 ...........................................................................126

Table 9-1. Assignment of Human Toxicity Factor Values using RfCs ..................................................................134

Table 9-2. Weight of Evidence Categories .............................................................................................................135

Table 9-3. EPA Weight of Evidence Guidelines ....................................................................................................136

Table 9-4. Assignment of Human Toxicity Factor Values using Cancer Slope Factor or Inhalation Unit Risk ....136
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List of Appendices

Appendix A – Inventory of Subsurface Intrusion Sites 

Appendix B – Chemical Factor Analysis

Appendix C – Conceptual Site Modeling

Appendix D –Scoring Information for Test Sites (Tier 1) 

Appendix E – Summary of Peer Review Comments and Responses
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Acknowledgements

The U.S. Environmental Protection Agency’s technical support document for the proposed addition of a 

subsurface intrusion component to the Hazard Ranking System (HRS) was prepared by staff from the Office of 

Land and Emergency Management (OLEM) and staff from EPA Regions 3, 4 and 6. Terry Jeng, OLEM, served 

as overall Rulemaking Lead, and Brenda Nixon Cook, Region 6, served as Overall Rulemaking Co-lead and 

Technical Lead. Lorie Baker, Region 3, served as Expert technical advisor and Jennifer Wendel served as Lead 

for Regulatory Impact Analysis as a part of this overall rulemaking effort. Additionally, staff from Computer 

Sciences Corporation (CSC) assisted in the development and writing of this document. Additional EPA staff 

members from OLEM, Office of Research and Development, Office of the Administrator, Office of Enforcement 

and Compliance Assurance, Office of General Counsel, and Regions 2, 3, 5, 8 and 9 provided assistance in the 

development of this document.
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1.0 Introduction and Rationale for the Addition of a Subsurface Intrusion (SSI) 

Component to the HRS

1.1 Introduction

This technical support document (TSD) discusses the technical and policy considerations that are the basis for the 

proposed addition of a subsurface intrusion component to the Hazard Ranking System (HRS). This TSD 

supplements sections of the proposed rule and preamble by providing a more detailed discussion of the basis for 

decisions made regarding the proposed addition to the HRS. The preamble and the proposed rule comprise the 

primary documents for understanding the addition. The purpose of this document is to provide the rationale for 

the addition and offer detailed background information on the proposal. Various existing guidance documents 

discussing how to perform site inspection (SI) activities are identified in Section 10.1 of this document. 

This TSD describes and explains the additions to the Soil Exposure Pathway (section 5 of the 1990 HRS)1

, as well 

as issues affecting multiple pathways (section 2 of the 1990 HRS). The supporting information in this document 

has been divided into 10 sections as follows: 

• Section 1 describes subsurface intrusion and the rationale for adding a new component to the 1990 HRS

(40 CFR 300, Appendix A).

• Section 2 details the options for evaluating the threat posed by subsurface intrusion in deciding what sites

are placed on the National Priorities List (NPL) – two optional methods are described as alternatives on

how to include threats from subsurface intrusion in the 1990 HRS.

• Section 3 describes the options for adding subsurface intrusion into the soil exposure pathway and the

basic outline of the proposed restructured pathway.

• Sections 4 through 8 describe the subsurface intrusion component structure in the context of the factor

categories (likelihood of exposure, waste characteristics and targets); the scoring methodology; and

testing of the proposed addition.

• Section 9 provides the proposed updates that affect specific sections of the 1990 HRS.

• Section 10 provides a full list of references used in the development of the proposed rule.

• The appendices provide supplemental information and data sources.

1.2 What is Subsurface Intrusion?

Subsurface intrusion is defined as the migration of hazardous substances, pollutants, or contaminants2

 from the 

subsurface environment, or more specifically, the unsaturated zone and/or the surficial ground water, into 

overlying structures. Subsurface intrusion can result in people being exposed to harmful levels of hazardous 

substances and can cause negative health effects over a short- or long-term time period. The most commonly 

identified form of subsurface intrusion is vapor3

 intrusion. However, EPA is also considering in this proposed 

1 For the purposes of this document, the term “1990 HRS” refers to the version of the Hazard Ranking System as published 

on December 14, 1990. (40 CFR Part 300). 2 For the purpose of this document, the term “hazardous substances, pollutants or contaminants” will be referred to simply as 

“hazardous substances.” See section 1.1, of the current 1990 HRS for the definition of a hazardous substance.

3 The terms ‘gas’ and ‘vapor’ refer to the gaseous state of matter, as distinguished from the liquid or solid state. Whereas 

“vapor’ refers to a volatile chemical that may comprise only a portion of the total volume, ‘gas’ refers to the entire volume. 
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addition sites with another forms of subsurface intrusion, that is, sites with contaminated ground water intruding 

into regularly occupied structures. 

1.3 Rationale for the Addition of Subsurface Intrusion to the Hazard Ranking System

a. Regulatory Rationale (CERCLA/SARA)

In 1986, Congress passed the Superfund Amendments and Reauthorization Act (SARA) (Pub. L. 99-499), which 

added section 105(c)(1) to the Comprehensive Environmental Response, Compensation, and Liability Act 

(CERCLA) of 1980, requiring the Environmental Protection Agency (EPA or agency) 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." To meet this statutory 

requirement, the HRS was last revised on December 14, 1990 (55 FR 51532). Prior to this revision, the issue of 

applying the HRS to assess the relative threat posed by releases to indoor air was evaluated. However, adequate 

knowledge of key variables needed to evaluate this pathway was not available or understood. Some examples 

include: 1) indoor air health-based benchmark values; 2) proven indoor air monitoring techniques; and 3) 

associations between contaminated ground water and soil vapors. In the intervening time period, the agency 

considers that the scientific knowledge and technical expertise required to evaluate the relative threat posed by 

subsurface intrusion contamination has become available.

Without an evaluation of threats posed by subsurface intrusion, the HRS is not a complete assessment of human 

exposure to contamination. EPA considers the addition of subsurface intrusion to the HRS to be consistent with 

CERCLA section 105 because it will improve the agency’s ability to identify hazardous waste sites that most 

warrant further investigation and will help EPA, in dialogue with other federal agencies and the states and tribes, 

determine the most appropriate state or federal authority to address those sites. As is currently the case, EPA often 

defers to other state and federal cleanup authorities based on the site assessments and HRS evaluations. While 

some states/tribes have programs to address subsurface intrusion contamination, they often have limited authority 

and resources, and variable remediation criteria. The availability of the federal remedial authority and the more 

comprehensive site assessment program should complement and strengthen these other programs. 

In fact, in a May 2010 report4

, the Government Accountability Office (GAO) concluded that if vapor intrusion 

sites “are not assessed and, if needed, listed on the National Priorities List (NPL), some seriously contaminated 

hazardous waste sites with unacceptable human exposure may not otherwise be cleaned up.” The GAO 

recommended that to better identify potential sites for placement on the NPL, EPA consider vapor intrusion as 

part of the NPL process. Additionally, the scientific knowledge and technology relevant to evaluating subsurface 

intrusion threats have sufficiently improved and now exist to support this addition to the HRS. With the addition 

of a subsurface intrusion component, a site with subsurface intrusion may qualify for the NPL, whereas it may not 

qualify using the threats evaluated in the 1990 HRS. Therefore without this addition, those sites that pose the 

greatest risk to human health and the environment may not be identified or listed on the NPL. 

For the purposes of this TSD and the proposed subsurface intrusion component, the term “vapor intrusion,” includes the 

intrusion of both vapors and gases and the two terms are used interchangeably for this purpose.

4 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.
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Furthermore, outreach activities were conducted to determine the level of interest and support from the public. 

These included a Notice of Opportunity for Public Input (76 FR 5370, January 31, 2011) and four public listening 

sessions held across the country. The remarks by a majority of speakers at the listening sessions were supportive 

of the potential addition of a subsurface intrusion component to the HRS. More than 40 written comments were 

received during the public comment period. The comments came from a diverse group of private citizens, 

businesses, states, American Indian tribes, environmental action groups and other governmental agencies. With 

the exception of several opposing comments from industry groups and concerns raised by the Department of 

Defense (DoD), the large majority of commenters were in favor of adding a subsurface intrusion component to 

the HRS. Five states and two tribes submitted comments - all in support of the addition. The Association of State 

and Territorial Solid Waste Management Officials (ASTSWMO) compiled and presented input from 14 states – 

all but one favoring the addition of subsurface intrusion to the HRS. 

b. Identification and Classification of Sites with Potential Subsurface Intrusion

Contamination

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.” 

An evaluation of the need for and development of this proposed addition to the 1990 HRS was undertaken to 

identify the scope of the subsurface intrusion contamination threat. This evaluation included efforts to identify 

and classify sites that may pose a subsurface intrusion threat. The evaluation identified 1,080 sites that may have 

evidence of subsurface intrusion issues (1,073 with suspected vapor intrusion issues and 7 sites with possible 

contaminated ground water intrusion issues). The magnitude of this number indicates the national significance of 

this issue. The data for this evaluation was compiled from federal, state, and other data sources discussed below in 

section 1.3.b.i. Although some of the sites in this inventory are currently listed in EPA’s Superfund database5

, it is 

also important to emphasize that the inventory of sites compiled (where subsurface intrusion has been identified 

as a possible issue) does not represent a list of sites that may be eligible to be placed on the NPL. Rather, this 

limited review of a selected set of databases indicates that subsurface intrusion is an issue of national significance 

and it is worthwhile to explore how EPA can make full use of Superfund to address this issue. EPA recognizes 

that, in many instances, additional information is needed to verify the presence, and to determine the nature and 

extent, of a subsurface intrusion problem. As such, the inventory should not be considered a list of NPL candidate 

sites. 

i. Source of Data

The list of 1,073 vapor intrusion sites evaluated to identify the scope of the subsurface intrusion contamination 

threat was compiled from the following sources: 

• EPA Superfund database key word searches, as well as EPA or state self-identification, but for which no

sampling data were obtained

• Preliminary assessments and site inspection (PA/SI) reports

• Sites that have undergone an HRS evaluation

5 EPA Superfund program is in the final stages of replacing the Comprehensive Environmental Response Compensation and 

Liability Act Information System (CERCLIS) data management system for Superfund site information, with the Superfund 

Enterprise Management System (SEMS) (hereinafter referred to as EPA’s Superfund database). 
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• Resource Conservation and Recovery Act (RCRA) Corrective Action sites

• Removal sites

• Sites previously examined for further evaluation without consideration of subsurface intrusion issues

• State program sites

• Agency for Toxic Substance and Disease Registry (ATSDR) sites

• EPA’s Indoor Air Vapor Intrusion database (IAVI)

The EPA site assessment programs identified the 7 additional sites where intrusion of contaminated ground water 

is a potential issue that cannot be evaluated using the current 1990 HRS. 

ii. Methodology of Analysis

Potential subsurface intrusion sites were identified and classified based on whether the information available for a 

site was sufficient to allow testing of the subsurface intrusion addition algorithm. These sites are not presently on 

the NPL and are not excluded statutorily under CERCLA or by agency policy. 

Sites were categorized by tier as follows:

Tier 4 – Sites identified as having a suspected subsurface intrusion threat based on EPA’s Superfund database and 

Agency for Toxic Substances and Disease Registry (ATSDR) key word searches, and EPA or state selfidentification, but for which no sampling data were obtained. 

Tier 3 – Sites identified as having characteristics or evidence that indicate subsurface intrusion (e.g., volatile 

hazardous substance in ground water) may have occurred or will occur.

Tier 2 – Sites identified as having a subsurface intrusion threat documented by subslab, crawl space, or indoor air 

samples, but insufficient HRS-required evaluation factors to qualify for the NPL. 

Tier 1 – Sites identified as having a subsurface intrusion 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.

iii. Results

Approximately 1,073 sites not currently on the NPL that could have vapor intrusion issues have been identified 

and categorized by the above methodology. Of these sites: 

• Tier 4 – 328 sites are identified as having a suspected subsurface intrusion threat based on EPA’s

Superfund database and ATSDR key word searches and EPA or state self-identification, but for which no

sampling data were obtained.

• Tier 3 – 532 sites are identified as having characteristics or evidence that indicate subsurface intrusion

(e.g., volatile hazardous substance in ground water) may have occurred or will occur.
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• Tier 2 – 202 sites are identified as having a subsurface intrusion threat documented by subslab, crawl

space, or indoor air samples, but insufficient HRS-required evaluation factors to qualify for the NPL.

• Tier 1 – 11 sites are identified as having a subsurface intrusion 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.

The identification of these 1,080 sites with a possible subsurface intrusion threat does not represent a 

comprehensive assessment of the potential scope of the subsurface intrusion threat. The identification of these 

sites is based on available information and serves as the basis for initially assessing the subsurface intrusion 

problem in general. In the case of vapor intrusion, certain states undertook comprehensive efforts to identify and 

evaluate subsurface intrusion threats, which resulted in the identification of a proportionately higher number of 

sites within those states than in other states. In the specific case of ground water intrusion, the extent of the issue 

is still emerging and information on these types of sites is not as readily available as is information for sites with 

vapor intrusion issues. For these reasons, there is a degree of inherent uncertainty associated with compiling an 

inventory of sites with potential subsurface intrusion problems. Additional analysis would be necessary to further 

refine the site inventory, especially in cases where little information exists. As additional information is gathered 

and new sites are added to EPA’s Superfund database to undergo the site assessment process, the number of sites 

with subsurface intrusion threats is likely to change. Nevertheless, the aforementioned illustrates that a large 

number of sites currently have significant actual or potential human exposure due to subsurface intrusion that 

cannot be evaluated by the current 1990 HRS. 
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1.4 Why Should Subsurface Intrusion Be Considered When Identifying Sites for Placement 

on the NPL?

Under the Superfund remedial program, subsurface intrusion is currently only addressed at sites placed on the 

NPL based on threats from other pathways. That is, if the presence of threats from other pathways indicates that 

the site has a high priority release, then subsurface intrusion issues may be addressed later in the remedial process 

after the site has been placed on the NPL. Sites without threats from other pathways that can be evaluated under 

the current 1990 HRS, but do have subsurface intrusion issues are not considered for placement on the NPL, 

because the current 1990 HRS does not have a component for evaluating subsurface intrusion issues. 

Additionally, even if sites have other pathway threats and subsurface intrusion issues, they may not be addressed 

because they do not qualify for placement on the NPL based on threats posed by other pathways. As GAO stated 

in its May 2010 report6

: 

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 otherwise be cleaned up. 

6 EPA’s Estimated Costs to Remediate Existing Sites Exceed Current Funding Levels, and More Sites Expected to be Added 

to the National Priorities List, GAO Report to Congressional Requesters, GAO-10-380, May 2010.
15

2.0 Evaluation Options

Several options for addressing the threat from subsurface intrusion were evaluated. This section discusses the 

options considered for placing a site on the NPL (see Section 2.1) and alternative methods considered for 

addressing subsurface intrusion within the 1990 HRS (see Section 2.2). 

2.1 Methods for a Site to be Placed on the NPL

a. Current Methods

CERCLA required revision of the National Oil and Hazardous Substances Pollution Contingency Plan (NCP) to 

establish procedures and standards for responding to releases of hazardous substances, pollutants, and 

contaminants. The revised NCP provides three methods for placing a site (defined in the HRS as the area of a 

release) on the NPL. First, a site may be included on the NPL if it scores sufficiently high on the HRS. Second, 

each state may designate a single site as its top priority to be listed on the NPL, regardless of HRS score, States 

and territories may only designate one site through this mechanism. The third mechanism allows sites to be added 

in response to a health advisory from ATSDR. This mechanism requires both the issuance of a health advisory by 

ATSDR, a determination by EPA that the site poses a significant threat to public health, and a finding that it is 

more cost effective to clean up the site using remedial authority than removal authority.

i. Hazard Ranking System

The HRS serves as a screening tool to evaluate the potential of uncontrolled hazardous substances to cause human 

health problems or environmental damage at one site relative to other sites, and is the mechanism most frequently 

used to list sites on the NPL. The pre-remedial portion of the Superfund program—the portion prior to placing 

sites on the NPL—is intended to identify those sites that that most warrant further investigation and possible 

cleanup under CERCLA. (See Figure 2-1 for a general depiction of the Superfund Site Assessment Process.) 

During pre-CERCLA screening, which is the first step of the pre-remedial process, possible releases of hazardous 

substances are evaluated and EPA determines if a substance in the release is regulated by CERCLA, whether the 

release is being addressed, and whether any statutorily mandated or policy limitations prohibit conducting a 

response action in accordance with CERCLA. If EPA determines the release meets these requirements, then the 

suspected release is listed in EPA’s Superfund database. If, based on the results of a preliminary assessment (an 

assessment made using existing data to determine if there is evidence of a potential threat), EPA determines that a 

site warrants further investigation, a site inspection is initiated as specified in the NCP (40 CFR 300.420). The site 

inspection may include the collection of samples for chemical analysis. Such samples aid in ascertaining what 

substances are present at the site and whether they are being released. The purpose of the site inspection is to 

determine if there is 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 

evaluated using the HRS. 
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Figure 2-1. Superfund Site Assessment Process

The 1990 HRS (40 CFR 300, Appendix A) evaluates the possibility for exposure to hazardous substances via four 

pathways (air migration, ground water migration, surface water migration, and soil exposure), further explained in 

Section 2.2 of this document, to determine the relative threat to human health and the environment posed by the 

release or threatened release of hazardous substances. 

The HRS score is the primary criterion for determining whether a site is eligible to be included on the NPL. The 

NPL (40 CFR 300, Appendix B) includes those sites that appear to pose the most serious threats to human health 

and the environment and that may warrant remedial investigation and possible cleanup under CERCLA. Only 

sites on the NPL are eligible for Superfund-financed remedial actions. Emergency removal and enforcement 

actions can be conducted at any site, whether or not the site is on the NPL. 
17

ii. State Designation of a Site

A site can be placed on the NPL when a state or territory designates the site as a top priority site. Each state or 

territory can designate one site to the NPL as a top priority site regardless of its HRS score this can be done only 

once. Therefore, once a state has designated a site as a top priority, it may not designate any other site to be

included on the NPL. (40 CFR 300.425(c)(2)). 

iii. Agency for Toxic Substances and Disease Registry (ATSDR)

Section 300.425(c) of the NCP identifies that a site is eligible for placement on the NPL when the following three 

conditions are met: 

1. The ATSDR has issued a health advisory that recommends dissociating people from the site;

2. EPA determines the site poses a significant threat to public health; and

3. EPA anticipates it will be more cost-effective to use its remedial authority (available only at NPL

sites) than to use its emergency removal authority to respond to the site.

b. Alternative Methods Considered for Addressing Subsurface Intrusion Sites

EPA considered alternatives to this proposed regulatory action for addressing the need to evaluate subsurface 

intrusion threats as discussed below. However, no other authority consistently and comprehensively addresses 

subsurface intrusion across all potential non-federal sites, particularly when subsurface intrusion is the key 

exposure route. 

i. Develop Guidance

EPA considered developing guidance documents to respond to the threat posed by the intrusion of hazardous 

substances into occupied structures from the subsurface environment. This alternative was rejected because it 

does not align with CERCLA requirements to amend the HRS to assure that HRS assesses relative risk of sites to 

the maximum extent feasible, as discussed below. Additionally, existing guidance documents do not provide a 

mechanism for comprehensively evaluating the subsurface threat.

CERCLA directs EPA to create criteria to prioritize releases of hazardous substances, based on degree of risk, to 

the maximum extent feasible through the HRS. Specifically, CERCLA section 105 required the establishment of a 

national hazardous substance response plan, which included the creation of criteria to determine priorities among 

releases or threatened releases of hazardous substances inside the United States and the use of these criteria to 

develop a list of national priority sites for further evaluation. Additionally, the Superfund Amendments and 

Reauthorization Act (SARA) (Pub. L. 99-499) required the agency 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.” The four pathways used to evaluate actual or 

threatened releases of hazardous substances in the 1990 HRS do not specifically address subsurface intrusion 

contamination. Therefore, based on the aforementioned statutory requirements, it was decided that guidance 

documents alone cannot address the subsurface intrusion contamination issue. 
18

Furthermore, no existing guidance document provides a comprehensive mechanism for evaluating and 

remediating the threat posed by intrusion of hazardous substances into occupied structures from the subsurface 

environment. In 2012, the agency published a supplement to the Comprehensive Five-Year Review Guidance; this 

supplement provides recommendations for assessing the protectiveness of the chosen remedies regarding vapor 

intrusion threats at applicable sites. This guidance only applies to sites already placed on the NPL. In June 2015, 

the agency published OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from 

Subsurface Vapor Sources to Indoor Air. These guidance documents do not provide a mechanism to place sites on 

the NPL. This proposed addition provides the mechanism for placing subsurface intrusion sites on the NPL. 

Therefore, this proposed addition to the HRS, in combination with these guides will further the agency’s efforts to 

establish national consistency in evaluating and remediating vapor intrusion threats under CERCLA. 

The agency considers it appropriate, given the potential for direct human contact with contamination, to add the 

subsurface intrusion threat to the HRS because guidance documents alone do not fulfill the agencies mandate 

under CERCLA to amend the HRS to assure that HRS assesses relative risk of sites to the maximum extent 

feasible. 

ii. Utilize the State Designation of Sites Program

As of June 2015, 44 of 60 states and territories have designated a top priority site for placement on the NPL. 

Therefore, only 16 states, as of 2015, could designate a subsurface intrusion as a priority to be listed on the NPL 

(see CERCLA section 105(a)(8)(B)). As Section 1.3.b of this document describes, approximately 202 sites have 

been identified as having a subsurface intrusion threat based on the presence of actual subsurface intrusion 

sampling data. This mechanism would not feasibly allow EPA to meet its obligations under CERCLA to prepare a 

comprehensive list of national priorities among all known or threatened releases throughout the United States 

because only 16 states could possibly designate a site with a subsurface intrusion threat to the NPL. 

iii. Expand the use of the ATSDR Listing Mechanism

EPA considered expanding the use of the ATSDR listing mechanism to list sites with subsurface intrusion threats 

on the NPL. As of April 2014, 13 out of 1,694 final NPL sites have been placed on the NPL utilizing this method. 

The agency decided that using this approach to address subsurface intrusion contamination of occupied structures 

is too limiting for the reasons described below. 

The ASTDR listing mechanism (50 FR 37625, September 16, 1985) was created for sites with unique conditions 

that render the site unsuitable for HRS evaluation. The ATSDR mechanism was designed to be used only when 

the ATSDR designated the threat found warranted immediate dissociation from the release and other criteria are 

met. As stated above, three requirements must be met to use this listing option. The first requirement is that 

ATSDR must determine that it is necessary to dissociate individuals from the release. Historically, the time 

required for an ATSDR evaluation to be conducted often limits the ability to proactively address exposure threats 

in a timely manner, particularly if the information necessary to complete this evaluation has not been previously 

collected. The limited data available during the listing stage of the Superfund process may not be sufficient to 

document the presence of contamination at levels requiring the dissociation of people from the site in question 

(i.e., the first condition of this listing mechanism). Due to the variable nature in rates of subsurface intrusion and

resultant indoor air concentrations, as well as the time-limited nature of an HRS site evaluation, actual 

representative-over-time indoor exposure data may not be available or may not represent actual conditions at the 

time of the listing decision. Therefore, collecting sufficient data needed for an ATSDR designation may require a 
19

time and resource intensive site investigation. This may limit the number of sites that can be evaluated sufficiently 

to support an ATSDR designation. 

The second condition requires ASTDR to determine that the release poses a significant threat to public health. 

While this condition may not require further data collection, it does require the agency to define what constitutes a 

significant threat to public health using a means other than an HRS evaluation. This may require site-specific 

determinations that do not have established procedures. 

The third condition required to place a site on the NPL using this mechanism entails a determination by EPA that 

it will be more cost-effective to use its remedial authority, instead of its emergency removal authority, to respond 

to the threat. It is also often difficult to determine the cost-effectiveness of all alternative actions without thorough 

investigation of each of the actions. 

For these reasons, the ATSDR option for addressing subsurface intrusion contamination may be a viable option at 

some individual sites, but it is not considered the most effective mechanism in all situations to address the extent 

of subsurface intrusion issues at a large number of sites. 

iv. Use of Existing Federal and State Programs

EPA considered whether existing federal or state programs would adequately address the risks posed by 

subsurface intrusion contamination. As described in more detail below, EPA determined that no individual 

authority or program consistently and comprehensively addresses subsurface intrusion threats across all potential 

non-federal sites, particularly when subsurface intrusion is the key exposure pathway. 

Other EPA programs have limited authority and ability to address all subsurface intrusion threats. For example, 

EPA RCRA Corrective Action/Enforcement program is only applicable at sites subject to RCRA permitting or 

sites reachable by RCRA enforcement activities. That is, only those sites with active RCRA permitting may be 

addressed under RCRA authority, whereas the Superfund program may address both active and abandoned 

facilities, as well as sites where the originating source of contamination may be unknown.

Using the Superfund Removal Program exclusively to address sites with subsurface intrusion contamination 

threats would not fully address the scope of the environmental problems that often accompany a subsurface 

intrusion situation. EPA presently uses the removal program, as envisioned by the CERCLA, to address the 

immediate public health threat associated with subsurface intrusion by breaking the exposure chain to 

contamination. However, removal actions conducted by EPA—largely due to current limitations on the extent and 

cost of an individual removal action (see CERCLA section 104(e)(1))—are often not able to permanently address 

the source of the contamination (e.g., extensive soil or ground water contamination) that is often associated with 

subsurface intrusion issues. CERCLA’s system of coupling the removal and remedial programs to achieve 

effective cleanup solutions is critical for subsurface intrusion sites where the exposure is often immediate but the 

underlying contamination is multimedia in nature and often significant. For example, installing vapor mitigation 

systems for the most contaminated structures at a ground water subsurface contamination site may be a short-term 

response that may not serve as a method for addressing the long term issues at the site. Depending on site-specific 

conditions, EPA may consider vapor intrusion mitigation systems installed to promptly reduce threats to human 

health to be “interim” or “early” response actions. Installation and operation of vapor intrusion mitigation systems 

can control exposure to hazardous substances entering structures from the subsurface, but these systems fail to 
20

address the source of the problem. Therefore, EPA’s removal program alone may not adequately address all 

subsurface intrusion threats. 

EPA also considered the potential for state Superfund programs or voluntary cleanup programs to address the 

subsurface intrusion threat. The 2004 Interstate Technology and Regulatory Council’s Vapor Intrusion Team 

developed and conducted an online survey of state, federal and tribal agencies regarding vapor intrusion 

regulations, policy, and guidance. Ninety-six percent of survey respondents considered vapor intrusion a concern; 

however, only eleven percent had a procedure for evaluating vapor intrusion codified into law, while a larger 

number of states have developed or are developing guidance for addressing vapor intrusion issues. Furthermore, a 

study titled, The 2009 Vapor Intrusion Pathway: A Guide for State and Territorial Federal Facilities Managers,

also surveyed state and territorial subsurface intrusion programs. According to this study, no state has a statute 

that directly addresses vapor intrusion or identifies requirements for assessing the risk. Nine states have 

regulations that address vapor intrusion specifically; three states have regulations under development. Thirty-four 

states either have published guidance for addressing vapor intrusion or are in the process of developing guidance. 

In those states that have programs, the programs may have limited authority, resources, and variable remediation 

criteria. This may inhibit their ability to respond to all applicable sites. In response to an EPA Notice of 

Opportunity for Public Input (76 FR 5370, January 31, 2011), ASTSWMO also compiled and presented input 

from 14 states to EPA—all but one state favored the addition of a subsurface intrusion component to the HRS. 

The use of Superfund enforcement actions also may not effectively address all subsurface intrusion sites. 

Enforcement actions can only occur at previously identified removal or remedial sites. Given that sites with only 

subsurface intrusion issues cannot be qualify for enforcement actions because they currently cannot be added to 

the NPL, a pre-requisite for conducting remedial actions, sites would need to qualify as removal sites posing an 

imminent threat for enforcement actions to take place. Therefore, enforcement actions could not be used at sites 

with no documented imminent threat. Furthermore, successful enforcement actions can only be achieved if 

potential responsible parties exist. That is, a site with subsurface intrusion contamination, but no potentially 

responsible parties could not be addressed. Finally, some potentially responsible parties who may be eligible to 

participate in state Superfund programs, state voluntary programs, enforcement actions or agreements, may be 

less inclined to enter into such programs if the site at issue cannot be listed on the NPL. 

v. Create Independent Listing Mechanism for Subsurface Intrusion

EPA considered development of a separate listing mechanism for placing sites on the NPL that would only take 

into account the subsurface intrusion threat in making a listing decision. Such a separate listing mechanism would 

not be based on the 1990 HRS or covered by the ATSDR mechanism. However, this option precludes the agency 

from fulfilling its legislative mandate under SARA 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.” This option is deficient because the cumulative effects on targets from multiple 

migration and exposure pathways would not necessarily be considered when evaluating the site for placement on 

the NPL. For example, the exposure to residents from drinking contaminated ground water and from breathing 

contaminated air due to subsurface intrusion may not qualify a site for the NPL if the pathways were assessed 

independently, but the cumulative risk to human health from both these pathways may be sufficient to place the 

site on the NPL. 
21

The selection of this option would also entail the creation of a new HRS structure for evaluating the relative 

degree of risk that a site with subsurface intrusion contamination poses to human health, necessitating more 

substantive changes to the NCP than the minor technical modifications currently proposed. 

c. Proposed Method and Rationale for Expanding the 1990 HRS to Include Subsurface

Intrusion

Potential options were evaluated for incorporating the evaluation of subsurface intrusion into the 1990 HRS. The 

available options were to add the consideration of subsurface intrusion into one of the existing three migration 

pathways (ground water, surface water, or air), create a new pathway specific to subsurface intrusion, or add a 

new component addressing the threat to the existing soil exposure pathway. 

Based on EPA’s evaluations of all options considered, the preferred approach to address the threat posed by the 

intrusion of hazardous substances into occupied structures from the subsurface environment is to add a new 

component to the soil exposure pathway of the 1990 HRS. Based on that decision, the following six concepts 

were used to evaluate possible approaches to the HRS addition and the selection of the preferred approach: 

a. Limit the proposed addition to the existing HRS structure to avoid confusion by minimizing the portions

of the present HRS that would need to be revised.

b. Utilize the existing HRS basic structure and scoring algorithm, and maintain the relative weighting of the

different pathways.

c. Base technical decisions on sound and proven science.

d. Ensure the HRS acts as an effective screening tool and minimizes unnecessary resource expenditures,

while also minimizing the erroneous inclusion or exclusion of sites for possible NPL placement.

e. Assemble and rely upon conceptual site models, case studies, and sensitivity analyses to test the model.

f. Ensure that an HRS scoring evaluation of the soil exposure and subsurface intrusion pathway can be

completed using the information and level of effort that are typical of a site inspection or expanded site

inspection (ESI).

In the process of developing the proposed rule, multiple options were identified that would meet the above 

criteria; these options are discussed in Section 2.2. Based on the agency’s research and experience, literature 

reviews, and public input, the agency projected a range of scenarios for which the proposed addition might be 

applied. Using the basic structure of the 1990 HRS, each option was tested by simulating the scores for typical 

scenarios (see Section 8.1 of this document). Using the results of these studies, the option that best met the above 

criteria was selected. To verify that the selected option would provide expected results at actual sites, the scoring 

algorithm was tested using existing subsurface intrusion data from actual sites. The results of the analyses 

demonstrate that the proposed addition functions as expected (see Section 8.2 of this document). 

CERCLA section 105 mandates that EPA implement the HRS to take into account “to the extent possible the 

population at risk, the hazard potential of hazardous substances..., the potential for contamination of drinking 

water supplies and the potential for direct human contact.” When the HRS was last revised in 1990, sufficient 

studies had not been performed to ensure that subsurface detection and evaluation technology would be effective 

where indoor air contamination was projected to occur. 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 
22

vapors were not well understood. However, it is now possible for subsurface intrusion threats to be adequately 

evaluated. Therefore, it is now appropriate, given the potential risk that subsurface intrusion of contaminants 

presents for direct human contact, to incorporate the consideration of this threat into the HRS. 

2.2 Methods for Addressing Subsurface Intrusion in the 1990 HRS

a. Current Structure of the HRS (i.e., pathway level description)

The 1990 HRS (40 CFR 300, Appendix A) evaluates four pathways in projecting the relative threat a site poses:

• The ground water migration pathway evaluates the likelihood that hazardous substances will migrate to 

aquifers and to drinking water wells that draw on those aquifers to service associated populations. 

• The surface water migration pathway evaluates the likelihood that hazardous substances can enter surface 

water and affect people or the environment. Threats to human health and the environment included in this 

pathway are drinking water (DW), the human food chain (HFC) (i.e., hazardous substances accumulate in 

the aquatic organisms that humans in turn consume), and sensitive environments (ENV). 

• The air migration pathway evaluates the likelihood of release of hazardous substances into the atmosphere 

and the number of people and sensitive environments actually or potentially exposed to hazardous 

substances carried in the ambient (outdoor) air, including gases and particulates. The air migration 

pathway does not evaluate releases to indoor air originating from the subsurface. 

• The soil exposure pathway evaluates the potential threats to humans and terrestrial environments posed by 

direct physical contact with, and subsequent ingestion of, hazardous substances. This pathway includes 

threats to people living on property where hazardous substances are present in the surface/subsurface, 

including contaminated soils, and to people living nearby with access to the contaminated area. 

The surface water migration pathway is further divided into two components reflecting different mechanisms for 

contaminant transport within each component. The soil exposure pathway and each component of the surface 

water migration pathway are further divided into threats that reflect the different targets that are subject to 

contamination. 
23

Figure 2-2. Structure of the 1990 HRS

The scoring system for each pathway is unitless and based on a number of individual factors associated with 

risk-related conditions at the site. These factors are grouped into three factor categories (see rows in Figure 2-2) 

as discussed below. These categories include factors that are used to characterize the relative risk at the site: 

1. Likelihood of release/exposure (i.e., likelihood that hazardous substances have been released or potentially 

could be released from a source into the environment, or that people or sensitive environments can come into

contact with hazardous substances). 

2. Waste characteristics (e.g., toxicity, mobility and/or persistence of the substances in the environment and the 

quantity of the hazardous substances that have or could be released). 

3. Targets (e.g., people or sensitive environments actually or potentially exposed to the release). 

An HRS score is determined for a site by summing the score for the four pathways. Specifically, the score for 

each pathway is obtained by evaluating a set of factors that characterize the potential of the facility to cause harm 

via that pathway. The factors, such as toxicity of the hazardous substance(s) at a site, waste quantity, and 

population are multiplied by a weighting yielding the factor value; the factor values are used to assign factor 

category values. The factor category values are then multiplied together to develop a score for the pathway being 

evaluated. Finally, the unitless pathway scores are combined according to the root-mean-square equation 

presented below to determine the unitless HRS score for the site. 

SS = �SSgggg

2 + SSssgg2 + SSssss2 + SSaa2

4

S = site score

Sgw = ground water pathway score 

Ssw = surface water pathway score

Sa = air pathway score

A site meeting the HRS threshold of 28.50 is eligible for placement on the NPL. By using this formula to assign a 

site score, the HRS score is low if all pathway scores are low. However, the final score can be relatively high if 
24

even one pathway score is high. This approach was chosen in development of the 1990 HRS to ensure that the site 

scores do not deemphasize single pathway threats, underestimating the risk posed by the site. This is an important 

consideration for the HRS scoring methodology because some sites exhibiting a high risk pose threats through 

only one pathway. 

It should be emphasized that while the existing pathways can address subsurface contamination if it enters into 

ground water (in the ground water migration pathway), if it enters into surface water (in the surface water 

migration pathway), if it enters into ambient air from the soil surface (in the air migration pathway), or if it leads 

to surface soil contamination (in the soil exposure pathway). However, none of these scenarios address intrusion 

from the subsurface into occupied structures. 

Finally, it should also be emphasized that the HRS score does not represent a specific level of risk at a site. 

Rather, the score serves as a screening-level indicator of the relative risk among sites reflecting the hazardous 

substance releases or potential releases at sites based on the criteria identified in CERCLA. 

b. Alternative Methods Considered for Incorporating Subsurface Intrusion into the 1990 

HRS

EPA considered several alternative methods for incorporating the subsurface intrusion component into the 1990 

HRS. For each alternative below, a brief description as to why the alternative was or was not the chosen is 

provided with pros and cons for that alternative. Please note that the discussion of the following alternative 

options for incorporating an evaluation of subsurface intrusion contamination into the 1990 HRS is focused on 

vapor intrusion, as it is considered the most common subsurface intrusion exposure route. 

i. Establishment of a New Subsurface Intrusion Pathway

The option to create a separate pathway to address subsurface intrusion was rejected mainly because this approach 

would involve changing the HRS from a four-pathway to a five-pathway scoring scenario, thereby requiring 

alteration of the core algorithm for combining pathway scores to generate an overall HRS score. This option 

would involve the development of a new HRS pathway that either evaluates the migration of contaminants from a 

source into an occupied structure or evaluates whether vapor intrusion has been identified in a contaminated area. 

This option could also require a re-evaluation of the HRS score necessary for a site to qualify for the NPL. 

General Pros:

• A new subsurface intrusion pathway would address the subsurface intrusion threat without the 

complexities that would be involved in combining this threat with other pathways that are designed to 

consider other threats.

• Adding a new subsurface intrusion pathway would not require a change to the existing HRS pathways. 

• The pathway components could be designed to focus exclusively on subsurface intrusion exposure. 

• The HRS pathway scoring structure for likelihood of release could be maintained because the likelihood 

of release could be based on a release into an occupied structure. Similarly, the other HRS scoring 

components (waste characteristics and targets) could also be maintained. This would enable a more direct 

comparison of threats across pathways. 
25

General Cons: 

• Addition of subsurface intrusion as a fifth pathway would change the basic HRS structure and scoring 

algorithm.

• Addition of a fifth pathway would no longer maintain the relative weighting amongst the four currently 

existing pathways. 

• This approach could necessitate a re-evaluation of the HRS site score that qualifies a site for placement on 

the NPL.

ii. Inclusion in the Air Migration Pathway

While the air migration pathway may intuitively seem like a logical pathway for inclusion of a subsurface 

intrusion component—because inhalation exposure is a concern with vapor intrusion and inhalation is an issue 

already considered in the air migration pathway—this option was rejected because the air migration pathway only 

considers migration and dilution of contaminants as gases, or particulates, in the open atmosphere to a receptor, 

not migration in the subsurface or migration from the subsurface into structures. 

General Pros:

• Vapor is gaseous, or is a suspension in gas, and the exposure route for the air pathway is through 

inhalation. The air pathway addresses targets that are exposed to contamination through inhalation by 

migration through ambient air; thus the primary exposure route would be the same. 

• The public perceives vapor intrusion as an issue for the air pathway because the exposure route is through 

inhalation. 

General Cons: 

• The air pathway considers migration and dilution of contaminants as gases or particulates in the open 

atmosphere, not migration via ground water or soil gas in the subsurface or migration from the subsurface 

into structures.

• The concentration of contaminated air in occupied structures (confined spaces) can actually be the result 

of accumulation in a structure, while the air pathway assumes immediate three-dimensional concentration 

dilution when weighting targets, thereby potentially resulting in an underestimate of risk.

• The exposure scenario in the air pathway is for outdoor (ambient) air only and is not the same exposure 

scenario for indoor air. For example, high risk populations such as children and the elderly spend more 

time indoors and thus may be exposed to concentrated levels of contamination for longer durations than 

other populations. 

• Evaluation of vapor intrusion in the air pathway would not include threats from the intrusion and 

exposure to contaminant forms other than vapors such as solids and liquids, which can also be of concern.

iii. Inclusion in the Ground Water Migration Pathway

Inclusion of a subsurface intrusion consideration in the ground water migration pathway was rejected as an option 

mainly because this pathway considers contaminant migration via ground water, not contaminant migration 

moving from ground water via soil vapor to occupied structures. The ground water pathway only considers 

releases to, and migration of, contaminants within aquifers used for drinking water and weights targets based on 

the threat posed by drinking water from contaminated wells and the threat via oral exposure. 
26

General Pros:

• Most vapor intrusion appears to be the result of volatilization of contaminants from ground water 

contamination plumes. 

• Evaluation of subsurface intrusion in the ground water pathway could include the threat posed by vapor 

intrusion and contaminated ground water entering directly into an occupied structure. 

General Cons: 

• The ground water pathway considers contaminant migration via ground water, not contaminant migration 

moving from ground water via soil vapor to occupied structures. 

• The ground water pathway only considers releases to and migration of contaminants within aquifers used 

for drinking water and weights targets based on the threat posed by drinking water from contaminated 

wells; subsurface intrusion contamination from surficial ground water not used for drinking water is not 

presently considered in this pathway. 

• To add consideration of the threat posed by vapor intrusion to this pathway, it would be necessary to 

develop a method to evaluate the potential for migration of vapors in soil from an aquifer into multiple 

occupied structures in a large area, new target delineation areas, and an associated weighting system for 

these targets exposed to subsurface intrusion contamination. 

• A method would need to be devised to combine the two threats and their targets into a single pathway 

score. 

• Evaluation of vapor intrusion in the ground water pathway would require changing how the waste 

characteristics factor category is calculated to reflect mobility of vapors in the subsurface environment. 

• Evaluation of vapor intrusion in the ground water pathway would require consideration of a different 

exposure route for determining target weightings (e.g., it would have to consider exposure via drinking 

water (i.e., ingestion) and exposure by inhalation of vapors). 

• Using the ground water migration pathway to address subsurface intrusion contamination would not 

address threats from non-ground water sources, such as contamination that originates directly from 

sources, enters the unsaturated zone, and then enters occupied structures. 

• Revising the ground water pathway to include a subsurface intrusion component would require 

restructuring the pathway, which would change the basic structure and algorithm of the HRS. 

iv. Inclusion in the Surface Water Pathway

The surface water migration pathway was rejected as an option for including the evaluation of subsurface 

intrusion because there is no relationship between surface water migration and subsurface intrusion entering an 

occupied structure. 

General Pros:

• The surface water pathway could possibly address the intrusion of contamination into an occupied 

structure by surface water flooding.

General Cons: 

• There is no relationship between surface water migration and vapor intrusion entering an occupied 

structure via the subsurface.
27

• Evaluation of vapor intrusion resulting from surface water migration would only include threats from 

intrusion and exposure to contaminants by surface water flooding.

v. Inclusion in the Soil Exposure Pathway 

This option would involve creating a component within the existing soil exposure pathway to evaluate areas 

where vapor intrusion, and potentially other forms of subsurface intrusion, has occurred. In this option, the soil 

exposure pathway would be revised to include two components. The scores for the two components of the 

expanded pathway (soil exposure and subsurface intrusion) could be summed to provide a pathway score. 

Including the new component in the soil exposure pathway does not require changes to the overall four-pathway 

HRS structure or the algorithm for combining pathway scores to a single site score. 

General Pros:

• This option would affect only the soil exposure pathway and would leave the surface water, ground water, 

and air pathway evaluation processes unchanged. 

• This option does not require changes to the overall four-pathway HRS structure or the algorithm for 

combining pathway scores to a single site score.

• The approach would minimize the changes to the soil exposure pathway by adding a new component but 

not changing the two threats that already exist in the soil exposure pathway. 

• The threat posed by intrusion and exposure to contaminants could be evaluated not only in vapor form but 

also as solids, liquid, and particulates. That is, a subsurface intrusion threat could be evaluated through 

multiple routes, not just contamination migrating to a structure via a single route. For example, migration 

could occur via multiple routes such as soil gas and/or ground water migration through a substructure 

(e.g., permeable or cracked buried basement wall)). 

• The methods for demonstrating that vapor intrusion has occurred are more advanced than predicting 

vapor migration from the subsurface into occupied structures. 

• Inclusion of a subsurface intrusion component in the soil exposure pathway would create a twocomponent structure parallel to that of the surface water pathway structure (see Figure 3-1). 

General Cons: 

• The HRS evaluation would need to be based on current conditions (the 1990 HRS soil exposure pathway 

evaluation is based on current conditions) and could not easily consider the future threat of subsurface 

intrusion. 
28

c. Proposed Method and Rationale for Including Subsurface Intrusion in the Soil Exposure 

Pathway

Threats due to subsurface intrusion are proposed to be evaluated using a new component of the HRS as a part of 

an expanded soil exposure pathway. The pathway is proposed to be renamed the soil exposure and subsurface 

intrusion pathway. This alternative best meets the decision criteria for selecting the most effective alternative as 

described in Section 2.1.c of this TSD. Structurally, subsurface intrusion fits best in this pathway because both 

have receptors coming into contact with the contamination and consider the relative risk posed by direct contact 

with existing contamination areas. Specifically, the focus of the threat is not migration of the subsurface 

contamination to the receptors, but instead is due to the receptors coming in contact with (i.e., exposure to) the 

contamination in specific locations where intrusion has occurred or is likely to be occurring. This contrasts with 

the migration pathways, in which the threat is due to the migration of the contamination to the receptors. In 

addition, adding the subsurface intrusion component to the soil exposure pathway will maintain the overall HRS 

structure and the scoring algorithm (see Section 7.0, Overview of Scoring Methodology for New Soil Exposure 

and Subsurface Intrusion Pathway, of this TSD). 
29

3.0 Addition of New Component to a Renamed Soil Exposure Pathway

3.1 1990 Structure of the Soil Exposure Pathway (i.e., factor category level description)

The soil exposure pathway of the 1990 HRS evaluates the potential threats to humans and terrestrial environments 

posed by direct, physical contact with, and subsequent ingestion of, hazardous substances. This pathway includes 

threats to people living on property where hazardous substances are present in the surface or within two feet of the 

surface, including contaminated soils, and to people living nearby with access to the contaminated area. 

This pathway consists of two threats, the resident population threat and the nearby population threat. In the 

resident population threat, targets are subject to direct exposure to contaminants where they live, attend school or 

day care, or work. To be eligible for evaluation, an area of observed contamination needs to be present within the 

relevant property boundary and within 200 feet of the respective residence, school, day care center or workplace. 

Resources and sensitive terrestrial environments are also eligible to be scored as targets subject to direct exposure 

to hazardous substances. In the nearby population threat, targets (only human populations are eligible) not 

qualifying as resident individuals and that live or attend school within one mile (travel distance) of an area of 

observed contamination at the site in question, are eligible for evaluation. Each of these threats is scored using the 

following three factor categories:

• Likelihood of Exposure: This factor category evaluates whether there has been or could be exposure to 

hazardous substances in an area of contamination.

• Waste Characteristics: This factor category reflects the characteristics (i.e., toxicity and quantity) of the 

hazardous substances that are found in areas of surficial contaminants associated with targets. 

• Targets: This factor category is based on the number of people (e.g., resident population, nearby 

population, workers) and sensitive environments that are either exposed to or have access to an area of 

observed contamination. 

As in the other existing pathways of the 1990 HRS, the three factor category values within each threat are 

multiplied together and the results summed, and then normalized using a value of 82,500 to calculate the pathway 

score (subject to a maximum score of 100). 

3.2 Alternative Methods Considered for Including Subsurface Intrusion in the Soil Exposure 

Pathway

Five alternatives were considered for including subsurface intrusion in a soil exposure and subsurface intrusion 

pathway. The methods considered for the incorporation of a subsurface intrusion component into the HRS 

involved whether and how to structure the component to include the evaluation of: 1) actually exposed targets 

(i.e., extent of documented exposure), 2) targets overlying subsurface contamination (i.e., extent of subsurface 

contamination) and 3) targets overlying areas where contamination could migrate in the future (i.e., extent of 

potential future exposure). Actually exposed targets are those populations residing in structures that are subject to 

observed exposure to contaminants. Targets overlying subsurface contamination are those individuals residing in 

structures that overlie subsurface contaminant plumes, but have not been demonstrated to be exposed to 

contamination. Targets overlying areas where contamination could migrate in the future are those potential targets 

that do not overlie subsurface contaminant plumes presently, but there is a potential for the plumes to migrate to 

these targets in the future. One concern was whether these three categories of targets should be evaluated as a 

single threat or multiple threats within the new pathway component. Several options for assigning threats were: 
30

• Separate threats for each of the three categories of targets; 

• A single threat for all three categories of targets;

• Actually exposed targets in one threat and the other two target groups in a second threat; 

• Actually exposed targets, targets overlying subsurface contamination in one threat and potential future 

targets in a second threat and

• Actually exposed targets and targets overlying subsurface contamination in one threat, no consideration 

of potential future targets.

The option selected places actually exposed targets and targets overlying subsurface contamination in one threat 

within the new pathway component. This threat would evaluate both actually exposed targets and targets 

overlying documented areas of subsurface contamination, but would not evaluate potential future targets. (See 

Section 6.1 of this document for additional discussion about target areas.) This option was selected because it 

reflects the relative risk currently posed by the site (current conditions). This consideration of current conditions is 

consistent with the existing soil exposure pathway, unlike the three migration pathways (ground water, surface 

water, and air) that also evaluate the potential for hazardous substance to migrate and contaminate targets in the 

future. 

EPA decided not to consider potential future targets because science and methods available for adequately 

predicting the rate and extent of future horizontal migration of contaminants through the subsurface environment 

and into the indoor air environment with confidence require site-specific information to be collected that is 

beyond the scope of a Superfund site inspection and therefore unlikely to be available during an HRS evaluation. 

The scope of a Superfund site inspection (including an expanded site inspection) is limited to collecting 

information necessary for screening sites using the HRS to determine if further investigation is necessary. 

3.3 Proposed Method and Rationale for Adding a Single Subsurface Intrusion Component

Based on the information and rationale discussed in previous sections of this document, EPA is proposing to add 

the subsurface intrusion threat to the existing soil exposure pathway, which considers direct exposure to receptors, 

as depicted in Figure 3-1. The soil exposure pathway is proposed to be restructured and renamed the soil exposure 

and subsurface intrusion pathway. The restructured pathway would retain the existing two soil exposure threats 

(resident population and nearby population) as one component. The threat posed by subsurface intrusion would be 

added as a new component. The internal structure of the soil exposure component would not be changed by this 

addition, and the two soil exposure threats that now comprise one of the two components in the subsurface 

intrusion pathway would remain unchanged. 
31

Figure 3-1. Structure of the HRS with the Proposed Addition

* The hatched area represents the proposed subsurface intrusion component. 

The score for the restructured soil exposure and subsurface intrusion pathway will be a combination of the two 

component scores, i.e., the soil exposure component score will be added to the subsurface intrusion component 

score to determine the pathway score. The two component scores are proposed to be additive because it is 

possible for populations to be subjected to exposures via both routes: the soil exposure component reflects 

exposures to people when outside a structure and focuses on ingestion; the subsurface intrusion component 

reflects exposures to people 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 does not count the relative 

risk twice. 

Achieving a sufficient score for qualifying a site for the NPL 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 listing threshold. Because the scoring of the soil exposure component is not being altered, the soil exposure 

component would contribute the same score to the overall site score as it would prior to the addition of the 

subsurface intrusion component. 
32

Sections 4 through 6 of this document describes in more detail the rationale and procedures for evaluating the 

subsurface intrusion component of the soil exposure and subsurface intrusion pathway. This includes a discussion 

of the factor categories (likelihood of exposure, waste characteristics, and targets) and the scoring methodology 

for non-radioactive and radioactive hazardous substances. Each aspect of the proposed subsurface intrusion 

component structure is discussed in Sections 4 through 6 in the following order: 

a. Explanation of the topic 

b. How the 1990 HRS addresses the topic, or if it does not address the topic, an explanation of how the topic 

is conceptually addressed within other pathways 

c. The suggested addition 

d. Options considered 

e. Rationale for the selected option and consideration of other options

Radioactive substances are considered hazardous substances under CERCLA but are evaluated differently than 

non-radioactive substances in some sections of the HRS scoring. Minor technical modifications are proposed to 

be incorporated into Section 7.0 of the HRS to account for the subsurface intrusion of radioactive substances into 

occupied structures. Section 104(a)(3)(A) of CERCLA limits addressing naturally releasing substances in their 

unaltered form, or when altered solely through naturally occurring processes or phenomena, from a location 

where it is found naturally. As is done currently, sites with subsurface intrusion of radioactive substances are 

evaluated using the pathway instructions specified in Sections 2.0 through 6.0 of the HRS, supplemented with 

additional instructions in section 7.0 of the 1990 HRS. 

3.4 External Peer Review of the Proposed Subsurface Intrusion Component

This proposed rule consists of narrow technical modifications to, and an expansion of the 1990 HRS, which was 

previously peer-reviewed by the agency’s Science Advisory Board (SAB). The 1988 SAB review was 

comprehensive in scope and addressed the basic structure and concepts of the HRS. This proposed addition 

adheres to the basic structure and concepts of the 1990 HRS, and thus, is consistent with the recommendations of 

the SAB. The 1988 SAB report (Review of the Superfund Hazard Ranking System) is available in the public 

docket for this proposed rulemaking. The 1988 SAB report focused on the following issues (these issues were 

either specifically referred to SAB by the agency or selected by the SAB for review): 

• The overall algorithm for the HRS 

• The inclusion of exposure in the HRS 

• How the HRS could be evaluated in the future 

• Work which could be done to provide better documentation for the next revision of the HRS 

• The types of toxicity the HRS should address and how it should do so 

• Distances from an uncontrolled hazardous waste site that are relevant when considering air pollutants 

from sites 

• Feasibility of including waste concentration in the HRS and whether large volume waste sites had been 

treated differently than others in the HRS 

During development of the proposed addition of a subsurface intrusion component to the HRS, EPA determined 

that several of its proposed approaches for evaluating and assigning HRS factor values warranted external 

independent scientific peer review. These topics were selected for review as they either represent the application 
33

of scientific concepts not addressed in the 1990 HRS or topics of which the scientific understanding has 

significantly advanced in recent years. The resulting information would then be utilized to ensure the proposed 

addition of a subsurface intrusion component to the HRS has a sound and credible basis. As a result, the following 

approaches for assigning the HRS factor values underwent peer review:

• The approach for evaluating and assigning values to the potential for exposure (intrusion) into regularly 

occupied structures from the subsurface 

• The general approach for evaluating and assigning values to the amount of hazardous substances that 

have intruded into regularly occupied structures (i.e., the hazardous waste quantity), evaluated by the 

subsurface intrusion component

• The approach for evaluating and assigning values to populations potentially exposed to subsurface 

intrusion when subslab samples documented contamination attributable to the site in single-tenant 

structures

7

 

• The approach for evaluating and assigning values to populations actually exposed to subsurface intrusion 

and for evaluating and assigning values to the amount of hazardous substances (i.e., the hazardous waste 

quantity) that had entered multi-story and multi-tenant8

 structures 

• The approach for assigning target values for workers

For each approach above, a specific charge question (introduced below) was developed for peer review. A peer 

review panel consisting of six individuals from the academic, government, and private sectors representing a 

range of expertise in public health risk assessments, contaminated soil and ground water investigations, and vapor 

intrusion was assembled to evaluate these questions and provide comments to EPA. Comments received from the 

peer reviewers were used to further refine the proposed component to more accurately evaluate the relative risk to 

human health posed by subsurface intrusion. The more significant modifications made to the proposed subsurface 

intrusion component are introduced below. 

The results of the 2011 peer review of the proposed addition are discussed in the Summary of Peer Review 

Comments and Responses on the Addition of a Subsurface Intrusion Component to the HRS, which is available in 

Appendix E of this TSD. Additional information, such as the official charge questions and other documents 

provided to the 2011 peer reviewers and the original peer review comments received on this proposed addition to 

the HRS are also available in the public docket for this proposed rulemaking. The proposed addition of a 

subsurface intrusion component to the HRS reflects modifications made as a result of EPA’s peer review process.

The peer review comments on most charge related issues were supportive of the procedures used in the proposed 

addition, but requested modifications to specific aspects of the evaluation methodologies. All of the comments 

were reviewed and evaluated. Many of these suggested modifications were incorporated into the proposed 

addition to the HRS. For example, adjustments made to the weighting of target populations in an area of 

subsurface contamination were based on issues raised by the peer reviewers and their suggestions to consider the 

most recent vapor intrusion scientific studies. 

 

7 The proposed subsurface intrusion SsI addition to the HRS has undergone revision since it was submitted for peer review in 

2011. The current version of the proposed component replaces “multi-tenant” with “multi-subunit”. This terminology was 

changed to more accurately define subsections within a structure. 

8 See footnote 7. 
34

Charge Question 1 asked if EPA’s approach for evaluating and assigning proportional weighting factors 

considered important in the evaluation of the potential for subsurface contamination to intrude into a regularly 

occupied structure was scientifically reasonable. While four factors are considered in the HRS evaluation, the peer 

reviewers were asked to review only three of the factors. The specific factors reviewed were: 

• The potential for subsurface contamination to enter a regularly occupied structure. This is proposed to be 

based mainly on the structure’s type of construction and any mitigating measures taken to prevent 

intrusion (i.e., structure containment). (See Section 5.2.1.1.2.1, Structure containment, of the proposed 

addition to the HRS and Section 4.3.c, Selection and Weighting of Structure Containment Features, of 

this TSD for additional information.)

• The distance the subsurface contamination had to travel to enter an occupied structure. This is based on 

the depth between an occupied structure and subsurface contamination (i.e., depth to contamination). (See 

Section 5.2.1.1.2.2, Depth to contamination, of the proposed addition to the HRS and Section 4.3.d.i, 

Depth to Contamination, of this TSD for additional information.) 

• The ease of migration of the substances through the site-specific subsurface geologic materials between 

an occupied structure and the subsurface contamination (i.e., vertical migration factor). (See Section 

5.2.1.1.2.3, Vertical migration, of the proposed addition to the HRS and Section 4.3.d.ii, Vertical 

Migration, of this TSD for additional information.) 

The fourth factor, the volatility of the subsurface contaminants and the corresponding assigned value (i.e., the 

vapor migration potential factor), is also evaluated in assigning the potential for exposure but was not specifically 

raised for comment in the peer review. (See Section 5.2.1.1.2.4, Vapor migration potential, of the proposed 

addition to the HRS and Section 4.3.d.iii, Vapor Migration Potential, of this TSD for additional information.) 

Regarding the structure containment factor, several comments were received questioning the building 

characteristics used to assign the structure containment factor value (Table 5-12, Structure Containment, of the 

proposed addition to the HRS). Based on these comments, EPA modified Table 5-12 to no longer consider 

construction and foundation types in the evaluation, except for regularly occupied structures with foundations 

raised to a height greater than six feet and the gap between the foundation and the land surface is open to the 

atmosphere (e.g., a house on stilts). The agency also modified the assigned values in the table regarding the 

presence of passive vapor mitigation systems, which now includes a consideration of whether information is 

available documenting their maintenance and continued functionality and for structures for where no subsurface is 

possible based on structure-specific factors. (See Section 4.3.c, Selection and Weighting of Structure Containment 

Features, of this TSD for additional information.)

Regarding the depth to contamination factor, comments were received questioning the scientific basis for 

including depth as a factor, as well as the associated range of assigned values. After review of this issue, the 

agency did not modify the depth to contamination factor based on the comments received, as it was confirmed 

that depth to contamination is a significant factor in predicting vapor intrusion and sufficient evidence exists to 

show that there is a difference in the migration potential in the depth categories considered. (See Sections 4.3.d, 

Selection and Weighting of Key Route Parameters, and 4.3.d.i, Depth to Contamination, of this TSD for 

additional information.)

As for the vertical migration factor, comments were received questioning the categories of soil types used in this 

factor’s evaluation, as well as their associated factor values. Additionally, comments were received stating that 
35

instead of soil type, soil moisture content should be considered in the evaluation of the vertical migration factor. 

However, after further review, the agency determined that although soil moisture content can indeed be a 

significant factor, this information would not be effectively collected during a typical time-limited site inspection. 

(See Sections 4.3.d, Selection and Weighting of Key Route Parameters, and 4.3.d.ii, Vertical Migration, of this 

TSD for additional information.) 

Comments were also received identifying biodegradation of a hazardous substance as a significant factor to be

considered when evaluating the potential for vapor to intrude into a regularly occupied structure. In response, the 

agency did not modify the potential for exposure evaluation, but did add a degradation factor to another 

component of the HRS scoring algorithm, the determination of the waste characteristics factor category value.

Since relevant subsurface conditions that significantly impact the degradation rate of subsurface contaminants,

such as soil moisture, nutrient availability, and oxygen content, can be extremely variable and site-specific, it 

would be beyond the scope of a typical time-limited site inspection to measure or predict with any accuracy. The

agency determined that a biodegradation factor would not be appropriate for the potential for exposure evaluation. 

Instead, a degradation factor based on whether the hazardous substance meets observed exposure or observed

release criteria, the depth to contamination, the half-life of the hazardous substance, and biodegradation and 

hydrolysis relationships of hazardous substances was added to the waste characteristics factor category evaluation. 

The degradation factor value was added to the waste characteristics factor category evaluation because substancespecific properties of hazardous substances associated with a site are considered in this part of the HRS. (See

Section 5.1.b, Degradation, of this technical support document for additional information.). 

Additionally, a peer reviewer recommended that the potential for exposure factor value evaluation reflect the 

possibility that non-volatile hazardous substances may be present. Such non-volatile hazardous substances pose a 

reduced likelihood of intrusion into an occupied structure. The agency incorporated this recommendation by 

adding the instruction to assign a potential for exposure factor value only if a volatile hazardous substance is 

present. With this change, only those substances with a vapor pressure greater than or equal to 1 torr, or a Henry’s

Law constant greater than or equal to 10-5 atmosphere-meter cubed per mole (atm m3 mol-1) are considered when 

evaluating the potential for exposure factor value. (See Section 4.3, Potential for Exposure (and all applicable 

subsections), of this TSD for additional information.) 

Charge Question 2 inquired if EPA’s approach for assigning a factor value reflecting the amount of a hazardous 

substance that has entered a regularly occupied structure (i.e., the hazardous waste quantity factor value) was 

appropriate. Comments were received questioning the comprehensiveness of the evaluation of hazardous waste

quantity based only on the regularly occupied structures located in an area of observed exposure. Specifically, 

peer reviewers suggested that the quantity of hazardous substances that could enter into regularly occupied 

structures within areas of subsurface contamination and/or are present in the subsurface be considered in the

evaluation of a site’s hazardous waste quantity. Based on these comments, the agency revised the proposed

evaluation to also consider the quantity of hazardous substances that could enter regularly occupied structures in 

areas of subsurface contamination. (See Section 5.2.1.2.2, Hazardous waste quantity, of the proposed addition to 

the HRS and Sections 5.3, Determination of “Exposure” Waste Quantity (and all applicable subsections) and 5.4, 

Considerations in Evaluating Multi-subunit Structures, of this TSD for additional information.). 

Comments were also received on a proposed method of comparing specific subsurface sample types (i.e., crawl

space or subslab samples) to health-based benchmarks when assigning a minimum hazardous waste quantity

factor. This method was to be used when a complete estimate of the hazardous waste quantity could not be 
36

determined). Because the rate of vapor intrusion into an occupied structure is extremely variable, and potentially 

not accurately represented by using crawl space or subslab samples from a time-limited sampling event, their use 

in determining a minimum hazardous waste quantity was removed. Instead, the magnitude of a minimum 

hazardous waste quantity factor value is proposed to be dependent on whether an actual exposure is documented, 

as is done for other HRS pathway evaluations. (See Section 5.3. c, Component Hazardous Waste Quantity 

Minimum Values, of this TSD for additional information). 

Comments were also received on the consideration of biodegradation of hazardous substances in the evaluation of 

the hazardous waste quantity factor value. As identified in the discussion of charge question 1 above, the agency 

included a degradation factor in the waste characteristics factor category evaluation of the HRS scoring algorithm. 

Since relevant site-specific subsurface conditions that significantly impact the biodegradation rate of subsurface 

contaminants, such as soil moisture, nutrient availability, and oxygen content, can be extremely variable and are 

beyond the scope of a typical time-limited site inspection to measure or predict with any accuracy, a 

biodegradation factor would not be appropriate for consideration in the potential for exposure evaluation. (See 

Section 5.1, Inclusion of a Toxicity/Degradation Factor, of this TSD for additional information on the topics 

discussed above.). 

Charge Question 3 asked whether it was appropriate to consider individuals/populations located in an area of 

subsurface contamination to be at a higher relative risk in situations when subslab contaminant concentrations 

were 10 times or greater than an applicable health-based benchmark. Several comments were received 

questioning the rationale for the proposed approach. Specifically, the reviewers questioned if the attenuation 

factors used to support the assigned target weighting factors were the most recent EPA values and asserted the 

need to utilize the most recent EPA study results to generate the corresponding weighting factors. In addition, the 

reviewers questioned the risk level assumptions used to establish the health-based benchmarks. The reviewers did 

suggest alternative approaches and different weighting values for use in assigning the population within an area of 

subsurface contamination. 

Based on these comments and analyses of EPA’s most recent version of its vapor intrusion database, the agency 

modified the methodology used to evaluate and weight these targets. The original proposal suggested weighting

targets based on the ratio of the contaminant concentrations documented in subslab and crawl space samples 

compared to an appropriate health-based benchmark was removed. This action was taken because it was decided 

that the recently documented extreme variability in vapor intrusion rates over time did not provide the ability to 

predict with confidence that indoor air concentrations would exceed a benchmark based on the limited samplings 

performed for an HRS evaluation. 

Instead, based on an analysis9 of the most recent EPA Vapor Intrusion Database, there are statistically significant 

correlations among sample type, depth, and the attenuation ratios. For example, the highest correlation between 

contaminant concentrations found in indoor air and subsurface samples (i.e., a ratio closest to 1) was determined 

to be found in crawl space and indoor air samples. These comparison ratios (termed “attenuation factors”) were 

found to decrease as indoor air samples were compared to subslab, subsurface soil or soil gas samples, and ground 

water samples. Additionally, as the depth to contamination increased, the ratio of contaminant concentrations 

9 EPA’s Vapor Intrusion Database: Evaluation and Characterization of Attenuation Factors for Chlorinated Volatile 

Organic Compounds and Residential Buildings. Dr. Helen E. Dawson, U.S. Environmental Protection Agency's Office of 

Superfund Remediation and Technology Innovation (OSRTI), served as the principal author, with Dr. Richard B. 

Kapuscinski, USEPA OSRTI, as coauthor. March 16, 2012.
37

comparing subsurface to indoor air samples also decreased. These findings document that as depth increases, the 

amount of the contamination at that depth (or sample type), that will intrude into occupied structures, will most 

likely decrease. These results were used to develop a revised weighting system for eligible populations in an area 

of subsurface contamination. As a result, in areas delineated by contaminated crawl space samples, the highest

target weighting is assigned. Decreasing weightings are applied to eligible populations in areas of subsurface 

contamination delineated by samples with increasing depth to contamination and different sample media. (See

Section 5.2.1.3, Targets and all applicable subsections of the proposed addition to the HRS and Section 6.3.d, 

Weighting of Population Targets within the ASC, of this TSD for additional information.). 

Comments regarding the screening concentrations for cancer and non-cancer risk levels used to establish the 

health-based benchmarks (which are, in turn, used to assign the target weightings for actually exposed 

populations), were also received. These comments generally suggested adjusting the risk levels from a one-in-amillion risk level to one-in-ten-thousand for cancer effects and from a hazard index of 1 to 10 for non-cancer 

effects. The one-in-a-million risk level for possible cancer effects and a hazard index of 1 for non-cancer effects

were retained. The risk levels in the proposed addition to the HRS are built into the overall HRS structure, are 

currently used for all benchmarks in the 1990 HRS, and are consistently used by EPA for screening risk 

assessments. (See Section 2.5.2, Comparison to benchmarks, of the proposed addition to the HRS and Section 

6.3.a, Assigning Level of Exposure to Structures, of this TSD for additional information). 

Charge Question 4 asked if it was scientifically reasonable when calculating the waste quantity and population to

evaluate populations in multi-story/multi-tenant10 buildings based only on the populations inhabiting the intruded 

space and the floor above and below the intruded space. (The charge question provided in the peer review

mistakenly stated that populations on a floor with contamination, as well as populations on the floor above and 

below that floor would be considered. However, the proposed addition to the HRS only considers targets on all

floors below a floor with an actual exposure and one floor above that floor). 

Peer reviewers generally supported the concept used to evaluate multi-story/multi-tenant11 occupied structures in 

the proposed addition. However, the reviewers did raise several issues for consideration by the agency. Many of 

these issues were related to site-specific considerations on the application of the evaluation process. For example, 

the reviewers stressed that the evaluation of regularly occupied multi-story or multi-tenant structures using the 

proposed component should consider preferential pathways between floors or tenant spaces enabling the 

migration of contamination. In response to these comments, the agency added the phrase “unless available 

information indicates otherwise” to the proposed addition to enable the consideration of site-specific situations.

Such situations may include when contaminant transport within an occupied structure might be impacted by 

specific building features (e.g., preferential pathways such as elevator shafts and centralized heating, ventilation, 

and air conditioning systems). 

Additional comments were received questioning the discounting (i.e., reduced weighting factor) of occupied 

spaces (or floors) located below or adjacent to an occupied space (or floors) with a documented observed 

exposure, and occupied spaces above an unoccupied basement with an observed exposure. The agency clarified

the wording in the proposed addition to state that all populations on floors below a documented observed 

exposure will be evaluated as actually exposed (i.e., Level I or Level II exposure), while the population one floor 

above will be evaluated as potentially exposed, unless available information indicates otherwise. This potentially

10 See footnote 7. 11 See footnote 7.
38

exposed population would receive the highest potential target weighting (i.e., 0.9). Furthermore, if a multi-story 

structure with subunits, having no documented observed exposure is located in a delineated area of observed 

exposure, populations within regularly occupied spaces on the lowest floor would be assigned Level II target 

weightings. If a multi-story structure with subunits with no documented observed exposure is located in a 

delineated area of subsurface contamination, populations within regularly occupied spaces on the lowest floor 

would be weighted based on the distance to contamination and sample media. (See Sections 6.2.b.iii, Populations, 

and 6.3, Weighting of Targets, of this TSD for additional information.). 

Charge Question 5 addressed if it was reasonable to assign a three times greater weighting to workers exposed to 

subsurface intrusion within the workplace, than to workers exposed to outdoor contamination, as in the existing 

(1990 HRS) soil exposure component evaluation. Peer reviewers generally agreed that it was reasonable to weight 

workers in structures exposed to subsurface intrusion at a higher relative risk than workers directly exposed to 

outdoor contamination. 

Several comments questioned the rationale and evidentiary support for assigning workers a threefold higher range 

of factor values than those used in the soil exposure component, based on a greater exposure time to 

contamination. The agency agrees that many different site-specific factors, such as contaminant concentration, the 

type of work, and the design of the structure could affect the relative risk of exposure to workers. However, this 

type of information would likely not be readily available at all workplaces at the screening stage of an HRS 

evaluation. No peer reviewer challenged the assumption that workers projected length of exposure to indoor air 

contaminants is at least three times greater than those exposed to contaminated soil in the outdoor environment. In 

response to other peer review comments, the agency did modify the evaluation of workers under the proposed 

component to include workers located in an area of subsurface contamination and workers located in an area of 

observed exposure. This modification was made to reflect that the typical time-limited site inspection used to 

investigate a potential site may underestimate the threat posed by vapor intrusion to workers in an area of 

subsurface contamination. This weighting method used is generally consistent with how EPA determines the 

appropriate Regional Screening Levels for chemical contaminates in commercial and industrial landuses. This 

varies from the assumed outdoor worker exposure assigned in the current soil exposure pathway because the 

proposed addition evaluates workers using health based benchmarks and removes the maximum limit that 

workers can contribute to the site score. (See Sections 6.3.c, Weighting of Populations Within the AOE and 6.3.d 

Weighting of Populations Within an ASC, of this TSD for additional information). 

Additionally, comments not related to a charge-question were also received stating that the proposed addition to 

the HRS is inconsistent with vapor intrusion related guidance documents that was developed by EPA’s Office of 

Land and Emergency Management (OLEM) and Office of Underground Storage Tanks (OUST). In response to 

these comments, the agency noted that the workgroup developing the proposed addition to the HRS worked with, 

or reviewed material created by, the various other vapor intrusion workgroups developing EPA-OLEM and EPAOUST guidance documents to ensure consistency between the relevant documents where appropriate. However, 

in some cases, because the HRS is applied at a different stage in the Superfund process, when only limited data is 

available, the HRS methodology may be different than methods in guidance documents designed for different 

stages in the risk assessment and remediation process.
39

4.0 Component Structure: Likelihood of Exposure

This section discusses the inclusion of a likelihood of exposure factor category in the subsurface intrusion 

component as outlined below: 

• A basic overview of the likelihood of exposure concept and how it is evaluated as part of the 1990 HRS, 

• The methods for establishing an observed exposure, including by direct observation and chemical 

analysis, and

• A discussion of potential for exposure and the factors used to evaluate the threat associated with 

subsurface intrusion when actual indoor air data is not available.

4.1 Basic Concept and Structure of the Likelihood of Exposure Factor Category – See Sections 

2.3 and 5.2.1.1 of Proposed HRS Addition. 

The subsurface intrusion component evaluation includes the threat from two areas of subsurface contamination in 

which exposure to populations in regularly occupied structures above subsurface intrusion contamination exists or 

is likely to exist. This includes: 1) areas of observed exposure (AOE)—areas in which contamination has been 

documented to have entered into occupied structures by either contaminated ground water or vapor intrusion; and 

2) areas of subsurface contamination (ASC) —areas in which subsurface contamination underlying occupied 

structures (such as in surficial ground water or soil vapor) has been documented, but at which either sampling has 

not documented that subsurface contamination has entered an occupied structure or no sampling of indoor air has 

been undertaken. See Section 6.3, Proposed Option, of this document for further discussion regarding how these 

two areas are established. 

A key consideration in the evaluation of all pathways and components of the HRS relative risk ranking process is 

whether any exposure (or release) has occurred and if not, whether there is a probability that one could occur. 

This consideration is termed the likelihood of exposure for the subsurface intrusion component. For purposes of 

an HRS evaluation, for the likelihood of exposure to be assigned a non-zero factor value, not only must 

subsurface intrusion have occurred or be likely to occur into a structure, but the structure must also be regularly 

occupied. 

Consistent with other HRS pathways and components, likelihood of exposure is evaluated in two steps within the 

proposed subsurface intrusion component. The first step is to determine whether contamination has entered a 

regularly occupied structure. If this has occurred, then an “observed exposure” is established. If this can be 

demonstrated in at least one such structure, the likelihood of exposure category value is assigned the highest 

possible value. If an observed exposure has not been documented, the next step in the HRS likelihood of exposure 

process is to evaluate whether there is a “potential for exposure.” The potential for exposure reflects site-specific 

conditions that impact the transport of subsurface contamination from the subsurface into an occupied structure. 

The potential for exposure factor value is lower than the value assigned when an observed exposure has been 

documented. 

The instructions for assigning the likelihood of exposure factor category value are provided in this section. 

Section 4.2 describes the two mechanisms for documenting an observed exposure. Section 4.3 describes the 

methodology for determining the potential for exposure (if an observed exposure cannot be established). 
40

4.2 Observed Exposure – See Section 5.2.1.1.1 of the Proposed HRS Addition. 

For HRS purposes, an observed exposure is established if it can be documented that a hazardous substance from 

the site being evaluated has entered at least one regularly occupied structure via the subsurface by either 

contaminated ground water or vapor intrusion. When it can be documented that subsurface intrusion has occurred, 

the likelihood of exposure is assigned its maximum value. The method for documenting that this observed 

exposure (or observed release in the migration pathways) has occurred is consistent with how this is performed in 

all HRS pathways, components, and threats. This process is discussed in section 2.3 of the 1990 HRS. (See 

section 2.6 of the 1988 Revised HRS TSD for supplemental information). Consistent with the existing 1990 HRS 

structure, it is proposed that an observed exposure in the subsurface intrusion component be established by one of 

the two methods: 1) by evidence of “direct observation” of subsurface intrusion, or 2) by “chemical analysis” of 

contaminant levels, and attribution of any increase in the contaminant levels to the site being evaluated.

a. Evidence by Direct Observation (Observed Exposure by Direct Observation)

The identification of an observed exposure by direct observation can be based on a solid, liquid, or gaseous 

hazardous substance (including precipitates) attributable to the site being observed or known to have entered an 

occupied structure from the subsurface. This finding will generally require the observation that a solid, liquid, or 

gas is entering the structure via the subsurface, and it can be documented (e.g., by sampling of the medium as it 

intrudes) that the medium contains a hazardous substance from the site being evaluated. 

For example, an observed exposure by direct observation can be documented if hazardous substances are found in 

a gaseous sample or in a liquid moving from an open sump into an occupied structure or moving from a 

foundation crack or opening into an occupied structure. The gaseous or liquid sample collected from the sump or 

foundation crack represents an observed exposure by direct observation in that hazardous substances are observed 

to be entering a living space from the subsurface. The sump and foundation cracks represent direct entry points 

from the subsurface. 

In addition to the measurement of the contamination entering the structure, it must also be documented that the 

hazardous substances found in the sump or foundation crack samples are due to the release(s) from the site being 

evaluated, as opposed to lifestyle factors within the structure that may also contain hazardous substances (e.g., 

household cleaners). The examples depicted in Figures 4-1 and 4-2 demonstrate that the release is coming from 

the subsurface and entering the structure by collecting a second sample from the living space, at a location away 

from the foundation crack or sump. These samples show significantly lower concentrations, indicating that the 

hazardous substances within the structure are emanating from the subsurface through the foundation crack and 

open sump, and into the living space of the regularly occupied structure. Other evidence to document that the 

contamination in the samples is entering the structure may include measuring the rate of air flow into the structure 

via the sump or foundation crack.12 Other methods may also be used for establishing direct observation depending 

on site-specific conditions. It should be noted that in the instances when intrusion into an occupied structure 

 

12 It should be noted however, that, in Figure 4-1, the samples taken away from the sump or crack do not represent 

“background” samples as required when establishing an observed exposure by chemical analysis, which is discussed further 

in Section 4.2.b of this TSD. The relative concentrations in these samples help confirm that the substance is coming from the 

sump into the structure and not from indoor sources Unlike chemical analysis, for a direct observation, it is not necessary to 

document that an observed exposure has occurred by comparing the concentration in the release sample to a background 

level.
41

results from a contaminated ground water table rising and flooding into the structure, the subsurface intrusion 

component proposes to only consider documented observed exposure by direct observation. 

Figure 4-1 depicts one example of observed exposure by direct observation where a Trace Atmospheric Gas 

Analyzer (TAGA) mobile lab has collected indoor air samples from a structure’s basement and also directly from 

the basement sump.(Note the two basement samples are not background samples as used in establishing an 

observed exposure by chemical analysis. These samples would be used to show what concentration would be in 

the structure if it were not for the release being evaluated and the release sample would have to meet a significant 

increase criteria. In this example these samples are used to show the release is coming from the sump and not 

form indoor sources.). 

Figure 4-1. Observed Exposure by Direct Observation Example 1

Observed Exposure via an Air Sample Collected from a Ground Water Sump 

TAGA

680

68 6

Air Sample 

from Sump

Field Laboratory

Air Sample 

Contaminant 

Concentrations

Contaminated 

Ground Water 

Plume

Ground 

Surface

Basement

Wind Gispersion

Figure 4-2 presents a second scenario for observed exposure by direct observation. In this example, an indoor air 

sample is collected in conjunction with a sample taken directly from a crack in the basement foundation using 

Summa canisters. As in the example above, the decrease in contamination levels with distance supports the 

conclusion that the contamination is entering the structure from the subsurface, but other information supporting 

that the flow is outward from the sump would be recommended, such as measurement of a net pressure 

differential forcing air to be drawn up and out of the structure. 
42

Figure 4-2. Observed Exposure by Direct Observation Example 2 

Observed Exposure via an Air Sample Collected from a Crack in the Foundation Slab 

Basement

Contaminated 

Ground Water 

Plume

62

Air Sample from 

Crack in the 

Foundation Slab

Indoor Air Sample 

Concentration 12

Ground 

Surface

Wind Dispersion

Another example of documentation of an observed release by direct observation would be if a hazardous 

precipitate is found in basements subject to ground water flooding or if a sample of contaminated ground water is 

collected from the basement during a flood period. This finding would be coupled with information that in both 

cases it is known that a contaminant plume of the same substance is present in ground water below the structure 

and evidence that the substance’s presence is not from indoor sources. This example is depicted in Figure 4-3. 
43

Figure 4-3. Observed Exposure by Direct Observation Example 3

Observed Exposure via Intrusion of Contaminated Ground Water 

b. Evidence by Comparison to Background (Observed Exposure by Chemical Analysis)

Observed exposure by “chemical analysis” is established by comparing hazardous substance concentrations in 

background and release samples based on chemical analysis and documenting that the increase is due to a release 

from the site being evaluated. This observed exposure is performed in two steps: 1) the concentration of one or 

more hazardous substance(s) in one or more indoor air sample(s) taken from an occupied structure (termed the 

“release sample”) is compared to the concentration(s) at appropriate background locations and under appropriate 

background conditions and there is a significant increase in a hazardous substance concentration; and 2) 

demonstration that at least part of the significant increase can be shown to be attributable to a release(s) from the 

site being evaluated. Both steps must be achieved to document an observed exposure by chemical analysis. This 

option for establishing observed exposure differs from observed exposure by direct observation in that there must 

be a comparison of the hazardous substance concentration in a release sample to a background level and that there 

must be a significant increase in that hazardous substance concentration above the background level. 

Additional discussion of this method for establishing observed exposure by chemical analysis is outlined in detail 

below. 

i. Significant Increase and Background 

The first step in determining if observed exposure has occurred is documenting that the difference between the 

background level concentration and the release sample concentration is of sufficient magnitude to rule out the 

possibility that the difference is not simply due to variation in site conditions or in the replicability and precision 

of the sampling and analytical procedures. The magnitude required for this “significant increase” was established 
44

for all HRS pathways based on studies peer reviewed by the Science Advisory Board when the HRS was last 

revised in 1990. (See section 2.6 of the 1988 Revised HRS TSD for supplemental information.) 

A significant increase is generally considered to have occurred if the release sample hazardous substance 

concentration is at least three times a site-specific background level, provided that the background sample 

concentrations for the hazardous substance are found above appropriate detection limits (see section 2.3 of the 

1990 HRS and section 2.6 of the 1988 Revised HRS TSD). If the hazardous substance background level is below 

detection, any level of the hazardous substance detected in the targeted structure above the sample quantitation 

limit is considered to have a concentration significantly above background. 

Background levels for establishing a significant increase, in some cases, may be determined using chemical 

analysis of samples from similar structures collected from outside the area impacted by the release(s) from the site 

being evaluated. While the appropriate sample locations to be used to establish this background level will vary 

based on site-specific conditions, appropriate background air contamination levels need to account for both 

outdoor air concentrations from ambient sources (usually collected within the plume boundary) and indoor air 

concentrations in structures of similar construction type (usually collected outside the plume boundary – e.g., 

basement, slab-on-grade) within the vicinity, and other factors that could influence the contaminant concentration 

(e.g., presence of indoor sources other than the release, use of dry cleaning services, structure age, construction 

and maintenance, similar HVAC systems). This requirement ensures that the background level represents the 

concentration of a hazardous substance in the absence of the subsurface intrusion. In some cases it may be 

possible to use published studies on typical background concentrations in establishing an appropriate background 

level or to compare with site specific estimates. Additionally, alternate or preferred methods for establishing 

background levels and methods of attributing the release to the site may become available as the science for 

addressing subsurface intrusion progresses. For example, a recent scientific study suggests that positive 

ventilation sampling may produce relatively quick and accurate background data for structures containing indoor 

sources of hazardous substances (when conducted after removal of the source). 

ii. Attribution

The second step in determining if observed exposure by chemical analysis has occurred involves documenting, 

using multiple lines of evidence, that the significant increase must be attributed at least in part to a release from 

the site being evaluated. This step is required in all HRS pathways, including establishing observed releases in the 

three migration pathways, establishing observed contamination in the soil exposure component, and establishing 

observed exposure in the proposed subsurface intrusion component. (See section 2.3 of the 1990 HRS, Section 2.3 

of the proposed addition and section 2.6 of the 1988 Revised HRS TSD for supplemental information.). This step 

is taken to ensure that the increase is due to the release being evaluated and not from possible contaminant sources 

in the vicinity or in the structure being evaluated. (See section 4.1 of this document for further discussion). 

For the proposed subsurface intrusion component, consideration of other sources and decay/breakdown products 

of hazardous substances is particularly critical because many of the contaminants typically encountered by 

subsurface intrusion are solvents, particularly chlorinated solvents. Solvents are found in various household and

commercial cleaning products, as well as various consumer goods found in occupied structures. These products 

present a substantial challenge when identifying the contribution of contamination from the release that is being 

evaluated. Therefore, it is critical that a significant increase in these types of hazardous substances be documented 

as coming from the subsurface and not from substances that are not CERCLA eligible substances (e.g., 
45

commercial or consumer products being used in the structure). In addition, it should be documented that the 

volatile substances are not coming from contaminated outdoor air into the occupied structures. 

It is suggested that the documentation to support attribution to subsurface intrusion employ multiple lines of 

evidence, including outdoor air hazardous substance concentrations, identifying the hazardous substance at the 

source facility, site or release being investigated, and detecting the hazardous substance in subsurface samples. 

(See EPA 2004 User’s Guide for Evaluating Subsurface Vapor Intrusion Into Buildings for additional information 

regarding lines of evidence). In addition, actions should be taken to ensure that sources of the hazardous 

substances inside a structure (e.g., household chemicals) have been removed from the structure prior to sampling. 

Several EPA regional offices and state environmental agencies use surveys/questionnaires or checklists that 

provide guidance when conducting indoor air sampling and discuss how to account for potential anthropogenic 

(lifestyle) sources. (See Section 10 of this document for a list of references including several of these checklists 

that were considered in the proposed HRS additions). Establishing attribution to the site in some situations, 

however, may be straightforward to document. For example, when the hazardous substance is manmade, unique, 

and not used in consumer products and no contamination of the substance of concern exists in ambient air.

Therefore, there may be situations where not all the steps identified above will be necessary to establish

attribution. Future advancement in technical methods and increases in practical experience documenting the 

source of indoor contamination may aid in establishing attribution (e.g. isotopic fingerprinting of substances). 

One example of establishing an observed exposure by chemical analysis is presented in Figure 4-4. In this 

example, four background samples are collected from the indoor air of structures that are located outside of the

plume boundaries and one ambient air sample is collected within the area of observed exposure. None of these 

samples indicate that significant concentrations of the contaminant exist outside of the plume boundaries. Several

indoor air samples are collected from other occupied structures with hazardous substance concentrations 

significantly above the highest concentration found in any of the background samples are identified. In addition, 

the increase in the concentrations of contaminants is attributed to the intrusion of the subsurface contamination, 

because the contaminants found in the subsurface plume under the structures with increased contamination are the 

same. Furthermore, for this example, each structure was inventoried prior to the sampling and no anthropogenic 

indoor source of the contamination (e.g., household cleaners) was found in any of the structures. 
46

Figure 4-4. Observed Exposure by Chemical Analysis

In summary, if it is demonstrated that there is a significant increase above background in hazardous substance 

concentrations in an occupied structure, and it is demonstrated that the significant increase of the contamination is 

in part due to subsurface intrusion, then an observed exposure by chemical analysis is established. If one or more 

observed exposures occur in one or more occupied structure(s), then a likelihood of exposure value of 550 is 

assigned.

c. Special Considerations for Radioactive Hazardous Substances

As identified in Table 7-1 of the proposed HRS addition, observed exposures are documented differently for 

radioactive substances than nonradioactive substances. For the subsurface intrusion component, an observed 

exposure by direct observation is demonstrated if a material that contains one or more radionuclides has been 

observed entering an occupied structure via the subsurface or is known to have entered an occupied structure via 

the subsurface. The method for establishing observed exposure by chemical analysis for radionuclides is 

presented in Section 7.1.1 of the proposed HRS addition. The method for establishing a significant increase in 

radionuclides is statistically based. It should be noted that the method proposed for evaluating potential for 

radionuclide exposure is the same as presented in Section 4 of this TSD for all hazardous substances. Also, when 

evidence supports the inference of subsurface intrusion of a material that contains one or more radionuclides by 

the site into an occupied structure, demonstrated adverse effects associated with that release may be used to 

establish observed exposure by direct observation. Please refer to Section 7.0 of the proposed HRS addition for 

additional information regarding the evaluation of radionuclides at subsurface intrusion sites. 
47

d. Other Option Considered for Establishing Observed Exposure – Use of Subslab and/or 

Crawl Space Sampling to Document Observed Exposure

Establishing an observed exposure with subslab or crawl space sampling was considered as an alternative. The 

strong correlation between subslab, crawl space, and indoor contamination is discussed in greater detail in Section 

6 of this TSD. One alternative considered for establishing observed exposure was whether to consider crawl space 

and possibly subslab sampling sufficient to document an observed exposure by chemical analysis. However, 

because these areas of a structure are not regularly occupied, it was determined these samples would not 

document actual exposure to these contaminants. It should be noted that higher weightings are given to targets 

within occupied structures where subslab or semi-enclosed crawl space sampling has demonstrated elevated levels 

of contaminant concentrations (see Section 6 of this TSD). This reflects the higher probability of some portion of 

the contamination at these locations enter into the overlying structures (as the attenuation factor for these sample 

types is higher than the attenuation factor for samples from greater depths). See Section 6 of this TSD for 

additional information. 

4.3 Potential for Exposure – See Section 5.2.1.1.2 of the Proposed HRS Addition. 

When an observed exposure has not been established, the potential for exposure resulting from subsurface 

intrusion within structures located in an ASC is evaluated. (The potential for ground water intrusion is not 

included in the evaluation). This potential for exposure is based on the physical features of the structures, the 

subsurface geology, and the presence and physical properties of volatile hazardous substances associated with the 

site being evaluated. The following discussion outlines the proposed approach for evaluating potential exposure 

and the rationale for including this method as part of the proposed HRS addition. 

a. Rationale for Including Potential for Exposure Risks in the Subsurface Intrusion Addition

Potential for exposure is proposed to be evaluated at sites with documented subsurface contamination where no 

observed exposure in regularly occupied structures has been documented. Considering the potential for exposure 

is necessary because the lack of documented observed exposure data does not mean that subsurface intrusion is 

not occurring at times other than at the time of sampling, or will not occur in the future. Potential for exposure 

may also be used to evaluate a site when indoor sampling data is not available for periods of time when 

subsurface intrusion has occurred. This may include situations such as when sampling activities may be limited

during the site investigation for various reasons (e.g., when access to a site is limited, or subsurface intrusion is 

not present at the time of sampling due to spatial and temporal variability). 

Nevertheless, even if indoor air sampling was not done or has not documented an observed exposure, crawl space 

or subsurface sample data (e.g., ground water, soil, soil gas, subslab sampling data) may be available that depicts 

the presence of a contaminant plume underlying structures. 

The presence of crawl space or subsurface contamination demonstrates that a potential exists for vapor intrusion 

into the overlying structure and exposure in the overlying structures may have occurred or will occur in the future. 

Actual exposure may not be documented because sampling may not have been performed in these structures— 

possibly due to restriction of access given by the home owner or limited time and resources during sampling 

events.
48

Furthermore, the factors impacting when intrusion will occur and the rate of subsurface intrusion are extremely 

time-, site-, structure-, and climate-specific. Even if indoor sampling is performed, especially since site 

inspections for HRS evaluations are by necessity, screening events of limited duration, the sampling may have 

occurred during conditions where the intrusion was not occurring or where intrusion may have been occurring at 

levels not detectable or that cannot be differentiated from background sources of the contaminant. It may also be 

impractical, due to resource constraints, to perform multiple sampling events at every structure that may be 

overlying the contaminant plume under all conditions that might prove favorable to finding subsurface intrusion. 

For these reasons, and consistent with other HRS pathways, a potential for exposure factor value is being 

proposed for inclusion in the subsurface intrusion component. Excluding consideration of the potential for 

exposure could result in an unacceptable underestimation of the actual threat posed at a site.

b. Overview of Potential for Exposure Structure

The potential for exposure for the proposed subsurface intrusion component uses the same concept and structure 

used to estimate the potential to release in other pathways in the 1990 HRS (section 2.3). This structure involves 

predicting the probability of exposure (or releases) in the ASC based on factors such as structural containment 

features of the occupied structure and the route characteristics within the subsurface. Route characteristics include 

factors such as the physical and chemical properties of the hazardous substances and physical subsurface 

properties at the ASC that influence the probability that intrusion is occurring. 

The factors chosen for consideration in assigning a potential for exposure factor value are deterministic factors 

identified in most proposed models for projecting vapor intrusion, and are likely to be quantifiable at the site 

screening stage when an HRS evaluation is likely to be performed. The potential for exposure factor value reflects 

the probability that vapor intrusion into an occupied structure will occur, but it does not directly reflect the 

concentration of the hazardous substances that may intrude. The projection of actual indoor concentrations cannot 

be accurately predicted, especially when based on information typically available at sites at the time of the HRS 

evaluation (i.e., due to a limited scope during the preliminary site inspection stage). 

The generic structure used throughout the HRS for determining the potential for exposure/release was first 

presented and subject to review and comment as part of the original 1982 HRS and adopted in the current 1990 

HRS. Consistent with other HRS pathways and components, the potential for exposure factor value can vary 

between 0 and 500, whereas the observed exposure value is either 550 or zero. 

Consistent with other potential to release determinations in the HRS, the potential for exposure factor value for 

this component is calculated by summing the proposed route characteristic factor values (Depth to Contamination, 

Vertical Migration, and Vapor Migration Potential) and multiplying them by the structure containment factor 

value of the least contained structure. 
49

Figure 4-5 depicts an overview of the proposed containment and route characteristic factors used when evaluating 

potential for exposure. Sections 4.3.c and 4.3.d below discuss the individual factors comprising the potential for 

exposure calculation. Table 4-1 below presents the scoring of the potential for exposure factor in the subsurface 

intrusion component. This scoring uses the following ranges:

• Structure containment factor values range from 0 to 10; 

• Depth to contamination factor values range from 0 to 10; 

• Vertical migration factor values range from 1 to 15; and 

• Vapor migration factor values range from 0 to 25. 

These ranges for each factor value are consistent with how potential for exposures is calculated in other 1990 

HRS pathways such as the ground water migration pathway. Because potential for exposure is limited to a 

maximum value of 500, the maximum sum of the route characteristics is limited to 50 and the maximum structure 

containment factor value is 10. 

Table 4-1. Excerpt of Subsurface Intrusion Scoresheet (Table 7-1 of this document)

Factor Categories and Factors Maximum 

Value 

Value 

Assigned

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 * (2b+2c+2d), subject to a maximum of 500) 500

 

In addition, a potential for exposure score may only be determined when an ASC contains a hazardous substance 

with a vapor pressure greater than or equal to 1 torr or a Henry’s Law constant greater than or equal to 10-5 atm m3

mol-1. That is, only those substances likely to evaporate or volatilize within the subsurface are being considered 

when evaluating the potential for exposure in an overlying occupied structure. For example, a ground water plume 

containing benzo(g,h,i)perylene could be evaluated only if additional, more volatile substances are also present 

because the vapor pressure for benzo(g,h,i)perylene is 10-10 torr. 

Figure 4-5 presents an overview of the structure containment and route characteristic factors proposed to be 

included as part of the potential for exposure evaluation. Sections 4.3.c and 4.3.d provide additional discussion of 

structure containment features and route characteristics. 
50

Figure 4-5. Potential for Exposure: Overview

Section 5.2.1.1.2 of the Proposed Addition

Structure Containment / Route Characteristics

Clay Particles

Pore Space

Sump

Contaminated Ground Water Plume

Regularly Occupied Residence

Clay Layer

Ground 

Surface

Depth to Contamination Factor

Vertical Migration Factor

Vapor Migration Potential Factor

Potential for Exposure = Structure Containment x (Depth to Contamination + Vertical Migration + Vapor Migration Potential) Unsaturated Zone

Unsaturated

Zone

Contaminants

Contaminated Soil Gas Plume

Regularly Occupied Residence with a 

Preferential Pathway

Structure Containment Factor

(e.g., mitigation system, preferential pathway)

c. Selection and Weighting of Structure Containment Features

The structure containment factor reflects whether a volatile substance can enter an occupied structure after the

substance has migrated from the subsurface to the base of the structure. The assigned containment factor depends

primarily on the presence of structural features (e.g., preferential pathways such as cracks in a foundation) and

mitigation actions that have been taken (e.g., active or passive ventilation systems). This is parallel to the 

evaluation of containment in other parts of the HRS. Within the HRS migration pathways, the containment factor 

is considered a measure of the manmade structures to minimize or prevent the movement of hazardous substances 

from the release location. (See section V.D.1. of the preamble to the 1988 Revised HRS (53 FR 51985, 

December 23, 1988) for supplemental information.) For the soil exposure pathway, containment (such as the 

presence of impenetrable cover or restrictions to the access to surficial contamination) (see sections 5.0.1, 

5.2.1.1, and 5.2.1.2 of the 1990 HRS) is included when determining the areas of exposure. For the proposed 

subsurface intrusion component, the structure containment features take into account structural features or other 

methods that minimize or prevent the exposure of hazardous substances resulting from subsurface intrusion by 

blocking their movement into an occupied structure. Consistent with the1990 HRS, in the proposed addition, 

structure containment factor values range from 0 to 10. A low containment factor value indicates a low chance 

for intrusion into a structure. For example, Table 5-12 in the proposed HRS addition specifies that a regularly 
51

occupied structure with an engineered, active vapor mitigation system (e.g., active venting) without documented 

institutional controls (e.g., deed restrictions; funding in place for ongoing operation, inspection, and maintenance) 

has a lower structure containment value (structure containment value of 2) than a regularly occupied structure

with an engineered vapor migration barrier system that does not address all preferential pathways (structure 

containment value of 7) because the vapor mitigation barrier system is more likely to minimize exposure of 

hazardous substances. The factors evaluated and factor values assigned are shown in Table 4-2 below. 

Table 4-2. Structure Containment (Table 5-12 in Proposed HRS Addition)

No. Evidence of Structure Containment Assigned 

value

1. Regularly occupied structure with evidence of subsurface intrusion, including documented 

observed exposure or sampling of bio or inert gases, such as methane and radon. 10

2. Regularly occupied structure with open preferential pathways from the subsurface (e.g., sumps, 

foundation cracks, unsealed utility lines). 10

3. Regularly occupied structure with an engineered vapor migration barrier system that does not 

address all preferential pathways. 7 

4.

Regularly occupied structure with an engineered passive vapor mitigation system without

documented institutional controls (e.g., deed restrictions) or evidence of regular maintenance 

and inspection.

6 

5. Regularly occupied structure with no visible open preferential pathways from the subsurface 

(e.g., sumps, foundation cracks, unsealed utility lines). 4 

6.

Regularly occupied structure 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.

3 

7.

Regularly occupied structure 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.

2 

8. Regularly occupied structure with unknown containment features. 2 

9.

Regularly occupied structure 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. This does not include mitigation 

systems installed as part of a removal or other temporary response by federal, state or tribal 

authorities. 

0 

10.

Regularly occupied structure with a foundation raised greater than 6 feet (e.g., structure on 

stilts) or structure that has been built, and maintained, in a manner to prevent subsurface 

intrusion

0 
52

In developing the proposed structure containment table, EPA reviewed the available literature, and identified 

multiple structural and mitigation features that may possibly influence the likelihood that volatile hazardous 

substances present at the base of an occupied structure could enter the structure. These features include, but are 

not limited to: 

• Evidence that gases or vapors have been entering the structure; 

• The presence of preferential pathways such as foundation cracks, utility conduits, open sumps, and 

unsealed crawl space access locations; and 

• The presence of passive and active mitigation measures such as vapor barriers, passive and active 

ventilation systems, along with evidence that systems are maintained and operating.

Structural and mitigation features installed as temporary remedies that do not fully address the long term threat of 

hazardous substances should not be considered a containment feature. The proposed structure containment Table 

5-12 was developed using these parameters along with input received during the peer review process (see 

Appendix E of this TSD for additional discussion regarding the specific input received as part of the peer review 

process). Containment factor values based on building parameters such as structure type were considered (e.g., 

basement, slab-on-grade, crawl space); however, it was determined that recent studies have shown construction 

type is not a discriminating factor in whether subsurface intrusion can occur. Selected structure containment 

examples are presented in Figure 4-6. 

Figure 4-6. Potential for Exposure: Structure Containment Factor

See Section 5.2.1.1.2.1 of the Proposed HRS Addition

Open Preferential Pathways

ConPaminaPed Ground WaPer or Soil Gas Plume

Ground 

Surface

Sump

Wind 

Dispersion

Above Ground 

Structure (min. 6’)

Basement Basement

Active Subslab

Ventilation System

Indoor Air Sampler 

(e.g., Evacuated Canister)

Example 1 – Regularly Occupied 

residence wiPO evidence of 

subsurface inPrusion (eBgB, 

observed exposure)

SPrucPure ConPainmenP = 10

(Table D-12 NoB 1)

Example 2 – Regularly occupied 

sPrucPure wiPO open preferenPial 

paPOways from POe subsurface 

(eBgB, sumps, cracks, unsealed 

uPiliPy lines

SPrucPure ConPainmenP = 10

(Table D-12 NoB 2)

Example 3 – Regularly occupied sPrucPure 

wiPO an engineered, acPive vapor 

miPigaPion sysPem (eBgB, acPive venPing) 

wiPOouP documenPed insPiPuPional conProls 

(eBgB, deed resPricPions) and funding in 

place for on-going operaPion, inspecPion 

and mainPenance

SPrucPure ConPainmenP = 2

(Table D-12 NoB 8)

Example 4 – Regularly occupied 

sPrucPure wiPO a foundaPion 

raised greaPer POan 6 feeP (eBgB, 

sPrucPure on sPilPs)

SPrucPure ConPainmenP = 0

(Table D-12 NoB 10)
53

d. Selection and Weighting of Key Route Parameters (Route Characteristics)

The HRS uses “route characteristics” (i.e., factors that impact the movement of gases through the subsurface) to

estimate the probability of contaminants moving to the foundation of occupied structures, such as from ground 

water through the soil to immediately below a foundation. These route characteristics represent the physical and 

chemical properties of the specific hazardous substances and the media in which they must have moved through 

or could move through. To determine what route characteristics are appropriate for evaluating potential exposure 

to subsurface contaminants, present modeling analyses (e.g., Identification of Critical Parameters for the Johnson 

and Ettinger (1991) Vapor Intrusion Model – Published May 2002) that estimate the levels of contaminant

exposure (based on numerous route characteristics along with the relationship between the relevant input 

variables) were examined. In addition, input received during the peer review process (see Appendix E of this TSD

for additional discussion) was evaluated during peer-evaluation of the selected route characteristics. Careful

consideration was also given to ensure that route characteristic factors may be available, measured or calculated, 

on a site-specific basis within the scope of a Superfund site inspection. 

To evaluate the intrinsic relationships between the examined route characteristics, a limited sensitivity analysis 

was performed to identify those factors that are likely available at most sites during a Superfund site inspection 

and have the greatest impact on potential for exposure. Further, the sensitivity analysis helped to identify the 

weightings that should be associated with the range of factors. This analysis was conducted using a simplified

version of the widely accepted Johnson and Ettinger (J&E) model. The online model calculates an estimated

attenuation factor and necessary contaminant concentration to impact indoor air based on the chemical properties 

of the contaminant, soil characteristics, average soil/ground water temperature, and depth to contamination. Based 

on the agency’s analysis, three factors represented the greatest impact on potential for exposure for which

sufficient site-specific information could be collected during a site inspection. These three factors are: depth to 

contamination, vertical migration, and vapor migration potential. Standard parameters for chemical, soil, building, 

and exposure parameters were obtained from EPA’s 2004 User's Guide For Evaluating Subsurface Vapor 

Intrusion Into Buildings. (See Appendix C of this document for further details on the sensitivity analysis.). 

As depicted in Table 4-3, the attenuation factors were projected for multiple scenarios and used in assigning 

possible weighting to correspond with the factors included in the HRS evaluation. For example, Figure 4-7 

illustrates the impact of depth on attenuation factors using naphthalene13 contamination in a clay soil as the 

scenario. The single variable altered for this scenario was the depth to contamination from a building’s 

foundation. As shown, both the soil gas and ground water attenuation factors reflect the percent of contaminants 

in subsurface soil gas or ground water, respectively, to intrude into a structure. As expected, the attenuation 

factors decrease with depth. Attenuation factors for both ground water and soil gas were obtained using the J&E 

model (http://www.epa.gov/vaporintrusion/epa-spreadsheet-modeling-subsurface-vapor-intrusion) and those 

results are presented in Table 4-3. A plot of those resulting attenuation factors and associated depths is presented

in Figure 4-7. As this figure shows, the attenuation factors for both ground water and soil gas begin to level off

around 150 feet below the surface. This shows that vapor intrusion could occur at least to this depth but the 

amount reaching a structure from this depth and deeper is quite low. Therefore, consideration of contamination at

depths up to 150 feet warrants consideration and has been included in the depth to contamination value, although 

the resulting potential to release category value will be low.

13 Naphthalene was chosen due to its relatively low volatility to not confound the effect of depth with the effect of volatility.
54

Figure 4-7. Depth to Contamination Attenuation Factor Trends

Based on Johnson and Ettinger Modeling Scenarios

0.0E+0

1.0E-4

2.0E-4

3.0E-4

4.0E-4

5.0E-4

6.0E-4

7.0E-4

8.0E-4

9.0E-4

1.0E-3

0.0E+0

5.0E-5

1.0E-4

1.5E-4

2.0E-4

2.5E-4

0 50 100 150 200 250 300

Soil Gas Attenuation Factor

Ground Water Attenuation Factor

Depth to Contamination (ft)

Ground Water

Soil Gas

Based on this projection, the proposed depth to contamination factor value is weighted lowest for all 

contamination with a depth to contamination of 150 feet or greater below a structure’s foundation (see Table 5-13 

of the proposed HRS addition). Additional discussion is presented below detailing the methods proposed for 

evaluating depth to contamination. 

Table 4-3. Effect of Depth to Contamination

Depth to 

Contamination 

(feet)

Contaminant Soil 

Type

Ground 

Water 

Temperature 

(Fahrenheit) 

Soil Gas 

Attenuation 

Factor 

(αSG) 

Ground 

Water 

Attenuation 

Factor 

(αGW) 

5 Naphthalene Clay 50 8.80E-04 2.18E-04

10 Naphthalene Clay 50 5.08E-04 1.84E-04

20 Naphthalene Clay 50 2.75E-04 1.41E-04

50 Naphthalene Clay 50 1.16E-04 8.28E-05

100 Naphthalene Clay 50 5.90E-05 4.90E-05

150 Naphthalene Clay 50 3.96E-05 3.48E-05

200 Naphthalene Clay 50 2.98E-05 2.70E-05

250 Naphthalene Clay 50 2.39E-05 2.21E-05

300 Naphthalene Clay 50 1.99E-05 1.86E-05

As noted above, the weightings for other route characteristic factors were also evaluated using the online J&E 

model. Specifically, total porosity, unsaturated zone moisture content, capillary zone moisture content at air-entry 

pressure, height of capillary zone, and soil-gas flow rate into a building were considered during the analysis. 

Standard parameters for clay, sand, and loam (published in EPA’s 2004 User’s Guide for Evaluating Subsurface 

Vapor Intrusion into Buildings) were used in the study. Table 4-4 indicates the proportional impact a soil type 
55

plays in evaluating subsurface intrusion. Additional information regarding the proposed method for incorporating 

porosity and permeability into the potential for exposure factor is provided below. 

Table 4-4. Effect of Soil Type (i.e., Porosity/Permeability)

Soil Type Porosity Contaminant 

Depth to 

Contamination 

(feet)

Ground 

Water 

Temperature 

(Fahrenheit) 

Soil Gas 

Attenuation 

Factor 

(αSG) 

Ground 

Water 

Attenuation 

Factor 

(αGW) 

Clay 0.459 Naphthalene 50 50 1.16E-04 8.28E-05

Loam 0.399 Naphthalene 50 50 1.65E-04 1.21E-04

Sand 0.375 Naphthalene 50 50 3.91E-04 3.28E-04

A contaminant’s volatility was also considered in the limited sensitivity analysis. Table 4-5 indicates the soil gas 

and ground water attenuation factors for trichloroethylene (TCE) and naphthalene. The variations between the two 

attenuation factors reflect the range in attenuation factors for common volatile substances. This variation is the 

basis for the proposed weighting within the potential for exposure factor values outlined in more detail below. 

Table 4-5. Effect of Contaminant (i.e., Volatility)

Contaminant Volatility Soil 

Type

Depth to 

Contamination 

(feet)

Ground 

Water 

Temperature 

(Fahrenheit) 

Soil Gas 

Attenuation 

Factor 

(αSG) 

Ground 

Water 

Attenuation 

Factor 

(αGW) 

Naphthalene 0.2 Clay 50 50 1.16E-04 8.28E-05

TCE 1 Clay 50 50 1.52E-04 1.92E-05

The rationale for the route characteristics selected to be included in the proposed subsurface intrusion component 

is discussed further in Sections 4.3.d.i through 4.3.d.v below. 

i. Depth to Contamination – See Section 5.2.1.1.2.1.2 of the Proposed HRS Addition.

The depth to contamination factor represents the vertical distance between the highest known point of subsurface 

contamination and the lowest horizontal point of an occupied structure (e.g., a basement floor). This distance 

reflects how much farther the contaminant has to travel to enter an occupied structure. It does not reflect the 

sample media type. The probability of exposure decreases as this depth increases. EPA considers depth to 

contamination a significant route characteristic based on a review of available literature along with input received 

during the peer review process. In addition, a review of EPA’s Vapor Intrusion Database, as of 2012, documents 

that depth to contamination is a significant factor.

The proposed depth to contamination factor values range from zero to 10, where increasing depth results in a 

lower factor value. As part of EPA’s sensitivity analysis in developing route characteristics, it was determined 

that at depths of 150 feet or greater the attenuation factor did not change significantly. Therefore, the potential for 

exposure correspondingly did not significantly change with depths greater than 150 feet. As a result, the 

maximum depth considered in Table 5-13 of the proposed HRS addition was selected to be 150 feet (see Section 

5.2.1.1.2.1.2 of the proposed addition). (Note however, if subsurface intrusion is documented from contamination 

at any depth regardless of whether it is less than, or greater than, 150 feet at indoor concentrations that meet 
56

observed exposure criteria, the likelihood of release category value is based on observed exposure, not on a 

potential for exposure score.). 

The depth to contamination factor has been designed to rely on data available at the time of a site inspection. At 

the SI stage, the subsurface plume may not be fully characterized. For example, an occupied structure with 

contamination meeting observed release criteria found in a ground water sample collected at a depth of 50 feet 

may or may not also have soil gas samples collected at shallower depths although the contaminated soil gas is 

present at that depth. However, without actual information documenting contamination at shallower depths, the 

evaluation must rely on the existing sample to determine depth to contamination. If both samples are available, 

that sample result giving the highest factor value is used. (However, sites may always be re-evaluated in the future 

if additional data become available.). 

In addition, special consideration is given when assigning the depth to contamination factor value to locations 

with channelized flow features, such as fractured bedrock or karst, and situations where the contamination 

meeting observed release criteria is present directly below the slab for the structure. The presence of these features 

increases the probability of exposure because there is less resistance to contaminant transport in the subsurface. 

Figures 4-8, 4-9 and 4-10, present examples for determining depth to contamination. The first example, Figure 4-

8, is based on a contaminant ground water plume. In the first example (Figure 4-8), the total measured depth from 

the building’s foundation (in this case, the slab-on-grade) to the ground water plume is 35 feet. However, a karst 

layer approximately 10 feet thick is present between the ground water plume and the structure. Therefore, the 

resultant depth to contamination is 25 feet (35 – 10). Using Table 5-13 of the proposed HRS addition, a depth to 

contamination factor value of 6 is assigned. 
57

Figure 4-8. Potential for Exposure: Depth to Contamination Factor (Ground Water Plume)

See Section 5.2.1.1.2.2 of the Proposed HRS Addition

In the second example (Figure 4-9), the total measured depth from the building’s foundation (in this case, the 

slab-on-grade) to the soil gas plume is 20 feet. However, there is a karstic layer approximately 7 feet thick 

between the soil gas plume and the structure. Therefore, the resultant depth to contamination is 13 feet (20 – 7). 

Using Table 5-13 of the proposed HRS addition, a depth to contamination factor value of 8 is assigned. 
58

Figure 4-9. Potential for Exposure: Depth to Contamination Factor (Soil Gas Plume)

See Section 5.2.1.1.2.2 of the Proposed HRS Addition

Figure 4-10 is an example showing where subslab contamination is present. Therefore, the depth to contamination 

is less than 10 feet, and a factor value of 10 is assigned.
59

Figure 4-10. Potential for Exposure: Depth to Contamination Factor (Subslab Contamination)

See Section 5.2.1.1.2.2 of the Proposed HRS Addition

ii. Vertical Migration – See Section 5.2.1.1.2.1.3 of the Proposed HRS Addition.

The vertical migration factor considers the geologic makeup of materials between an occupied structure and the 

contaminant plume and the rate at which substances are likely to move or to have moved through the materials. 

EPA is proposing to base vertical migration on two factors: effective porosity (or equivalently, the permeability) 

of geologic materials and the thickness of the least porous layer.

As depicted in Table 4-6, factor values for effective porosity (as it relates to permeability) of geologic materials 

range from one to four and are based solely on the typical range of porosity of subsurface materials (e.g., gravel, 

sand, silt and clay). The effective porosity of these materials represents the open pore space available for 

contaminant vapors to migrate through. These factor values are used in conjunction with the thickness of the layer 

with the least porosity (greater than one foot in thickness) to establish a vertical migration factor value, ranging 

from 1 to 15. Similar to the ground water migration pathway, the top 10 feet is not considered when evaluating 

the vertical migration factor because the structure of the soil in this layer is typically impacted by human activities 

such as utility lines and by roots from vegetation that can result in channelized flow. 
60

Table 4-6. Effective Porosity/Permeability of Geologic Materials (Table 5-14 in the Proposed HRS Addition)

Type of Material

Assigned 

Porosity/Permeability 

Value

Gravel; highly permeable fractured igneous and metamorphic rocks; permeable basalt; 

karst limestones and dolomites. 1 

Sand; sandy clays; sandy loams; 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.

2 

Silt; loams; silty loams; loesses; 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.

3 

Clay; low permeability till (compact unfractured till); shale; unfractured metamorphic 

and igneous rocks. 4 

Studies have demonstrated that effective porosity largely reflects moisture content and is an important factor in 

projecting vapor intrusion. Long-term measurements of effective porosity are beyond the scope of an HRS 

evaluation in that they vary with time, climate, and depth. Soil type reflects, at least in, part long-term moisture 

porosity conditions and the HRS uses soil type as a surrogate for effective porosity.

To reflect soil moisture in EPA’s assessment of potential for exposure, “average soil moisture content” values for 

specific soil types, published by EPA, were used to develop effective porosity/permeability factor values (see 

EPA’s 2004 User’s Guide for Evaluating Subsurface Vapor Intrusion into Buildings). 

Factor values for effective porosity (as it relates to permeability) of geologic materials range from one to four, and 

the values are based on the typical range of porosity of subsurface materials (e.g., gravel, sand, silt and clay). The 

effective porosity of these materials represents the open and interconnected pore space (i.e., the effective) 

available for contaminant vapors to migrate through. 

It should be noted that some lower porosity materials such as clay can also be very effective vapor barriers; 

however, the horizontal continuity of layers is often incomplete and can allow for vapor migration through the 

resulting gap. Incomplete continuity can result from differential deposition during formation, erosion after 

deposition, or manmade preferential pathways that inhibit the horizontal continuity. Similarly, bedrock often 

contains cracks and fractures that function essentially as open channels for vapor migration. This phenomenon is 

especially important when the low porosity layers are relatively thin. Therefore, Table 5-15, Vertical Migration 

Factor Values, of the proposed addition to the HRS assigns a high value in these situations. However, at greater 

thickness these layers tend to be more competent over a wider area, and thereby hinder vapor migration more 

effectively, and are assigned a lower value in these situations.

Furthermore, competent clay layers are likely to also influence the depth to contamination factor value as well as 

the vertical migration value. The presence of any low porosity layer would likely correspond with a greater depth 

to contamination value and thus also a lower potential for exposure factor value. 
61

Figures 4-11 and 4-12 present two examples for determining vertical migration factor values, the first based on a 

contaminant ground water plume and the second based on a contaminant soil gas plume. In the first example 

(Figure 4-11), there is an approximate six foot thick clay layer that represents the lowest porosity/permeability 

range (highest Table 5-14 factor value from the proposed HRS addition). Using table 5-14 factor value for clay 

(i.e., 4) and the measured thickness, the vertical migration factor value is obtained from Table 5-15 of the 

proposed HRS addition (i.e., 12). 

Figure 4-11. Potential for Exposure: Vertical Migration Factor (Ground Water Plume)

See Section 5.2.1.1.2.3 of the Proposed HRS Addition

Sand Layer

Depth Below Ground 

Surface (BGS)

Contaminated Ground Water Plume

Limestone (Karstic) Layer

5’

10’

15’

20’

25’

30’

35’

40’

Clay Layer

0’

1

4

1

2

Effective Porosity / Permeability Factor Values

(From HRS Table 5-14)

Assigned 

Porosity/

Permeability 

Value

Thickness of Lowest Porosity Layer(s)(feet)

0 to 5 Greater 

than 5 to 

10

Greater 

than 10 to 

20

Greater 

than 20 to 

50

Greater 

than 50 to 

100

Greater 

than 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

Table 5-15 Vertical Migration Factor Values

Layer with the lowest effective porosity / 

permeability (i.e., highest Table 5-14 Factor 

Value); Thickness = 6’ (min. of 1’).*

Topsoil Layer

Regularly 

Occupied 

Residence

Vertical Migration 

Factor Value = 12

* Do not consider layers or portions 

of layers within the first 10 feet of 

the depth to contamination.

6’

In the second example (Figure 4-12), there is an approximate six foot thick sand layer that represents the lowest 

porosity/permeability (highest proposed HRS addition Table 5-14 factor value). Using the Table 5-14 factor value 

for sand (i.e., 2) and the measured thickness, the vertical migration factor value is obtained from the proposed 

HRS addition Table 5-15 (i.e., 14).
62

Figure 4-12. Potential for Exposure: Vertical Migration Factor (Soil Gas Plume)

See Section 5.2.1.1.2.3 of the Proposed HRS Addition

Depth Below Ground 

Surface (BGS)

Contaminated Ground Water or Soil Gas Plume

20’

35’

40’

Effective Porosity / Permeability Factor Values

(From HRS Table 5-14)

Assigned 

Porosity/

Permeability 

Value

Thickness of Lowest Porosity Layer(s)(feet)

0 to 5 Greater 

than 5 to 

10

Greater 

than 10 to 

20

Greater 

than 20 to 

50

Greater 

than 50 to 

100

Greater 

than 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

Table 5-15 Vertical Migration Factor Values

Layer with the lowest effective porosity / 

permeability (i.e., highest Table 5-14 Factor 

Value); Thickness = 6’ (min. of 1’).*

Regularly 

Occupied 

Residence

Vertical Migration 

Sand Layer Factor Value = 14

Ground Water Aquifer

Topsoil Layer 5’

Clay Layer

0’

Contaminated Soil Gas Plume

Limestone (Karstic) Layer 15’

25’

30’

2

1

1

Soil Gas 

Sampling Point

* Do not consider layers or portions 

of layers within the first 10 feet of 

the depth to contamination.

10’

6’ iii. Vapor Migration Potential – See Section 5.2.1.1.2.1.4 of the Proposed HRS Addition.

The vapor migration potential factor is based on hazardous substance-specific parameters, including both the 

vapor pressure and Henry’s Law constant values for contaminants associated with the site. The vapor migration 

potential factor evaluates the volatile nature of these contaminants and, based on a limited sensitivity analysis of 

subsurface migration modeling (see Appendix C, Conceptual Site Modeling), is projected to be the most 

influential route characteristic factor for calculating potential for exposure. When calculating the vapor migration 

potential factor, values are assigned to each volatile hazardous substance. Because vapor pressure and Henry’s 

Law constant value are directly related to a substance’s volatility, vapor pressure and a Henry’s Law constant 

values are determined for each substance and then summed. The highest of those resulting values is used to assign

the vapor migration potential factor value. 

Figure 4-13 presents an example for determining vapor migration potential. In the example, four CERCLA 

hazardous substances are associated with the site: trichloroethylene, tetrachloroethylene, benzene, and 

naphthalene. Vapor pressure and Henry’s constant values are used to assign a value from the proposed HRS 

addition Table 5-16. Those two values are then summed for each contaminant and assigned a value from the 

proposed HRS addition Table 5-17. The hazardous substance with the highest resulting value is selected and its 

vapor migration potential value is assigned as the vapor migration potential factor value. In the example below, 

trichloroethylene, tetrachloroethylene, and benzene all have an assigned value of 25, and naphthalene has an 

assigned value of 15. Therefore, the vapor migration potential factor value would be 25.
63

Figure 4-13. Potential for Exposure: Vapor Migration Potential Factor (Contaminated Ground Water 

Plume)

See Section 5.2.1.1.2.4 of the Proposed HRS Addition

iv. Other Route Characteristic Options Considered

Soil Moisture

As noted previously, soil moisture content is also known to impact a contaminant’s potential for movement 

through the subsurface. However, this factor cannot be evaluated adequately in the limited period of a site 

inspection. Temporal and seasonal fluctuations in rainfall/runoff rates over multiple seasons affect the soil 

moisture content at a specific location. As a result, soil moisture content can vary temporally and with depth. (See 

e.g., Swartjes, F. A., (2011) Dealing with Contaminated Sites—From Theory towards Practical Application, 

chapter 10—Vapour Intrusion). For these reasons, a site inspection level sampling event of limited duration is 

unlikely to appropriately produce a measure of soil moisture content that reflects all the possible conditions that 

could occur over time at a site. Therefore, soil moisture was not included as a route characteristic in the projection 

of the potential for exposure in the proposed subsurface intrusion component. However, soil moisture content is 

usually considered directly correlated to soil type, a factor which is included in projecting the probability of 

vertical migration. (See Identification of Critical Parameters (discussing the Johnson and Ettinger Model)). 

Ground Water Temperature

The limited sensitivity analysis on J&E parameters also included evaluating the impact that ground water 

temperature and soil temperature in general have in establishing potential for exposure. As depicted in Table 4-7, 
64

varying ground water temperatures ranging from 35 to 77 degrees Fahrenheit resulted in attenuation factors 

varying roughly two percent. Thus, ground water and soil temperature plays a role in evaluating a contaminant’s 

potential for movement in the subsurface. However, EPA decided not to include temperature as a factor in the 

evaluation because a search for temperature information only identified generic ranges based on limited 

measurements. Furthermore, it is not always possible to measure the range of temperatures over time at a site 

during a typical site inspection. The infeasibility of collecting long term data, coupled with the minimal variance 

in impact on intrusion rates, supports the decision to omit site-specific ground water temperature as a potential for 

exposure factor.

Table 4-7. Effect of Ground Water Temperature

Ground 

Water 

Temperature 

(Fahrenheit) 

Contaminant Soil 

Type

Depth to 

Contamination 

(feet)

Soil Gas 

Attenuation 

Factor 

(αSG) 

Ground 

Water 

Attenuation 

Factor 

(αGW) 

35 Naphthalene Clay 50 1.17E-04 9.90E-05

50 Naphthalene Clay 50 1.16E-04 8.28E-05

77 Naphthalene Clay 50 1.15E-04 5.21E-05

Degradation/Attenuation

Degradation can play a major role in subsurface intrusion, in that some hazardous substances may degrade 

significantly within the subsurface prior to entering a structure. Determining if a substance will degrade depends 

on both the substance characteristics and site-specific conditions. While the ability of a substance to degrade in 

the environment can be predicted based on its structural characteristics, whether it actually degrades is dependent 

on site-specific conditions. The site-specific conditions that can affect the rate of degradation can be extremely 

variable (e.g., oxygen content, moisture) and it is beyond the scope of an SI to measure or predict these conditions 

with accuracy. Therefore, a degradation factor was not added as a route characteristic. Instead, degradation is 

considered in conjunction with toxicity as part of the waste characteristics factor category, which reflects the 

chemical properties of the substances at a site. See Section 5.0 of this TSD for additional discussion regarding the 

inclusion of degradation as a factor value as part of the waste characteristics factor category.

v. Consideration of Potential for Ground Water Contaminant Intrusion

EPA considered including ground water contaminant intrusion in evaluating potential for exposure in the 

proposed subsurface intrusion component. As with the potential for vapor intrusion (i.e., potential for exposure), 

the standard containment and route characteristic approach was applied to the potential for ground water intrusion. 

Consideration was given to the depth to contaminated ground water and the fluctuations in ground water 

elevations. However, the available science and methods are considered beyond the scope of a site inspection to 

predict this potential for intrusion with a significant level of confidence.

vi. Assigning a Likelihood of Exposure Factor Value

As previously discussed, when an observed exposure is documented at a site, either through direct observation or 

by chemical analysis, a likelihood of exposure category value of 550 is assigned. However, if an observed 
65

exposure is not established, then the potential for exposure is evaluated. The likelihood of exposure category 

value is the resulting product of the containment factor value and the sum of the route characteristics factor 

values. These values range from zero to 500. For example, if it is determined that using the methods outlined 

above a site has the potential for exposure factor values depicted in Table 4-8, then the resulting potential for 

exposure factor value—and therefore the likelihood of exposure factor category value—would be [10 * (6 + 12 + 

25)], or 430.

Table 4-8. Example Score Sheet - Potential for Exposure Factor Value Score

Factor Categories and Factors Maximum 

Value 

Value 

Assigned

2. Potential for Exposure

 2a. Structure Containment 10 10

 2b. Depth to contamination 10 6 

 2c. Vertical Migration 15 12

 2d. Vapor Migration Potential 25 25

3. Potential for Exposure (lines 2a * (2b+2c+2d), subject to a maximum of 500) 500 430
66

5.0 Component Structure: Waste Characteristics

The waste characteristics category value uses factor values that are related to three of the relative risk 

considerations included in the basic HRS structure: 1) the toxicity or other potential adverse effects to a receptor 

from the releases; 2) the potential for a substance to degrade in the subsurface prior to intruding into an overlying 

occupied structure and thereby reducing the possible dose; and 3) the expected magnitude and duration of the 

exposure. The factors considered in determining the waste characteristics category value are the toxicity of the 

hazardous substances, whether the substance is likely to degrade before intruding into an occupied structure, and 

an estimate of the quantity of the hazardous substances to which occupants could be exposed. Consistent with the 

HRS, the assigned toxicity/degradation and hazardous waste quantity factor values are multiplied together and 

then scaled to determine this category value for the subsurface intrusion component. (See sections 2.2 and 2.4 of 

the 1988 Revised HRS TSD for further discussion on the structure of this factor category and how it fits within 

the overall HRS structure.) 

Throughout the evaluation of the waste characteristics factor category, only those regularly occupied structures 

that can be assigned a structure containment factor value greater than zero (proposed HRS addition Table 5-12) 

and only those wastes, waste-streams, and hazardous substances that can be attributed (either directly or 

indirectly) to the site being evaluated may be considered. A regularly occupied structure with a structure 

containment factor of zero represents a structure that effectively eliminates the threat associated with vapors 

intruding from the subsurface. Hazardous substances meeting observed exposure criteria at the site are eligible 

for consideration in the site evaluation. If a hazardous substance does not meet the observed exposure criteria, but 

the hazardous substance concentration within the ASC meets observed release criteria and has a vapor pressure 

greater than or equal to 1 torr, or a Henry’s Law constant of 10-5 atm m3 mol-1 or greater, then the substance can be 

considered eligible in assigning a Waste Characteristics category value. How and why these factors are proposed 

to be included in this containment factor category is discussed in Sections 5.1 through 5.3 below. 

5.1 Inclusion of a Toxicity/Degradation Value – See Section 5.2.1.2.1of the Proposed HRS Addition 

The proposed toxicity/degradation factor is a combination of both a toxicity and degradation factor value. These 

factors are hazardous substance-specific and represent both the magnitude of toxicity to humans and the potential 

for a substance to at least partially degrade within the subsurface environment prior to entering an overlying 

structure. A description of each factor and the rational for their inclusion is provided below.

a. Toxicity

The human toxicity factor (as compared to the ecosystem toxicity factor) in the overall HRS structure reflects the 

toxicity of a hazardous substance associated with a release or exposure and is assigned the same factor value for 

all the pathways and components in the 1990 HRS. This same approach is taken in the proposed HRS addition. 

The subsurface intrusion component is assigned the same corresponding factor value as the other components and 

pathways of the HRS. The rationale for the assignment of the factor value is discussed in section 2.3 of the 1988 

Revised HRS TSD. In developing the1990 HRS, guidance was sought from the Science Advisory Board (SAB) 

regarding the inclusion of toxicity as a factor. It was determined in 1990 that the inherent toxicity of a substance 

should be included independent of the mode of migration (via water, air, or by direct exposure). The toxicity 

factor within the HRS algorithm reflects that hazardous substances can come into contact with the receptor 

through different exposure routes (e.g., by inhalation, orally by ingestion of solids and drinking of liquids, by 
67

dermal exposure). All other factors in the HRS evaluation of the individual pathways and components (e.g., 

likelihood of release, mobility, persistence, containment) are based on the probability of the hazardous substances 

migrating to a receptor—or in the soil exposure component, a receptor moving to the contamination. 

Specifically, the toxicity factor for the subsurface intrusion component is based on the toxicity of eligible 

substances present at a site that are available to intrude into an occupied structure (and therefore to a receptor). 

The factor value is driven by the magnitude of each hazardous substance’s acute and chronic toxicity to humans. 

The toxicity factor value is directly related to the concentration at which the hazardous substance is known to 

have an adverse health effect—the more toxic the chemical, the higher the toxicity value. The method for 

assigning this value is contained in section 2.4.1.1 of the 1990 HRS (40 CFR 300, Appendix A) and is discussed 

in section 2.3 of the 1988 Revised HRS TSD. 

Special Considerations for Assigning Toxicity of Radioactive Hazardous Substances

As outlined in Section 7.2.1 of the proposed HRS addition, human toxicity factor values are assigned to those 

radionuclides that meet observed exposure criteria; the assigned values are based on quantitative dose-response 

parameters for cancer risks. As indicated in Table 7-1 of the proposed HRS addition (HRS factors evaluated 

differently for radionuclides), the method for establishing toxicity for radionuclides requires additional 

instructions outlined in Section 7.1.1 of the proposed HRS addition. The radionuclide toxicity value is assigned on 

the basis of carcinogenicity. If all radionuclides available to the subsurface intrusion component are assigned a 

human toxicity factor value of 0 (i.e., no slope factor is available for all the radionuclides), then a default human 

toxicity factor value of 1,000 is assigned as the human toxicity factor. These differences also apply to radioactive 

hazardous substances for establishing toxicity in the proposed subsurface intrusion component. Please refer to 

Section 7.2.1 of the proposed HRS addition for additional information regarding the establishment of toxicity 

factors for subsurface intrusion sites with radionuclides, including instructions addressing sites containing mixed 

radioactive and other hazardous substances.

b. Degradation

The degradation factor represents the potential for an eligible substance to degrade within the subsurface prior to 

intruding into an overlying occupied structure. Degradation of hazardous substances has the potential to 

significantly reduce the concentration of the hazardous substances in the subsurface and thus reduce the threat that 

vapors will intrude into a structure. However, the rate of degradation relies heavily on environmental conditions 

in the subsurface and the specific properties of the hazardous substances, including depth to contamination, the 

half-life of the hazardous substance, and biodegradation and hydrolysis relationships of hazardous substances. 

For example, simple single-ringed hydrocarbon compounds, such as benzene, may completely biodegrade in 

aerobic soils. However, under anaerobic or low oxygen conditions many substances, including benzene, may 

biodegrade slowly or not at all. Also, under anaerobic conditions benzene can be transformed biologically into 

methane related compounds that may be considered more likely to intrude into a structure. Some hazardous 

substances can degrade into an even more toxic substance. An example is tetrachloroethylene may biodegrade 

into the more toxic vinyl chloride.

Since degradation can significantly impact the rate of intrusion from a contaminant subsurface plume, it is 

considered as a factor in the proposed HRS addition. However, because degradation rates can vary depending on 
68 

factors that may not be fully identified based on a time-limited site inspection, only three values are proposed to 

be assigned to this factor, 1, 0.5, or 0.1, as discussed below. 

All substances that meet criteria for an observed exposure are assigned a degradation factor value of 1, i.e., no 

degradation assumed, because, in the instance of a documented observed exposure, there is no longer any 

consideration of whether the substance could potentially intrude making any potential degradation of the 

substance irrelevant. The proposed HRS addition also assigns a degradation factor of 1 for all contaminants: 

• at a depth of less than 10 feet,

• if available evidence suggests there is less than 10 feet of biologically active between the subsurface 

contamination and an occupied structure at the site, or 

• where the depth to contamination at the site is greater than or equal to 10 feet but less than 30 feet, or 

available evidence suggests that at least 30 feet of biologically active soil is not present between the 

subsurface contamination and an occupied structure at the site, and the hazardous substance has a half-life 

greater than 30 days.

• with a half-life of 100 days or more.

All substances with a half-life between 30 and 100 days, and with a depth to contamination of at least 30 feet, are 

assigned a degradation factor value of 0.5 to account for potential degradation likely to occur to vapors migrating 

through such a thick column of soil. Substances with a half-life of 30 days or less, and with a depth to 

contamination of at least 10 feet, are assigned a degradation factor value of 0.1. 

The depth of 10 feet represents a vertical distance great enough to allow for effective degradation of most simple 

petroleum/petroleum-like substances prior to entering an overlying structure as a vapor if other subsurface 

conditions support degradation. EPA’s Technical Guide for Addressing Petroleum Vapor Intrusion at Leaking 

Underground Storage Tank Sites (June, 2015) identifies that six feet of biologically active soil along with 

sufficient oxygen and moisture levels are generally sufficient to breakdown vapors resulting from dissolved 

petroleum hydrocarbons from underground storage tanks. However, because it is unlikely that information for 

determining whether soil conditions at the site are sufficient for effective degradation of simple petroleum 

products could be determined during typical Site Investigation activities, the proposed HRS addition uses a more 

conservative depth of 10 feet for substances with half-lives less than 30 days. For substances with a longer halflife than petroleum and petroleum-like substances 30 feet represents a vertical distance great enough to allow for 

vapor degradation in the absence of soil-specific conditions. A 2010 report (Evaluation of vapor attenuation at 

petroleum hydrocarbon sites published in Soil and Sediment Contamination: An International Journal) suggests 

that contamination found at depths less than 30 feet (approximately 10 meters) from a building’s foundation 

required additional evaluation in order to consider factors that may impact the rate of biodegradation (e.g., 

oxygen, carbon dioxide and methane within the vadose zone). Alternatively, some studies have suggested a 

shallower depth to contamination (e.g., 15 feet) be used to evaluate vapor intrusion when the contaminants of 

concern are known to readily biodegrade, but not in all situations. Due to the limited investigative nature of a Site 

Investigation which likely prevents proper evaluation of factors contributing to attenuation and degradation of 

vapors, the proposed HRS SsI addition uses the conservative depth of 30 feet. 

Effective biodegradation of certain hazardous substances can occur in situations where the soil column is 30 feet. 

The rate of this biodegradation depends on the chemical composition, subsurface environment, and the state of the 

compound being considered (e.g., liquid, gas). These conditions can vary greatly at any specific site due to

climatic and diurnal conditions. In particular, the oxygen content can be influenced by the presence of 
69

impermeable surface barriers (e.g., pavement, snow cover). Therefore, it cannot be assumed based on a limited 

site inspection that conditions are adequate for degradation to sufficiently reduce all threats posed by the 

contamination. 

Any hazardous substances meeting observed release criteria that are not assigned a degradation factor value of 1, 

based on the criteria above, are assigned a factor value based upon the substance’s half-life. In the proposed HRS 

addition the half-life of a hazardous substance is estimated by taking the multiplicative inverse of the sum of the 

multiplicative inverses of the hydrolysis half-life and the biodegradation half-life of the substance. For substances 

with a half-life of 30 days or less, and a depth to contamination of at least ten feet, a degradation factor value of 

0.1 is assigned. A degradation factor value of 0.5 is assigned to a substance with a half-life of 100 days or less, 

and a factor value of 1 is assigned for a half-life of more than 100 days. If no half-life information is available for 

a substance it will be assigned a factor value of 1, unless it is a straight-chain or simple-ring structure which is

assigned a degradation factor value of 0.5. An evaluation of various hazardous substances associated with vapor 

intrusion determined that common chlorinated solvents and other volatile substances with complex structures had 

half-lives well in excess of 100 days, whereas most simple structure (straight chain or simple ring chain structure) 

volatile compounds had half-lives shorter than 30 days; 30 days was set as the breakpoint because total petroleum 

hydrocarbons have a half-life of less than 30 days. Therefore, the 100 day cut-off point was chosen for 

determining the degradation factor value based on half-life. A substance with a half-life of greater than 100 days 

is considered to be present in the subsurface for a sufficient period of time to allow vapor migration into occupied 

structures under most situations. 

Additionally, a hazardous substance present at the site that meets observed released criteria and can be linked to a 

parent-daughter degradation relationship, such as the breakdown of tetrachloroethylene into vinyl chloride, is

assigned a degradation factor value using different criteria. If a parent substance, (e.g., tetrachloroethylene) is not 

present in the same media of concern (e.g., soil, soil gas, ground water) from the same relative depth or shallower, 

as the daughter substance (e.g., vinyl chloride) then the daughter substance is assigned a degradation factor value 

based on its half-life. If the parent substance is present in a sample from the same media of concern and from the 

same relative depth or shallower than the daughter substance, then the half-life based degradation factors for the 

two substances should be compared. The daughter substance is then assigned the higher of the two degradation 

factor values. 

The purpose of evaluating parent-daughter degradation substances in such a manner is to properly account for 

hazardous substances that may otherwise biodegrade in biologically active soil. Vinyl chloride is one such 

substance. Vinyl chloride may biodegrade in soil and could potentially attenuate prior to intruding into a structure 

if no parent source of the vinyl chloride exists. However, if tetrachloroethylene is present, it will continue to 

degrade to vinyl chloride as the existing vinyl chloride attenuates. Therefore, vinyl chloride may continue to be 

found if a parent substance is present. 

If no half-life information is available and the substance is not already assigned a degradation value of 1, unless 

information indications otherwise, all straight-chain and simple-ring structure substances will be assigned a halflife of less than 30 days. For other substances the half-life will be assumed to be greater than 100 days. 
70

Assigning Degradation for Radioactive Hazardous Substances

EPA proposes the degradation factor for radionuclides considered in the subsurface intrusion component be based 

solely on half-life. To determine the degradation factor value for each radionuclide, take the multiplicative inverse 

of the multiplicative inverse of the radioactive half-life of the radionuclide. 

c. Establishing a Combined Toxicity/Degradation Factor Value

A toxicity/degradation factor value is established by multiplying the toxicity and degradation factor values 

together for each eligible hazardous substance present at a site. The hazardous substance with the highest 

combined toxicity/degradation value is assigned as the factor value in calculating the subsurface intrusion threat. 

5.2 Other Option Considered – Inclusion of a Mobility Factor

Inclusion of a mobility factor was considered in the determination of a waste characteristics factor value. The 

ground water and air migration pathways incorporate a mobility factor value to address the likelihood of 

contaminant migration via ground water or the air, respectively. For the proposed subsurface intrusion 

component, the mobility of the substance in the ground water and/or in the unsaturated zone is not considered. 

The ability of a substance to migrate horizontally (migration based solely on the substance’s physical/chemical 

properties) beyond the known plume boundaries is not considered in the waste characteristics factor value because 

the subsurface intrusion component is based on current conditions only (i.e., the component evaluates only the 

hazardous substances that have already become located in or under occupied structures). However it should be 

noted, that the ability of a substance to move vertically in the unsaturated zone based on both the substance’s 

properties and the subsurface conditions, is already considered in determining the likelihood of exposure. This 

was discussed in more detail in Section 4 of this document. 

5.3 Determination of “Exposure” Waste Quantity – See Section 5.2.1.2.2 of Proposed HRS Addition. 

In the basic HRS algorithm used in all pathways and component evaluations, the hazardous waste quantity factor 

reflects the risk consideration related to the magnitude and duration of the release for a migration pathway or the 

extent of the exposure area for an exposure pathway. In other words, for an exposure pathway (i.e., soil exposure), 

the risk posed by a release of hazardous substances is directly related to the amount of hazardous substances that 

receptors are, or may be, exposed to.

As explained in the preamble to the 1990 HRS and in the 1988 Revised HRS TSD, an estimate of the waste 

quantity associated with a site was determined to be the best surrogate available at all sites to represent the 

amount of hazardous substances to which receptors could be exposed. EPA also considered that the duration of 

the exposure is probably correlated to the magnitude of the exposure. In the three migration pathways (i.e., ground 

water, surface water, air), the hazardous waste quantity factor reflects the total amount of hazardous substances 

placed in, or released from, sources at the site. This action considers both contamination that has already migrated 

from the sources in the past, or is migrating in the present, and any future releases of contamination from the 

sources. 

For the soil exposure pathway, however, the estimate does not include the total amount of hazardous substances

in, or released from, the site sources. It instead considers the amount of hazardous substances which receptors are 
71

currently likely to be exposed. This includes the amount of hazardous substances located in: 1) the top two feet of 

contaminated soil sources, and 2) the surficial areas of other sources in an area of observed contamination (i.e., 

AOCs) that an individual might come into contact with (see section 5.0.1 of the 1990 HRS). 

It is proposed that the waste quantity factor value for the Subsurface Intrusion component should, as in the 1990 

HRS soil exposure pathway, reflect only the amount of hazardous substances that people are currently exposed to 

(that is, the amount in occupied structures within an AOE), along with the amount of hazardous substance that 

people are likely being exposed to (that is, the amount in an occupied structure within an ASC), because the 

subsurface intrusion component focuses on exposure and not the amount of hazardous substances that might 

migrate to targets in the future. While it would result in a better estimate of relative risk to have the HRS 

algorithm reflect the concentration of hazardous substances in an observed exposure at a site, particularly for 

subsurface intrusion, it is unlikely that representative site contaminant concentrations can be obtained using data 

collected during a typical time-limited site inspection. Studies have shown that the rate of vapor intrusion varies 

significantly over time and that the magnitude of variation cannot be estimated with sufficient accuracy from a 

limited site inspection. Recent studies have identified it may be possible to determine long-term averages and/or 

worst-case scenarios for vapor intrusion concentrations over a short time frame using structure depressurization 

techniques, but further research of these techniques is ongoing. Therefore, it is not likely that a measurement 

taken during a short time period would accurately depict dose or the actual amount of hazardous substances that

populations are exposed to. However, the proposed addition is written to use such information if the procedure 

becomes accepted, as such sampling results could demonstrate the structure should be considered within an area 

of observed exposure. 

A four-tiered hierarchical approach for estimating the hazardous waste quantity is proposed, consistent with the 

1990 HRS (See section III.C of the preamble of the 1990 HRS (55 FR 51542, December 14, 1990)). In addition, 

minimum waste quantity factors are proposed when insufficient information is available to accurately estimate the 

actual waste quantity (see section 2.4.2 of the 1990 HRS). This approach and the proposed minimum values to be 

used when the data is incomplete are explained below.

a. Summary of Tiered Approach

The 1990 HRS includes tiers used to document the hazardous waste quantity for each source in migration 

pathways, or area of observed exposure in the soil exposure pathway, in the following hierarchy: 

• Tier A – Hazardous Constituent Quantity 

• Tier B – Hazardous Wastestream Quantity

• Tier C – Volume 

• Tier D – Area

This same four-tiered structure is proposed to be used to estimate waste quantity for the subsurface intrusion 

component and is proposed to be based on the amount of hazardous substances that may have entered occupied 

structures. In performing an HRS evaluation of the waste quantity at a site, a site assessor moves down through 

the above tiers, assigning values using the information that is readily available. If the information available is 

sufficient to evaluate all the wastes using a certain tier, then the assessor does not continue to evaluate lower tiers. 

For each AOE or ASC, the highest value of tiers A through D or exposure location is selected as the hazardous 

waste quantity value. However, because, as identified below, for the subsurface intrusion threat, Tiers A and B are 
72

based on measuring contamination entering into a regularly occupied structure, the waste quantity for an ASC can 

only be estimated using Tiers C or D. 

For a site evaluated using the proposed subsurface intrusion component, it is likely that multiple occupied 

structures are being evaluated as part of the subsurface intrusion component. It is also possible that there may be 

multiple AOEs and ASCs. Therefore, when determining the component hazardous waste quantity factor, the 

hazardous waste quantity of each identified regularly occupied structure in all of the AOEs and ASCs is 

determined and subsequently added together to determine the component hazardous waste quantity. The 

structures may also be evaluated using different tiers, as each regularly occupied structure requires an individual 

evaluation and the amount of information known about each structure is likely to vary at the time of an HRS 

evaluation. 

With the exception of Tier A, a divisor for each tier is used, which reflects the possible amount of hazardous 

substances associated with the waste in each pathway, component, or threat. These divisors are used to 

standardize the hazardous waste quantity that is determined for each tier. As stated above, a site may evaluate 

structures using multiple tiers, and these divisors allow for the “standardized” hazardous waste quantities to be 

summed together to determine a hazardous waste quantity factor value for the site being evaluated.

The divisor values being proposed for the subsurface intrusion component (see Table 5-18 of the proposed HRS 

addition) are consistent with those values currently used in other parts of the HRS (see Table 2-5 of the 1990 

HRS). These divisors were originally determined as part of the 1990 HRS, and are based on a survey of typical 

concentrations of hazardous substances surveyed in various wastestreams. (See section 2.5 of the 1988 Revised 

HRS TSD for additional discussion regarding the development of hazardous waste quantity divisors.) Further 

discussion regarding the evaluation of the individual tiers is provided below. The following subsections include 

examples for calculating hazardous waste quantity by each tier. 

i. Tier A – Hazardous Constituent Quantity – See Section 5.2.1.2.2 of the Proposed HRS Addition

Tier A is used to determine the waste quantity factor value for a component or pathway when sufficient data are 

available to predict, with reasonable confidence, the total mass of individual CERCLA hazardous substances (not 

including pollutants and contaminants) in each regularly occupied structure. In addition, Tier A includes only the 

pounds of CERCLA hazardous substances, not the mass of pollutants or contaminants. (Note that Tier A could 

not be used for structures in an ASC as no measurement of intrusion would be available.)

Figure 5-1 depicts an example of the estimate of the mass (in pounds) of CERCLA hazardous substances that has 

entered into a single occupied structure, based on the concentrations in samples collected from each room of the 

occupied structure. In this example, it is assumed that the samples were collected over a 24-hour period and are 

statistically representative. A mass (in μg) is calculated by multiplying the concentration (in μg/m3

) by the volume 

of the room where the sample was collected (in m3

). Using the universal gas constant as a conversion factor, the 

pounds (lb) of pure hazardous substance are determined. If the hazardous constituent quantity for the occupied 

structure is adequately determined (that is, the total mass of all CERCLA hazardous substances in the area of 

observed exposure is known or is estimated with reasonable confidence), Tiers B, C, and D are not evaluated and 

a value of 0 is assigned to these tiers. If the hazardous constituent quantity is not adequately determined, a partial 

estimate is assigned to each occupied structure. Tier B is then evaluated, as described below. It should be noted 

that this example is only a partial estimate for one day and assumes the concentration is continuous for the entire 
73

day in each room and that there is no air flow through the rooms. It also does not consider the air exchange rate 

for the structure for that day. If multiple, regularly occupied structures were present at this site, this process would 

be repeated and the hazardous waste quantities of each structure would then be added together.

Figure 5-1. Hazardous Waste Quantity: Tier A

ii. Tier B – Hazardous Wastestream Quantity – See Section 5.2.1.2.2 of the Proposed HRS Addition

The Tier B estimate reflects the amount of hazardous substances that may have intruded into an occupied 

structure over a period of time (not its volume), that is—the hazardous wastestream that flows into the structure 

from the subsurface. The Tier B estimate is based on the total wastestream flow, not just the CERCLA hazardous 

substances, but also pollutants and contaminants and the associated waste material that contains the hazardous 

substances. The divisor accounts for the amount of CERCLA-eligible hazardous substances, pollutants and 

contaminants in a typical wastestream. In the example presented below in Figure 5-2, the hazardous waste 

quantity within a single regularly occupied structure is determined using the building volume and an average 

building air exchange rate. (See EPA’s 2004 User's Guide for Evaluating Subsurface Vapor Intrusion into 

Buildings). This example assumes that samples demonstrating observed exposure criteria have been collected 

over a one-week period, an air exchange rate of 0.45 hr-1 for one week and a universal gas constant of 2.8516 

lb/m3

. If the hazardous wastestream quantity for the occupied structure is adequately determined—that is, the 

total mass of all hazardous wastestreams and CERCLA pollutants and contaminants for the occupied structure is 

known or estimated with reasonable confidence—Tier B is used and Tiers C and D are not evaluated. Otherwise, 

the evaluation proceeds to Tier C, which is described below. This example estimate is considered to be a partial 
74

estimate in that it is only a wastestream estimate for a period spanning a single week. (Note that Tier B could not 

be used for structures in an ASC because no measurement of intrusion would be available.). 

Figure 5-2. Hazardous Waste Quantity: Tier B

iii. Tier C – Volume – See Section 5.2.1.2.2 of the Proposed HRS Addition

Tier C is based on the capacity in the living space of a structure, not on the total volume of the occupied structure. 

In the example presented below in Figure 5-3, the volume waste quantity for a single regularly occupied structure 

is determined based on the actual volume of the building (i.e., footprint for each occupied floor multiplied by the 

ceiling height). The example structure is 2,100 ft2 with 8-ft ceilings. After converting to cubic yards and applying 

a “normalizing” divisor of 2.5, the volume hazardous waste quantity is determined. The divisor of 2.5 is used in 

other pathways of the 1990 HRS when evaluating sources that are considered to be tanks or containers. When 

compared to the existing 1990 HRS source types, a structure is conceptually most similar to a container with gas 

in it, therefore, the container divisor of 2.5 was chosen for the subsurface intrusion component when evaluating 

Tier C. 

For the subsurface intrusion component, a value of 8 feet is proposed for use if the actual ceiling height within a 

structure is unknown when calculating the volume of a structure. However, the actual basal footprint (i.e., the 

outline of the building’s foundation) of any occupied structures within an AOE or ASC should be known to 

calculate Tier C. The value of 8 feet for ceiling height is referenced in numerous documents including the 2015

OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor 
75

Sources to Indoor Air and the 2004 Johnson and Ettinger model parameter study (see 2004 User's Guide for 

Evaluating Subsurface Vapor Intrusion into Buildings). If the volume of the occupied structure cannot be 

determined, the area of exposure is assigned a value of zero for the volume measure and the evaluator proceeds to 

Tier D, which is described below. If the volume can be determined, Tier C is used and Tier D is not evaluated. 

Note that this height can be used for each level in a multistory structure. 

Figure 5-3. Hazardous Waste Quantity: Tier C

When an intrusion into a structure by contaminated ground water has occurred, only the volume of water that has 

intruded into an occupied structure is considered when using the volume tier. Figure 5-4 depicts an example Tier 

C hazardous waste quantity calculation at a residential structure where the basement has flooded as a result of a 

rising ground water table. In this example, the basal footprint of the building is 700 square feet and the water has 

risen to a maximum height of 4 feet in the designated living space. The resulting volume is 700 ft2

 x 4 ft, or 2,800 

cubic feet. The divisor of 2.5 is used and after converting to cubic yards, a Tier C hazardous waste quantity factor 

value of 41.48 is determined for this structure.
76

Figure 5-4. Hazardous Waste Quantity: Tier C

iv. Tier D – Area – See Section 5.2.1.2.2 of the Proposed HRS Addition

Similar to the method for evaluating Tier C, Tier D uses actual data based on the building parameters of each 

occupied structure within an AOE being evaluated. In Figure 5-5 presented below, a default value of 1,740 ft2 is 

assumed for each residential structure in an AOE being evaluated; however, if actual values can be determined, 

they are to be used in this calculation. It is important to note that 1,740 ft2 represents the 50th percentile basal 

footprint for each residential structure assuming a ceiling height of 8 feet (see Table 19-8 of EPA Exposure 

Factors Handbook: 2011 Edition). In other words, if a single family home has 3,000 ft2 of living area but includes 

multiple stories, then only the basal footprint (i.e., the outline of the building’s foundation) is used in determining 

area. After calculating the basal square footage of all structures being evaluated, a divisor of 13 is applied to 

calculate the hazardous waste quantity for that AOE. The divisor of 13 is used in other pathways of the 1990 HRS 

when evaluating sources that are considered to be surface impoundments. Because the basal footprint of a 

structure is measured the same as the area of a surface impoundment, the divisor of 13 was chosen for the 

subsurface intrusion component when evaluating Tier D.
77

Figure 5-5. Hazardous Waste Quantity: Tier D

b. Assigning a Hazardous Waste Quantity Value

In determining the hazardous waste quantity factor value for a site, each structure within an AOE and ASC with a 

non-zero containment factor value is evaluated using the tiered approach outlined above. The highest of the tier 

estimates for each structure is selected. If there are multiple occupied structures being evaluated, each structure 

may be evaluated using a different tier if appropriate. After the hazardous waste quantity for all occupied 

structures has been selected, the hazardous waste quantities are summed together to determine the component 

hazardous waste quantity using HRS Table 2-6. If the hazardous constituent quantity (i.e., Tier A) is not 

adequately determined, the hazardous waste quantity factor value from Table 2-6 is assigned to the subsurface 

intrusion component. In some instances, which are discussed further below, the component hazardous waste 

quantity minimum values may be applied to ensure the hazardous waste quantity factor is not underestimated due 

to a lack of readily available information.

c. Component Hazardous Waste Quantity Minimum Values

The minimum component waste quantity factors are included because of insufficient information at many sites to 

adequately estimate waste quantity with confidence, as discussed in section I of the preamble to the 1990 HRS (55 

FR51533, December 14, 1990). In developing the 1990 HRS, pilot studies were conducted to evaluate the 

efficacy of this method for establishing hazardous waste quantity (among other HRS factors). Because it was 

found to be cost prohibitive, and in some cases technically infeasible, to obtain the necessary data, the 1990 HRS 
78

included the use of hazardous waste quantity minimum factor values in specific situations where an accurate 

waste quantity estimate cannot be obtained. For these situations, the 1990 HRS establishes a minimum waste 

quantity factor value for each pathway or component. The minimum hazardous waste quantity factor value is 10 

for sites with no actually contaminated targets and 100 for the migration pathways if observed exposure has been 

documented (see section 2.4.2 of the 1990 HRS). These same minimum values are proposed for the subsurface 

intrusion component. 

In estimation of waste quantity for the subsurface intrusion component, as proposed, if an AOE has been 

established (that is, there is a documented exposure to receptors), a minimum hazardous waste quantity factor 

value of 100 is used. Given the limited time available to conduct sampling during a site inspection (to collect the 

data for an HRS evaluation), a single minimum factor of 10 may significantly underestimate the waste quantity in 

some situations. For example, if contamination is found at levels that document an observed exposure (and an 

AOE is established), it is considered reasonable to assign a minimum component waste quantity factor value of 

100. Assigning a minimum component waste quantity factor value of 100 if an observed exposure is present is 

consistent with the minimum value assigned for other HRS pathways and components with actually exposed 

targets. 

It is proposed that a minimum waste quantity factor value of 10 should apply for sites where only an ASC exists 

(no AOE), or when the hazardous constituent quantity is not adequately determined and the component waste 

quantity (the sum of waste quantities for all occupied structures in the AOE and ASC, as assigned by HRS Table 

2-6) is below 100. This is a comparable approach and relative weighting to that taken in other pathways and 

components. Minimum waste quantity values are also assigned when sites have been subject to removal actions. 

For all migration pathways, if the component waste quantity factor value is 100 or greater before the removal 

action is completed, the minimum component waste quantity factor value is also 100.

d. Special Considerations for Radioactive Hazardous Substances

As indicated in Table 7-1 (HRS Factors Evaluated Differently for Radionuclides) of the proposed HRS addition, 

the method for establishing hazardous waste quantity for radionuclides includes additional instructions outlined in 

Section 7.1.1 of the proposed HRS addition. As outlined in Section 7.2.5 of the proposed HRS addition, the 

hazardous waste quantity factor values for subsurface intrusion sites with radionuclide contamination are 

evaluated using only two measures in the following hierarchy (these measures are generally consistent with Tiers 

A and B for nonradioactive hazardous substances in Sections 2.4.2.1.1 and 2.4.2.1.2 of the 1990 HRS): 

• Radionuclide constituent quantity (Tier A) 

• Radionuclide wastestream quantity (Tier B)

However, it should be noted that for the subsurface intrusion component, in estimating Tier B, radionuclide 

wastestream quantity, the total wastestream volume is estimated for all regularly occupied structures located 

within AOEs with observed or inferred intrusion and within ASCs. The volume of each regularly occupied 

structure is determined based on actual data, and if unknown, a default ceiling height of 8 feet may be used. 

In addition, the hazardous radioactive waste quantity is added to the hazardous non-radioactive waste quantity to 

obtain the component (or pathway) hazardous waste quantity. Please refer to Section 7.2.5 of the proposed HRS 

addition for additional instructions regarding the methods for establishing both radionuclide constituent and 
79

wastestream quantities, specifically for the subsurface intrusion component. These proposed additions to section 

7.0 of the 1990 HRS represent only minor technical modifications and the proposed methods for evaluating 

radionuclides at subsurface intrusion sites are consistent with the approach in the 1990 HRS. 

e. Other Options Considered – Hazardous Waste Quantity 

In developing the proposed subsurface intrusion addition, several additional options were considered for 

establishing hazardous waste quantity including: 

• Considering the original source quantity,

• Considering the quantity known to be located within a soil gas or ground water plume,

• Including only occupied structures in the AOE,

• Including the areas between structures within an AOE, 

• Including the areas between structures within an ASC, and

• Assigning a minimum hazardous waste quantity when subslab concentrations exceed 10x the value of a 

benchmark.

After considering these options, it was determined most appropriate to consider only those substances that are

documented as entering a regularly occupied structure, or those substances likely to intrude in areas with 

documented subsurface contamination (in an ASC), when assigning the waste quantity factor value. (Note that

this approach alleviates the need to assign a minimum hazardous waste quantity based on subslab concentrations, 

because structures associated with subslab concentrations are included in the ASC evaluation.) Because not all of 

the contaminants located in the original source, or in plumes, will intrude into a structure, it was determined that 

including other locations of contamination would not accurately reflect the actual amount available to enter, or 

likely to enter, into an occupied structure. Also, including contamination in areas between occupied structures 

would not reflect the amount entering into structures, as most of this contamination would directly enter the 

atmosphere. 

5.4 Considerations in Evaluating Multi-Subunit Structures

The proposed method for determining hazardous waste quantity within multi-subunit structures is similar to that 

of a structure with no subunit. The same four-tiered approach is applied based on the information available at the 

time of the HRS evaluation. The subunit spaces identified as being located within an AOE and/or an ASC are 

included in the hazardous waste quantity calculations. If specific dimensions are not available to calculate the 

hazardous waste quantity for the volume tier, a minimum value of an 8-feet ceiling is applied as the height of each 

level within a structure subunit space within the AOE and/or an ASC. If specific dimensions are not available to 

calculate the hazardous waste quantity for the area tier, a minimum value of 1,740 ft2 is applied per structure or 

subunit space14. As discussed in further detail in Section 6.1, Delineation of Target Areas, of this TSD, the 

individual floors of a multi-story building are identified to be within an AOE or an ASC based on the location of 

samples documenting observed exposure and/or an observed release. For example, an observed exposure sample 

collected from a subunit space on the third floor of a five-story building indicates that all regularly occupied 

subunit spaces on the third floor are inferred to be within an AOE. In addition, those regularly occupied subunit 

 

14 A minimum value for the area of structures and subunit spaces was derived from building characteristic values from EPA’s 

Exposure Factors Handbook: 2011 Edition.
80

spaces on all floors below the floor with the observed exposure (i.e., first and second floors) are also inferred to be 

within an AOE. The regularly occupied subunit spaces on the floor immediately above the observed exposure, in 

this case the fourth floor, are inferred to be within an ASC. For the purposes of determining the hazardous waste 

quantity in this example, all regularly occupied subunit spaces on the first, second, third, and fourth floors are 

determined individually using the tiered approach described above. Only the fifth floor in this scenario would not 

be considered as being part of an AOE or ASC, and would not be evaluated when estimating hazardous waste 

quantity. 

If there are structure-specific factors that indicate that some occupied subunit spaces are, or are not, appropriate to 

be evaluated in estimating the hazardous waste quantity, then documentation of those factors can be used in the 

calculation of hazardous waste quantity. For example, some multi-subunit structures have a centralized heating 

ventilation and air conditioning (HVAC) system that circulates throughout the entire building. In this case, all 

subunit spaces in the structure could be considered actually contaminated if the HVAC system intake is located in 

a contaminated area. Whereas, when a building is partially commercial and partially residential, there may be two 

different HVAC systems present that may prevent air communication between the two areas. In this example, 

contamination would not necessarily be in both areas. See also Section 6.3.b, Populations (i.e., Individuals Living, 

Working, or Attending School or Daycare), of this TSD for an additional discussion of how multi-story buildings 

may be evaluated under the proposed subsurface intrusion component. 

5.5 Determination of Waste Characteristics Factor Category Value

The subsurface intrusion component waste characteristics factor category value is proposed to be calculated using 

the same approach used in all pathways and components in the 1990 HRS. The hazardous waste quantity value is 

multiplied by the combined toxicity/degradation factor value. Then, using Table 2-7 of the proposed HRS 

addition, this product is assigned as the component waste characteristics factor category value.

5.6 Method to Determine the Waste Characteristics Factor Value

The proposed method for determining a waste characteristics factor category value is consistent with the approach 

used throughout the 1990 HRS. As discussed in Section 3.4 of this TSD, this approach has undergone peer review 

by the Science Advisory Board, including review of the divisors and standardizing tables used in the 1990 HRS. 

These same divisors and standardizing tables (used in the 1990 HRS) are proposed for use in this HRS addition. 

Additionally, EPA’s approach to base the hazardous waste quantity on the amount of contaminants that can be 

found in the intruded structures was also subject to a separate external peer review (also discussed in Section 3.4 

and Appendix E of this TSD) conducted as part of the proposed HRS addition development. This external review 

considered this approach to be reasonable. 

Additionally, a sensitivity analysis was conducted to ensure that the proposed method of calculating the 

component waste characteristics factor category value is consistent with the 1990 HRS structure and 

methodology. This limited sensitivity analysis used a combination of waste characteristic category values and 

population densities, and typical factor values from previous HRS assessments to ensure that the same level of 

relative threat within the subsurface intrusion component would receive a similar score as another pathway with 

similar relative threat levels. Results from the limited sensitivity analysis confirmed that this goal was met; these 

results are presented in Appendix C of this document. 
81

6.0 Component Structure: Targets

The Targets factor category in the HRS pathway structure is based on estimates of the expected dose to each

receptor and the number and type of receptors present. In environmental risk assessments it is critical to

understand the nature and extent of exposure to individuals, populations, resources, and sensitive environments. 

The relative risk assessment embodied within the HRS uses the targets factor as an index of the nature and extent

of exposure to individuals, populations, resources, and sensitive environments.

6.1 Delineation of Target Areas – See Section 5.2.0 of the Proposed HRS Addition 

The delineation of target areas within the HRS is a necessary step in identifying those receptors or targets (e.g.,

populations) that are exposed to, or may become exposed to, hazardous substance contamination. As discussed in 

Section 2.2.a of this TSD, a pathway or component score is the product of three factor categories: likelihood of 

release/exposure, waste characteristics, and targets. Whereas likelihood of release/exposure and waste 

characteristics are subject to maximum factor values, the targets factor category value is unlimited. (Target factor

category values were unlimited to give emphasis to sites with populations actually exposed to contamination.) 

Because the targets value is unlimited while the other two category values are capped, the Targets factor category

is likely to be the most influential, i.e., the most likely to have a substantial impact on the ultimate HRS site score.

Furthermore, the emphasis on targets is consistent with the SARA amendments to CERCLA. 

Below is a detailed description of how the 1990 HRS delineates target areas. In general, all pathways assign

higher weighting to targets meeting observed release or observed exposure criteria. Whereas the three migration

pathways (ground water, surface water, and air) also consider those targets that may become impacted in the 

future as a result of “contaminants” migrating, the soil exposure pathway considers those “populations” that 

“migrate” (i.e., move) to the site and become exposed to contamination. 

a. Approach within the 1990 HRS

The migration pathways (ground water, surface water, and air) delineate target areas based on levels of exposure 

within a target distance limit (TDL). The TDL may also be subdivided into separate distance categories subjected

to distance-based weightings of targets within each division. For example, targets within the air migration 

pathway are first identified based on two levels of contamination: actual or potential. The criterion for

documenting actual contamination is that an observed release to air must be established at the target location. The 

level of actual contamination assigned to targets in each area (see Section 6.0 of this document), is based on the 

contaminant concentration in comparison to an appropriate health-based benchmark (see section 3.3.2.1 of the 

1990 HRS). After identifying targets meeting actual contamination criteria, the air migration pathway considers 

targets in locations not actually contaminated but within the TDL as potentially contaminated. For the air

migration pathway, the TDL is 4 miles in all directions from the boundary of each source unless there is an 

observed release at a greater distance. Targets within the TDL (excluding any previously identified to meet 

observed release criteria) are then weighted based on the level of contamination and their actual distance 

categories from the source boundaries (i.e., 0 to 1⁄4 mile, 1⁄4 to 1⁄2 mile, 1⁄2 to 1 mile, 1 to 2 miles, 2 to 3 miles, and 3 

to 4 miles). The actual distances and directions used to establish a TDL vary depending on the migration pathway

(e.g., the surface water migration pathway considers a standard distance (15 miles) downstream from the most

downstream probable point of entry (PPE) unless observed releases are established at a greater distance). The

ground water migration pathway evaluates targets using locations of wells tapping the aquifer being evaluated 
82

within a 4-mile TDL if no aquifer discontinuity restricts contaminant transport within the TDL. The level of 

contamination and the weighting factors are assigned on a well-by-well basis.

Alternatively, the soil exposure pathway delineates target areas by establishing areas of contamination (AOCs). 

An AOC is based on sampling locations where hazardous substances meet observed contamination15 criteria. A 

TDL is not established when evaluating the resident population threat of the soil exposure pathway, because it is 

not expected that surface contamination will significantly migrate in the future. However, the soil exposure 

evaluation does consider the nearby population that may “migrate” (i.e., move) to an AOC and subsequently 

become exposed to contamination. That population is limited to an area of 1 mile travel distance beyond the 

boundaries of an AOC, and is subdivided into three travel distance categories (i.e., 0 to 1⁄4 mile, 1⁄4 to 1⁄2 mile, and 

1⁄2 to 1 mile). 

In the soil exposure, surface water migration, and air migration pathways, inference of contamination (i.e., the 

assumption that contamination is continuous among sampling locations), is based on the location of observed 

exposure or observed release samples. In the soil exposure pathway, 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. The air migration pathway uses inference when assigning population 

values based on distance categories. For example, in the air migration pathway, if an observed release sample falls 

within the 1 to 2 mile distance category, then that distance category and all those distance categories closer to a 

source (i.e., 0 to 1⁄4 mile, 1⁄4 to 1⁄2 mile and 1⁄2 to 1 mile) are considered to be actually contaminated. In the air 

migration pathway, the level of contamination assigned to a target is based on the level of contamination assigned 

to the entire distance category. The soil exposure pathway assumes all inferred targets to be subject to Level II 

concentrations, even if there are some Level I targets within the AOC, based on sampling at the target location. In 

the surface water pathway, all targets between the PPE associated with an observed release and the observed 

release sample locations are assumed contaminated at the same level of contamination as the most contaminated 

downstream sample.

b. Proposed Option

The subsurface intrusion component evaluation includes two areas in which exposure due to subsurface intrusion 

contamination exists or is likely to exist: 1) areas of observed exposure (i.e., areas where contaminant intrusion 

into occupied structures has been documented); and 2) areas of subsurface contamination (i.e., areas where 

subsurface contamination underlying occupied structures, such as in surficial ground water or soil vapor, has been 

documented, but either sampling has not documented that subsurface contamination has entered an occupied 

structure or no sampling of indoor air has been performed). See Figure 6-1 for an illustration of these two areas. 

 

15 As discussed in Section 4.0 of this TSD, the three migration pathways evaluate whether a “release” has occurred while the 

soil exposure pathway evaluates whether an actual “contamination” of hazardous substances has occurred. The terminology 

differences reflect that an observed “release” documents migration of contamination in the medium, whereas observed 

“contamination” documents actual contamination at a target location. However, in both cases, a significant increase in 

contaminant levels and attribution of the increase to the site are required.
83

Figure 6-1. Proposed Method for Delineating Target Areas in the Subsurface Intrusion Component

i. Area of Exposure (AOE)

As outlined in Section 5.2.0 of the proposed HRS addition, an AOE is defined as: 

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 which are either contaminated or inferred to be within an area of observed 

exposure based on samples meeting observed exposure criteria contaminated (see section 

5.2.1.1.1 Observed Exposure). 

An area of observed exposure is identified based on the location of occupied structures that have documented a 

significant increase in contamination resulting from subsurface intrusion that can be attributed to the site. The area 

encompassed by such structures constitutes the AOE. Other occupied structures within this encompassed area (or 

areas) will also be considered as part of the AOE unless information indicates otherwise. Populations occupying 

structures within the AOE are considered exposed to subsurface contamination for HRS scoring purposes and are 

therefore included in the HRS evaluation.16 Unless information indicates otherwise, receptors in occupied 

structures, including any subunits, within an AOE are inferred to be actually exposed, even if contaminated indoor 

samples are not associated with every structure. In the case of a multistory multi-subunit structure inferred to be in

the AOE, only the lowest level is inferred to be within the AOE. 

 

16 As discussed in Section 6.1.a of this document, inferring contamination among contaminated sample locations is an 

established concept in the HRS.
84

In the example depicted in Figure 6-1, the AOE is delineated based on the location of five structures with 

documented contamination identified as red or orange. One of these structures, in red, has contamination levels 

above a benchmark. Within the AOE there are seven additional structures, including three known to be 

contaminated, shown in red or orange (including one structure with contamination concentrations above 

benchmark values). One unsampled structure (in white) is inferred to be contaminated based on its location within 

the AOE boundaries and no information suggests that it is not contaminated (e.g., similar construction type, 

structure containment factor value, and topography). 

Additionally, there are three structures (in yellow) that have been sampled, and the samples did not identify any 

actual contamination. This sampling information from a cluster of structures could represent “information 

indicating otherwise” for not inferring contamination. Therefore, in this example, these structures are not inferred 

to be contaminated and are not considered to be part of the AOE. This decision could be altered if additional 

information indicates that contamination does exist. For example, if sample data from a different time period 

indicates that contamination meets observed exposure criteria, then this information may be used to include this 

structure within the AOE. Alternatively, these three structures could be evaluated as part of an ASC as discussed 

in Section 6.1.b.ii, Area of Subsurface Contamination (ASC), below. 

An AOE can also be delineated based on locations where contaminated ground water has intruded into a structure 

and represents a significant threat. The intrusion of contaminated ground water into a structure, which may occur 

as a result of a rising water table, is an observed exposure by direct observation and therefore can be used to 

delineate an AOE. Additionally, if an unsampled structure is located within an area between structures where 

there is an observed exposure by direct observation of intruding contaminated ground water, the structure can be 

inferred to be actually contaminated unless information indicates otherwise. As Figure 6-2 depicts, factors such as 

structure construction (e.g., basement vs. slab-on-grade) and topography impact on the water table can impact the 

potential for contaminated ground water to intrude from the subsurface into structures. These factors are examples 

of “information indicating otherwise” that may lead to inferring ground water contamination exists. 
85

Figure 6-2. Inference of Ground Water Intrusion

ii. Area of Subsurface Contamination (ASC)

As outlined in Section 5.2.0 of the proposed HRS addition, an ASC is defined as: 

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. 

EPA proposes identifying an ASC as an area where SsI sample17 contamination has been documented at levels 

meeting observed release criteria and that this area is not within the AOE. Observed release criteria requires 

contamination exists at levels significantly above background and this significant increase can be attributed, at 

least in part, to the site (see section 2.3 of the 1990 HRS for observed release criteria). In the case of multistory 

multi-subunit structures in the ASC, only the lowest level is inferred to be within the ASC. 

 

17 An SsI sample is a sample that meets the criteria for an observed release in a structure within, or in a sample from below, 

an area of observed exposure that meets the criteria for an observed release at the site. This sample may be, but not limited to, 

a subslab sample, soil gas sample, ground water sample, or indoor air sample.
86

For a multistory multi-subunit structure inferred to be in the AOE (as is discussed in section 6.1.b.i, Area of 

Exposure (AOE), above), the level above the highest level considered to be within the AOE is inferred to be an 

ASC.

It is proposed that sites which have the potential for ground water to rise into a structure not be evaluated as part 

of an ASC. Rather, only sites that have the potential for vapor to intrude into overlying regularly occupied 

structures may be considered when delineating an ASC. Populations in occupied structures within this area are 

considered likely to be exposed to contamination. The factor value assigned to populations within an ASC is 

weighted based on sample depth and media, as described further in Section 6.3.b.ii, Weighting of Populations 

within the ASC, of this document. 

It is also proposed that substances meeting observed release criteria with a vapor pressure equal to, or greater 

than, 1 torr, or a Henry’s Law constant greater than or equal to 10-5 atm m3 mol-1 be used in delineating an ASC. 

In addition, if at the same site, a substance has been found in an observed exposure then that substance can be 

used in delineating an ASC because it has been shown to have entered the structure at the site. These minimum 

vapor pressure values are meant to identify only those hazardous substances with the potential to volatilize and 

migrate into a structure through vapor intrusion. This is likely to exclude, for example, certain metals and nonvolatile compounds from being considered as part of an ASC. When present in the subsurface, these low-volatile 

compounds are unlikely to enter into overlying structures as a vapor or gas. 

In the example depicted in Figure 6-1, the ASC is delineated based on the location of 10 subsurface samples 

meeting observed release criteria. The targets associated with this ASC include the five sampled structures with 

no evidence of contamination (including the three structures discussed above in Section 6.1.b.i, Area of Exposure

(AOE), and the one unsampled structure that falls within the ASC boundaries, but is not a part of the AOE. 

The ASC is proposed to be included in the HRS evaluation because this method will allow for evaluation of an 

area with existing subsurface contamination below structures to be evaluated for the potential intrusion of the 

contamination into the overlying structures. Numerous studies, including several studies presented by members of 

the scientific community on current vapor intrusion research at the AEHS co-sponsored Annual International 

Conference on Soil, Water, Energy, and Air (https://iavi.rti.org/WorkshopsAndConferences.cfm) have discussed 

the extremely variable nature associated with sampling and detecting the rate of vapor intrusion. For example, a 

research site in Indianapolis, Indiana was monitored in two adjoining duplex residential units over an extended 

period of time. Factors such as diurnal temperature, seasonal climate, and the presence of building features such 

as an HVAC system or subslab mitigation venting system were shown to dramatically affect the rate of intrusion 

into a structure (See Indoor Air and Soil Gas Temporal Variability: Effects on Sampling Strategies under 

Controlled and Uncontrolled Conditions). As science advances, the methods used by site assessment practitioners 

to collect data to delineate AOEs and ASCs may evolve and results from these methods will be incorporated into 

HRS evaluations. 
87

c. Other Options Considered

In developing the proposed subsurface intrusion component, EPA considered several alternatives for how to 

delineate target areas. Those options considered and the rationale for why they were rejected is discussed below 

and includes the following:

• Consideration of actually exposed targets only, without inference of contamination,

• Consideration of actually exposed targets only and those targets with inference of contamination, and

• Consideration of establishing a potential migration zone.

i. Consideration of Only Actually Exposed Targets without Inference of Contamination

One consideration for the subsurface intrusion component was to only include sites in the HRS evaluation where 

targets are considered actually exposed (i.e., via direct observation or chemical analysis). This option is 

considered the least protective because there is no possibility that targets would be counted that have not been 

documented as exposed to subsurface intrusion. Figure 6-3 below depicts a conceptual scenario in which only 

those structures with Level I (red houses) or Level II (orange houses) contamination would be considered when 

counting targets. 

Limiting targets to only include actual exposures, as illustrated in Figure 6-3, could fail to identify high priority 

releases because of the significant possibility that not all exposures would be identified during the SI. As 

discussed in Section 6.1.b.ii, Area of Subsurface Contamination (ASC), of this TSD, the nature and rate of vapor 

intrusion is highly variable based on factors such as temperature and structural characteristics. This approach was 

also considered to be generally inconsistent with the 1990 HRS methods for evaluating targets because all other 

pathways consider potentially threatened targets. Exclusion of those targets overlying the contaminated plume 

could significantly underestimate the threat at a site.

Figure 6-3. Consideration of Only Actually Exposed Targets without Inference of Contamination
88

ii. Consideration of Only Actually Exposed Targets and Those with Inference of

Contamination

A second option considered for delineating target areas for the proposed subsurface intrusion component was to 

define AOE boundaries using those occupied structures with observed levels of exposure and to infer 

contamination in occupied structures that lie within those defined boundaries, unless information indicates 

otherwise, such as a structure with a structure containment factor value of zero (e.g., structure with a foundation 

raised greater than six feet). In Figure 6-4, there are three occupied houses located within the proposed AOE 

boundaries that have been sampled but show no evidence of contamination. Under this option, these houses and 

the areas between them would be excluded from the AOE when identifying targets. In this example, a single 

indoor air sampling event that failed to detect actual contamination would not provide sufficient information to 

exclude these structures from being inferred to be contaminated. Additional information including the close 

grouping of the structures and/or a different construction type (e.g., these structures have an open crawl space 

while other structures in the AOE have a basement foundation) would be required to suggest that the structures 

may not be inferred as actually contaminated in the absence of multiple sampling events failing to detect 

contamination within the structures. However, for the one house that was not sampled, since no information exists 

to indicate it was not likely to be contaminated, it is inferred to be contaminated. As with the method for inferring 

contamination in the soil exposure pathway, those targets inferred to be contaminated would only be considered 

subject to Level II concentrations when counting targets. 

It should be noted that this option also includes an exception for multi-subunit structures (e.g., apartment 

building). Similar to the method proposed in the HRS addition (described in Section 6.3 of this TSD), in this 

option, contamination would be inferred for all individuals within a subunit on any level that contaminant 

concentrations were measured at observed exposure levels and for all levels below that level. However, unlike the 

option proposed and described in TSD Section 6.3, this alternative does not give consideration to targets in any 

occupied space lying one level above a floor with actual contamination. 

Similar to the option considering only actually exposed targets without inference of contamination, this approach 

may also underestimate the level of relative risk when identifying high priority releases because populations in 

structures inferred to be within the AOE cannot be assigned Level I exposure. Given the significant variability in 

contaminant intrusion occurrences, it is possible that intrusion into occupied structures at the site could be 

occurring under different climatic conditions. 
89

Figure 6-4. Consideration of Only Actually Exposed Targets and Those with Inference of Contamination

iii. Consideration of Establishing a Potential Migration Zone (PMZ)

An alternative option considered expanding the proposed method for delineating target areas by including within 

the subsurface intrusion component an approach for consideration of populations subject to future migration 

(beyond those targets in the ASC), similar to that used in evaluating targets in other HRS pathways and 

components (e.g., the ground water migration pathway). Figure 6-5 depicts one variation of this alternative. The 

potential approach includes a standard 4-mile distance (modified if site-specific geologic information indicates 

otherwise) radiating either in all directions or only in the probable downgradient direction from each source at a 

site to establish the future migration zone. This approach may account for the possibility of future horizontal 

migration of substances in either contaminated ground water or in soil gas beyond the demonstrated boundaries of 

the subsurface contamination and subsequently into occupied structures (i.e., a potential future migration zone). 

This might happen, for example, if hazardous substance plumes expand or migrate downgradient within ground 

water. Other factors that could cause changes in the extent of the plume may be the additional release of 

hazardous substances, the plume shifting side-to-side due to ground water gradient changes resulting from 

seasonal variations or changing direction due to the sequencing of dry and wet years, or drawdown due to 

pumping at municipal water supply well fields or industrial ground water use. Additionally, natural and 

anthropogenic influences, such as utility corridors, fracture patterns, karst features, buried stream channels, or 

other geologic heterogeneities may alter or enhance hazardous substance migration. The example presented in 

Figure 6-5 includes the potential migration zone (PMZ) as depicted by the green area, which indicates where the 

contaminant subsurface plume may migrate to in the future. In this example, the PMZ expands outward to a set 

TDL and those targets associated with structures in this area are included as part of the HRS evaluation. This 

method of establishing a PMZ with a set TDL boundary is similar to how the ground water migration pathway 

considers future migration of hazardous substances in a contaminated aquifer to wells that have the potential to 

become impacted in the future as contaminants migrate horizontally through the subsurface. 

However, the confidence in the present science to accurately project or predict hazardous substance migration 

through the unsaturated zone is limited. There are several fate and transport models used to project possible 

horizontal vapor migration. However, most vapor intrusion models, including the J&E model, typically focus only 

on the vertical migration of contaminant vapors and do not consider the potential for horizontal migration. The 

ability of a site assessor to accurately evaluate the potential future horizontal migration of subsurface hazardous 
90

substances relies heavily on the ability to gather extensive site-specific data in all areas of future migration in the 

relatively short time period and minimal resources available during an SI. Review of existing models indicates 

that in most instances, extensive site-specific data collection efforts and often multiple rounds of site 

investigations are required to develop an accurate model for predicting the future horizontal extent of vapor 

migration, especially in the unsaturated zone. As discussed in section 2.5 of 1988 Revised HRS TSD (prepared to 

support the 1990 HRS revisions), 

[The] misapplication of a model or the use of incomplete data would, of course, result in less 

accuracy... [and] a very conservative model may also increase the frequency with which sites that 

do not pose significant risks are placed on the NPL... 

Vertical profiles of soil gas concentration can be very different underneath buildings compared to locations 

exterior to the building and soil gas concentrations may not be uniform laterally, particularly in the vicinity of the 

building, even when the vapor source is a laterally extensive plume of contaminated ground water. Therefore, 

after thorough review of this option, the agency has chosen not to include the consideration of future subsurface 

contaminant migration as part of the proposed subsurface intrusion component. It may be possible to place sites 

on the NPL based on projections that do not demonstrate risk of actual exposure. The exclusion of this option in 

the proposed HRS addition does not directly prevent a site from being considered for listing on the NPL based on 

demonstrated intrusion, nor does it restrict future investigations from expanding the site boundaries or preclude 

re-evaluating a site if further studies, or new information, indicate that the extent of contamination at a site may 

have increased due to migration. 

Figure 6-5. Consideration of Actually Exposed and All Potentially Exposed Targets
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d. Rationale for the Selected Option

As stated above, the proposed approach for delineating targets includes establishing an AOE and/or an ASC. For 

the AOE, each pathway within the 1990 HRS identifies and counts targets subject to actual exposures in the soil 

exposure pathway and subject to observed release criteria in the three migration pathways. The proposed method 

for identifying AOE targets based on two levels of actual contamination (i.e., Levels I and II) is consistent with 

every existing 1990 HRS pathway. The inference of contamination between points of actual exposure is also 

consistent with the current soil exposure pathway and the surface water and air migration pathways.

A parallel method to the establishment of an ASC does not exist in the current soil exposure pathway. The current 

soil exposure pathway does not consider migration of contamination from the existing zone of contamination into 

other areas (although the nearby threat, one of two threats within the soil exposure pathway, does consider the 

potential for off-site targets to migrate on-site). However, this situation is different for subsurface intrusion 

because, due to the time and resource limitations inherent to the Superfund site inspection process and the 

variability in subsurface intrusion rates with time, it is unlikely and impractical to expect that all occupied 

structures in which subsurface intrusion is occurring would be identified during a site inspection sampling event. 

Therefore, it is also possible that those structures with subsurface intrusion that pose the greatest risk to human 

health would not be identified during a site inspection. The air migration pathway addresses this variability in air 

releases with time by considering those targets located “on a source” as most likely exposed even if sampling has 

not shown them to be and weights those targets accordingly. By defining the ASC to include targets above 

subsurface contamination, the proposed method for delineating target areas in a subsurface intrusion evaluation is 

consistent with the consideration of targets located “on a source” in the air migration pathway of the 1990 HRS. 

Unlike the air migration pathway, the proposed subsurface intrusion component includes the qualifier “unless 

available information indicates otherwise” when establishing an ASC. Numerous studies have discussed the 

extremely variable nature of sampling and detecting vapor intrusion. Because of this variability, there may be 

scenarios where intrusion may or may not be occurring based on site-specific information. For example, a 

building with a structure containment factor value of zero, such as a structure raised six feet above the ground 

surface, represents information indicating that intrusion of contaminated vapors is not likely to occur. 

One of the main considerations when establishing the proposed method for delineating target areas was to 

minimize the potential numbers of false positives (sites with low relative risk) and false negatives (sites with high 

relative risk). In addition to the proposed approach being consistent with the methods used throughout the 1990 

HRS, a sensitivity analysis was conducted based on population densities and typical factor values from previous 

HRS site evaluations. In this analysis the target factor values were evaluated to ensure that the same level of 

relative threat within the subsurface intrusion component would receive a score similar to other pathways and 

components with similar relative threat levels. Results from the limited sensitivity analysis are presented in 

Appendix C of this document. These results show that while not the only factor to impact an HRS site score, the 

targets factor category and, more specifically, the population density of the site being evaluated generally have the 

greatest impact on an HRS site score. 

6.2 Selection of Eligible Targets – See Section 5.2.1.3 of the Proposed HRS Addition 

This section describes how the 1990 HRS evaluates eligible targets and the method for identifying and selecting 

targets in the proposed subsurface intrusion component. In the overall HRS structure, following the delineation of 

target areas, the 1990 HRS identifies those targets that are eligible for consideration. Specifically, targets are 
92

eligible for consideration if they are, or are likely to be, exposed to hazardous substances due to releases—in this 

case as a result of exposure to released hazardous substances via subsurface intrusion—into occupied structures. 

This step is necessary to identify what targets should be included when calculating the relative risk a site poses for 

HRS purposes via subsurface intrusion. Eligible targets vary among HRS pathways, and are intended to include 

those persons or environments that may be impacted as a result of the release. EPA proposes the subsurface 

intrusion component maintain this overall structure in selecting eligible targets. 

a. Approach within the 1990 HRS

The 1990 HRS evaluates the targets factor category based on:

• reasonable maximum exposure (RME) factor, 

• population factor, 

• resources

• other pathway-specific target factors, and

• level of contamination.

Descriptions of each of these factors and how they are evaluated in the 1990 HRS are discussed below. 

i. Reasonable Maximum Exposure (RME) – See Section 5.2.1.3.1 of the Proposed HRS Addition.

All HRS pathways and components consider a reasonable maximum human exposure (RME) factor (the surface 

water migration pathway also includes assessment of environmental threats) that contributes to the overall HRS 

score based on the relative magnitude of the RME. Including this factor is consistent with EPA’s standard 

procedures for evaluation of human risk levels as documented in EPA’s Risk Assessment Guidance for Superfund, 

Volume 1, Human Health Evaluation Manual (Part A). The RME is defined as the highest exposure that is 

reasonably expected to occur at a site. In the 1990 HRS, RMEs are estimated for individual pathways or 

components. The goal of the RME is to estimate a health protective exposure case (i.e., well above the average 

exposure scenario) that is still within the range of possible exposures. Uncertainty is evaluated under this 

approach. Within the 1990 HRS, the RME is evaluated as follows:

• Ground water migration pathway – Nearest well (see section 3.3.1 of the 1990 HRS); 

• Surface water migration pathway: Drinking water threat – Nearest intake (see section 4.1.2.3.1 of the 

1990 HRS); 

• Surface water migration pathway: Human food Chain threat – Food chain individual (see section 4.1.3.3.1 

of the 1990 HRS); 

• Soil exposure pathway: Resident population threat – Resident individual (see section 5.1.3.1 of the 1990 

HRS);

• Soil exposure pathway: Nearby population threat – Nearby individual (see section 5.2.3.1 of the 1990 

HRS); and

• Air migration pathway – Nearest individual (see section 6.3.1 of the 1990 HRS). 
93

ii. Population – See Section 5.2.1.3.2 of the Proposed HRS Addition.

Each pathway and component within the 1990 HRS considers the population factor, with the exception of the 

surface water migration pathway – human food chain and environmental threats. The human population factor 

includes those persons subject to actual or potential contamination. Those persons eligible for consideration and 

evaluation are pathway specific, but generally include a person living at the site, attending school or daycare at the 

site, or working at the site. The population factor in the 1990 HRS is an indicator of the population at risk or 

potentially at risk. The population factor for all pathways and components includes both Level I and Level II 

contamination. The migration pathways also include a consideration of potentially contaminated populations. 

iii. Resources – See Section 5.2.1.3.3 of the Proposed HRS Addition.

A resources factor is evaluated in the ground water migration pathway, the surface water migration pathway –

drinking water threat, soil exposure pathway – resident population threat, and the air migration pathway. A factor 

value of 5 is assigned for each pathway or component if at least one resource is present; a factor value of zero is 

assigned if no resource is present. Eligible resources are pathway-, component-, or threat-specific. These 

resources represent economic uses of, or public exposure to, a contaminated medium or area where exposures 

occur and are not covered by the other identified targets. For example, resources within the air migration pathway 

include commercial agriculture or silviculture and major/designated recreation areas. Resources in the surface 

water migration pathway include certain water recreation facilities.

iv. Other Pathway-Specific Targets

Each pathway and component within the 1990 HRS also considers other pathway-specific targets when evaluating

the relative risk posed by a site. The evaluated targets vary between pathways and components and include the 

following: 

• Ground water migration pathway

o Wellhead protection area

• Surface water migration pathway: Environmental threat

o Sensitive environments

• Soil exposure pathway: Resident population threat

o Terrestrial sensitive environments

• Air migration pathway

o Sensitive environments

Each of these pathway-specific targets includes a variation of sensitive environments that may be subject to actual 

or potential contamination. Because the subsurface intrusion component only considers the relative threat posed to 

indoor spaces, an environmental target factor was not considered for inclusion in the proposed HRS addition. 

v. Level of Contamination

The levels of contamination within the 1990 HRS include actual contamination and potential contamination. The 

application of each type of contamination is pathway-and-component-specific. Actual contamination indicates 

that a target is associated either with a sampling location that meets the criteria for an observed release or 
94

observed contamination, or with the location of an observed release based on direct observation. Actual 

contamination is subdivided into two levels: Level I and Level II. 

Level I contamination of targets is assigned based on whether media-specific concentrations for the target meet 

the criteria for an observed release or exposure by chemical analysis and are at or above media-specific 

benchmarks (see section 2.5 of the 1990 HRS). These benchmarks include both screening concentrations and 

concentrations specified in regulatory limits. The surface water pathway human food chain threat also considers 

concentrations in tissue samples from aquatic human food chain organisms when identifying Level I 

contamination. Contamination established based on an observed release by direct observation is not eligible to be 

considered Level I.

Level II contamination of targets is determined based on whether media-specific concentrations for the target 

meet the criteria for an observed release or exposure by chemical analysis but are less than media-specific 

benchmarks or when no benchmark exists (see section 2.5 of the 1990 HRS). For observed releases based on 

direct observation or those targets inferred to be contaminated (e.g., soil exposure, resident population threat), 

Level II contamination is assigned based on pathway-specific procedures.

Potential contamination of targets includes targets that are considered subject to a potential release based on 

pathway-specific requirements, but not associated with actual contamination for that pathway or component.

6.3 Proposed Option

The following subsections outline the methods for identifying and selecting targets for the proposed subsurface 

intrusion component. The targets being considered as part of the subsurface intrusion component include an 

exposed individual, populations that are subject to actual exposure (within an AOE), populations that are probably 

exposed (within an ASC), and common indoor community resources. 

a. Assigning Level of Exposure to Structures

The proposed addition considers both actual and potential contamination of regularly occupied structures with 

individuals living in, attending school or daycare, and workers working in the structures, in parallel with other 

pathways and components of the 1990 HRS (as discussed above). The proposed addition subdivides actually 

contaminated populations (i.e., populations at locations with an observed release/contamination or direct 

observation) into Level I and Level II concentrations using health-based media-specific benchmarks for cancer 

and non-cancer effects, as is discussed in this TSD. Those targets living, working, or attending a school/daycare in 

one or more structures or subunits where observed exposure by chemical analysis criteria are met (in an AOE), 

and media concentrations meet or exceed the appropriate benchmark are considered Level I. Those targets in one 

or more structures or subunits where observed exposure by chemical analysis criteria have been met (in an AOE), 

but the media concentrations are not above benchmarks, are considered Level II. Those targets where observed 

exposure by direct observation have been established, and those targets inferred to be actually contaminated when 

establishing an AOE, are considered subject to Level II contamination consistent with other HRS pathways and 

components. 
95

Populations in Structures with no Subunits

Populations regularly occupying a structure with no subunits (e.g., single-family home) that lies within an AOE 

will be weighted according to the applicable Level I or Level II benchmarks associated with that structure. 

Likewise, populations regularly occupying a structure with no subunits that lies within an ASC will be weighted 

based on the potential as described in Section 6.3.b.ii, Populations within the ASC, of this TSD.

The current scoring framework used to weight populations throughout the HRS is proposed to be retained, with 

actually exposed targets being weighted more heavily than potentially exposed targets. The proposed subsurface 

intrusion component will evaluate populations based on the number of individuals located within an identified 

AOE (i.e., those populations weighted based on exposure to Level I and Level II concentrations) and the number 

of individuals located within an ASC (i.e., populations in occupied structures above subsurface contamination will 

be weighted based on factors influencing the probability of exposure), as is described below in Section 6.3.b.i, 

Populations in the AOE (Level I and Level II), and Section 6.3.b.ii, Populations Within the ASC. 

Populations within structures on an ASC are weighted based on the sample media and depth to contamination. 

Section 6.3.b.ii of this TSD discusses the weighting of populations on an ASC. These targets are weighted less 

than those subject to actual contamination. See Section 6.3 of this TSD for additional description of the 

methodology and rationale for weighting targets. 

Populations in Multi-Subunit Structures

The weighting of populations of eligible individuals (residents, workers, students, or individuals attending day 

care) occupying multi-subunit buildings is different than the weighting applied to populations residing in 

structures with no subunits. Not all populations in a multi-subunit structure may actually be exposed to observed 

subsurface contamination in one unit or floor of the structure, or populations might be exposed to different levels 

of contamination if their unit is on a different floor than where the subsurface contamination is located. In 

addition, structure designs can vary considerably and may impact the distribution of vapors (e.g., buildings with 

more than one story, open staircases, separate outdoor access to each unit, or individual or zoned HVAC systems). 

Therefore, a generic scoring approach is proposed, with the express ability to incorporate site-specific situations 

into the target evaluation. This approach is presented in Section 5.2.0 of the proposed HRS addition and is 

illustrated in the examples presented below. 

It is proposed that for a multi-subunit structure, such as a multi-story apartment building that has contaminant 

concentrations meeting observed exposure criteria, populations occupying on the floors with observed exposure 

concentrations and populations residing on all floors below will be included within an AOE. Populations residing 

one floor above the floor with the observed exposure may be considered as being within an ASC. A site assessor 

may also consider “information indicating otherwise” when establishing an ASC in a multi-subunit building. For 

example, a multi-story apartment building may have multiple HVAC systems that are zoned throughout the 

building; if it is known that air is received from a non-contaminated area, this may indicate that targets within a 

given HVAC system, even if they are on the same floor or are below a floor with an observed exposure, may not 

be exposed to contamination resulting from subsurface intrusion. 
96

Additionally, because vapor intrusion includes a vertical migration component, originating below the foundation 

of the structure and conceptually rising upward through the structure, all populations on a level with a measured 

observed exposure and all levels below that level are inferred to be contaminated. Only individual subunit spaces 

with measured Level I concentrations are considered Level I. All inferred subunit spaces within an AOE are 

considered subject to Level II concentrations. 

Furthermore, because vapor intrusion includes a vertical migration component, the floor directly above the floor 

where actual contamination is detected is considered to be in an ASC. Those populations one floor above actual 

contamination are treated as being within an ASC and are weighted accordingly. 

An exception to this method of counting the floor above a floor with actual exposure (in a multi-story, multisubunit structure) to be within the ASC is when the contaminant intrusion occurs via ground water. In this 

situation, only the contaminated building level is considered in the scoring. Only the contaminated level is 

considered because it is unlikely ground water would rise to an upper floor; and if the upper floor was flooded 

with contaminated ground water, then the upper floor would be considered within the AOE. 

Figure 6-6 depicts four conceptual scenarios for identifying and weighting target populations in multi-subunit 

structures when the structure is the only target. For example, the multi-subunit structure in Example 3 of Figure 6-

6 below has two measured indoor exposures detected in two separate subunit spaces on the first floor. In this 

example, the two samples exceed the appropriate health-based benchmarks and populations within these two 

subunit spaces are considered subject to Level I actual exposures. One additional subunit space on the first floor 

and all three occupied subunit spaces on the basement level may be inferred to be actually exposed at Level II 

concentrations. The entire basement and first floor are therefore considered as part of an AOE. The populations in 

the three subunit spaces on the second floor are considered to be part of an ASC as those individuals are probably 

exposed to contaminants intruding from the subsurface and migrating up and into the living space. 
97

Figure 6-6. Potential Scenarios for Evaluating Target Weighting in Multi-Subunit Structures

Figure 6-7 depicts a scenario in which a multi-subunit structure is located within an AOE but no observed 

exposure has been documented in the building. In this situation, an observed exposure is inferred due to its 

location among residential structures within an AOE. Therefore, occupants on the lowest level are considered to 

be within an AOE if no available information indicates otherwise. 

Figure 6-7. Inferring Level of Exposure in Occupied Multi-Subunit Buildings within an Area of Observed 

Exposure

Figure 6-8 presents a situation in which a multi-subunit structure is in an ASC and no observed exposure has been 

documented in any regularly occupied subunit space. In this case, only those regularly occupied subunit spaces on 

the lowest level are considered to be within an ASC, unless available information indicates otherwise. If there are 
98

no regularly occupied spaces in the lowest level (a basement), there would be no target population considered in 

the ASC associated with the structure. An example of “information indicating otherwise” may be the existence of 

a forced pressurized air HVAC system that uses an outdoor air source to exchange the air in the basement 

apartments and prevents any vapor intrusion from occurring.

Figure 6-8. Inferring Level of Exposure in Occupied Multi-Subunit Buildings within an Area of Subsurface 

Contamination

Figure 6-9 presents a situation in which an observed exposure has been documented based on an intruded liquid 

or particulate sample. In this case, any regularly occupied subunit spaces above the level where an observed 

exposure has been documented are not considered to be within an ASC. 

Figure 6-9. Surficial Ground Water Intrusion into an Occupied Multi-Subunit Building

In addition, it should be noted that building construction and mechanical system configurations have the potential 

to significantly impact the rate of intrusion from the subsurface as well as the migration route. This is particularly 

evident in multi-story buildings where features such as elevator shafts and HVAC systems may play a significant 
99

role. It may be necessary to evaluate a multi-story building on a site-specific basis, because the construction of 

these multi-story buildings is likely to vary depending on the date of construction, style, and geographical 

location. When inferring levels of contamination within a multi-story building, site-specific information such as 

the location of HVAC intakes and zoned circulation paths may be considered as “information indicating 

otherwise.” An additional example of “information indicating otherwise” is a building with a structure 

containment factor value of zero, such as a structure with an active subslab ventilation system approved by EPA 

as a permanent remedy or a structure raised greater than 6 feet above the ground surface with open space between, 

as shown in Example 2 of Figure 6-10 below. 

Figure 6-10. Example of “Information Indicating Otherwise” - Structure Containment Factor Value of Zero

Weighting and Indoor Air Benchmarks

The same target weighting structure, based on whether or not exposure levels are above benchmarks in the 

subsurface intrusion component, is proposed to use the same weighting structure used in other HRS components 

(see Sections 5.2.1.3.1 and 5.2.1.3.2 of the proposed HRS addition). Receptors regularly occupying structures in 

the AOE are considered actually exposed. Whereas, receptors regularly occupying structures in the ASC are 

considered potentially exposed. Structures in the AOE are divided into two categories based on whether the 

hazardous substance concentrations are above the same standard health-based benchmarks. If the concentration of 

a hazardous substance is above one or more benchmark, then the receptors are considered to be exposed to Level I

concentrations. If the concentration of a hazardous substance is below all benchmarks, then they are identified as 

exposed to Level II concentrations. The receptors on an ASC are also weighted based on the sample media and 
100

the depth to the shallowest contaminated samples. The benchmarks proposed for the subsurface intrusion 

component are: 

• Screening concentrations for cancer, and

• Screening concentrations for non-cancer toxicological responses.

As stated in section III.H of the 1990 HRS: 

[T]argets [are] based on actual and potential exposure to contamination across all pathways and

threats, including those for which benchmarks were not originally proposed, because [the] EPA

believes that this approach improves the ability of the HRS to identify sites that pose the

greatest threat to human health and the environment and increases the internal consistency of

the HRS... [the] EPA [also] concluded that the HRS is improved by including concentrations

based on nationally uniform standards, criteria, or toxicity values as health-based or ecologicalbased benchmarks in all pathways and threat... All health-based benchmarks are set in

reference to the major exposure concern [i.e., component specific]... All benchmarks are set in

reference to uniform exposure assumptions that are consistent with RI/FS procedures...

Reasonable Maximum Exposure Individual (Exposed Individual Factor) 

The reasonable maximum exposure (RME) is defined for the subsurface intrusion component as the highest 

exposure to an individual resulting from subsurface intrusion that is reasonably expected to occur at a site. An

RME factor within the subsurface intrusion component is proposed and is consistent with the 1990 HRS and with 

standard EPA risk assessment procedures. This factor is known as the Exposed Individual factor in the proposed 

addition. See also sections 3.5.1, 4.4.3.1 and 5.6.2 of the 1988 Revised HRS TSD for additional justification 

regarding the inclusion of an RME factor. 

The basic scoring approach used for assigning a factor value throughout the 1990 HRS to evaluate the RMEI, an 

exposed individual, factor is retained for the subsurface intrusion component. As is consistent with all pathways, a 

value of 50 points is assigned if any individual in a regularly occupied structure is exposed to Level I

concentrations; or a value of 45 points is assigned if there is any individual exposed to Level II concentrations 

(see sections 6.3.c of this TSD and 5.2.1.3.1 of the proposed addition for further explanation of assigning Level I 

and II concentrations.) If there are no individuals in structures exposed to Level I or Level II concentrations, but at 

least one individual is living, working, or attending school or day care in a structure in an ASC, a value of 20 is 

assigned. (See section 2.5 of the 1990 HRS for supplemental information regarding how exposed individuals are 

addressed within the HRS structure.) 

b. Populations (i.e., Individuals Living, Working, or Attending School or Daycare)

This section details how populations (made up of exposed individuals) are identified in the proposed subsurface 

intrusion component with particular consideration of the difference between evaluating a structure with no 

subunits and structures with multiple subunits and/or multiple stories. 

Those persons living, working, or attending school or daycare (such as child- and elder-care facilities) in an AOE

or ASC are proposed to be evaluated in the population factor. Residents are those individuals most exposed to 
101

subsurface intrusion contamination based on the duration of time these populations spend in their respective 

structures. Students in general are considered to be more susceptible than workers or teachers based on age. The 

weighting of the population factor is proposed to be evaluated using media-specific, health-based benchmarks as 

discussed below. Workers are considered in the population factor evaluation, but are weighted two-thirds less than

residents and those attending school and daycare to account for the length of the work day. This weighting is 

generally consistent with how EPA determines the appropriate Regional Screening Levels for chemical

contaminates.

i. Populations in an AOE (Level I and Level II)

Those exposed individuals living, working, or attending a school/daycare in one or more structures or subunits 

where observed exposure by chemical analysis criteria are met (in an AOE), and media concentrations meet or

exceed the appropriate benchmark are considered Level I. Those exposed individuals in one or more structures or

subunits where observed exposure by chemical analysis criteria have been met (in an AOE), but the media 

concentrations are not above benchmarks, are considered Level II. Those exposed individuals where observed

exposure by direct observation have been established, and those exposed individuals in structures inferred to be 

actually contaminated when establishing an AOE, are considered subject to Level II contamination consistent

with other HRS pathways and components. 

Weighting of Populations in an AOE

Consistent with the weighting of populations (exposed individuals) in structures documented as exposed to 

released hazardous substances throughout the HRS (see section 2.5 of the 1990 HRS), the proposed subsurface 

intrusion component identifies and weights exposed individuals occupying structures in the AOE subject to Level 

I contaminant concentrations by a factor of 10 and weights exposed individuals in structures subject to Level II 

contaminant concentrations by a factor of 1. 

For example, if a single residence occupied by a family of four is observed to be exposed to hazardous substance 

concentrations above a media-specific, health-based benchmark, the number of residents is multiplied by 10 for a 

Level I factor value of 40. However, if that same family is exposed to a hazardous substance at concentrations 

below the applicable benchmark but above a limit of detection, the number of residents is multiplied by 1 for a 

Level II factor value of 4. To determine the overall actual exposure factor value, Level I and Level II factor values 

for each regularly occupied structure are summed.

Figure 6-11 presents an example scenario with both Level I and Level II targets located within an AOE. In this 

example, 8 structures have Level I contamination and 9 structures have Level II contamination. While in practice,

a door-to-door survey of population per household would be strongly recommended, for purposes of this example 

population per household is estimated using the county average of 2.5 people per occupied house. Therefore, the 8 

Level I structures represent 20 people or 200 target points (20 * 10). The 9 Level II structures represent 22.5 

people or 22.5 target points (22.5 * 1).The total target population within the AOE is 222.5 target points (200 + 

22.5). 

Note that number of workers that are exposed to Level I and II concentrations are divided by three and six for 

full- and part-time workers, respectively, and then weighted according to assigned level of contamination (i.e., 

Level I or Level II). If information is unavailable to classify a worker as full- or part-time, evaluate that worker as 
102

being full-time. The rationale for the difference between residents and workers is based on the difference in

exposure time for full-time workers of 8 hours a day (or 4 hours for part-time workers) versus the typical

exposure scenario for residents of as high as 24 hours a day. The agency recognizes that this ratio is not exact for

all situations, and that to determine the most appropriate ratio of exposure would require site specific information

beyond that collectable during a typical site inspection. This consideration of less exposure for workers than 

residents is consistent with Superfund’s risk assessment exposure scenarios. 
103

Figure 6-11. Example Calculation of Actually Exposed Targets

As discussed in Section 6.3 of this document, the proposed HRS addition includes within the AOE populations in 

occupied structures for which sampling has not established observed exposures, if these structures are surrounded 

by occupied structures in which observed exposures have been identified, unless information indicates otherwise. 

This action is proposed because it is considered likely that if sampled during the correct conditions, observed 

exposure would be identified in these structures at levels similar to those in surrounding structures. However, 

numerous studies (see Section 10.0 of this TSD) have discussed the extremely variable nature associated with 

sampling and detecting the rate of vapor intrusion. Because of this variability, there may be scenarios where 

intrusion may or may not be occurring based on site-specific information. The ability to infer contamination when 

actual data is not available—but, when all surrounding structures have documented actual exposure—prevents the 

evaluator from underestimating the relative threat posed by a site and minimizes the cost of sampling.

Individuals inferred to be exposed to this contamination will be weighted as Level II because there are no actual 

sample results to compare against benchmarks. However, the proposed “information indicates otherwise” 

provision included in this evaluation allows for situations where site-specific conditions clearly document that 

there could be no observed exposures in these structures. This inferred exposure concept is also included in the 

existing soil exposure pathway and in the air migration pathway (see sections 5.0.1 and 6.3 of the 1990 HRS). 

As previously stated, the proposed method is different for evaluating targets when multi-subunit structures, such 

as an apartment building, are present within the boundaries of an AOE. Not all persons residing in a multi-subunit 

building overlying a plume may be considered actually or potentially exposed. The proposed method for 
104

evaluating multi-subunit buildings is discussed in further detail in subsection 6.3.a, Assigning Level of Exposure 

to Structures, of this document.

Figure 6-12 depicts an example of when the inference of contamination exists in structures within an AOE. In this 

example, there are 8 residential structures with Level I contamination, 8 residential structures with Level II 

contamination, and 1 residential structure that has not been sampled. Using the same methods presented above, 

the 8 Level I structures represent 21 residents or 210 target points (21 * 10). Because the unsampled structure lies 

in between structures known to be contaminated (either Level I or Level II) it is inferred to be contaminated at 

Level II concentrations. Therefore, the 8 known Level II structures and the 1 inferred Level II structure represent 

24 residents or 24 target points (24 * 1).The total target population within the AOE is 224 target points (200 + 

24). 

Figure 6-12. Example Calculation of Actually Exposed Targets Including Inference of Contamination

Figure 6-13 presents a conceptual scenario where an area lying within an AOE is determined not to be 

contaminated based on “information suggesting otherwise.” It should be noted however, that although these 

targets may not be considered Level I or Level II, they may be evaluated as potential exposures and part of an 

ASC. In this example, there are 7 structures with Level I contamination, 8 structures with Level II contamination 

and 3 structures that have been sampled and do not have concentrations meeting Level I or Level II criteria. Using 

the same methods presented above, the 7 Level I structures represent 18 people or 180 target points (18 * 10). 

Because evidence indicates otherwise, the 3 structures with sample concentrations not meeting Level I or Level II 

criteria cannot be inferred to be contaminated at Level II concentrations. Therefore, the 8 Level II structures 

represent 20 people or 20 target points (20 * 1).The total target population within the AOE is 200 target points 

(180 + 20).
105

Figure 6-13. Example Calculation of Actually Exposed Targets Excluding Areas Where Information 

Indicates Otherwise

ii. Populations within the ASC

Populations within structures on an ASC are weighted based on the sample media and depth to contamination. 

These targets are weighted less than those subject to actual contamination. An ASC is defined by documented SsI 

samples meeting observed release criteria. These areas are included in the subsurface intrusion component due to 

the possibility that limited sampling conducted during a site inspection may not identify that subsurface intrusion 

is occurring due to the high temporal and spatial variability associated with detecting subsurface intrusion. 

Temporal and spatial differences can significantly impact the rate at which volatile hazardous substances enter a 

structure. However, when an ASC has been defined, that area represents a location where subsurface hazardous 

substances have a high probability of intruding into a structure. The extent of screening activities during the 

Superfund site inspection process is time-limited, and a single indoor air sampling event is unlikely to identify the 

full threat posed by subsurface intrusion. 

Weighting of Populations in an ASC

Consistent with the 1990 HRS, it is proposed to weight these potentially exposed targets at a value less than those 

targets that have been identified to be actually exposed. Due to the high 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. Using EPA’s vapor intrusion attenuation factors published in 2012, 
106

EPA developed a relatively proportional weighting for targets within an ASC based on the probability of a target 

to be exposed as a result of contaminant intrusion from the subsurface in a variety of likely sampling scenarios. 

The target weighting factors presented in the proposed HRS addition do not directly correspond to attenuation 

factors; instead, the relative weighting between these values is based on the published attenuation factors and 

other factors identified as playing a key role in the probability of intrusion. For situations where samples have 

been collected from multiple media, the sample that results in the highest target weighting factor is used in 

calculating the population within an ASC. The proposed weighting factors for structures within an ASC are 

outlined in Table 5-20 of the proposed HRS addition and presented below. 

Note that the population of workers located within the ASC are divided by three and six for full- and part-time 

workers, respectively, then assign a further weighting based on Table 6-1. 

Table 6-1. Weighting Factor Values for Populations within an Area of Subsurface Contamination

Eligible Populationsa in Structuresb within an Area of Subsurface Contamination

Population 

Weighting 

Factor

Population in a structure with levels of 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

0.9

Population in a structure where levels of contaminants meeting observed release criteria are 

found in any sampling media at or within 5 feet horizontally or vertically of the structure 

foundation 

0.4

Population in a structure where levels of contaminants meeting observed release criteria are 

found or inferred based on any underlying non-ground water subsurface sample at a depth less 

than or equal to 30 feet

or 

Population in a structure within an area of subsurface contamination where levels of 

contaminants meeting observed release criteria are inferred based on semi-enclosed or 

enclosed crawl space samples in surrounding structures 

0.2

Population in a structure where levels of contaminants meeting observed release criteria are 

found or inferred based on underlying ground water samples greater than 5 feet from the 

structure foundation 

or

Population in a structure where levels of contaminants meeting observed release criteria are 

found or inferred based on any underlying sample at depths greater than 30 feet 

0.1

a Eligible populations include residents, workers, and students attending school or daycare (see 1990 HRS Section 5.2.1.3.2.3). b Eligible structures may include single- or multi-subunit structures where eligible populations reside, work or attend school or daycare. They may 

also be mixed use structures.

Figures 6-14 and 6-15 are example scenarios for weighting targets in multi-subunit structures and structures 

without subunits. In Figure 6-14 potential population weighting factors are assigned based on such factors as 

sample media and the depth or distance to contamination. Figure 6-15 presents the situation where an observed 

exposure has been documented on the first floor of a multi-story multi-subunit building and the resulting 

weighting factor applied to the floor located within the ASC. 
107

Figure 6-14. Example Weightings of Targets for Structures without Subunits within an Area of Subsurface 

Contamination
108

Figure 6-15. Example Weightings of Targets for a Multi-story Multi-subunit Structure within an Area of 

Subsurface Contamination

These weighting factors reflect that contaminants found in a crawl space, for example, are less likely to attenuate 

less before entering an overlying structure, and thus more likely to pose a threat, as opposed to those found in a 

shallow ground water sample. An example population calculation for targets located within an ASC is presented 

below in Figure 6-16. In this example, there is a combination of ground water, soil gas, crawl space, and subslab 

sampling data that is used to: 1) delineate the area of subsurface contamination and 2) assign appropriate target 

weightings. Figure 6-16 depicts a total of eight structures that have ground water samples that meet observed 

release criteria. In addition, there are four structures within a contaminated soil gas plume, one of which is known 

to have subslab contamination and one is known to have crawl space contamination. As shown in Table 5-20 of 

the proposed HRS addition (Table 6-1 above), the population in the structure with crawl space contamination is 

weighted by 0.9. The subslab sample represents contamination found within 5 feet of the structure’s foundation 

and that population is weighted by 0.4. Populations associated with the remaining two structures located within 

the identified soil gas plume are weighted by 0.2, because the contamination is found in unsaturated soil vapor 

samples at a distance greater than 5 feet from a structure’s foundation, but at a depth less than or equal to 30 feet 

below ground surface. Finally, the population in the four structures that overly the ground water plume, but which 

have no additional sampling data, are weighted by 0.1. Assuming a national average of 2.6 people per home, the 

population associated with the ASC depicted in Figure 6-16 is calculated to be 5.46 after applying the appropriate 

weighting factors. 
109

Figure 6-16. Example Calculation of Residential Populations within an Area of Subsurface Contamination

EPA considered other options when determining an appropriate weighting factor for occupied structures located 

in an ASC including: 

• Assigning a weighing of 0.1 to all targets located within an ASC;

• Assigning a higher weighting to targets located in structures with documented subslab concentrations

exceeding 10x an appropriate benchmark (0.1, 0.5, 0.75 and 1.0); and

• Assigning a higher weighting to targets located in structures based on subsurface soil type.

However, as stated above, the weighting factors were chosen based on the most recent attenuation factors 

published by EPA and other factors known to impact the probability of vapor intrusion. It should be noted that the 

higher weighting of structures with soil gas, soil, or crawl space contamination—as opposed to ground water 

contamination—reflects that contamination has volatilized out of a liquid state and it is documented that the

contamination has begun to migrate vertically. 
110

c. Resources

For the subsurface intrusion component, regularly occupied structures in which there are significant populations 

that could be exposed to hazardous substances due to subsurface intrusion are considered “resources” if they are 

located within a defined area of exposure (as previously discussed in section VI.B.2 of the preamble to the 1990 

HRS) and the populations may be exposed to contamination due to subsurface intrusion on a regular basis. For 

example, most indoor public recreation facilities, libraries, community centers, religious facilities, and tribal

cultural facilities could qualify as eligible resources if they are regularly occupied (e.g., occupied daily). It is 

proposed that any identified resource will be assigned a resource target value of 5, as is consistent with the

method for counting resources under the existing soil exposure pathway. 

Weighting of Resources

A resources target factor is assigned in all pathways in the 1990 HRS. In the proposed addition a factor value of

five is assigned if at least one resource is present within an AOE or ASC, and a factor value of zero is assigned if

no resource is present. Eligible resources are specific to the pathway, component, or threat. These resources 

represent economic uses, public exposure to a contaminated medium, or areas where exposures occur and are not 

covered by the other identified targets. For example, resources within the air migration pathway include 

commercial agriculture or silviculture and major/designated recreation areas. The resident population threat of the

1990 HRS soil exposure pathway considers commercial livestock production or grazing that could lead to 

contamination of human food supplies. (See section III.I of the preamble to the 1990 HRS (55FR51549, 

December 14, 1990) for supplemental information.) 

d. Special Consideration for Radioactive Hazardous Substances when Identifying and

Evaluating the Targets Factor Category

As indicated in Table 7-1 (HRS Factors Evaluated Differently for Radionuclides) of the proposed HRS addition, 

the method for evaluating the exposed individual and population factors for radionuclides includes additional 

instructions outlined in Section 7.1.1 of the proposed HRS addition. As is stated in Section 7.3 of the proposed 

HRS addition, the targets factor category is evaluated in the same manner as discussed in above in this section of 

this TSD, except that Level I and Level II concentrations at sampling locations are established as outlined in 

Sections 7.3.1 and 7.3.2 of the proposed HRS addition. These sections identify the appropriate benchmarks for 

comparison when evaluating the subsurface intrusion component. In addition, targets within an ASC established 

based on gamma radiation are assigned a weighting of 0.9. These differences also apply to radioactive hazardous 

substances for establishing target-exposed individual and populations. It should be noted however, that for 

instances where naturally occurring radioactive substances (such as radon) are intruding into an occupied 

structure, these intrusions are not considered to be a release unless they are a direct result from human activity 

(such as mine tailings). Please refer to Section 7.0 of the proposed HRS addition for additional information 

regarding the evaluation of radionuclides at subsurface intrusion sites.
111

7.0 Overview of Scoring Methodology for New Soil Exposure and Subsurface 

Intrusion Pathway

As previously discussed in Section 3.3 of this TSD, an HRS score with the proposed subsurface intrusion addition 

is the compilation of four individual pathways, which include the three migration pathways (i.e., ground water, 

surface water, and air) and the renamed soil and subsurface exposure pathway. This section provides a discussion 

on how the three factor categories (likelihood of exposure, waste characteristics, and targets) and the multiple 

factor values that make up each factor category are combined to generate a subsurface intrusion component score. 

This section also describes how that component score is combined with the soil exposure component score and 

the other pathway scores to achieve an overall HRS site score.

Table 7-1 of this document (Table 5-11 in the proposed HRS addition) presents the subsurface intrusion 

scoresheet proposed to be used when preparing an HRS package for a site in which a subsurface intrusion threat is 

evaluated. As previously mentioned, the subsurface intrusion component is based on three factor category scores: 

1) Likelihood of Exposure (Line 4), 2) Waste Characteristics (Line 7), and 3) Targets (Line 12). The values for 

these three factor categories are multiplied together and divided by 82,500. This divisor is considered a 

“normalizing” factor and is consistent with all pathways and components in the 1990 HRS. The resultant value is 

then subject to a maximum value of 100 (Line 13) and represents the subsurface intrusion component score. Table 

5-11 in the proposed HRS addition also includes the instructions for combining the soil exposure and subsurface 

intrusion components together to obtain the soil exposure and subsurface intrusion pathway score (Line 14 of 

Table 7-1 of this TSD). 
112

Table 7-1. Subsurface Intrusion Component Scoresheet (Table 5-11 in Proposed HRS Addition)

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 * (2b+2c+2d), subject to a maximum of 500) 500

4. Likelihood of Exposure (higher of lines 1 or 3) 550

Waste Characteristics:

5. Toxicity/degradation (a) 

6. Hazardous Waste Quantity (a) 

7. Waste Characteristics (subject to a maximum of 100) 100

Targets:

8. Exposed Individual 50

9. Population:

9a. Level I Concentrations (b) 

9b. Level II Concentrations (b) 

9c. Population within an Area(s) of Subsurface Contamination (b) 

9d. Total Population (lines 9a + 9b + 9c) (b) 

10. Resources 5 

11. Targets (lines 8 + 9d + 10) (b) 

Subsurface Intrusion Component Score

12. Subsurface Intrusion Component (lines 4 x 7 x 12)/82,500c (subject to a 

maximum of 100) 100

Soil Exposure and Subsurface Intrusion Pathway Score

13. Soil Exposure Component + Subsurface Intrusion Component (subject to a 

maximum of 100) 100

a

Maximum value applies to waste characteristics category.

b

Maximum value not applicable.

Do not round to the nearest integer.

It is not a requirement that a site be evaluated for both the subsurface intrusion and soil exposure components to 

obtain a pathway score for the soil exposure and subsurface intrusion pathway. It is proposed that a site’s 

subsurface intrusion component score (Line 13 of the scoresheet) be added to the site’s soil exposure component 

score, if both have been scored (see Table 5-1 of the proposed HRS addition for the soil exposure component 

score sheet). The rationale for adding these two scores is that the subsurface intrusion component includes 

exposures inside a structure and the soil exposure component includes only exposures outside of structures. 
113

Therefore, EPA considered the two components to be additive and, therefore representing the total exposure 

threat.

The sum of the subsurface intrusion component score (Line 13) and the soil exposure component score is also 

subject to a maximum value of 100 and represents the overall soil exposure and subsurface intrusion pathway 

score (Line 14 of the scoresheet). 

Because it is not necessary to evaluate the soil exposure component, it is also not necessary to evaluate any of the 

three migration pathways to obtain an overall HRS site score. However, each migration pathway may be 

evaluated and the four pathway scores (ranging from 0 to 100) then combined for the site using the following 

root-mean-square equation to determine the overall HRS site score, which also ranges from 0 to 100. Please see 

also Section 2.2 of this TSD for additional discussion regarding the HRS root-mean-square algorithm, including 

the justification for its inclusion.

SS = �SSgggg

2 + SSssgg2 + SSssssssssss

2 + SSaa

2

4
114

8.0 Testing of the Proposed HRS Addition

This section discusses EPA’s testing and evaluation of the proposed HRS addition. It addresses the overall 

approach as well as individual factor category weightings and values. Specifically, this section includes the 

following: 

• A discussion regarding the development of example site scenarios for the purpose of establishing estimate 

site scores based on the initial options considered for identifying eligible targets and delineating target 

areas (Section 8.1),

• An evaluation of actual sites based solely on existing, readily available information for the purpose of 

testing the proposed HRS addition (Section 8.2), and 

• A discussion of planned future pilot study efforts to be conducted for the specific purpose of generating 

an HRS site score that includes a subsurface intrusion component score (Section 8.3).

8.1 Development of Example Site Scoring Scenarios

In evaluating alternatives for the HRS addition, extensive conceptual simulations were performed to project the 

effectiveness and appropriateness for each option identified for assigning factor values. This evaluation entailed 

the development of numerous example site scenarios. The example site scenarios were developed in three phases 

and are described below. Note that the tables cited in this discussion represent example sites only and do not 

represent actual sites identified by EPA.

a. Example Site Scenarios – 1st Phase

The initial testing phase established estimated site scores based on initial options considered for identifying 

eligible targets and delineating target areas. The testing was conducted using factor values similar to other 

existing HRS pathways, components, and threats—along with factor category values common to historically 

listed sites. These options included: 1) consideration of only actual exposed targets without inference of 

contamination (exclude structures without an observed exposure, even though they fall within the boundaries of 

an AOE), 2) consideration of only actually exposed targets with inference of contamination, 3) consideration of 

actually exposed targets with inference of contamination and potentially exposed targets overlying a known 

contaminant plume (in an ASC), and 4) consideration of establishing a potential migration zone (PMZ) that 

accounts for the future horizontal migration of a subsurface contaminant plume. Please refer to Section 6.1 of this 

TSD for a detailed discussion of each option considered. 

Specifically, these initial site scenarios were developed to ensure relative risk is evaluated appropriately; such that 

the number of sites potentially eligible for listing on the NPL do not result in the identification of a significant 

number of false positives (listing sites that have a low level of individual risk) or false negatives (not listing sites 

that have a high level of individual risk). The characteristics needed for a site to be placed on the NPL are 

illustrated in Tables C-1 and C-1a of Appendix C of this TSD. Please see also Sections 4.0 through 6.0 of this 

TSD for a more detailed discussion of each factor and proposed weighting values. 

b. Example Site Scenarios – 2nd Phase

A second testing phase was conducted following the selection of the proposed option for identifying target areas 

by delineating AOEs and ASCs described in Sections 4.0 through 6.0 of this TSD. This analysis was conducted to 
115

test the proposed HRS addition and to illustrate the types of sites that may qualify for the NPL considering vapor 

intrusion contamination under various scenarios based on different site characteristics. Scoring for hundreds of 

example site scenarios was completed with various modified individual factor values. These scoring models are 

presented in Tables C-2 through C-6 in Appendix C of this document. Many scenarios were used to select the 

relative size of the impacted area and level of subsurface intrusion contamination necessary for a site to qualify 

for the NPL. A brief discussion of the parameters modified for each table is provided below. It is important to 

understand that these tables were used to develop the proposed target weighting factor values; the values 

presented within these tables do not represent the proposed option. The proposed option is reflected in Table C-7. 

The range of discrete values for the factors was derived from projected typical values based on EPA’s experience 

conducting HRS evaluations. 

• Table C-2 illustrates the baseline scenario first considered for purposes of evaluating weighting options. It 

presents sample HRS scores for example sites where no observed exposure has been identified and only 

an ASC is being scored based on potential for exposure. This analysis includes the modification of the 

following parameters: 

o Population Density: 100, 250, 500, 1,000 and 10,000 (people per square mile) (2.6 people per 

residence)

o Likelihood of Exposure: 180, 320, 500 

o Hazardous Waste Quantity: Based on assumed volume of all structures Toxicity: 1, 10, 100, 

1,000, 10,000

o Target Weighting: 0.1, 0.2, 0.5, 0.75 (no distinction between structures with subslab 

contamination) 

• Table C-3 illustrates the impacts of assigning 2 different hazardous waste quantity minimum values. It is 

based only on an ASC being scored. This analysis includes the modification of the following parameters:

o Population Density: 100, 250, 500, 1,000 and 10,000 (people per square mile) 

o Likelihood of Exposure: 180, 320, 500 

o Hazardous Waste Quantity: 10, 100 (minimum values considered)

o Toxicity: 1, 10, 100, 1,000, 10,000 

o Target Weighting: 0.1, 0.2, 0.5, 0.75 (no distinction between structures with subslab 

contamination) 

• Table C-4 illustrates the impact of contaminated subslab sample concentrations in comparison to healthbased benchmarks when weighting potential targets. It presents sample HRS scores for example sites 

based only on an ASC being evaluated. However, special weighting consideration to structures with 

positive subslab foundation sampling was considered in the following six scenarios:

o All targets located within the ASC and subslab data not available. Assumes a hazardous waste 

quantity minimum of 10. 

o All targets located within the ASC and subslab data not available. Assumes a hazardous waste 

quantity minimum of 100. 

o All targets located within the ASC: 20 percent having subslab contaminant concentrations 10 

times greater than an appropriate benchmark. Assumes a hazardous waste quantity minimum of 

10. 
116

o All targets located within the ASC: 20 percent having subslab contaminant concentrations 10 

times greater than an appropriate benchmark. Assumes a hazardous waste quantity minimum of 

100. 

o All targets located within the ASC: 50 percent having subslab contaminant concentrations 10 

times greater than an appropriate benchmark. Assumes a hazardous waste quantity minimum of 

10. 

o All targets located within the ASC: 50 percent having subslab contaminant concentrations 10 

times greater than an appropriate benchmark. Assumes a hazardous waste quantity minimum of 

100. 

• Table C-5 illustrates the impacts of having both actual and contaminated targets on site characteristics 

necessary for qualifying for the NPL. It presents sample HRS scores for three example site scenarios 

using a combination of Level I and Probable target populations. The scenarios include the following: 

o Four Level I residences within an AOE with the remaining homes located in an ASC where no 

subslab data is available.

o Four Level I residences within an AOE with the remaining homes located in an ASC where 20 

percent have subslab contaminant concentrations 10 times greater than an appropriate benchmark.

o Four Level I residences within an AOE with the remaining homes located in an ASC where 50 

percent have subslab contaminant concentrations 10 times greater than an appropriate benchmark.

• Table C-6 illustrates the characteristics of sites necessary for qualifying for the NPL when more than one 

pathway at a site poses a threat. It includes sample HRS scores for 10 example scenarios that evaluate the 

combination of a ground water pathway score with a subsurface intrusion score. The scenarios included 

the following:

o Four residences subject to Level I ground water concentrations, potential ground water targets, 

and no SsI component evaluated. 

o Four residences subject to Level II ground water concentrations, potential ground water targets, 

and no SsI component evaluated.

o Four residences subject to Level I ground water concentrations, potential ground water targets, 

and SsI component evaluated for ASC populations only. Assumes none of the ASC homes have 

subslab contaminant concentrations 10 times greater than an appropriate benchmark and a 

hazardous waste quantity minimum of 10 for the SsI component. 

o Four residences subject to Level II ground water concentrations, potential ground water targets, 

and SsI component evaluated for ASC populations only. Assumes none of the ASC homes have 

subslab contaminant concentrations 10 times greater than an appropriate benchmark and a 

hazardous waste quantity minimum of 10 for the SsI component. 

o Four residences subject to Level I ground water concentrations, potential ground water targets, 

and SsI component evaluated for ASC populations only. Assumes 20 percent of the ASC 

residences have subslab contaminant concentrations 10 times greater than an appropriate 

benchmark and a hazardous waste quantity minimum of 10 for the SsI component. 

o Four residences subject to Level I ground water concentrations, potential ground water targets, 

and SsI component evaluated for ASC populations only. Assumes 50 percent of the ASC 

residences have subslab contaminant concentrations 10 times greater than an appropriate 

benchmark and a hazardous waste quantity minimum of 100 for the SsI component. 

o Four residences subject to Level I ground water concentrations, potential ground water targets, 
117

four residences subject to Level I SsI concentrations, and no ASC evaluated. 

o Four residences subject to Level I ground water concentrations, potential ground water targets, 

four residences subject to Level I SsI concentrations, and ASC evaluated assuming none of the 

ASC residences have subslab contaminant concentrations 10 times greater than an appropriate 

benchmark and a hazardous waste quantity minimum of 100 for the SsI component. 

o Four residences subject to Level I ground water concentrations, potential ground water targets, 

four residences subject to Level I SsI concentrations, and ASC evaluated assuming 20 percent of 

the ASC residences have subslab contaminant concentrations 10 times greater than an appropriate 

benchmark and a hazardous waste quantity minimum of 100 for the SsI component. 

o Four residences subject to Level I ground water concentrations, potential ground water targets, 

four residences subject to Level I SsI concentrations, and ASC evaluated assuming 50 percent of 

the ASC homes have subslab contaminant concentrations 10 times greater than an appropriate 

benchmark and a hazardous waste quantity minimum of 100 for the SsI component. 

• Table C-7 illustrates the characteristics of sites necessary for qualifying for the NPL using the final 

proposed addition. It uses approximately the same ranges of factor values as Tables C-2 through C-6 to 

make this illustration. The weighting values used for this table includes the changes made in the proposed 

addition to reflect the peer review comments. 

Please refer to Appendix C of this document for actual site scenario scores and a list of the assumptions made as 

part of these calculations. 

c. Example Site Scoring Scenarios – 3rd Phase

To illustrate the proposed subsurface intrusion component and weighting of factor values, three comprehensive 

site scoring scenarios were created. In the first scenario a situation was considered in which a site would not 

qualify for placement on the NPL (score below 28.50). In another scenario, the site would marginally qualify for 

the NPL (score of about 28.50), and in the final scenario the site would exceed the scoring criterion for the NPL 

(site score considerably above 28.50).

A brief summary of the three site scenarios and resulting scores is presented below. Each site scoring scenario 

includes a figure depicting the example site and an example scoresheet that is based on Table 5-11 of the 

proposed HRS addition. 

Example Site Scoring Scenario No. 1 

The first example site scenario consists of a ground water plume contaminated with a hazardous substance with 

moderate toxicity (HRS toxicity of 1,000). Initial ground water sampling has delineated the horizontal extent of 

the ground water plume as underlying approximately three acres of a residential neighborhood at depths between 

10 and 15 feet below ground surface. All of the ground water samples were collected from locations greater than 

five feet from a structure’s foundation. The neighborhood consists of 12 single-family detached homes. As part of 

an expanded site investigation (ESI) indoor air samples have been collected from inside two adjacent homes 

where occupants had previously reported foul odors. Indoor air concentrations within the two homes contained 

hazardous substance concentrations above laboratory detection limits, but below the applicable benchmark. The 

homes were checked for indoor sources of hazardous substances prior to sampling. 
118

The hazardous substance was also detected in subslab samples collected from the two homes discussed above. 

Several other occupied structures were sampled for indoor air and subslab contaminant concentrations; however, 

no other detections of hazardous substances were observed. Outside ambient air samples were collected near the 

two homes with identified indoor contaminant concentrations; however, contaminant concentrations were not 

detected in the ambient air samples.

In calculating the HRS score for this scenario, a likelihood of exposure factor category value of 550 was assigned 

based on an observed exposure by chemical analysis. The indoor air contaminant concentrations in the two homes 

were sufficiently elevated to document a significant increase above that in other homes outside the plume area. By 

considering the lack of indoor sources with the collected ground water, subslab and ambient air samples, the 

increase in measured indoor air concentrations was attributed to the contaminated ground water plume underlying 

the residences.

The Waste Characteristics factor category is the product of the toxicity/degradation factor and the hazardous 

waste quantity factor values. Only one hazardous substance of a moderate level of toxicity is associated with this 

site. A toxicity value of 1,000 is assigned from HRS Table 2-4 based on the oral reference dose for this 

contaminant. The degradation factor value is assigned a value of 1 due to the presence of an observed exposure. 

Hazardous waste quantity is calculated based on the dimensions of the structures within the area of observed 

exposure (AOE) and the area of subsurface contamination (ASC). For this site, the AOE is defined by the two 

adjacent homes with measured indoor air contaminant concentrations. No other structures may be inferred as 

actually exposed. The ASC is delineated by contaminated ground water samples meeting observed release criteria. 

There is not sufficient data to calculate hazardous constituent or wastestream quantities, but the actual dimensions 

of each structure are unknown. The 10 homes within the ASC have a basal footprint of 1,700 ft2

 each, and the two 

homes in the AOE have a basal footprint of 1,940 ft2

 each. Therefore, the total area of structures within the AOE 

and ASC can be calculated to be 20,880 ft2

. Using the proposed HRS addition Tables 2-6 and Table 5-18, the 

hazardous waste quantity factor value is assigned 100 and the resulting waste characteristics factor category value 

is 18 (Table 2-7). 

The targets factor category value is based on the populations in the AOE and ASC. The exact number of people 

living within each home in the AOE was obtained during sampling activities. However, the number of residents in 

the ASC is estimated based on the county-wide census average of 2.6 people per home.

Because an observed exposure has been established at Level II concentrations (i.e., concentrations below the 

applicable benchmark but meeting observed exposure criteria), the exposed individual factor value assigned is 45. 

The population factor category value for the AOE is then determined by the number of individuals subject to 

Level I concentrations (zero) and Level II concentrations (2 homes – 5 people). 

In this example, the population factor for the ASC includes those populations subject to potential exposure—that 

is, those populations living in structures overlying the ground water plume but not within the AOE. Based on 

HRS Table 5-20, and a weighting of 0.1, the 26 residents are assigned a value of 2.6. 

The population value for this scenario is 7.6 [5(AOE) + 2.6(ASC)]. The targets factor category value is thereby 

determined to be 52.6 (45 + 7.6). 
119

The resulting subsurface intrusion component score is established by multiplying the three factor category values 

and dividing by a scaling factor (82,500). A subsurface intrusion component score of 6.31 and an HRS site score 

of 3.15 were determined for this example site scoring scenario. Therefore, this site would not be considered 

eligible for listing on the NPL. Table 8-1 provides the scoresheet for the subsurface intrusion component scored in 

this example scenario.

Figure 8-1. Example Site Scoring Scenario No. 1
120

Table 8-1. Scoresheet for Example Site Scoring Scenario No. 1

Factor Categories and Factors Maximum

Value

Value

Assigned

Subsurface Intrusion Component

Likelihood of Exposure:

1. Observed Exposure 550 550

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 * (2b+2c+2d), subject to a maximum of 

500) 500 - 

4. Likelihood of Exposure (higher of lines 1 or 3) 550 550

Waste Characteristics:

5. Toxicity/degradation (a) 1,000

6. Hazardous Waste Quantity (a) 100

7. Waste Characteristics (subject to a maximum of 100) 100 18

Targets:

8. Exposed Individual 50 45

9. Population:

 9a. Level I Concentrations (b) 0

 9b. Level II Concentrations (b) 5

9c. Population within an Area(s) of Subsurface Contamination (b) 2.6 

 9d. Total Population (lines 9a + 9b + 9c) (b) 7.6

10. Resources 5 0

11. Targets (lines 8 + 9d + 10) (b) 52.6

Subsurface Intrusion Component Score:

12. Subsurface Intrusion Component (lines 4 x 7 x 11)/82,500c (subject to a 

maximum of 100) 100 6.31 

Soil Exposure and Subsurface Intrusion Pathway Score:

13. Soil Exposure Component + Subsurface Intrusion Component (subject 

to a maximum of 100) 100 6.31 

HRS Site Score:

15. Total of Sgw

2 + Ssw2 + Ssessi2 + Sa2 39.81

16. HRS Site Score (divide the value on line 15 by 4 and take the square 

root) 50 3.15 

a

Maximum value applies to waste characteristics category.

b

Maximum value not applicable.

c

Do not round to the nearest integer.
121 

Example Site Scoring Scenario No. 2 

The second example site scoring scenario also consists of a mixed-use residential and commercial area above a

shallow ground water plume contaminated with a hazardous substance of moderate toxicity (HRS toxicity of 

1,000). Initial ground water sampling has documented a depth to ground water of 35 ft. below ground surface and 

delineated the horizontal extent of the ground water plume as underlying approximately 20 acres of a residential 

neighborhood, a daycare facility, and a single-story office building. The neighborhood consists of 29 singlefamily detached homes. As part of an expanded site investigation (ESI), indoor air samples were collected inside 

19 homes; one daycare facility having approximately 25 enrolled children, 6 full-time workers, and 2 part-time

workers; and the office building with 18 full-time workers. Indoor air concentrations above the applicable 

benchmark were detected in 14 of the homes sampled; indoor air concentrations above detection limits but below 

the applicable benchmark were found in the remaining 5 sampled homes and daycare facility. All homes and the 

daycare facility were checked for indoor sources of hazardous substances prior to sampling. 

The second example site scoring scenario receives a likelihood of exposure value of 550 because an observed

exposure has been established by chemical analysis in 19 homes, one daycare facility, and one office building. 

The contaminant levels are significantly above background levels established based on concentrations in homes 

not above the ground water plume. The increase in contaminant levels is attributed to subsurface intrusion based

on multiple lines of evidences. Outside ambient air samples were collected near the homes with identified indoor

contaminant concentrations. However, contaminant concentrations were not detected in the ambient air samples.

In addition, the ground water contamination contained the same substance as those in the samples taken from the 

residences, day care, and office and no indoor sources of the contaminant were found. 

The waste characteristics factor category is a product of the toxicity/degradation factor and hazardous waste 

quantity factor values. For this site, the AOE is defined by the 19 homes, the daycare facility, and the office 

building with measured indoor air contaminant concentrations meeting observed exposure criteria. Based on the 

location of these structures, the boundary of the AOE does not encompass any additional structures; therefore, no

other structures are inferred as actually exposed. The ASC is delineated by contaminated ground water samples 

meeting observed release criteria. Ten homes have been identified as overlying this ground water plume excluding 

those found within the AOE. Sufficient data does not exist to calculate hazardous constituent or wastestream

quantities. The actual dimensions of each structure within the AOE and ASC are known and used to calculate the 

hazardous waste quantity area. Ten homes have a basal footprint of 1,700 ft2

 each. Eighteen homes have a basal

footprint of 1,760 ft2

 each, and one home has a basal footprint of 1,780 ft2. The daycare facility is documented as 

having a basal footprint of 4,600 ft2

. The office building is documented as having a basal footprint of 12,000 ft2. 

Therefore, the total area of structures within the AOE and ASC is 67,060 ft2

. Only one hazardous substance of a 

moderate level of toxicity is associated with this site. A toxicity value of 1,000 is assigned from HRS Table 2-4 

based on the oral reference dose for this contaminant. The degradation factor value is assigned a value of 1 due to

the presence of an observed exposure. Using the proposed HRS addition Tables 2-6 and 5-18, the hazardous waste 

quantity factor value of 100 is assigned using a toxicity/degradation value of 1,000 for the hazardous substance of 

moderate toxicity and the presence of an observed exposure. The resulting waste characteristics factor category

value is 18 (Table 2-7). 

The targets factor category value is based on the population in the AOE and ASC. Because an observed exposure 

has been established at Level I concentrations in 14 occupied homes, the site receives an exposed individual factor

value of 50. The number of receptors in the AOE significantly impacts the population factor for this site. The 

number of residents in the AOE is known. The exact number of people living within each unsampled home of this 
122

site scenario was unavailable, but the county-wide census average number people per home is 2.6. In addition to 

the targets subject to Level I contamination (14 homes – 37 people), there are five homes, one daycare facility, 

and one office building subject to Level II contamination (13 residents, 25 children attending daycare, 6 full-time 

and 2 part-time workers at the daycare, and 18 full-time workers at the office building). The 10 homes with 26 

residents overlying the ground water plume (ASC) have been sampled for subslab contamination and have been 

found to have concentrations meeting observed release criteria. Each of the 10 homes had a subslab sample 

collected within five feet of the structure’s foundation; therefore, the populations within are weighted by 0.4 

based on the proposed HRS addition Table 5-20, resulting in a value of 10.4. 

The different levels of exposure and the presence of a daycare facility and office building between the first and 

second example site scoring scenarios increases the site score significantly. This second scenario has a component 

score of 58.00 and a corresponding HRS site score of 28.60, which qualifies the example site for listing on the 

NPL. Table 8-2 provides the scoresheet for the subsurface intrusion component scored in this scenario. 

Figure 8-2. Example Site Scoring Scenario No. 2
123

Table 8-2. Scoresheet for Example Site Scoring Scenario No. 2

Factor Categories and Factors Maximum

Value

Value

Assigned

Subsurface Intrusion Component

Likelihood of Exposure:

1. Observed Exposure 550 550

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 * (2b+2c+2d), subject to a maximum of

500) 500 - 

4. Likelihood of Exposure (higher of lines 1 or 3) 550 550

Waste Characteristics:

5. Toxicity/degradation (a) 1,000

6. Hazardous Waste Quantity (a) 100

7. Waste Characteristics (subject to a maximum of 100) 100 18

Targets:

8. Exposed Individual 50 50

9. Population:

 9a. Level I Concentrations (b) 370

 9b. Level II Concentrations (b) 46.33

9c. Population within an Area(s) of Subsurface Contamination (b) 10.4 

 9d. Total Population (lines 9a + 9b + 9c) (b) 426.73

10. Resources 5 0

11. Targets (lines 8 + 9d + 10) (b) 476.73

Subsurface Intrusion Component Score:

12. Subsurface Intrusion Component (lines 4 x 7 x 11)/82,500c (subject to a

maximum of 100) 100 57.20 

Soil Exposure and Subsurface Intrusion Pathway Score:

13. Soil Exposure Component + Subsurface Intrusion Component (subject

to a maximum of 100) 100 57.20 

HRS Site Score:

15. Total of Sgw

2 + Ssw2 + Ssessi2 + Sa2 3271.84

16. HRS Site Score (divide the value on line 15 by 4 and take the square

root) 50 28.60 

a

Maximum value applies to waste characteristics category.

b

Maximum value not applicable. c

Do not round to the nearest integer.

Example Site Scoring Scenario No. 3 

The third example site scoring scenario consists of a mainly residential area above a ground water plume 

contaminated with a highly toxic hazardous substance (HRS toxicity of 10,000) based on its cancer slope factor. 

The depth to ground water at this site is approximately between 10 and 15 feet below ground surface. Initial 

ground water sampling has delineated the horizontal extent of the ground water plume as underlying 

approximately 25 acres of a residential neighborhood and a daycare facility. The residential neighborhood 
124

consists of single-family detached homes. As part of an expanded site investigation (ESI) Level I contaminant 

concentrations were detected in indoor air samples from 34 homes and a daycare facility with approximately 25 

children enrolled and 5 full-time workers. Level II contaminant concentrations were detected within 15 homes. 

The homes and daycare facility were checked for indoor sources of hazardous substances prior to sampling. 

Outside ambient air samples were collected near the homes with identified indoor contaminant concentrations. 

However, contaminant concentrations were not detected in the samples collected. Subslab contaminant 

concentrations meeting observed release criteria were found in samples collected at 5 additional homes.

This example site scoring scenario receives a likelihood of exposure of 550 because an observed exposure has 

been established by chemical analysis in 49 homes and one daycare facility. The contaminant level was 

significantly above background levels based on samples from nearby residences and is attributed to subsurface 

intrusion based on multiple lines of evidence including the lack of an indoor source, and contamination found in 

subslab and ground water samples. 

The waste characteristics factor category is the product of the toxicity/degradation factor and the hazardous waste 

quantity factor values. For this site, the AOE is defined by the location of the 49 homes and the daycare facility 

with measured indoor air contaminant concentrations meeting observed exposure criteria. Based on the location of 

these structures, the boundary of the AOE does not encompass any additional structures; therefore, no structures 

are inferred as actually exposed. Ten additional homes are located within the ASC, which is delineated by ground 

water samples meeting observed release criteria. Data are not sufficient to calculate hazardous constituent or 

wastestream quantities. The actual dimensions of each structure are known. Each of the 49 homes located in the 

AOE has a basal footprint area of 1,750 ft2

, and the 10 homes in the ASC have a basal footprint of 1,700 ft2. The 

daycare facility has a basal footprint of 4,600 ft2

. Therefore, the total area of structures within the AOE and ASC 

is 107,350 ft2

. A hazardous substance having a high level of toxicity is associated with this site. A toxicity value 

of 10,000 is assigned from HRS Table 2-4 based on the cancer slope factor for this contaminant. The degradation 

factor value of 1is assigned due to the presence of an observed exposure. Using the proposed HRS addition Tables 

2-6 and 5-18, the hazardous waste quantity factor value is calculated to be 100. Using a toxicity/degradation 

factor value of 10,000, for the highly toxic hazardous substance and the presence of an observed exposure of that 

substance, the resulting waste characteristics factor category value is 32 (Table 2-7). 

The targets factor category value is based on the population in the AOE and ASC, and the workers at the day care 

facility. The number of residents in the AOE is known. The exact number of people living within each unsampled 

home of this site scenario was unavailable, but the county-wide census average number people per home is 2.6. 

Because an observed exposure has been established at Level I concentrations in 34 homes and one daycare 

facility, the site receives an exposed individual factor value of 50. 

The number of targets in the AOE significantly impacts the population factor for the site. The total number of 

targets subject to Level I contamination (34 homes and 1 daycare – 115.06 people) and to Level II contamination 

(15 homes – 39 people) is 154.06 targets residing in residences or attending daycare within an AOE.

The 10 homes in the ASC have been sampled for subslab contamination and the contamination levels in five of 

the subslab samples have been found to meet observed release criteria. The five subslab samples meeting 

observed release criteria were collected within five feet of the structure’s foundation. Therefore, the population 

residing in the five homes with subslab concentrations meeting observed release criteria are assigned a weighting 

factor of 0.4 based on HRS addition Table 5-20 (5 homes – 13 people)—resulting in a value of 5.2. The remaining 
125

5 homes within the ASC are overlying a ground water plume defined by samples that meet observed release 

criteria and were collected from locations greater than five feet from a structure’s foundation. Therefore, the 

populations residing within the ASC are weighted by 0.1 based on HRS Table 5-20 (5 homes – 13 people)— 

resulting in a value of 1.3. 

The population value for this scenario is 1,196.1. . The resulting maximum subsurface intrusion component score 

of 100 (uncapped value of 265.83) and a corresponding HRS site score of 50.00, qualifies the example site for 

listing on the NPL. Table 8-3 provides the scoresheet for the subsurface intrusion component in this scenario.

Figure 8-3. Example Site Scoring Scenario No. 3
126

Table 8-3. Scoresheet for Example Site Scoring Scenario No. 3

Factor Categories and Factors Maximum

Value

Value

Assigned

Subsurface Intrusion Component

Likelihood of Exposure:

1. Observed Exposure 550 550

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 * (2b+2c+2d), subject to a maximum of 

500) 500 - 

4. Likelihood of Exposure (higher of lines 1 or 3) 550 550

Waste Characteristics:

5. Toxicity/degradation (a) 10,000

6. Hazardous Waste Quantity (a) 100

7. Waste Characteristics (subject to a maximum of 100) 100 32

Targets:

8. Exposed Individual 50 50

9. Population:

 9a. Level I Concentrations (b) 1,150.6

 9b. Level II Concentrations (b) 39

9c. Population within an Area(s) of Subsurface Contamination (b) 6.5 

 9d. Total Population (lines 9a + 9b + 9c) (b) 1,196.1

10. Resources 5 0

11. Targets (lines 8 + 9d + 10) (b) 11,246.1

Subsurface Intrusion Component Score:

12. Subsurface Intrusion Component (lines 4 x 7 x 11)/82,500c (subject to a 

maximum of 100) 100 100 

Soil Exposure and Subsurface Intrusion Pathway Score:

13. Soil Exposure Component + Subsurface Intrusion Component (subject 

to a maximum of 100) 100 100 

HRS Site Score:

15. Total of Sgw

2 + Ssw2 + Ssessi2 + Sa2 10,000

16. HRS Site Score (divide the value on line 15 by 4 and take the square 

root) 50 50.00 

a

Maximum value applies to waste characteristics category.

b

Maximum value not applicable.

c

Do not round to the nearest integer.
127

8.2 Tier 1 Sites (Test Sites) with Existing Vapor Intrusion Data

As discussed previously in Section 1.3.b of this report, an evaluation of the need for this proposed addition was 

undertaken to identify and classify sites that may pose a vapor intrusion threat. This evaluation resulted in the 

identification of 1,073 sites that may or may not qualify for the NPL, but could have vapor intrusion issues. 

Of those 1,073 sites identified, 11 were considered to have a subsurface intrusion threat based on the presence of

actual sampling data and other HRS-required data to confirm that these sites would not score sufficiently high 

enough to qualify for the NPL without the addition of a subsurface intrusion component to the HRS. 

The identification of these sites is based on currently available information only, and no sampling or site-specific 

investigations were conducted to expand the data available for the purposes of this analysis. Below is a synopsis 

of site information for each of 11 actual sites. More detailed site-specific information, including subsurface 

intrusion component scoresheets and figures for each of the 11 test sites, is included in Appendix D of this 

document. 

Test Site No. 001 

Test site 001 consists of a chlorinated solvents release from an operating drycleaner situated adjacent to a daycare 

and a private school. The U.S. Environmental Protection Agency (EPA) determined that exhaust from the 

drycleaner was causing elevated levels of tetrachloroethylene (PCE) and trichloroethylene (TCE) to be taken into

nearby building ventilation systems. EPA has collected indoor air samples from the drycleaner, daycare, and

private school and has also collected soil gas samples from beneath the foundations (i.e., subslab) of surrounding 

residential and commercial buildings, including the drycleaner, daycare and private school. Additionally, EPA has 

also collected outdoor air samples. 

Test Site No. 002 

Test site 002 is located in central Maryland, within an area of commercial and residential land use types. The 

subject property consists of an L-shaped shopping center strip and three pad sites occupied by multiple 

commercial tenants. Historical operations resulted in a release of chlorinated solvents to shallow ground water 

and soils immediately surrounding the facility. Dry cleaning facilities are documented to have been ongoing for 

approximately 40 years—the current tenant ceased on-site dry cleaning operations in 2004. This site has been 

investigated on multiple occasions, including indoor air sampling in January, July, and November of 2013. 

Additionally, ground water samples have been collected on multiple occasions dating back to November 2007. 

Test Site No. 003 

Test site 003 is located in the District of Columbia, within an area of residential homes and a church. Previous 

investigations to assess a gasoline spill from a service station have identified ground water contaminated with

PCE and other chlorinated and non-chlorinated volatile organic analytes. The contamination appears to be

traceable to a dry cleaning operation. The PCE contamination was first detected in 2002 and subsequent 

investigations in 2003 and 2004 were conducted to determine the extent of the PCE contamination and to evaluate 

the extent of vapor intrusion into overlying residential structures. 

Test Site No. 004 

Test site 004 is located in central Maryland, within an area containing a mixture of residential, commercial, and 

industrial land use types. Historical operations at a former precious metal recovery operation resulted in a release 
128

of chlorinated solvents to shallow ground water and soils immediately surrounding the facility. The site includes a 

residential area comprised of row houses located between two noncontiguous property parcels. The two 

noncontiguous property parcels were previously the location of a former precious metal recovery operation. In 

1981, the State of Maryland reported the property parcels to EPA and requested assistance after discovering 

deteriorating and damaged tanks, containers, and drums that were leaking their contents into the environment. 

EPA assessed the site and commenced an emergency response action, which was completed in December 1981. 

In 2002 the Maryland Department of the Environment (MDE) collected air samples from locations on or adjacent 

to the two noncontiguous property parcels. Results of these samples indicated the presence of volatile organic 

compounds (VOCs) inside the building located at the state-owned parcel. Further air testing at the site in 

November 2003 confirmed this finding and indicated the presence of VOCs in the air inside a building adjacent to 

the vacant property parcel. In November and December 2005, MDE conducted a more detailed investigation of 

the ground water and soil vapor beneath the homes separating the two noncontiguous property parcels. Results of 

this investigation showed that ground water contamination from the parcels was migrating beneath many of the 

intervening homes. Soil gas and indoor air testing conducted for MDE between June 2007 and August 2008 in six 

of the twenty row houses indicated vapor intrusion has occurred in all six tested houses. As of 2010, installation 

of vapor intrusion mitigation systems in three homes had begun.

Test Site No. 005 

Test Site 005 is located in southeastern Pennsylvania, within an area of mixed residential, commercial, and 

industrial use. Historical operations at nearby facilities have resulted in a release of chlorinated solvents to 

shallow ground water and soils surrounding commercial and residential properties. EPA initiated a removal 

assessment at the Site in September 2003 and continued investigations at the site through January 2006. The 

results of the removal assessment revealed the presence of trichloroethylene (TCE) in ground water, soil gas 

vapor, and indoor air at both commercial and residential properties in the area of investigation. The EPA Removal 

Program installed indoor air mitigation systems at three private residences which are impacted by the Site. The 

site was originally scored using only the ground water migration pathway, but would not score high enough to be 

considered NPL eligible based on this score alone.

Test Site No. 006 

Test site 006 is located in north-central Texas within an area of mixed residential, commercial, and industrial use. 

Test Site 006 consists of a release of hazardous substances from a facility that historically conducted machining 

and metal forging operations, and regularly occupied residential and commercial structures are located within 

documented areas of observed exposure (AOE) and an area of subsurface contamination (ASC) surrounding the 

AOE. TCE contamination was first identified at the facility during an environmental property assessment in 2002. 

Additional investigations identified ground water contaminated, predominantly with TCE, underlying the facility 

and adjacent residential areas. In 2008, EPA conducted a vapor intrusion sampling event at 18 residential and 

commercial structures overlying the ground water plume, which documented indoor air samples containing levels 

of TCE and PCE meeting observed exposure criteria. 
129

Test Site No. 007 

Test site 007 is located in the District of Columbia, within an area of commercial and residential land use types 

and is immediately adjacent to a hospital complex. Dry cleaning operations have reportedly been ongoing for 

approximately 60 years at the facility. These historical operations have resulted in a release of chlorinated 

solvents to shallow ground water and soils immediately surrounding the facility, as has been documented in 

several environmental investigations conducted in 2013 and 2014. 

Test Site No. 008 

Test site 008 is located in south-central Montana within an area of residential and commercial land use types.

Historical releases possibly associated with two nearby dry cleaning operations have resulted in a release of

chlorinated solvents to ground water in the area; the plume encompasses about 140 acres (1,000 ft wide by 8,000 

ft long). State-lead preliminary assessments (PA), site inspections (SI), and expanded site inspections (ESI) were 

conducted at this site in Montana from 1993 to 2002. These investigations have been under the direction of the 

EPA Removal program since 2006. 

Test Site No. 009 

Test site 009 is located in south central Wisconsin within an area of commercial, industrial, and residential land 

use types. It consists of an AOE delineated by indoor air samples meeting observed exposure criteria and an ASC, 

delineated by samples from multiple media, meeting observed release criteria. Among the hazardous substances 

documented to be present in the AOE and ASC were tetrachloroethylene (PCE), TCE, vinyl chloride, and cis-1,2-

dichloroethene.

Test Site No. 010 

Test site 010 is located in eastern Missouri within an area of residential housing. EPA discovered a ground water

plume in a residential neighborhood and delineated the extent over multiple sampling events beginning in the 

summer of 1998. Two hazardous waste facilities are known to exist in the immediate vicinity of the ground water

plume, including a former metal shaping facility and a life insurance facility (where small scale dumping

historically occurred in the mid 1960’s). The ground water plume consists of chlorinated solvents (TCE, PCE, 

CIS-1,2-DCE, Vinyl Chloride, etc.) and underlies a residential neighborhood consisting of a mix of single family 

residences and several single-story, multi-tenant residential buildings. Indoor air sampling has identified VOC’s 

within the living space of numerous residential structures, including several contaminant concentrations 

exceeding cancer/non-cancer benchmarks. In addition, subslab sampling has been conducted and elevated 

concentrations have been documented to be underlying several of residential structures.

Test Site No. 011 

Test site 011 is located in southeastern Minnesota within a primarily residential neighborhood. In 2009, the 

Minnesota Pollution Control Agency (MPCA) conducted an investigation to identify potential sources for a large 

contaminant ground water plume. Part of one study area, Area A, was documented to have elevated 

concentrations of chlorinated solvents in ground water surrounding a single-family residential structure. Although 

this residential area with elevated contaminant ground water concentrations was determined not to be a likely 

source of the contamination, subsequent soil gas and indoor air sampling events detected contaminants within 11 

of the surrounding residences that were sampled. Sufficient information to identify the originating source of

contamination for this area is not available.
130

8.3 Planned Future Efforts

In addition to the conceptual and test site scoring discussed in Sections 8.1 and 8.2 of this report, workplans were 

developed for a number of sites across the country that will evaluate the proposed subsurface intrusion 

component. At the time of publication for this report, there are currently a minimum of 10 actual sites that have 

been identified as having actual or potential subsurface intrusion for this effort. Figure 8-4 below depicts the 

approximate spatial distribution of pilot sites identified at the time of drafting this proposed HRS addition, 

although additional sites may be identified later. These pilot studies validate the proposed HRS addition and 

evaluate the application of selected weighting factor values as well as the efficacy for accurately identifying sites 

with significant relative risk. These pilot studies are not intended to identify sites for listing on the NPL, and not 

all sites considered for listing on the NPL achieve an HRS score greater than (or equal to) 28.50. However, by 

collecting actual data for the purposes of generating an SsI component score, EPA ensures the proposed HRS 

addition considers subsurface intrusion threats appropriately. These pilot studies were conducted concurrently 

with the development of the proposed SsI addition. 

The VI pilot study was conducted to ensure the proposed HRS addition was an exposure pathway that is 

structurally consistent with the 1990 HRS, and a conceptual site model for VI was developed. The pilot study 

identified sites to test this model, assess the current capabilities and technology available to assess the threat from 

VI, determined the associated costs with performing VI investigations, and the site-specific data needed to test the 

proposed model. Sites for testing the proposed HRS SsI addition were identified by EPA regional offices during 

the development of this rulemaking. Data from past and ongoing VI site assessments were provided by the EPA 

regional offices and used to assess the parameters and limitations associated with actual VI site assessments. The 

HRS VI Pilot Study and outcomes of this study are further identified and discussed in the HRS VI Pilot Study 

Final Report.
131

Figure 8-4. Vapor Intrusion Pilot Study Site Distribution
132

9.0 Terminology Updates Affecting Specific Sections of the HRS (Sections 1, 2, 5 & 6)

Several terms and procedures have evolved in the establishment of health and environmental risk-based criteria 

since the HRS was last revised in 1990. As part of the proposed addition to the HRS, the terminology and tables 

in the HRS are proposed to be updated to reflect this evolution. 

These updates include: 

• Identification of Ambient Water Quality Criteria,

• Use of Reference Criteria (RfC) for non- cancer effects from inhalation exposures,

• Use of Inhalation Unit Risk (IUR) for cancer effects from inhalation exposures, and

• Revised Weight-of-Evidence Groupings for designation of a substance as a human carcinogen.

These terminology updates are explained below and do not result in any substantive change in the application of

the HRS.

a. Identification of Ambient Water Quality Criteria:

Ambient Water Quality Criteria (AWQC) are used in the HRS for two purposes in the environmental threat of the

Surface Water Migration Pathway. The first use is in assigning an environmental toxicity factor value for

individual hazardous substances. (See HRS section 4.1.4.2.1.1, Ecosystem toxicity). The second use is in 

assigning weighting for aquatic sensitive environments exposed to releases (i.e., considered “actually 

contaminated”). (See HRS sections 2.5.2, Comparison to Benchmarks, and 4.1.4.3., Environmental threat 

targets). 

These criteria are listed in the table identified as National Recommended Water Quality Criteria (NRWQC) on an 

EPA website (See EPA web page http://www.epa.gov/wqc/national-recommended-water-quality-criteria). To 

ensure clarity, the definition for Ambient Water Quality Criteria in HRS section 1.1, Definitions, is proposed to be 

revised to reflect this terminology.

In addition, the terms “acute” and “chronic” are also referred to at times using different terminology than in the 

1990 HRS definition section. The term “acute” is also referred to as the Criteria Maximum Concentration (CMC). 

The CMC is an estimate of the highest concentration of a material in surface water to which an aquatic 

community can be exposed briefly without resulting in an unacceptable effect. The “chronic” criteria are also 

referred to as the Criterion Continuous Concentration (CCC). The CCC is an estimate of the highest concentration

of a material in surface water to which an aquatic community can be exposed indefinitely without resulting in an 

unacceptable effect. (See EPA web page http://www.epa.gov/wqc/national-recommended-water-quality-criteriaaquatic-life-criteria-table). The definition of Ambient Water Quality Criteria in the HRS is also proposed to be 

revised to reflect this alternative nomenclature.

b. Use of Reference Concentration (RfC) Criteria

The 1990 HRS uses the Reference Dose (RfD) for determining the human toxicity factor value and the human 

health benchmark for non-cancer effects for all exposure routes in all four HRS pathways. (See 1990 HRS 

sections 2.4.1.1, Toxicity Factor, and 2.5.2, Comparison to benchmarks.) Since 1990, EPA has adopted the use of 
133

Reference Concentrations (RfCs) instead of Reference Doses (RfDs) when determining non-cancer related risk 

levels for inhalation exposures. 

The term RfD is defined by EPA in the 1990 HRS in HRS section 1.1 Definitions and on EPA web page: 

http://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?detail

s=&glossaryName=IRIS%20Glossary#formTop. An RfD is an estimate (with uncertainty spanning perhaps an 

order of magnitude) of a daily oral exposure to the human population (including sensitive subgroups) that is likely 

to be without an appreciable risk of deleterious effects during a lifetime. The term RfC is also defined by EPA on 

the following EPA web page: 

http://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?detail

s=&glossaryName=IRIS%20Glossary#formTop. It is defined there, as an estimate (with uncertainty spanning 

perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive 

subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. As illustrated in 

these definitions, the two values are roughly equivalent in their purpose, and the addition of the use of an RfC is 

not a change in concept or function, only nomenclature. 

In the 1990 HRS, when assigning a human toxicity to hazardous substances based on an RfD, the factor value is 

directly related to the magnitude of the RfD (see 1990 HRS Table 2-4, Toxicity Factor Evaluation). An additional 

table has been developed to direct the assignment of a human toxicity factor value based on RfCs. The process for 

developing the 1990 table is discussed in section VI-2 of the 1988 Revised HRS TSD for the 1988 proposed HRS. 

The RfD of 229 hazardous substances were examined and found to be log normal in distribution. The values were 

binned into 6 categories corresponding to the 6 possible factor values. This same process was duplicated for 

assigning factor values based on 81 RfCs obtained from EPA IRIS database. The boundaries of each bin were 

established so that they resulted in roughly the same proportional distribution of factor values as in 1988 using 

RfDs. 

This was done by first calculating the Standard Normal cumulative probabilities of the log-transformed RfCs. 

Because these cumulative probabilities estimate the proportion of analytes with RfCs below a given 

concentration, they could then be used to determine RfC boundaries for each bin that would assign analytes to 

each category at frequencies approximately equal to the 1990 table. The distribution was also verified by 

examining the substances that fell in each category using both procedures. The frequency of values and the 

suggested assigned HRS factor values are presented in the table below.
134

Table 9-1. Assignment of Human Toxicity Factor Values using RfCs

Reference Concentration (RfC) (mg/m3) Assigned 

value

Frequency of 

RfC Value

RfC < 0.0001 10,000 8 

0.0001 ≤ RfC < 0.006 1,000 25

0.006 ≤ RfC < 0.2 100 24

0.2 ≤ RfC < 2.0 10 11

2.0 ≤ RfC 1 13

RfC not available 0 0 

The list of hazardous substances and the RfCs used in developing this table is contained in Appendix B of this 

document. 

c. Use of Inhalation Unit Risk (IUR)

The 1990 HRS uses the Cancer Slope factor for determining the human toxicity factor value and the human health 

benchmark for cancer effects for all exposure routes in all four HRS pathways. (See 1990 HRS sections 2.4.1.1, 

Toxicity Factor, and 2.5.2, Comparison to benchmarks.) Since 1990, EPA has adopted the use of an Inhalation 

Unit Risk (IUR) instead of the Cancer Slope Factor when determining cancer-related risk levels for inhalation

exposures.

The term Slope factor is defined by EPA on EPA web page 

http://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?detail

s=&glossaryName=IRIS%20Glossary#formTop as an upper bound, approximating a 95 percent confidence limit, 

on the increased cancer risk from a lifetime exposure to an agent. This estimate, usually expressed in units of

proportion (of a population) affected per mg/kg-day, is generally reserved for use in the low-dose region of the 

dose-response relationship, that is, for exposures corresponding to risks less than 1 in 100. An Inhalation Unit 

Risk (IUR) is the upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an 

agent at a concentration of 1 μg/m3

 in air. The interpretation of IUR would be as follows: if unit risk = 2 × 10-6

 per 

μg/m3

, 2 excess cancer cases (upper bound estimate) are expected to develop per 1,000,000 people if exposed 

daily for a lifetime to 1 μg of the chemical per m3

 of air. The typical units for the IUR is (μg/m3)

-1

. As illustrated

in the definitions, the two values are roughly equivalent in their purpose, and the addition of the use of an IUR is 

not a change in concept or function. 

In the 1990 HRS when assigning a human toxicity to hazardous substances based on an Cancer Slope factor, the 

factor value is directly related to the magnitude of the factor and the Weight-of-Evidence that a substance is a 

carcinogen(see 1990 HRS Table 2-4, Toxicity Factor Evaluation). An additional table has been developed to 

direct the assignment of a human toxicity factor value based on the IUR and the Weight-of-Evidence. The binning 

of the IURs into categories was performed in a parallel manner as that for the binning of RfCs discussed in the 

above section of this document, except that IURs were also subcategorized based on their Weight-of-Evidence 

criteria as well as their magnitude. The bins were sized again so that they had the same proportional distribution 

as previously when the binning was based on Cancer Slope Factors. Because of the limited number of analytes in 
135

Weight-of-Evidence categories A and C, IUR ranges were determined following the approach used with RfCs for 

Weight-of-Evidence category B, only. After this was completed, the resulting Weight-of-Evidence B ranges were 

then adjusted downward and upward by an order of magnitude for Weight-of-Evidence categories A and C, 

respectively, to be consistent with the 1990 factors. 

Table 9-2. Weight of Evidence Categories

Weight-of-Evidence A Weight-of-Evidence B Weight-of-Evidence C

IUR Range Frequency IUR Range Frequency IUR Range Frequency

> 4.0E-05 8 > 4.0E-04 16 > 4.0E-03 0 

1E-05 – 4.0E-05 1 1E-04 – 4.0E-04 6 1E-03 – 4.0E-03 0

1E-06 – 1.0E-05 2 1E-05 – 1.0E-04 7 1E-04 – 1.0E-03 1

< 1.0E-06 0 < 1.0E-05 11 < 1.0E-04 3 

No Value 0 No Value 0 No Value 0 

How these groupings are combined with Weight-of-Evidence groupings is discussed in the following section of 

this document. The list of hazardous substances, their Weight-of-Evidence and their IUR is contained in

Appendix B of this document. 

d. Weight-of-Evidence Groupings:

The 1990 HRS and the proposed HRS addition utilize EPA Weight-of-Evidence (WOE) groupings for 

categorizing the information on the carcinogenicity of a substance. As defined in the 1990 HRS section 1.1, 

Definitions, and in IRIS, the WOE groupings are a system used by EPA to characterize the extent to which the 

available data support the hypothesis that an agent causes cancer in humans. Under EPA's 1986 risk assessment

guidelines, the WOE was described by categories "A through E," Group A for known human carcinogens through 

Group E for agents with evidence of noncarcinogenicity. 

In 2005 EPA revised the groupings. The approach outlined in EPA's guidelines for carcinogen risk assessment 

(2005) considers all scientific information in determining whether and under what conditions an agent may cause 

cancer in humans, and provides a narrative approach to characterize carcinogenicity rather than categories. Five 

standard WOE descriptors are used as part of the narrative. The designation of a substance as a human 

carcinogen has been incorporated into the HRS algorithm for determining the toxicity factor value. (See U.S. 

EPA. 2005. Guidelines for Carcinogen Risk Assessment.) The table below compares the two sets of guidelines. 
136

Table 9-3. EPA Weight of Evidence Guidelines

1986 Guidelines

Weight-of-Evidence Category

2005 Guidelines

Weight-of-Evidence Category

A Human Carcinogen Carcinogenic to humans

B1 Probable human carcinogen - based on 

limited evidence of carcinogenicity in humans 

and sufficient evidence of carcinogenicity in 

animals Likely to be carcinogenic to humans

B2 Probable human carcinogen – based on 

sufficient evidence of carcinogenicity in 

animals

C Possible Human Carcinogen Suggestive evidence of carcinogenic potential

D Not classifiable as to human carcinogenicity Inadequate information to assess 

carcinogenic potential

E Evidence of non-carcinogenicity for humans Not likely to be carcinogenic to humans

The definition of Weight-of-Evidence in HRS section 1.1 of the 1990 HRS has been proposed to be revised to 

identify both sets of weights of evidence. Additionally, portions of HRS Table 2-4 have been revised to reflect 

both sets when assigning a human toxicity factor using the Cancer Slope factor or an IUR, as is presented below. 

Table 9-4. Assignment of Human Toxicity Factor Values using Cancer Slope Factor or Inhalation Unit Risk

Carcinogenicity (Human)

Weight-of-Evidence/Slope factor (mg/kg-day)-1

A or Carcinogenic to 

humans

B or Likely to be 

carcinogenic to humans

C or Suggestive 

evidence of carcinogenic 

potential

Assigned 

value

0.5 ≤ SFb 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-Evidencea/Inhalation unit risk (μg/m3) 

A or Carcinogenic to 

humans 

B or Likely to be 

carcinogenic to humans

C or Suggestive 

evidence of 

carcinogenic potential

Assigned 

value

0.00004 ≤ IURc 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 

available.

Inhalation unit risk not 

available. 0 
137

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) a value of 0 for carcinogenicity.

b SF = Slope factor.

c IUR = Inhalation Unit Risk.
138

10.0 Resources

The following is a list of resources consulted for the purposes of the development and testing of the proposed 

Subsurface Intrusion Component addition to the Hazard Ranking System.

10.1 EPA Resources

1. United States Environmental Protection Agency. (1982). Uncontrolled Hazardous Waste Site

Ranking System: A User’s Manual (HW-10). Published in the Federal Register on July 16, 1982.

2. United States Environmental Protection Agency. (1986). Memorandum: Science Advisory Board

Review of Hazard Ranking System. Office of Emergency and Remedial Response. December 9, 1986.

3. United States Environmental Protection Agency. (1987) Analysis of the Air Target Distance Limit 

in the Hazard Ranking System. Office of Solid Waste and Emergency Response. July 15, 1987.

4. United States Environmental Protection Agency. (1987). Chemicals Contributing to Estimated Risks

at Superfund Sites. Science Advisory Board. July 16, 1987.

5. United States Environmental Protection Agency. (1987). Discussion of Options for Revising the

Hazard Ranking System (HRS) Toxicity Factor. Office of Emergency and Remedial Response. May

4, 1987.

6. United States Environmental Protection Agency. (1987). Incorporating Mobility Factors into the

Hazard Ranking System. Office of Solid Waste and Emergency Response. August 20, 1987.

7. United States Environmental Protection Agency. (1988). 1988 Revised HRS Technical Support

Document.

8. United States Environmental Protection Agency. (1988). Review of the Superfund Hazard Ranking

system. (SAB-EC-88-008).Review by the Hazard Ranking System Review Subcommittee of the

Science Advisory Board. Office of the Administrator Science Advisory Board.

9. United States Environmental Protection Agency. (1989). Risk Assessment Guidance for Superfund

(RAGS) Part A. Office of Solid Waste and Emergency Response (OSWER). Available on-line at:

http://www.epa.gov/oswer/riskassessment/ragsa/index.htm

10. United States Environmental Protection Agency. (1993). Final Consensus Report of the National

Commission on Superfund.

11. United States Environmental Protection Agency. (2004). User’s Guide for Evaluating Subsurface

Vapor Intrusion into Buildings. Office of Emergency and Remedial Response.

12. United States Environmental Protection Agency. (2005). Review of Recent Research on Vapor

Intrusion. Tillman, F., Weaver, J.W. (EPA/600/R-05/106) Office of Research and Development.

13. United States Environmental Protection Agency. (2005). Guidelines for Carcinogen Risk Assessment.

(EPA/630/P-03/001F). March 2005. 
139

14. United States Environmental Protection Agency. (2008). Vapor Intrusion Database: Preliminary

Evaluation and Attenuation Factors. Office of Solid Waste.

15. United States Environmental Protection Agency. (2009). Vertical Distribution of VOCs in Soils from

Groundwater to the Surface/Subslab. Office of Research and Development National Exposure Research

Laboratory. Available on-line at: https://cluin.org/download/contaminantfocus/vi/Vertical%20Distribution%20of%20VOCs.pdf

16. United States Environmental Protection Agency. (2008). Quality System and Technical Procedures

for SESD Field Branches. EPA Region 4. Available on-line at: http://www.epa.gov/quality/qualitysystem-and-technical-procedures-sesd-field-branches

17. United States Environmental Protection Agency. (2009). Vapor Intrusion Framework (Interim Final).

Region 3. Hazardous Site Cleanup Division.

18. United States Environmental Protection Agency. (2010). EPA Region 9's "RARE" Opportunity to

Improve Vapor Intrusion Indoor Air Investigations. Proceedings to EPA update on Vapor Intrusion

Workshop. March 16, 2010. AEHS Annual Conference, San Diego, California.

19. United States Environmental Protection Agency. (2010). Risk-Based Screening Table – Generic

Tables. Available on-line at: http://www.epa.gov/risk/risk-based-screening-table-generic-tables

20. United States Environmental Protection Agency. (2010). Vapor Intrusion Guidebook. Region Five.

Superfund Division.

21. United States Environmental Protection Agency. (2011). Background Indoor Air Concentrations of

Volatile Organic Compounds in North American Residences (1990-2005): A Compilation of Statistics

for Assessing Vapor Intrusion. (EPA 530-R-10-001). Office of Solid Waste and Emergency

Response.

22. United States Environmental Protection Agency. (2011). Beneficial Effects of the Superfund

Program. (EPA Contract EP W-07-037). Office of Superfund Remediation and Technology

Innovation.

23. United States Environmental Protection Agency. (2011). Exposure Factors Handbook: 2011 Edition.

(EPA 600-R-090-052F). Office of Research and Development.

24. United States Environmental Protection Agency. (2011). Intensive Study of COVs and Radon in a

Historical Duplex - A First Look. Proceedings to USEPA Workshop: Addressing Regulatory

Challenges. AEHS West Coast Conference. Presented by Schumacher, B. March 15, 2011. San Diego

CA.

25. United States Environmental Protection Agency. (2011). Potential Addition of Vapor Intrusion

Component to the Hazard Ranking System. (EPA-HQ-SFUND-2010-1086; FRL-9260-1). Federal

Register, Vol.76: 5370-5373.

26. United States Environmental Protection Agency. (2011). Update on Vapor Intrusion Guidance and

Policy Activities at U.S. EPA. Proceedings to USEPA Workshop: Addressing Regulatory Challenges.

AEHS West Coast Conference. Presented by Foster, S. March 15, 2011. San Diego CA. 
140

27. United States Environmental Protection Agency. (2011). Vapor Intrusion (VI): EPA's Experiences

with Analogous Gases. Proceedings to USEPA Workshop: Addressing Regulatory Challenges. AEHS

West Coast Conference. Presented by Schuver, H. March 15, 2011. San Diego CA.

28. United States Environmental Protection Agency. (2011). VI + HRS + EXPOSURE = SHIVER.

Region 2. Presentation by Sivak, M.

29. United States Environmental Protection Agency. (2011). VI Assessment & Mitigation Decisions.

Proceedings to USEPA Workshop: Addressing Regulatory Challenges. AEHS West Coast

Conference. Presented by Schuver, H. March 15, 2011. San Diego CA.

30. United States Environmental Protection Agency. (2011). Groundwater/Subslab/Indoor Air

Relationships. Proceedings to USEPA Workshop: Addressing Regulatory Challenges. AEHS West

Coast Conference. Presented by Dawson, H. March 15, 2011. San Diego CA.

31. United States Environmental Protection Agency. (2012). Agency for Toxic Substances and Disease

Registry National Priorities List (NPL) Sites. Accessed on May 2013, at:

http://www.epa.gov/superfund/agency-toxic-substances-and-disease-registry-atsdr-national-prioritieslist-npl-sites

32. United States Environmental Protection Agency. (2012). Assessing Protectiveness at Sites for Vapor

Intrusion: Supplement to the “Comprehensive Five-Year Review Guidance.” Accessed at:

http://www.epa.gov/superfund/superfund-five-year-reviews-guidance-and-policy#pane-1

33. United States Environmental Protection Agency, Office of Superfund Remediation and Technology

Innovation (OSRTI). (2012). EPA’s Vapor Intrusion Database: Evaluation and Characterization of

Attenuation Factors for Chlorinated Volatile Organic Compounds and Residential Buildings. Dr.

Helen Dawson, U.S. Environmental Protection Agency’s Office of Superfund Remediation and

Technology Innovation (OSRTI), served as principal author with Dr. Richard B. Kapuscinski,

USEPA OSRIT, as coauthor.

34. United States Environmental Protection Agency. (2012). Screening Level Implementation of the

Johnson and Ettinger Vapor Intrusion Model. EPA on-line tools for site assessment and calculation.

Available on-line at: http://www.epa.gov/athens/learn2model/part-two/onsite/JnE_lite.html

35. United States Environmental Protection Agency. (2012). State-Designated Top-Priority National

Priorities List (NPL) Sites. Accessed on May, 2013, at:

http://www.epa.gov/superfund/sites/query/queryhtm/nplss.htm

36. United States Environmental Protection Agency. (2013). DuPont Works Site, Pompton Lakes, New

Jersey. Proceedings to USEPA Workshop: Vapor Intrusion Stakeholder-Involvement Forum:

Improving Communication, Outreach, and Decisions at Vapor Intrusion Sites. AEHS West Coast

Conference. Presented by Riggiola, L. March 18, 2013. San Diego CA.

37. United States Environmental Protection Agency. (2013). Protecting the Buncombe County (NC)

Schools Building from Vapor Intrusion. Proceedings to USEPA Workshop: Vapor Intrusion

Stakeholder-Involvement Forum: Improving Communication, Outreach, and Decisions at Vapor

Intrusion Sites. AEHS West Coast Conference. Presented by Baldwin, L. March 18, 2013. San Diego

CA. 
141

38. United States Environmental Protection Agency. (2013). Stakeholder’s Comments. Proceedings to

USEPA Workshop: Vapor Intrusion Stakeholder-Involvement Forum: Improving Communication,

Outreach, and Decisions at Vapor Intrusion Sites. Presented by Horton, J. March 18, 2013. San Diego

CA.

39. United States Environmental Protection Agency. (2013). When Both Aquifers and Government

Oversight are Toxic. Proceedings to USEPA Workshop: Vapor Intrusion Stakeholder-Involvement

Forum: Improving Communication, Outreach, and Decisions at Vapor Intrusion Sites. AEHS West

Coast Conference. Presented by Moss, B. March 18, 2013. San Diego CA.

40. United States Environmental Protection Agency. (2013). Analysis of Long-term Data from the CDOT

MTL and Redfield Sites. Proceedings to USEPA Workshop: Looking Beyond Natural Variation in

Vapor Intrusion: Understanding, Controlling, and Addressing Site Variables for Improved Practices

and Effective Protection Strategies. AEHS West Coast Conference. Presented by Kurtz, J. March 19,

2013. San Diego CA.

41. United States Environmental Protection Agency. (2013). Characterizing Variability in Indoor Air

Concentrations at Vapor Intrusion Sites. Proceedings to USEPA Workshop: Looking Beyond Natural

Variation in Vapor Intrusion: Understanding, Controlling, and Addressing Site Variables for

Improved and Effective Protection Strategies. AEHS West Coast Conference. Presented by Dawson,

H. March 19, 2013. San Diego CA.

42. United States Environmental Protection Agency. (2013). Comparison of Sun Devil and Indianapolis

Studies. Proceedings to USEPA Workshop: Looking Beyond Natural Variation in Vapor Intrusion:

Understanding, Controlling, and Addressing Site Variables for Improved Practices and Effective

Protection Strategies. AEHS West Coast Conference. Presented by Truesdale, R. March 19, 2013.

San Diego CA.

43. United States Environmental Protection Agency. (2013). Indoor Air and Soil Gas Temporal

Variability: Effects on Sampling Strategies under Controlled and Uncontrolled Conditions.

Proceedings to USEPA Workshop: Looking Beyond Natural Variation in Vapor Intrusion:

Understanding Controlling, and Addressing Site Variables for Improved Practices and Effective

Protection Strategies. AEHS West Coast Conference. Presented by Schumacher, B. March 19, 2013.

San Diego CA.

44. United States Environmental Protection Agency. (2013). Living with Variability: Development of

Effective and Protective Sampling Strategies. Proceedings to USEPA Workshop: Looking Beyond

Natural Variation in Vapor Intrusion: Understanding, Controlling, and Addressing Site Variables for

Improved Practices and Effective Protection Strategies. AEHS West Coast Conference. Presented by

Carr, B. March 19, 2013. San Diego CA.

45. United States Environmental Protection Agency. (2013). Multi-Year Monitoring of a House Over a

Dilute CHC Plume: Implications for Pathway Assessment Using Indoor Air Sampling and Forced

Under-Pressurization Tests. Proceedings to USEPA Workshop: Looking Beyond Natural Variation in

Vapor Intrusion: Understanding, Controlling, and Addressing Site Variables for Improved Practices

and Effective Protection Strategies. AEHS West Coast Conference. Presented by Holton, C. March

19, 2013. San Diego CA.

46. United States Environmental Protection Agency. (2013). OSWER’s Development of Final Vapor

Intrusion Guidance. Proceedings to USEPA Workshop: Looking Beyond Natural Variation in Vapor

Intrusion: Understanding, Controlling, and Addressing Site Variables for Improved Practices and
142

Effective Protection Strategies. AEHS West Coast Conference. Presented by Dawson, H. March 19, 

2013. San Diego CA. 

47. United States Environmental Protection Agency. (2013). Spatial and Temporal Variability: Order

Statistics for Indoor Air Concentrations from 2 Sites with Large Data Sets. Proceedings to USEPA

Workshop: Looking Beyond Natural Variation in Vapor Intrusion: Understanding, Controlling, and

Addressing Site Variables for Improved Practices and Effective Protection Strategies. AEHS West

Coast Conference. Presented by McAlary, T. March 19, 2013. San Diego CA.

48. United States Environmental Protection Agency. (2013). Temporal Variation of Soil Gas Intrusion

and Retention. Proceedings to USEPA Workshop: Looking Beyond Natural Variation in Vapor

Intrusion: Understanding, Controlling, and Addressing Site Variables for Improved Practices and

Effective Protection Strategies. AEHS West Coast Conference. Presented by Steck, D. March 19,

2013. San Diego CA.

49. United States Environmental Protection Agency. (2013). Vapor Intrusion Across Space and Time: A

Pilot Community Study in San Antonio. Proceedings to USEPA Workshop: Looking Beyond Natural

Variation in Vapor Intrusion: Understanding, Controlling, and Addressing Site Variables for

Improved Practices and Effective Protection Strategies. AEHS West Coast Conference. Presented by

Johnston, J. March 19, 2013. San Diego CA.

50. United States Environmental Protection Agency. (2013). OSWER Final Guidance for Assessing and

Mitigating the Vapor Intrusion Pathway from Subsurface Sources to Indoor Air (External Review

Draft). Office of Solid Waste and Emergency Response (OSWER).

51. United States Environmental Protection Agency. (2015). OSWER Technical Guide for Assessing and

Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air. Office of

Solid Waste and Emergency Response. June 2015. (EPA Publication No. 9200.2-154).

52. United States Environmental Protection Agency. (2015).Technical Guide for Addressing Petroleum

Vapor Intrusion at Leaking Underground Storage Tank Sites. Office of Underground Storage Tanks

(OUST).

53. United States Environmental Protection Agency. (n.d.). Rationale for the Potential Addition of a

Vapor Intrusion Component to U.S. EPA's Hazard Ranking System.

54. United States Environmental Protection Agency. (n.d.). What You Should Know About Vapor

Intrusion.

55. United States Environmental Protection Agency. (n.d.). National Recommended Water Quality

Criteria (NRWQC). Accessed from:

http://water.epa.gov/scitech/swguidance/standards/current/index.cfm

56. United States Environmental Protection Agency. (n.d.). Integrated Risk Information System Site Help

and Tools. Accessed from: http://www.epa.gov/iris/help_ques.htm. 
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10.2 Other Federal Agency Resources 

57. Agency for Toxic Substances and Disease Registry. (2011). Public Health Assessment for W.R. Grace

Superfund Site. (CERCLIS NO: MAD001002252). U.S. Department of Health and Human Services

Public Health Service.

58. Agency for Toxic Substances and Disease Registry. (2009). Vapor Intrusion Health Issues.

Presentation by Watters, M.

59. Gorder, K.A. (2010). Active Hunting for Background Sources. Environmental Restoration Branch.

Hill Airforce Base VI Program.

60. Naval Facilities Engineering Command. (2011). Guidance for Environmental background Analysis.

Volume IV: Vapor Intrusion Pathway. A Users Guide.

61. Naval Facilities Engineering Command. (2011). Navy Vapor Intrusion (VI) Assessment Tool.

Presentation for USEPA Briefing on March 23, 2011.

62. United States Department of Defense (DoD). (2008). Cost and Performance Report – Detailed Field

Investigation of Vapor Intrusion Processes. Environmental Security Technology Certification

Program (ESTCP). Available on-line at: http://www.frtr.gov/pdf/FRTR-2010-factsheet.pdf

63. United States Department of Defense (DoD). (2009). Vapor Intrusion Handbook. Prepared by the TriService Environmental Risk Assessment Workgroup.

64. United States Government Accountability Office. (2010). Superfund, 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. Report to Congressional Requesters. (GAO-10-380). 

10.3 Other Published References

65. Association of State and Territorial Solid Waste Management Officials. (2009). Vapor Intrusion

pathway: A Guide for State and Territorial Federal Facilities managers. ASTSWMO Policy and

Technology Focus Group.

66. Barrett, K., and Platt, A. (1982). Uncontrolled Hazardous Waste Site Ranking System (HRS) -

Response to Public Comments. Contract NO: 68-01-6278.

67. Barrett, K., Gerestein, R., Johnson, R. and Platt, A. (1985). Interconnection of Hydraulic Strata.

Project No: 1176A.

68. Barrett, K.W. (1983). Project Management Managing the National Priorities List.

69. Barrett, K.W., Chang, S.S., Haus, S.A., and Platt, A.M. (1982). Uncontrolled Hazardous Waste Site

Ranking System: A Users Manual.

70. Bradley, P.M. 2011. Reinterpreting the Importance of Oxygen-Based Biodegradation in

Chloroethane-Contaminated Groundwater. Ground Water Monitoring and Remediation 31(4): 50-55.
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71. Brown, R. D. (1986). Hazard Ranking System Issue Analysis: Use of Significance in Determining

Observed Release. (Contract NO: EPA-68-01-7054.)

72. Burger, C.K., and Kushner, L.M. (1986). Hazard Ranking System Issue Analysis: Target Distance

Limit for Surface Water. (Contract NO: EPA-68-01-7054.)

73. Carr, D.B., Levy, L.C., Horneman, A.H., (2011). Stylistic Modeling of Vadose Zone Transport.

Insight into Vapor Intrusion processes. Proceedings to USEPA Workshop: Addressing Regulatory

Challenges. AEHS West Coast Conference. March 15, 2011. San Diego CA.

74. Chang, S. and Holberger, R. (1983). National Priorities List Technical Data Base. (Contract No: 

68-01-6278.)

75. Chrostowski, P. (1986). Risk Assessment and the Hazard Ranking System.

76. Clark, J. N., Goodwin, D., O'Neill, H. and Odencrantz. J.E. (2008). Application of Passive Soil Gas

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Environment. Journal of Remediation-In Press: July 2008.

77. Connor, J. A., Ahmad, F., and Mchugh, T. E. (2008). Evaluation of Vapor Intrusion from a

Subsurface Diesel Plume Using Multiple Lines of Evidence. In Proceedings of Petroleum

Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Assessment, and Remediation

Conference, 90–104. November 6–7, Houston, Texas. Westerville, Ohio: National Ground Water

Association.

78. Coyne, L. S., Havalias, G., & Echarte, M. C. (n.d.). Vapor intrusion sampling options: performance

data for canisters, badges, and sorbent tubes for vocs. 1786(0904), 1-9. Available online at:

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79. Crook, A. and Morrison, B. (1987). Overview of the Remedial Action Priority System (RAPS).

80. Dawson, H. E., and McAlary, T. (2009). A Compilation of Statistics for VOCs from Post-1990 Indoor

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81. DeVaull, G., Ettinger, R., and Gustafson, J. (2002). Chemical Vapor Intrusion from Soil or

Groundwater to Indoor Air: Significance of Unsaturated Zone Biodegredation of Aromatic

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82. DeVaull, G., Ettinger, R., Salanitro, J. P., and Gustafson, J. (1997). Benzene, Toluene, Ethylbenzene

and Xylenes [BTEX] Degradation in Vadose Zone Soils during Vapor Transport: First-Order Rate

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83. DeVaull, G. E., Ettinger, R. A., Salanitro, J. P. and Gustafson, J. B. (n.d.). Benzene, Toluene,

Ethylbenzene, and Xylenes (BTEX) Degradation in Vadose Zone Soils During Vapor Transport: FirstOrder Rate Constants. Soil and Sediment Contamination: An International Journal Volume 11, Issue 

4, 2002.

84. DÖRR, H. and MÜNNICH, K. O. (1987). Annual variation in soil respiration in selected areas of the

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85. Elliot, J., Rigby, M. C., Mills, W. B., Sayenko, K. I., Brenner, D., Bell, D. C., and Strausbauch, S. V.

(2009). Investigation of Multiple Lines of Evidence for Vapor Intrusion Assessment. Proceedings of

the Air and Waste Management Association Specialty Conference: Vapor Intrusion. January 27-30,

2009.

86. Environmental security Technology Certification Program. (2008). Final Report: Detailed Field

Investigation of Vapor Intrusion Processes. (ESTCP Project ER-0423).

87. Fitzgerald, J.M. (1986). Hazard Ranking System Issue Analysis: Laboratory Limits of Detection.

(Contract NO: EPA-68-01-7054.)

88. Fitzgerald, J. (2009). One Regulatory Perspective on the Vapor Intrusion Pathway. Ground Water

Monitoring & Remediation 29, no. 1: 51-52.

89. Fitzpatrick, N. A., and Fitzgerald, J. J. (2002). An Evaluation of Vapor Intrusion into Buildings

through a Study of Field Data. Soil and Sediment Contamination (formerly Journal of Soil

Contamination) 11: 603-23.

90. Folkes, D. J., Kurtz, J. P., & Sanpawanitchakit, C. (2007). Vapor intrusion as a function of lateral

distance from a groundwater plume boundary. Paper presented at AWMA conference on vapor

intrusion.

91. Folkes, D. J., and Kurz, D.W. (2002). Efficacy of Sub-Slab Depressurization for Mitigation of Vapor

Intrusion of Chlorinated Organic Compounds. Proceedings to Indoor Air 2002 conference.

92. Fortune, A., and Tuday, M. (2008). The Importance of Air Sampling Media Cleanliness for Vapor

Intrusion Investigations. Presentation from National Environmental Monitoring Conference (NEMC),

August 10-16, 2008, Washington, DC.

93. Gerstein, R. E. (1986). Hazard Ranking System Issue Analysis: Geohydrology.

94. Goldfarb, A.S., Malone, G.A. and Vogel, G.A. (1992). Remedial Technology Design, Performance,

and Cost Study.

95. Gossett, J.M. 2010. Sustained Aerobic Oxidation of Vinyl Chloride at Low Oxygen Concentrations.

Environmental Science & Technology 44(4): 1405-1411.

96. Haus, S.A. (1987). Hazard Ranking System Issue Analysis: Containment Rating Factor.

97. Haus, S.A., and Wolfinger, T.F. (1986). Hazard Ranking System Issue Analysis: Review of Existing

Ranking Systems.

98. Hawley, J.K. (1985). Assessment of Health Risk from Exposure to Contaminated Soil.

99. Healey, S., Petrisor, I., and Morrison, R. (2004). Vapor Intrusion-Forensic Approaches and Recent

Developments. Environmental Claims Journal 16: 237-48.

100. Hers, I., Truesdale, R. (2011). Comparison of Empirical Attenuation Factors to USEPA Draft VI

Guidance Semi Site Specific Factors. Proceedings to USEPA Workshop: Addressing Regulatory

Challenges. AEHS West Coast Conference. March 15, 2011. San Diego CA.
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101. Hers, I., Zapf-Gilje, R., Johnson, P.C., and Li, L. (n.d.). (2003) Evaluation of the Johnson and

Ettinger Model for Prediction of Indoor Air Quality. Ground Water Monitoring and Remediation 23,

no. 2: 119–133.

102. Holton, C., Guo, Y., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., and Johnson, P. C. (2015).

Long-Term Evaluation of the Controlled Pressure Method for Assessment of the Vapor Intrusion

Pathway. Environmental Science and Technology 49(4), pp. 2091–2098.

103. Holton, C., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., and Johnson, P. C. (2013). Temporal

Variability of Indoor Air Concentrations under Natural Conditions in a House Overlying a Dilute

Chlorinated Solvent Groundwater Plume. Environmental Science and Technology 47 (23), pp.

13347–13354.

104. Howard, P.H. 1991. Handbook of Environmental Degradation Rates. Lewis Publishers, Chelsea, MI.

105. HRS Issue Analysis: Relationship Between Hazardous Waste Quantity and Hazardous Constituent

Quantity. (1986).

106. Industrial Economics, Inc. (1987). Preliminary Analysis of Alternative Models to Support Revisions

the CERCLA Hazard Ranking System. Proceedings of a Briefing for the U.S. Environmental

Protection Agency’s Science Advisory Board. July 16, 1987.

107. Industrial Economics, Incorporated. (2011). Assessment of the Potential Cost, Benefits, and Other

Impacts of the proposed Revisions to EPA's Underground Storage Tank Regulations. Prepared for

Policy and Standards Division. Office of Underground Storage Tanks. U.S. EPA.

108. Interstate Technology & Regulatory Council (ITRC). (2007). Vapor Intrusion Pathway: A Practical

Guidance.

109. Interstate Technology & Regulatory Council (ITRC). (2007). Vapor Intrusion Pathway: Investigative

Approaches for Typical Scenarios.

110. Interstate Technology & Regulatory Council. (2003). Vapor Intrusion Issues at Brownfield Sites.

Brownfields Team. December 2003. Available on-line at:

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111. Interstate Technology & Regulatory Council. (2007). Vapor Intrusion Pathway: A Practical

Guideline. Vapor Intrusion Team. January 2007. Available on-line at:

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112. Jen, C. Y., Benni, W., Song, S. and Washburn, S. (2003). Evaluation of Vapor Intrusion Pathway: A

Multifaceted Approach. Proceedings of the 2003 Petroleum Hydrocarbons and Organic Chemicals in

Ground Water: Prevention, Assessment, and Remediation. August 19-22, 2003. Costa Mesa, CA.

113. Johnson, P. C. (1991). Identification of Application-Specific Critical Inputs for the 1991 Johnson and

Ettinger Vapor Intrusion Algorithm. Ground Water Monitoring & Remediation 25, no. 1: 63–78.

114. Johnson, P. C., and Ettinger, R. A. (1991). Heuristic model for predicting the intrusion rate of

contaminant vapors into buildings. Environmental Science and Technology, 25:1445-1452. 
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115. Johnson, P.C., Fulton, I.A. (2010). Vapor Intrusion pathway Assessment: State-of-the-Practice and

opportunities for v3.0. Arizona State University School of Engineering Presentation.

116. Kushner, L.M. (1986). Hazard Ranking System Issue Analysis: Sites with Unknown Waste Quantity.

(Contract No. EPA-68-01-7054.)

117. Lutes, C.C., Studebaker, R.L., Uppencamp, R., Abreu, L., Weinberg, N. (2011). Efficient Assessment

and Mitigation of Vapor Intrusion (VI): Optimal Sub-slab Depressurization Design Strategies and

Handling the Problems of Variability with Long-term Integrated Samplers. Proceedings to USEPA

Workshop: Addressing Regulatory Challenges. AEHS West Coast Conference. March 15, 2011. San

Diego CA.

118. Lundegard, P.D., P.C. Johnson, and P. Dahlen, 2008. Oxygen transport from the atmosphere to soil

gas beneath a slab-on-grade foundation overlying petroleum-impacted soil. Environmental Science

and Technology 42(15): 5534–5540.

119. McAlary, T. (2009). Development of more Cost-Effective Methods for Long-Term Monitoring of Soil

Vapor Intrusion to Indoor Air Using Quantitative Passive Diffusive-Absorptive Sampling Techniques.

Geosyntec Consultants Inc. Federal Remediation Tehnologies Roundtable Presentation.

120. McAlary, T. (2010). Think outside the Summa: Time-Weighted Average and Volume-Weighted

Average Sampling to Manage Temporal and Spatial Variability in Vapor Intrusion Assessment Data.

Geosyntec Consultants, Inc.

121. McAlary, T., Bertrand, D., Nicholson, P., Wadley, S., Rowlands, D., Thrupp, G., Ettinger, R. (2011).

Pneumatic Testing, Mathematical Modeling and Flux Monitoring to Assess and Optimize the

Performance and Establish Termination Criteria for Sub-slab Depressurization Systems. Proceedings

to USEPA Workshop: Addressing Regulatory Challenges. AEHS West Coast Conference. March 15,

2011. San Diego CA.

122. McAlary, T., Ettinger, R., Johnson, P., Eklund, B., Hayes, H., Shields, T., Brady, R., et al. (2009). A

Review for the US Navy of Best Practices, Knowledge and Data Gaps And Research directions for

vapor Intrusion. Presented at the Environmental Monitoring and Data Quality Workshop. March

2009. San Antonio, TX.

123. McAlary, J. Provoost and H. Dawson, “Chapter 10—Vapour Intrusion,” In: F. Swartjes, Ed., Dealing

with Contaminated Sites—From Theory towards Practical Application, Springer Publishers,

Netherlands, January 2011, pp. 409-453.

124. McAlary, T., and Johnson, P.C. (2009). GWMR Focus Issue on Vapor Intrusion. Ground Water

Monitoring & Remediation 29: 40-41.

125. McAlary, T., Groenevelt, H., Górecki, T., Seethapathy, S., and Hayes, H. (2009). PDMS Membrane

Samplers for Quantitative Passive Monitoring of Soil Vapor Intrusion to Indoor Air. Proceedings to

an A&WMA Vapor Intrusion conference. January 27-30, 2009. San Diego, CA.

126. McAlary, T. (2008). Lessons from Petroleum Hydrocarbon and Chlorinated Solvent Sites Extensively

Monitored for Vapor Intrusion. EPA Workshop on Vapor Intrusion. March 14, 2008. San Diego, CA.

127. McDonald, G. J., and Wertz, W.E. (2007). PCE, TCE, and TCA Vapors in Subslab Soil Gas and

Indoor Air: A Case Study in Upstate New York. Ground Water Monitoring & Remediation 27, no. 4:

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128. McHugh, T.E., Beckley, L., Bailey, D., Gorder, K., Dettenmaier, E., Rivera-Duarter, I., Brock, and S., 

MacGregor, I. C. (2012). Evaluation of Vapor Intrusion Using Controlled Building Pressure.

Environmental Science and Technology 46 (9), pp. 4792–4799.

129. McHugh, T. E., Beckley, L., Kuder, T., Philp, P., Gorder, K., Rivera, I., Chadwik, B., et al. (n.d.)

Investigation methods to Distinguish between Vapor Intrusion and Indoor Sources of VOCs.

Presentation by ESTCP.

130. McHugh, T. E., R. Davis, G. DeVaull, H. Hopkins, J. Menatti, and T. Peargin. 2010. Evaluation of

vapor attenuation at petroleum hydrocarbon sites. Soil and Sediment Contamination: An

International Journal 19(6): 725–745.

131. McHugh, T.E., Connor, J.A., Ahmad, F. (2003). An Empirical Analysis of the Groundwater-toIndoor-Air Exposure Pathway: The Role of Background Concentrations in Indoor Air. Environmental

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132. McHugh, T. E. (2008). Recommendations for the Investigation of Vapor Intrusion. Environmental

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133. McHugh, T. E., DeBlanc, P.C. and Pokluda, R.J. (2006). Indoor Air as a Source of VOC

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134. Mickunas, D.B. (2009). Using the TAGA Mobile Laboratory to Resolve Vapor Intrusion IssuesInterpretation of Multiple Lines of Evidence for Vapor Intrusion. Proceedings to Vapor Intrusion

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135. Mills, W. B., Liu, S., Rigby, M.C. and Braemar, D. (2007). Time-Variable Simulation of Soil Vapor

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136. Odencrantz, J. (2008). Residential Vapor Intrusion to Indoor Air Comparative Study Near DoD

Facility: Canisters vs. Sorbent Tubes vs. Passive Diffusion Samplers. Beacon Environmental

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137. Odencrantz, J. (2010). Beyond Canisters (SUMMAs): Passive and Active Samplers and International

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138. Odencrantz, J. E. and O'Neill, H. (2009). Passive to active tie-in for soil gas surveys: Improved

technique for source-area, spatial variability, remediation-monitoring, and vapor-intrusion

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139. Phillips, S., Krieger, G., Palmer, R., and Waksman, J. (2004). Solvents and vapor intrusion pathways.

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140. RTI International. (2012). Indoor Air Vapor Intrusion Database (IAVI). Available on-line at: https://

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143. Schumacher, B., Zimmerman, J., Swanson, G., Hartman, B., Springer, D., Elliot, J. and Rigby, M.

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144. Siegel, L. (2010). Vapor Intrusion Assessment Stakeholders Perspective. Panel Discussion. Center 

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145. Siegel, L. (2009). Stakeholders’ Views on Vapor Intrusion. Ground Water Monitoring & Remediation

29: 53-57.

146. Steck, D. (2011). Factors affecting radon entry and indoor concentrations. Lessons for Chlorinated

VI. Proceedings to USEPA Workshop: Addressing Regulatory Challenges. AEHS West Coast

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147. Steinmacher, S. J., Hanover, R., EerNissse, P.A., Miller, J.P., and Case, J.D. (n.d.) The Effects of

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149. Tillman Jr., F.D., Weaver, J.W. (2007). Parameter sets for upper and lower bounds on soil-to-indoorair contaminant attenuation predicted by the Johnson and Ettinger vapor intrusion model.

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150. Tillman Jr., F.D., Weaver, J.W. (2007). Temporal moisture content variability beneath and external

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151. Weisel, C.P., Zhang, J., Turpin, B.J., Morandi, M.T., Colome, S., Stock, T.H., Spektor, D.M., et al.

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