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 , 201 62 

## 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 C onsidered 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. Ratio n ale 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

..................................................................... 703 5.3 Determination of “Exposure” Waste Quantity

....................................................................................... 70

a. Summary of Tiered Approach

................................................................................................................. 71

b. Assigning a Hazardou s 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 th e 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 Developme nt 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 Tie r 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

.................................................................................................................. 1434 Lis t of Figures

Figu re 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
Fi gu re 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 Wei ghtings 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 ..................................................................................................1195 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 ......................................................................................1316 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 Exampl e 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 ....1367 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 Responses8 

## 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 (O LEM ) and staff f rom EPA Regions 3, 4 and 6. Terry Jeng, O LEM , 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 Re search 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.9 

### 1.0 Introduction and Rationale for the Addition of a Subsurface Intrusion (SSI) Component to the HRS

1.1 Int roduction

Thi s 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 Haza rd 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 inspectio n (SI) activities are identified in Section 10.1 of this document.

Thi s 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:

• Sec tion 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 affe ct 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?

Subsur face intrusion is defined as the migration of hazardous substances , pollutants, or contaminants 2 from the subsurface environment, or more specifically, the unsaturated zone and/or the surficial ground water, into overlying structures. Subsurface intrusion can result in p eople 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 vapor 3 intrusion. However, EPA is also considering in t his 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 contami nants” 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

.10 addit ion sites with another forms of subsurface intrusion, that is, sites with contaminated ground water intruding into regularly occupied structures.

1.3 Rat ionale for the Addition of Subsurface Intrusion to the Hazard Ranking System

a. Re gulator y Rationale (CERCLA/SARA)

In 1986, Cong ress 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 an d 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 evalua ted. 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 c ontaminated 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.

With out 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 addre ss 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 ha ve 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 fa ct, in a May 2010 rep ort 4 , 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 subsur face 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

, ” includ es 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 Lis t, GAO Report to Congressional Requesters, GAO

- 10

- 380, May 2010.11 Fur thermore, 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 Ind ian 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
– al l but one favoring the addition of subsurface intrusion to the HRS.

b. Identi fication and Classification of Sites with Potential Subsurface Intrusion
Contamination

Congr ess 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 t his 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 s ome of the sites in this inventory are currently listed in EPA ’s Superfund database 5 , i t 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, thi s 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 Sup erfund site information, with the Superfund Enterprise Management System (SEMS) (hereinafter referred to as EPA’s Superfund database).12 • Resou rce 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 si tes where intrusion of contaminated ground water is a potential issue that cannot be evaluated using the current 1990 HRS.

ii. Methodology of Analysis

Pot ential 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 e xcluded statutorily under CERCLA or by agency policy.

Sit es 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 self

identification, 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

Approxi mately 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 d atabase 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.13 • 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 fo r 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 ide ntification 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 se rves 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 identificatio n 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 info rmation 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
refi ne 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 thre ats 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.14 1.4 Wh y Should Subsurface Intrusion Be Considered When Identifying Sites for Placement
on the NPL?

U nder 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 pre sence 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.

Addit ionally, 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 report 6 :

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
wh en 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

t o the National Priorities List , GAO Report to Congressional Requesters, GAO

- 10

- 380, May 2010.15 

### 2.0 Evaluation Options

Sever al 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. Curr ent Methods

CER CLA 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 , S tates 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 findin g that it is more cost effective to clean up the site using remedial authority than removal authority.

i. Hazar d 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 s ites 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 act ion 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 analy sis. 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.16 Figure 2-1 . S u perfund Site Assessment Process

Th e 1990 HR S (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 -f inanced remedial actions. Emergency removal and enforcement actions can be conducted at any site, whether or not the site is on the NPL.17 ii. St ate Designation of a Site

A sit e 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. Ag ency for Toxic Substances and Disease Registry (ATSDR)

Sec tion 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 onl y at NPL sites) than to use its emergency removal authority to respond to the site.

b. Alt ernative Methods Considered for Addressing Subsurface Intrusion Sites

EPA considered alternatives to this proposed regulatory action for addressing the need to eva luate 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. D evelop G uidance

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 CER CLA 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 thre at.

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 evalu ate 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 add ress the subsurface intrusion contamination issue.18 Furt hermore, 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 subsur face 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 n ot provide a mechanism to place sites on the NPL. This proposed addition provides the mechanism for placing subsurface intrusion sites on the NPL. Therefore, t his proposed addition to the HRS, in combination with these guides will further the agency’s effo rts to establish national consistency in evaluating and remediating vapor intrusion threats under CERCLA.

The ag ency 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 ass ure that HRS assesses relative risk of sites to the maximum extent
feasible.

ii. Ut ilize 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 201 5 , 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 throug hout the United States because only 16 states could possibly designate a site with a subsurface intrusion threat to the NPL

.

iii. Expand t he 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
i s 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 evaluat ion . The ATSDR mechanism was designed to be used o nly 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 necess ary 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 f rom 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 AT SDR designation may require a19 tim e and resource intensive site investigation . This may limit the number of sites that can be evaluated sufficiently to support an ATSDR designation.

The s econd 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 s ite

- specific determinations that do not have established procedures.

The t hird 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, t he 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.

Othe r EPA programs have limited authority and ability to address all subs urface 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.

Usi ng 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
immed iate 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 to20 addre ss 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 o r 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 develo pment. 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. Give n 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, enforcemen t actions or agreements, may be less inclined to enter into such programs if the site at issue cannot be listed on the NPL.

v. Creat e Independent Listing Mechanism for Subsurface Intrusion

EPA considered development of a separate listing mechanism for placi ng 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 a gency 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 sufficie nt to place the
site on the NPL.21 The s election 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 M ethod and Rationale for Expanding the 1990 HRS to Include Subsurface
Intrusion

Pot ential options were evaluated for incorporating the evaluation of su bsurface 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.

Bas ed on EPA’s evaluations of all options considered, the preferred approach to address the threat posed by the intrusion of hazardous substances into occupied structur es 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 a pproach:

a. Lim it 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 we ighting 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 site s 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 option s 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-b ased 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 soil22 vapors were not wel l 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 th e 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 hazardou s 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 poten tial 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 unitl ess HRS score for the site.

SS = � SS

gggg 2 + SS ssgg 2 + SS ssss 2 + SS aa 2

4

S = site score

S gw = ground water pathway score

S
sw = surface water pathway score

S
a = air pathway score

A site meeting the HRS threshold of 28.50 is eligible for placement on the NPL. By usi ng 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 if24 even one pathway score is high. This approach was chosen in development of the 1990 HRS to ensure that the site

s cores 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 potentia l 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 sepa rate 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 contam inated 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 b e 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 algor ithm.

• 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

a lready 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 structu res (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, n ot 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 pos ed 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, su ch 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 surf ace 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 rel ationship 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 w ould leave the surface water, ground water, and air pathway evaluation processes unchanged.

• Th is 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 subsu rface into occupied structures.

• Inclusion of a subsurface intrusion component in the soil exposure pathway would create a two

component 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 exp osure 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 E xposure 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 e valuates 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 prese nt 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 bo undary 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. E ach 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 su bject 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 th e 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 discussi on 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 migra tion 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 su bsurface 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 exp osure 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 . S tructure 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 scor e. 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 sub surface 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 t wice

.

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 thres hold. 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 throu gh 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 alt ered 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 stru cture 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 pollu tants
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 application33 of scientific concep ts 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 fro m 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

structure s 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- tenant 8 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 ex pertise 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
subsur face 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 t o 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 withi n 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 ent er 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). (S ee 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 assi gned 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 predicti ng 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 , o f 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 that35

instea d of s oil t ype, soil m oisture content s hould be c onsidered in the evaluation of t he v ertical m igration f actor. However, after further r eview, the agency de termine d that a lthough soil m oisture content ca n indeed be a significan t factor, this information would not b e effectively collected during a typica l time -limited site i nspection. (See S ections 4.3.d, Selection and Weighting of K ey R oute P arameters , a nd 4.3.d.ii, Vertical Migration

, of this

TSD f or a dditional i nformation.)

Comments w e r e a lso received identifying biodegradation of a hazardous s ubstanc e as a significant factor to be
considered when e valuating the potential f or vapor to intrude i nt o a r egularly oc cupie d structure. In response

, the

agency di d not m odify t he potential for e xposure e valuation, but d id add a de gradation factor to another
component of the HRS scoring algorithm, the de termination of the wast e c haracteristic s f actor category value.
Sinc e relevant subsurf

ace condition s t

hat significantly impact th e de gradation rat e of subsurfac e c ontaminants,
such a s s oil moisture, nutrient availability, and oxygen content, can b e e xtremely variabl e a nd site

- specific, it would be beyond the s cope of a typical t ime- limited sit e i nspection to measur e o r predict with any accuracy. The

agency de termine d that a b iodegradation f actor w oul d not be appropriate for the potential for exposure evaluation. Instead, a de gradation factor based on whether the h azardous substance m eet s o bserved exposur e or observed
releas e c riteria, the depth to co ntamination, the ha lf-life of t he ha zardous substance, and biodegradation and hydrolysis relationship s of hazardous substan ces w as a dded to th e w ast e c haracteristi cs factor category evaluation

. The degradation factor valu e was a dded to the w ast e characteristi cs factor category evaluation because substance

specifi c p roperties of hazardous s ubstance s a ssociated with a sit e ar e considered in thi s pa rt of the H RS. (See Section 5.1.b, Degradation

, of t his technical s upport do cument f or additional i nformation.).

Additionally, a p eer reviewer recommended that the potential for exposur e factor value evaluation reflect the possibility t hat non- volatile hazardous substances may be pr esent . Such non- volatile ha zardous substances pose a reduce d likelihood of intrusion int o an occupied structure

. The a gency i ncorporated this r ecommendation by adding t he i nstruction t o assign a po tential f or e xposure f

actor v alue o nly i f a v olatile ha zardous substance is
present

. Wit h this c hange, only t hose substances with a vapor pressur e g reater than or equal to 1 torr

, or a Hen ry’s

Law c onstant g reater than or e qual to 10 -5 a tmosphere-meter c ubed per m ole ( atm m 3 mol -1 ) a re considered when evaluating th e po tential for exposur e factor value. (S ee Secti on 4.3, Potential for Exposure (and all applicable subsections) , of this TSD for additional information.)

Charge Q uesti on 2 inquired i f E PA’s a pproach for a ssigning a f actor v alue r eflecting t he a mount of a ha zardous
substanc e t hat h as e ntered a regularl y oc cupie d structure ( i.e

., the h azardous w aste quantity f actor v alue) w as appropriate. Comment s w er e r eceived questioning the comprehensivene ss o f th e e valuation of hazardous w aste quantity ba sed only on the r egularly oc cupied structures l ocated in a n area of observed exposure. Specifically, peer reviewers s uggested that t he qu antity of hazardous s ubstances that c ould enter i nto r egularly oc cupied

structure s w ithin ar eas o f subsurfac e c ontamination and/or ar e pr esent in th e s ubsurfa ce be c onsidered in the
evaluation of a site’ s ha zardous w ast e q uantity. Based on the se c omments, the agency revised the p roposed
evaluati on to als o consider the qu antity of hazardous s ubstances t hat could enter r egularly oc cupied structures i n

area s of subsurfac e c ontamination. (See S ection 5.2.1.2.2, Hazardous w aste quantity , of t he pr oposed addition t o the H RS a nd S ections 5.3, Determinati on of “ Exposure” W

aste Q uantity (and all applicable s ubsections) a nd 5.4,
Considerations i n E

valuating Multi-subunit Structures , of t his TSD for a dditional information.).

Comment s w er e a lso received on a proposed method of comparing specifi c s ubsurf ace s ampl e t ype s ( i.e., crawl

space or subslab samples) to health

- based benchmark s w hen assigning a m inimum hazardous wast e qu antity
factor

. This method was t o be us ed when a c omplete e stimate o f t he ha zardous w aste qu antity c ould not be36 de termined). 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)

.

C omments 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 wast e 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 o xygen 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. Sp ecifically, 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.

B ased 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 weight ing
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.

I nstead, based on an analysis 9 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 subsurf ace 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 subsurfa ce to indoor air sampl es a lso decreased. Th ese finding s do cument that as depth increases, the

amount of t

he c ontamination at that de pt h (or sample t ype), t ha t w i ll intrude into occupied structures , w i ll m ost likely decrease. The se result s w er e us ed to develop a r evised weighting system for eligibl e po pulations in an area of subsurfac e c ontamination. A s a r esult, in are as de lineated by contaminated crawl spa ce s amples, th e h ighest
target weighting i s a ssigned. Decreasing weighting s a re applied to eligibl e p opulations i n ar eas o f subsurface contamination delineated by s amples w it h increasing d ept h to contamina tion and different sampl e m edia. (See Section 5.2.1.3, Targets a nd all a pplicable s ubsections of t he p roposed addition t o t he H RS a nd Section 6.3.d, Weighting of P opulation Targets w ithin the A SC , of t his T SD for additional i nformation.).

Comment s r egarding th e s creening concentration s f or cancer and non

- cancer risk level s u sed to establish the
health -based benchmarks ( which are

, i n turn, us ed t o assign the t arget w eightings for a ctually e xposed populations), were a ls o receive d. These c omme nt s g enerally suggested adjusting the r isk level s from a one- in- amillion risk leve l t o one -i n -t en -thousand for cancer effects and from a hazard index of 1 to 10 for non-c ancer effects. Th e on e -i n -a-m illion risk leve l for possible cancer effect s a nd a h aza rd index of 1 for non-c ancer effects

we re retained. The risk levels in the proposed addition to the HRS are built into the overall HRS structure, a re currently used for all benchmarks in the 1990 HRS, a nd ar e consistently used by EPA for screen ing risk

as sessments

. (See S ecti on 2.5.2, Comparis on to benchmarks , of t he p roposed additi on t o the H RS a nd Section 6.3.a, Assigning Level of Exposure to Structures , of this T SD f or a dditional i nformation).

Charge Q uesti on 4 a sked if it wa s s cientifically reasonabl e when calculating the waste quantity and population to
evaluate popu lations in multi- story/multi

- tenant 10 buildings ba sed only on the popu lations i nhabiting t he intruded

space a nd the floor a

bove a nd below t he i ntruded space. (The c harge que stion provided i n the p eer review mistakenly s tate d that popu lations on a floor w it h contamination, a s w ell a s p opulations on t he f loor above a nd
below that f

loor w oul d be c onsidered. However, the p ropose d additi on to the H RS onl y c onsiders target s on all
floor s b elow a f

loor w ith an a ctual exposure and one f loor a bove t hat f

loor).

Peer reviewer s generally s upporte d the c oncept used to evaluat e m ulti

- story/multi

- tenant 11 occupied structures i n the p roposed addition. However, the reviewer s d id rais e s everal issu es f or co nsideration by t he a gency. Many of
thes e i ssue s w er e r elated to site -specific considerations on the application of the e valuation process. For e xample,
the reviewer s s tressed that the e valuation of regularly occupied multi -story or m ulti-tenant s tructures us ing t he proposed component s hould consider pr eferential pathways be twee n floors or tenant s paces e nabling t he migration of contamination. In respons e to thes e c omments, the agency added the phras e “ unle ss a vailable
information indicates otherwise” to the p ropose d addition to enable t he c onsideration of s ite- specifi c s ituations.
Such situations may include w hen contaminant t ransport w ithin an occupied structure m ight be i mpacte d by specifi c bu ilding featur es (e.g., preferential pathway s s uch as elevator sh afts a nd centralized heating, ventilation,
and air c onditioning s ystems).

Additional c omments w ere received questioning t he d iscounting ( i.e

., reduced weighting f actor) of o ccupied

space s ( or floors) located below or adjacent to an occupied spac e ( or floors) w it h a doc umente d observed

exposure , and occupied spaces abov e a n unoccupied basement with an observed exposure. The agency clarified
the w ording i n the proposed addition to state t hat all p opulations on f loors be low a doc umente d observed

exposure w ill be e valuate d as a ctually e xpose d (i.e., Level I or L evel II e xposure)

, while the populati on one floor
abov e w ill be evaluated a s potentially exposed, unles s availabl e information indicates otherwise. Thi s 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-stor y structure with subunits , having no documented observed exposure is located in a delineated area of observed exposure, populations within regularly occupied spaces on the lowes t 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 floo r 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 reason able 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 hig her 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 o f 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 wo rkers 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 und er 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 poten tial 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 contaminate s 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

wor kers can contribute to the site score. (See Section s 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 (O LEM ) 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 EPA

OUST guidance documen ts 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 meth ods 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 intr usion 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) are as 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 contaminatio n 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 pa thways 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 structur e. I f 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 migratio n 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 sampl e 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
fr om 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 subsurfac e 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 Figur e 4

- 1, the sample s taken away from the sump or crack do not represent “background” sample s 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 thes e 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 contami nated 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 whe re 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 exposu re by chemical analysis. Th ese 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 G ispersion

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 ou tward 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 subjec t 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 pre sent 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 ba ckground 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) f rom 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 h azardous 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 th is “significant increase” was established44 f or 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 struc ture above the sample quantitation limit is considered to have a concentration significantly above background.

B ackground 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 c ontamination 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 boundar y – 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 hazardo us substances (when conducted after removal of the source).

i i. Attributi on

T he 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 ens ure 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).

F or 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 a re 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 be ing us e d in the s tructure). In additi on, it s hould be doc umented that the
volatile s ubstances a

re n ot coming f

rom c ontaminate d outdoor a ir into the oc cupied structures.

It i s s uggested that th e d ocumentati on t o support attributi on t o subsurface intrusion e mploy multiple l ines of
evidence

, including out door a ir ha zardous substance c oncentrations

, identifying t he ha zardous s ubstance a t t he

sourc e f acility, sit e o r relea se be ing investigated

, and detecting t he ha zardous substance i n subsurface s amples. (See EPA 2004 User’s Guide for Evaluating Subs urface Vapor Intrusion I n to Buildings f

or a dditional i nformation regarding l ines of evidence). In addition, actions should be t aken to ensure t hat s ources of the hazardous
substances inside a structure (e.g., household chemicals) have been removed from the structure prior to sampling.
Several EPA region al offices a nd stat e e nvironmental agencie s u se surveys/question nair es o r checklist s that provide g uidance w hen conducting i ndoor a ir sampling a nd discuss how to account f or po tential a

nthropogenic (lifestyle) sources. (S ee S ection 10 of thi s d ocument for a l ist of referenc es including several of th ese checklists
that w ere c onsidered in the proposed HRS a ddi tions)

. Establishing a ttributi on to the s ite in some s ituations, however, may be s traightforwar d to document. For example, when the hazardous s ubstance is m anmade, unique,
and not us ed i n consumer pr oducts and no contamination of the substance o f c oncern e xists in ambien t a ir.
Therefore

, ther e m ay be s ituations where not all th e s teps i dentified abov e w ill be necessary to establish
attribution. Futur e a dvancement in technical methods and increase s i n practical experienc e documenting t he

source of i ndoor contamination m ay aid in e stablishing a ttributi on (e.g. isotopic fingerprinting of s ubstances).

One exampl e o f establishing a n ob served exposur e by chemical analysi s i s p resented in Figur e 4

- 4. In this example, four ba ckground samples a re c ollected from t he indoor air of structures that a re located outside of the plume bounda

ries a nd one ambient a ir sample i s c ollected within the a rea of ob served exposure. None of these sampl es indicate tha t significan t concentrations of the contaminant exist outside of the plume bound aries . S everal
indoor a ir s amples are collecte d from ot her oc cupie d structures w ith hazardous substance concentrations significantly a bove t he h ighest concentrati on found i n any of t he b ackground samples a re i dentified. In addition,
the increase i n th e c oncentrations o f contaminant s is a ttributed to the i ntrusion of t he s ubsurface c ontamination,
becau se the c ontaminants found in t he s ubsurface pl ume unde r t he s tructures w ith increase d contaminati on are the
same. Further more , for t

his example, each structure w as i nventoried prior t o the s ampling a nd no anthropogenic
indoor s ource of the contaminati on (e.g., household cleaners) w as f ound in any of t he s tructures.46 Figure 4-4 . Observed Exposure by Ch emical 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 subsur face. 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 regularl y 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
ty

pes 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 esta blished, 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 fea tures 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 subs urface 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 ma y 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 varia bility).

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 occu r 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 undere stimation 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 ot her 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 actu al 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 p otential 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 c alculated 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 d epicts 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 pr esents 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;

• Verti cal 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 mig ration 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 m 3

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 . P o tential 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 W eighting of S tructure C ontainment Feat ures

The structure containment f actor reflects whether a volatil e sub stance can enter an occupied structure after the

s ubstanc e ha s migrated fro m t he subsurfac e to th e bas e of t he structure

. The assigned containment f actor depends

p rimarily on the presence of structural f eatur es (e.g., preferential pathways such as crack s in a f o undation) and mitigation actions that have been taken (e.g., active or passive ventilation sy stems). This is parallel to the
evaluation o f containme nt in othe r part s of th e HRS. Withi n t he HR S migrati on pathways, t he containme nt factor
is considered a measure of the manmade s tructures to minimize or prevent the movement of ha zardous 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 (s uch as the

presence of impenetrable cover or restriction s 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 c omponent, the st ructur e containment featur es take into account structural featur es 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 valu es ran ge 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 regularly51

occupied stru ctur e w ith an engineered, activ e v apor mitigation system (e. g., active venting) without documented
institutional c ontrols ( e.g., dee d restrictions ; funding i n place f or ong oing ope ration, inspection, a nd maintenance )
ha s a lower structur e c ontainment value ( structur e containment value of 2) than a regularly occupie d structure with an engineered vapor m igration barrier s ystem t hat does no t a ddress a ll pr eferential p athways ( structure
containment value of 7) becaus e the v apor mitigation barrier system is more likely to m inimize e xposure of
hazardous substances. The factor s e valuated and factor value s a ssigned are shown in Tabl e 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 maint enance. 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 t ribal 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 052 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 a n 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 mitigati on features installed as temporary remedies that do not fully address the long term threat of
hazardous substances should not be considered a contain me nt 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 pr esented 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

Con P amina P ed Ground Wa P er 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 wi PO evidence of subsurface in P rusion (e

B g

B ,

observed exposure)

S P ruc P ure Con P ainmen P = 10 (Table D -12 No

B 1) Example 2 – Regularly occupied

s P ruc P ure wi PO open preferen P ial

pa PO ways from PO e subsurface (e

B g

B , sumps, cracks, unsealed

u P ili P y lines

S P ruc P ure Con P ainmen P = 10 (Table D -12 No

B 2) Example 3 – Regularly occupied s P ruc P ure wi PO an engineered, ac P ive vapor mi P iga P ion sys P em (e

B g

B , ac P ive ven P ing)
wi PO ou P documen P ed ins P i P u P ional con P rols

(e

B g

B , deed res P ric P ions) and funding in place for on-going opera P ion, inspec P ion and main P enance

S P ruc P ure Con P ainmen P = 2 (Table D -12 No

B 8) Example 4 – Regularly occupied

s P ruc P ure wi PO a founda P ion raised grea P er PO an 6 fee P (e

B g

B , s P ruc P ure on s P il P s)

S P ruc P ure Con P ainmen P = 0 (Table D -12 No

B 10)53

d. Selection and W eighting of K ey R oute Parameter s (Route C haracteristics)

The HRS uses “route characteristics” (i.e., factor s t hat impact th e m ovement of g ases through the s ubsurface) to
estimate t he pr obability of contaminants m oving t o the f oundation of oc cupied structures

, suc h as from g round

water t hrough the soil to immediately be low a foundation. These r oute characteristics represent the physical a nd
chemical p roperties of the s pecific ha zardous s ubstances a nd the m edia in whic h they must ha ve m oved through

or c ould move t hroug h. To determine w hat rout e c haracteristi cs ar e appropriat e for evaluating potential exposure

to subsurfac e c ontaminants, present modeling analy ses (e.g., Identificati on of C ritical P arameters for t he J ohnson and Ettinger (

1991) V apor Intrusi on Model – Published M ay 2002) that estimat e t he l evel s of contaminant exposure (based on numerous r oute characteristics a long w ith the relationship between the r elevant i nput variables) wer e e xamined . In addition, input r eceived during t he pe er r eview pr ocess ( see A ppendix E of this TSD
for a dditional di scussion) w as e valuated during peer

- evaluation of th e s elected rout e c haracteristics. Careful
consideration wa s a lso given to ensur e t hat rout e c haracteristi c f

actor s m ay be a vailable, measured or calculated

,

on a s ite- specifi c ba sis w ithin the s cope o f a S uperfund s ite i nspection.

To evaluat e t

he intrinsi c r elationships b etween the e xamined rout e c haracteristics, a l imited sensitivity analysis

w as pe rformed to identify thos e factor s t hat ar e likely availabl e at most sit es du r ing a S uperfund site i nspection
and have t he g reatest i mpact on potential for e xposure

. Further

, the sensitivity a nalysis he lpe d to identify t he weighting s t hat should be associated with the rang e of factors. Thi s a nalysi s was c onducted using a s implified
version of the w idely accepted Johnson and Ettinger (J&E) model. Th e on lin e m odel calculate s a n estimated
attenuation factor and necessary contaminant concentration to impact indoor air based on th e c hemical properties
of th e c ontaminant, soil characterist ics, average s oil/ground water temperature, and depth t o contamination. Based

on the agency’ s a nalysis, thr ee factor s r epresented the greatest impact on potential for exposur e for which
sufficien t s ite -specific information could be collected during a s ite inspecti on. These t hree f actors are: de pt h to contamination, vertica l m igration

, and vapor m igration pot ential. Standard parameters f or c hemical

, soil, building, and exposure pa

rameters w ere o btained from E PA’s 2004 User's G uide F or E valuating Subsurface V

apor Intrusion Into Buildings

. (See A ppendix C o f t his doc ument f

or f urther de tails on the s ensitivity a nalysis.).

A s de picted in Tabl e 4 -3, the a ttenuati on factors w ere p rojected for m ultiple s cenarios a

nd used in assigning possible w eighting t o correspond wit h the factors i ncluded i n the H RS e valuatio n. For e xample, Figure 4- 7
illustrates t he i mpact of d epth on attenuation factors us ing na phthalene 13 c ontamination in a clay soil as the
scenario. The singl e v ariabl e a ltered for thi s s cenario w as t h e de pth to contamination from a bui lding’s foundation. As shown, both t he s oil g as a nd ground water attenuati on factors r eflect t he pe rcent of c ontaminants
in subsurfac e s oil g as or ground water, respectively, to intrude into a structure. As expected, th e a ttenuat ion
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-s preadsheet -modeling-subsurface-vapor-intrusion ) and those
result s a r e p resented in Tabl e 4 -3. A plot of those resulting attenuation factor s a nd associated depths i s p resented in Figure 4- 7. As t his figure s hows

, the attenuati on factors f or bo th ground water and soil g as b egin t o leve l off
around 150 feet be low t

he s urface

. Thi s show s that vapor intrusion could occur at least to thi s dept h but the

amount r eaching a s tructure f rom t his de pth and deeper i s qu ite l ow

. Therefore

, consideration of contamination at depths up to 150 feet warrants c onsiderati on and has b een include d i n the d epth t o c ontamination value

, although
the resulting potential to rel ease category value w ill 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 300Soil Gas Attenuation FactorGround 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 type55 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 substan ces. 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 Volat ility 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 co ntaminant 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 a vailable 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 meet56 observed exposure criteria, th e 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 de pths 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 av ailable, 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 fact or 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 (i n 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 poro sity (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

Tabl e 4 -6

. E ffective Porosity/Permeability o f Geologic M aterials (T able 5

- 14 i n th e P roposed H RS 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); moderat ely 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

Stu dies 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 Ev aluating Subsurface Vapor Intrusion into Buildings ).

Facto r 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 s hould 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 m igration. 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

t hickness 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.

Fur thermore, competent clay layers are likely to also influence the depth to contam ination 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 prese nt 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 LayerDepth Below GroundSurface (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) a nd 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 AdditionDepth Below GroundSurface (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
Factor Value = 14 Sand Layer

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. Th e 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 cal culating 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
value s are determined f

or 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
addi tion 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 wo uld 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. Tem poral and seasonal fluctuations in rainfall/runoff rates over multiple seasons affect the soil moisture content at a specific location

. As a result, s oil moisture content can vary temporally and with depth. (See e.g., Swartjes, F. A., (2011) Dealing with Contaminated Sites— From Theory towards Practical Application

,

c hapter 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 a ll 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 cont ent 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 inc lude 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. D etermining 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. Th erefore , a degradation factor was not added as a route characteristic. Ins tead, 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
inclu sion 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 lev el 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 ass igned. However, if an observed65 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 T able 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 2 a * ( 2b+2c+2d ), subject to a maximum of 500) 500 43066 

### 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 intrudi ng 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 sub stances, 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
contain ment 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 evaluati on. 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 m 3 mol -1 or greater , then the substance can be considered eligible in assigning a W aste 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 ratio nal 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 i nto contact with the receptor through different exposure routes (e.g., by inhalation, orally by ingestion of solids and drinking of liquids, by67 dermal exposure) . All other factors in the HRS evaluation of the individual pathways and components (e.g.,
likel ihood 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 su bsurface 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 toxicit y 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 a n 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 t he 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 re lated 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 t etrachloroethylene 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 on68 factors that may not be fully identified based on a ti me -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 as sumed, 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 t he depth to contamination at the site is g reater 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 valu e 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, becaus e it is unlikely that information for determining whether soil conditions at the site are sufficient for effective degradation of simple petroleum products coul d be determined during typical Site I nvestigation activities , the proposed HRS addition uses a m ore conservative depth of 10 feet for substances with half- lives less than 30 days. For substances with a longer half

life than petroleum and petroleum- like substances 30 feet represents a vertical distance great enough to allow for vapor degradation in t he 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 m ethane 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 . D ue 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 biode gradation 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, g as). 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 of69 impermeable surface barriers (e.g., pavement, snow cover). Therefore, it cannot be as sumed 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 valu e 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 in verses 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, unle ss 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 valu e using different criteria. If a parent substance , (e.g., tetrachloroethylen e) 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 biolog ically 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 half

life 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 inve rse 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 Fac tor

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

prope rties 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 expos ure 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 ex plained 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 are71 currentl y 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 co me in to contact with (see section 5.0.1 of the 1990 HRS).

It is proposed that the waste quantity factor value for th e 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 hazardo us 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 min imum 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 soi l 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 subsur face 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, assig ning 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 are72 based on measuring contamination entering into a regularly occupied stru cture, 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 informatio n 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 quantit ies 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 v alues 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 includ e 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 per iod and are statistically representative. A mass (in μg) is calculated by multiplying the concentration (in μg/m 3 ) by the volume of the room where the sample was collected (in m 3 ). 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 r easonable 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 entire73 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 al so 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 prese nted 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 . (S ee 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/m 3 . 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 . O therwise

, the evaluation proceeds to Tier C, which is described below. This example estimate is considered to be a partial74 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 determi ned 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 ft 2 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 concep tually most similar to a container with gas
in it , therefor e, 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 actu al ceiling height within a
structure is unknown when calculating the volume of a structure. H owever, 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 Ti er C. The value of 8 feet for ceiling height is referenced in numerous documents including the 20 15

OSWER Technical Guid e for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor75 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 ft 2 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 t his 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 ft 2 is assumed for each residential structure in an AOE being evaluated; however, if actual values can be determine d, they are to be used in this calculation. It is important to note that 1,740 ft 2 represents the 50 th 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 ft 2 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. Becau se 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 . Hazard ous 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, th e 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 HRS78 included the use of hazardous waste quantity minimum factor values in spec ific 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 si tes 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 wast e 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 grea ter 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
hazardo us 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

wa stestream 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 and79 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 Quantit y

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 enter ing 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 qu antity 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 f or the area tier

, a minimum value of 1,740 ft 2 is applied per structure or
subunit space 14 . 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 f rom 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 der ived 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 th is 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 t o 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 conditionin g (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 , incl uding 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 subsur face 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 Compo nent S tructure: T argets

The Target s f

actor category in the H RS pathway structur e i s ba sed on estimate s of the e xpected dose to each
receptor and th e num

ber and type of receptor s present. In environmental risk assessment s i t is critical to
understand the na ture a nd extent of exposure t o individuals

, populations, resources, and sensitive e nvironments.
The relativ e r isk assessment embodied within the H RS us es the target s factor as a n index of the natur e and extent
of e xposure t o individuals

, populations

, resour ces, and sensitiv e e nvironments.



### 6.1 Delineation o f T arget Areas – See S ection 5.2.0 of t he P roposed HRS A ddition

The delineation of target ar eas w ithin the H RS i s a ne cessary step in identifying thos e r eceptor s o r target s (e.g., populations) t hat are exposed t o, or m ay be come e xposed to, hazardous substance c ontamination. As d iscussed in
Section 2.2.a o f t his TSD

, a pa thway or component s core i s t he p roduct o f t hree f actor c ategories: l ikelih ood of release/exposure, wast e c haracteristics, and targets. Wherea s l ikelihood of release/exposur e a nd waste
characteristic s a

r e s ubject to maximum factor values, the t arget s factor category value i s u nlimited. (Target factor
category values were unlimited to give emphasis to sites with populations actually exposed t o contamination.)

Becaus e t h e t arget s v alue is unlimited whil e the o ther two category value s a r e c apped, th e Target s factor category
is likely to be the mos t i nfluential, i.e., the mos t l ikely to have a substantia l impact on the ultimate HRS site score. Furthermore, the e mphasis on targets is c onsistent w ith t he S ARA a mendments t o CERCLA.

Below i s a d etailed description of how the 199 0 HRS delineat es target areas. In general, all pathway s a ssign higher weighting to target s meeting observed releas e o r observed exposur e c riteria. Wherea s t h e t hree migration pathway s ( ground water, surfa ce w ater, and air) also consider thos e t arget s that may becom e i mpacted in the
futur e as a result of “contaminant s” m igrating, the s oil exposur e pa thway consider s t

hos e “p opulations” that “migrate” ( i.e., move) t o the s ite a nd become e xpose d to contamination.

a. Approach within t he 19 90 HRS

The migration pathway s (ground water, surfac e w ater, and air) delineat e t arget are as ba sed on level s of exposure within a targe t di st anc e limit (TDL). The TDL may also be s ubdivided into separat e d istanc e c ategori es s ubjected to distance-based weightings of targets w ithin each division. For e xample, targets within the a ir m igration pathway ar e f

irst identified based on two level s of con tamination : a ctua l o r potential. The criterion for documenting a ctual c ontaminati on is that a n obs erved release to air m ust b e e stablished at t

he target locati on. The
level o f actual contamination assigned to target s in each are a (see S ection 6.0 of thi s d ocument), 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 target s m eeting actual contamination criteria, the a ir migration pathway considers
target s i n location s not actually contaminated but within the TDL as po tentially contaminated. For th e a ir migration pathway, the TDL i s 4 miles i n a ll d irections f rom t he bounda

ry of e ach source un less t here i s a n observed releas e a t a g reater distance. Targets wit hi n the TDL ( excluding a ny pr eviously i dentified to meet
observed releas e c riteria) ar e t hen weighted based on th e l evel of contamination and their actual distance categories f

rom t he s ource 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). Th e a ctual distanc es and directions used to establish a TDL vary depending on th e m igration pathway (e.g., the surfac e w ater migration pathway consider s a standard distanc e (15 miles) downstream from the m ost
downstre am probabl e point of entry (PPE) unl ess observed releas es a r e established at a greater distance). The ground water m igration pa thway e valuates t argets u sing l ocations of w ells t apping t he a quifer be ing e valuated82 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 contamination 15 criteria. A TDL is not established when evaluating the resident population threat of the soil exposure pathway, because it is not expected that surface contaminat ion 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 m ile 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 contami nation (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 b ased 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 illustratio n 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 observe d “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 cont aminated (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 attrib uted 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

struct ures 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 A OE 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 in ferred 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 the re 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 contamin ation 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 Contaminatio n (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 observati on 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 imp act 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

co ntamination 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 sample 17 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 a ttributed, 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 meet s 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. Th is sample may be, but not limited to, a subslab sample, soil gas sample, ground water sample, or indoor air sample

.86

For a multistory mu lti -subuni t structur e inferred t o be in t he AO E (a s i s discusse d in section 6.1.b.i, Are a of Exposure (AOE) , above)

, the level above the hi ghes t leve l considere d to be withi n t he AO E i s inferred t o be an ASC.

I t i s propose d tha t site s which hav e th e po tential for groun d water to rise into a structure not be evaluated as part
of an ASC. Rather, onl y site s tha t hav e t he potentia l fo r va por to intr ude int o overlyi ng regularl y occupied

structure s ma y be considered when delineati ng an ASC. Populati ons in occ upied structures within thi s area are considere d likel y to b e expose d to contaminati on. T he fact or val ue assigne d to populatio ns within an AS C is weighted base d on sampl e dept h and media

, a s described furthe r in Section 6.3.b.ii, Weighting o f Populations
within t he ASC , o f thi s document.

I t i s als o propose d tha t substance s meetin g observed rel ease criteria with a vapor pressure e qua l t o, or greater than, 1 torr

, or a Henry’s Law constant greater than or equal to 10 -5 atm m 3 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 i n delineating an ASC because it has been shown to have en tered the structu r e at the site

. These minimum

vapor p ressu re values are mean t to identify only those hazardous substances with the potential to v olatilize and migrate into a structure through vapor intrusion. This is likely to exclude, for example, certain metals and nonvolatile compounds from being consi d ered as part of an ASC. When present in the subsurface, th ese low -v olatile

com pounds ar e unlikel y t o ente r into overlyi ng structure s a s a vapo r or gas.

In t he exampl e depicted i n Figur e 6-1, t he AS C i s delineated based on th e location of 10 subsurfac e s amples meeting ob served release criteria. The targets associated with this ASC include the five sampled structures with
no evidenc e of contamination (includi ng th e thre e structure s discussed abov e in Section 6.1

. b.i

, Are a of Exposure (AOE) , and t he one unsampled structur e tha t fall s within t he AS C boundaries, bu t i s not a par t o f t he AOE.

T he AS C i s propose d to be include d i n t he HR S evaluati on becaus e thi s method wil l all ow fo r evaluation o f an

area with existing subsurface co ntamination belo w stru ctur es to b e evaluate d fo r t he potentia l intrusion o f the
contamination int o t he overlyi ng structures. Numer ous studies

, includi ng severa l studie s presente d by member s 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 detectin g t he rat e of va por intrusion. Fo r example, a
research sit e in Indianapolis, India na wa s monitore d i n two adjoini ng duplex residentia l unit s ove r an extended

p erio d of time. Factors such as diurnal temperature, seasonal climate, and the presence of building featur es such

as an HVAC system or subslab mitigation ventin g system were shown to d ramatically affect the rate of intrusion
into a structur e (S ee Indo or Ai r and Soi l G as Tempora l Variability : Effect s on Sampling Strategie s under Controlled and Uncontrolled Conditions). A s scienc e advances, t he method s use d by sit e assessment practitioners

to collect data t o delineat e AOEs and ASCs may ev olve and result s fr om thes e method s wil l b e incorporate d into

HR S evaluations.87 c. Othe r Options Considered

In dev eloping 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:

• Consi deration 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. Consider ation of Only Actually Exposed Targets without Inference of Contamination

One c onsideration 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.

Lim iting 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.

Fig ure 6-3 . Consideration of Only Actually Exposed Targets without Inference of Contamination88 ii. Consi deration of Onl y Actually Exposed Targets and Those with Inference of
Contamination

A se cond 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 infor mation 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 n ot 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 conta minated, 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 s hould 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.

Sim ilar 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 . C o nsideration of Only Actually Exposed Targets and Those with Inference of Contamination

iii. Consi deration of Establishing a Potential Migration Zone (PMZ)

An al ternative 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). Figu re 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 direct ion 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 h eterogeneities 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 f uture. 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.

Howe ver, the confidence in the pr esent 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 hazardous90 subs tances 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 mo st 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...

Ver tical 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 pr oposed 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.

Fig ure 6-5 . Consideration of Actually Exposed and All Potentially Exposed Targets91 d. Rati onale for the Selected Option

As s tated 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 so il 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 par allel 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 conside r 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 in trusion 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 wit h 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 oth erwise” 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- specifi c 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 presente d 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

Thi s 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 id entifies those targets that are eligible for consideration. Specifically, targets are92 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 pathwa ys, 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 structur e 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 do cumented 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 199 0 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 migra tion 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. Popul ation – See Section 5.2.1.3.2 of the Proposed HRS Addition.

Eac h 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 r isk 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. Resour ces – See Section 5.2.1. 3.3 of the Proposed HRS Addition.

A reso urces 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 facto r 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 us es 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. Ot her Pathway

- Specific Targets

Eac h 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 w ater 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 potent ial 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 co ntamination 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 or94 obser ved 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.

Lev el I contamination of t argets 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

concent rations 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.

Pote ntial contamination of tar gets 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 Pr oposed Option

The f ollowing 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. Assigni ng Level of Exposure to Structures

The pr oposed addition considers both actual and potential contamination of regularly occupied structures with
indi viduals 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 di rect 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 o r 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 actuall y contaminated when establishing an AOE, are considered subject to Level II contamination consistent with other HRS pathways and
components.95 **Populat ions in Structures with no Subunits** 

Popula tions regularly occupying a structure with no subunits (e.g., si ngle

- 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 c urrent 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

.

Popula tions 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.

**Popu l**  **ations in Multi**

**- Subunit**  **Structures**

The w eighting 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 i s 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 establi shing 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 co mponent, 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 s ubunit 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 flo or 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 are98 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 significant99 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 “infor mation 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 o r 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 . W hereas, receptors regularly occupying structures in the ASC a re 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 ar e also weighted based on the sample media and100 t he depth to the shallowest contaminated samples. The benchmarks proposed for the subsurface intrusion
component are:

• S creening concentrations for cancer, and

• Screening concentrations for non- cancer toxicologi cal responses.

A s 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 th e

g reatest 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 toxi city 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 assump tions that are consistent with RI/FS procedures...

**R easonable Maximum Exposure Individual (**  **Exposed Individual Factor )** 

The r easonable m aximum e xposure ( RME) is d efine d for t he s ubsurface i ntrusi on component a s the hi ghest
exposure to a n individual r esulting from subsurf ace intrusion that i s r easonably expected to occur at a site. An

RME f actor w ithin t he s ubsurface intrusi on component i s p roposed and is c onsistent w ith the 199 0 HRS a nd with
standard EPA risk assessment procedures. Thi s factor i s k nown as the Exposed Individual f actor in the pr oposed addition. See also sections 3.5.1, 4.4.3.1 and 5.6.2 of t he 1988 Revised HRS TSD f or additional justification regarding t he i nclusion of a n RME f actor.

The basi c scoring approach used for assigning a f actor value throughout the 1990 HRS t o evaluate the R MEI, a n exposed individual

, factor is r etaine d for the s ubsurface i ntrusion c omponent. As i s c onsistent w ith all pathways, a
value of 5 0 points i s a ssigned if a ny i ndividual i n a r egularly oc cupie d structure i s exposed t o Level I
concentrations ; o r a v alue of 45 points i s a ssigned if there is a ny i ndividual e xposed to Level I I c oncentrations (see sections 6. 3.c o f t

his TSD a nd 5.2.1.3.1 of t he pr opose d addition for further e xplanation of a ssigning L evel I
and II c oncentrations.) I f t here a re no individuals in structur es expose d to Level I or L evel I I c oncentrations

, but a t
least on e i ndividual is living, working, or a ttending s chool o r da y c are in a structure i n an ASC, a v alue o f 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., I ndividual s Living, W orking, or Attendi ng School or D aycare)

This section details how populations ( made up of e xposed individuals) are identified i n the propose d subsurface intrusion component w ith particular c onsideration of t he di fference be tween evaluating a s tructure w ith no
subunits and structures w ith multiple s ubunits and/or multiple s tories.

Tho se p ersons living , working, or a ttending s chool or daycare (such as child- a nd elder- car e f acilities) in an AOE

or A SC a re pr oposed t o be evaluated in the popu latio n factor. Residents a re those i ndividuals m ost e xposed to101

subsurface intrusi on contamination based on the du ration of t ime t hese popu lations s pend in t heir r espective

structures. Student s in general are considered to b e m or e s usceptibl e t han worker s or teachers based on age. The weighting of the p opulation f actor is propose d to be e valuate d using m edia- specific, health

- based benchmark s a s discussed below. Workers are c onsidered in the popu lation factor evaluation, but a re w eighte d two- third s l ess than
residents and those a ttending s chool a nd daycare t o account for the length of the w ork da y. This w eighting i s

gene rally consistent with how EPA determine s t he a ppropriat e R egional Screening Level s f or chemical
contaminates.

i. Populations i n a n AOE (Level I a nd L evel II)

Those e xposed individuals living, working, or a ttending a s chool/daycare i n one or m ore s tructures o r s ubunits
wher e ob served exposur e by chemical analysi s criteri a ar e m et (in an AOE), and medi a concentrations m eet or exceed th e a

ppropriat e benchmark ar e c onsidered Level I. Thos e e xposed individual s in one o r mor e s tructur es o r subunits w here obs erved e xposur e by chemical analysi s c riteri a ha v e be en met (in an AOE), but th e m edia
concentrations ar e no t abov e be nchmarks, ar e c onsidered Level II. Thos e e xposed individual s w her e obs erved

exposure by di rect ob servation have be en establishe d, and those expose d individuals in structures i nferred t o be actually contaminated when establishing an AOE, ar e considered subject to Level II contamination consistent
with other H RS pa thways a nd components.

Weighting of Popu lations in an AOE

Consistent w it h the w eighting of popul ations (exposed individuals) in structures d ocumented as e xpose d to released hazardous substances throughout t he H RS ( see s ection 2.5 of the 19 90 HRS), the pr oposed subsurface intrusion component i dentifies and weights e xposed i ndividuals oc cupying s tructures in the A OE s ubject to Level

I c ontaminant c oncentrations by a f actor of 10 and w eights e xposed individuals i n s tructures s ubject to Level II
contaminant c oncentrations by a f actor o f 1.

For example, if a s ingl e r esidenc e occupied by a f amily of four i s ob served to be exposed to hazardou s s ubstance
concentrations abov e a m edia

- specific, health -based benchmark, the num ber of residents is m

ultiplied by 10 for a Level I factor valu e of 40. However, if that sam e f amily i s e xposed to a hazardou s s ubstan ce a t concentrations
below the a

pplicable be nchmark but a bove a l imit o f d etecti on, the num ber o f r esidents i s m

ultiplied by 1 for a
Level II factor value o f 4. To determine t he ov erall actual exposur e factor value, Level I and Level II factor values
for each regularly occupied structur e ar e s ummed.

Figure 6 -11 pr esent s an exampl e s cenario with both Level I and Level II target s l ocated within an AOE. In this example, 8 structures ha ve Level I c ontamination a nd 9 structure s ha v e L evel II contamination. While in practice,

a door

- to- door s urvey of po pulation per hous ehold would be strongly r ecommended, for pur poses of this e xample population per hou sehol d is e stimated using t he county a verage of 2.5 people p er o ccupied house

. Therefore, the 8
Level I s tructures r epresent 20 people o r 200 target po ints (20 * 10)

. The 9 Level I I s tructures represent 22.5 people or 22.5 target points ( 22.5 * 1).The t otal t arget population within the A OE is 222 .5 target po ints ( 200 + 22.5).

Note that number of worker s t hat ar e exposed to L evel I and II concentrations are divided by three and six for
full - and part-t im e w orkers, respectively , and then weighted accordin g to assig ned level of contaminatio n (i.e.,
Level I or Level II). If informatio n i s unavailabl e to classify a w orker as full - or pa rt- time, evaluat e that worker as102

being f ull

- time. The rational e f

or the d ifferenc e be tween resident s and worker s is b ased on th e di fferenc e in exposure time f

or fu ll

- time w orkers of 8 hours a da y (or 4 hours for part- tim e w orkers) versu s t h e t ypical

exposur e s cenario for resident s of a s h igh as 24 hour s a da y. Th e a gency recogniz es t hat thi s r atio i s no t exact for all situations, and that t o determine the m ost a ppropriate r ati o of exposure w oul d r equir e s it e s pecifi c i nformation beyond that collectab l e d uring a t ypical sit e inspection. This consideration of less exposure for workers than
resident s i s c on sistent with Superfund’ s r isk assessment exposur e s cen arios.103 Figure 6

- 11 . Example Calculation of Actually Exposed Targets

A s 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 co nditions, 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 det ecting 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.

I ndividuals inferred to be exposed to this contamination will be weighted as Level II bec ause 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).

A s previously stated, the proposed method is di fferent 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 expos ed. The proposed method for104 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. T herefore, 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. Population s 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 s ubject 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 tempo ral 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 a ctually 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 we ighting 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 Populations a in Structures b 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
Contamination108 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 populatio n 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 samp le 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 weighti ng factors.109 Figure 6

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

E PA considered other options when determining an appropriate weighting factor for occupied structures located in an ASC including:

• A ssigning a weighing of 0.1 to all targets located within an ASC;

• Assigning a higher weighting to targets located in structures with documented subslab concentration s

e xceeding 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 s tated above, the w eighting factor s w er e c hosen based on the m ost recent attenuation factors published by E PA a nd other f actors k nown to i mpact the pr obability of v apor intrusion. I t s hould be no te d that t he higher weighting of structures with soil gas, soil, or crawl sp ace contamination —as opposed to ground water
contamination —r eflects th a t c ontamination has volatilized ou t o f a liq uid state and it is docu mented tha t t he contamination has begun to migrate vertically.110

c. Resources

For the subsurface i ntrusion component

, regularly oc cupie d structures in which there a re significant populations
that could b e e xposed to hazardous s ubstanc es due t o subsurfac e i ntrusion ar e c onsidered “resource s” i f they are located withi n a de fined area of e xposure (as previously di scussed in secti on VI.B.2 of the p reamble to the 19 90 HRS) a nd the po pulations may be e xpose d t o contamination due t o subsurface i ntrusion on a r egular b asis. For example, most i ndoor pu

blic r ecreati on facilities, libraries

, community centers, religious f acilities, and tribal
cultural facilities could qualify as e ligibl e r esources if they ar e r egularly occupied (e.g., occupied daily). It is proposed that any identified resourc e w ill b e a ssigned a r esour ce target value o f 5, as is c onsis tent w ith the method for counting r esources un der the existing s oil e xposure pa thway.

Weighting of R esources

A resource s t arget factor i s assigned in all pathway s i n the 19 90 HRS. In the pr oposed addition a factor valu e of
fiv e i s assigned if at least one r esour ce i s p resent within an AOE or ASC, and a factor value o f zero i s a ssigned if

no resour ce i s p resent. Eligibl e resourc es a r e specifi c to the p athway, component, or threat. Th ese resources represent e conomic us es, public e xposure t o a c ontaminated m edium, or a reas w here e xposures oc cur a nd are not covered by t he other identifie d targets. For e xample

, resources w ithin the a ir m igrati on pathway i nclude
commercial agricultur e or silvicultur e and major/designated recreation areas. Th e resident populatio n threat of the

1990 HRS s oil e xposure p athway c onsiders c ommercial l ivestock pr oduction or g razing t hat c ould lea d 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. S pecial Consideration for Radioactive Hazardous Substances when Identifyi ng and
E valuating the Targets Factor Category

A s 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 id entify 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 hazard ous 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 site s.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 characteristi cs , 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

t he 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 t hree 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 + 1 0) (b)

Subsurface Intrusion Component Score

12. Subsurface Intrusion Component (lines 4 x 7 x 12)/82,500 c (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 sc ore (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 s ubsurface 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
th reat.

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 scor es (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 HR S root

- mean -square algorithm, including the justification for its inclusion.



### SS = � SS

gggg 2 

### + SS ssgg 2 

### + SS ssssssssss 2 

### + SS aa 2



### 4114 

### 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 e xample 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 Scena rios – 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 overl ying 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 di scussion 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
illustr ated 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 to115 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 pr oposed 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 co nsidered 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 o f 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 To xicity: 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 sce narios:

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 l ocated 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 concentr ations 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 haz ardous 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 impa cts 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 contam inant 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 conc entrations, 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 subje ct 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 ta rgets,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 ass uming 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 s ubslab 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

p roposed 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 margina lly 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 S coring 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 d etected 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 Wa ste 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 ft 2 each, and the two homes in the AOE have a basal footprint of 1,940 ft 2 each. Therefore, the total area of structures within the AOE and ASC can be calculated to be 20,880 ft 2 . Using the proposed H RS 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 de termined 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 t argets 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 compon ent scored in this example scenario.

Figure 8-1 . Example Site Scoring Scenario No. 1120 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 1 1 )/82,500 c (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 S gw 2 + S
sw 2 + S sessi 2 + S
a 2 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.12 1

Example S ite Scoring Scenario N o. 2
The s econd example s ite scoring s cenari o also consists of a mixed

- us e r esidential and commercial area abov e a shallow g round water pl ume c ontaminated with a h azardous substance o f m oderate t oxicity ( HRS t oxicity of 1,000). Initial g round water s ampling ha s do cumente d a de pth to g round water of 35 ft. be low g round surface a nd
delineated the horizontal extent of th e g round water plum e as u nderlying approximately 20 acr es o f a residential neighborhood, a daycare facility , and a single-story office building. The neighborhood c onsists of 29 singlefamily de tached homes. As part of an expanded site investi gation (ESI), indoor air sampl es w er e c ollected inside 19 homes; one daycare facility having approximately 25 enrolled c hildren , 6 f ull

- time workers , and 2 part- time workers; a nd the o ffice building w ith 18 full- tim e w orkers . Indoor a ir concentrations a bov e t h e a pplicable
benchmark w ere de tected i n 14 of the homes s ampled; i ndoor a ir concentrations a bove de tecti on limits b ut b elow
the a pplicable benchmark were f ound in the remaining 5 sampled homes a nd daycare facility. All hom es a nd t he daycar e facility we re c hecked for i ndoor sources of hazardous s ubstances p rior t o sampling.

The second exampl e s it e scoring scenario receiv es a likelihood of exposur e v alu e of 550 becaus e an observed

exposur e has b een established by chemical analysi s i n 19 homes , one da ycar e facility , and one office bu ilding.
The contaminant level s a

r e significantly abov e ba ckground level s e stablished based on concentrations in homes

not abov e t he g round water plume. Th e i ncre ase in contaminant levels i s attributed to subsurfa ce i ntrusion ba sed

on multipl e l ine s o f evidences. Outsid e a mbient air sampl es w er e collected near the hom es w ith identified indoor
contaminant concentrations . H owever, contaminant concentrations w er e no t detected in the a mbient air samples.
In additi on, the g round water contamination containe d the s am e s ubstanc e as thos e i n the s ampl es t aken from t he
residences

, day c are, a nd office a nd no indoor s ources of t he c ontaminant w ere f ound.

The wast e c haracteristic s f actor category i s a product of th e t oxicity/degradation factor and hazardou s w aste quantity f actor v alues. For t

his s ite, the A OE i s de fined by t he 19 homes, t h e da ycar e f acility , a nd the o ffice building w ith measure d indoor a ir contaminant concentrations m eeting obs erved exposure c riteria. Based on the
location of th ese structures, the bo undary of th e A OE doe s no t encompa ss a ny additional structures; therefore, no other structur es are inferred as a ctually exposed. The A SC i s de lineated by contaminated ground water samples meeting observed releas e c riteria. Ten hom es ha ve be en identified as ov erlying t his g round water p lume e xcluding
those f ound within t he A OE. S ufficien t da ta does n ot exist to calculat e h azardous constituent or wastestream quantities

. The actual di mensions of each structure w ithin the A OE a nd ASC a re k nown and use d to calculate the
hazardous w aste qua

ntity a rea

. Te n homes ha ve a ba sal f ootprint of 1,7 00 ft 2 e ach. Eighteen hom es ha v e a ba sal footprint o f 1,7 60 ft 2 eac h, and one home ha s a b asal f ootprint of 1,780 ft 2

. Th e d aycar e f acility i s documented as having a ba sal f

ootprint o f 4,600 ft 2

. The office bu ilding i s doc umente d as ha ving a ba sal footprint of 12 ,000 ft 2

. Therefore, the t otal a

rea of structures w ithi n the A OE a nd ASC is 67, 060 ft 2 . Only one ha zardous subs tanc e of a moderat e level of toxicity i s a ssociated with thi s site. A toxicity value of 1,000 is assigned from HRS Tabl e 2 -4
based on th e or al referenc e dos e for thi s c ontaminant. The degradation factor valu e i s assigned a v alu e of 1 due t o the p resence o f an observed e xposure. Using t he proposed HRS a dditi on Tables 2- 6 and 5- 18, the h azardou s waste quantity f actor v alue of 1 00 i s a ssigned using a t oxicity/degradation value of 1, 000 f or the hazardous s ubstance of
moderat e toxicity and th e p resenc e o f an ob served exposure

. T he resulting wast e characteristic s f actor category
value i s 18 (Tabl e 2 -7).

The target s factor category value i s based on the population in th e A OE and ASC. Becaus e a n observed exposure ha s be en established at Level I concentrations in 1 4 o ccupied homes, th e s it e receiv es a n exposed individual factor
value of 5 0. The n

umber of r eceptors i n the A OE s ignificantly i mpacts the po pulation factor for this s ite

. The
number of r esidents in the AOE i s k nown. The e xact number of pe ople living w ithin each unsampled home of t his122 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 groun d 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. 2123 Table 8-2 . Scoresheet for Example Site Sco r i ng 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 1 1 )/82,500 c (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 S gw 2 + S
sw 2 + S sessi 2 + S
a 2 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.

Exam ple 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 neighborhood124 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

. H owever, 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, an d 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 struct ures 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 ft 2 , and the 10 homes in the ASC have a basal footprint of 1,700 ft 2 . The daycare facility has a basal footprint of 4,600 ft 2 . There fore, the total area of structures within the AOE and ASC
is 107,350 ft 2 . 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 thi s contaminant. The degradation
factor value of 1 is 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 . U sing a toxicity/degradation
fa ctor 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 popul ation 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
fac tor of 0.4 based on HRS addition Table 5-20 (5 homes – 13 people) —resulting in a value of 5.2. The remaining125

5 hom es w ithin th e A SC ar e ov erlying a g round water plum e de fined by sampl es that meet observed release
criteria and were collected from locations greater than fiv e f eet from a s tructure’ s foundation. Therefore, th e populations residing within th e A SC ar e weighted by 0.1 based on HRS Table 5-20 (5 homes – 13 people)— resulting i n a v alue o f 1.3.

The popu lation v alue for this s cenario is 1,196. 1. . The r esulting m aximum s ubsurface intrusion component score
of 100 (uncapped value o f 265.83) and a c orresponding H RS s ite s core of 50 .00, qualifies the e xample s ite f or listing on the N PL

. Table 8 -3 provides t he s coresheet for t he s ubsurface i ntrusi on compone nt in thi s s cenario.

Figure 8-3

. E xample S ite Scoring Scenario N o. 3126 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) 1 1,246.1 Subsurface Intrusion Component Score:

12. Subsurface Intrusion Component (lines 4 x 7 x 1 1 )/82,500 c (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 S gw 2 + S
sw 2 + S sessi 2 + S
a 2 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 Sit es (Test S ites) with E xisting V apor Intrusion Data

As di scussed previously i n Section 1.3.b of t his report, a n evaluation of t he ne ed for t his propose d addition was
undertaken t o i d entify and cl assify si t es t hat m ay pos e a vapor i ntrusion t

hreat

. T hi s e valuation r esulted i n t he
identification of 1,0 73 sites t hat m ay or may not qua lify f or t he N PL, but could have v apor i ntrusi on issues.

Of t hose 1,073 s ites identified, 11 w ere considered to h ave a subsurf ace intrusion threat ba sed o n t he presence of
actual sam

pling da ta a nd o ther H RS-required data to confirm t hat these s ites w ould not score s ufficiently hi gh enough to qualify f

or t he N PL w ithout the a ddition of a s ubsurface intrusion component t o the H RS.

T he i dentification o f

t h ese sites is based

on currently av a i lable i nformation only, and no sampling or site- specific investigations were conducted t o expand

t he data availabl e for t he purposes of t hi s analysis. B elow i s a synopsis
of s it e information f

or e ach of 11 actual sites . M ore detailed s ite

- specific informa tion , inc luding s ubsurface intrusion component s coresheets a nd figures f or e ach of t he 11 test s ites

, is i ncluded in Appendix D of this
document.

Test S ite N

o. 001

Test site 001 con sist s of a chlorinated solvents releas e from an ope rating dr ycleaner s ituated ad jacent t o a daycare
and a p rivate s chool

. The U .S. Environmental P rotection Agency ( EPA) de termined that e xhaust f rom t he drycleaner

w as causing el evated

l evels of t etrachloroethylene (PCE) and trichloroethylene ( TCE) to b e taken i nt o nearby bu ilding v entilation sy stems. E PA ha s collected i ndoor a ir samples from t he drycleaner, da ycare, an d private sch ool and

ha s also collected soil g as sampl es f rom be neath t he foundations ( i.e., subslab) of surrounding residential and commercial buildings, including t he dr ycleaner, da ycare a nd private s chool. Additionally, EPA ha s

a ls o collected outdoor a ir s a mples.

Test S ite N

o. 002

Test site 002 i s l ocated

i n c entral Ma

rylan d, within an area of commercial a nd residential l and use t ypes

. The subject property cons ists of an L

- shaped

shop ping cen ter

s trip an d t hree pa d sit es oc cupied

by m ultiple

com mercial t enants. Historical operations resulted i n a releas e of

chl orinated solve nts to shallow g r ound water
and soils immediately surrounding the facility. Dry cleaning facilities are documented to have been ongoing for approximately 40 years— the current tenant ceased o n -site dry cleaning operations in 2004. This site has been investigated

o n m ultiple 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 S ite N

o. 003

Test site 003 i s l ocated

i n t he District of C o lumbia, withi n an area of residential homes a nd a c hurch. Previous investigations to as sess a gasoline spill f rom a servi ce station ha ve identified g r ound w ater co ntaminated

w ith

PCE a nd other chlorinate d and non-chlorinated volatile or ga nic analytes. The contamination ap pear s to be traceable to a dry c leaning ope rati on. The P CE c ontamination was f irst detecte d in 2002 and s ubs equent investigations i n 2003 a nd 2004 were c onducte d to determine t he e xtent of the P CE c ontamination a nd t o evaluat e the e xtent o f v apor intrusion into overlying r esidential structures.

Test S ite N

o. 004

Test site 004 i s l ocated

i n c entral Ma

ryland, w ithin an ar ea containing a mixture of r esidential, com

mercial

, and
indus trial l and

u se types. H i storical ope rations at a former pr ecious metal r ecovery ope ration resulted in a release128 of c hlorinated solvents to shallow ground water and soils immediately surrounding the facility. The site includes a
residential area c omprised 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 EP A and requested assistance after discovering deteriorating and damaged tanks, containers, and drums that were leaking their contents in to the environment. EPA assessed the site and commenced an emergency response action, which was completed in December 198 1. 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 build ing 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 th e 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.

Tes t 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 chlor inated 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.

Tes t Site No. 006
Test site 006 is l ocated 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 reside ntial 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 S ite N

o. 007
Test site 007 is located in the District of Columbia, within an ar ea of co mmercial an d residential l and use types
and i s immediately ad

jacent t o a hospital complex. D ry cl eaning o perations have reportedly b een ongoing for approximately 60 years at the facility. These historical ope rations have resulted i n a release of chlorinated

so lvent s to sh allow ground w ater an d so ils i mmediately su rrounding t he facility, as has been documented in
several environmental investigations conducted in 2013 and 2014.

Test S ite N

o. 008

Test si t e 00 8 i s l ocated in south- central Montana within an a rea of r es idential and com mercial land us e t ypes. Historical r eleases pos sibly asso ciated w i th t wo n earby dr y cl eaning ope rations have resulted in a release of chlorinated solvents to ground water i n the a rea; t he p lume e ncompasses a bout 140 acres ( 1 ,000 ft w ide by 8,0 00
ft l ong) . S tate

- lead

p reliminary asse ssment s (PA), s ite inspections (SI) , a nd expa nd ed

si t e inspections (ESI) w ere

c onducte d at this s ite i n M ontana f rom 1993 to 2002. T hese investigations h av e be en unde r t he d irection of t

he

EPA R emoval pr ogram s ince 2006.

Test S ite N

o. 009
Test site 00 9 is l ocated in south central W isconsin within a n area o f c ommercial

, industrial, and residential l and
use types. I t cons ist s of

a n AOE de lineated

by i ndoor air samples meeting obs erved expo sure criteri a and

an ASC

, delineate d by s amples f

r om m ultiple m edia, m eeting o bserved r elease criteria. A mong t he hazardous substances
doc umented t o be present i n the A OE a nd ASC w ere tetrachloroethylene ( PCE)

, TCE, vinyl c hloride

, and cis-1,2-
dichloroethene.

Test S ite N

o. 010

Test site 010 i s l ocated

i n e astern Mi ssouri w it hin a n a rea of r es idential ho using. E PA di scovered a ground water plume i n a residential neighborhood a nd delineated the extent ov er m ultiple s ampling e vents be ginning i n the

summer of 1998. Two h azardous waste facilities are known t o ex ist i n t he immediate vicinity of t h e ground

w ater plume, including a f

ormer metal s haping f

aci lity and

a l if e insurance facility ( where small scale dumping historically oc curre d in the mid 1960’s). The g round w ater pl ume c onsists of c hlorinated solvents ( TCE, PCE,
CIS -1,2-DCE, Vinyl C hloride, etc.) and underlies a r esidential n eighborhood consisting of a m ix of s ingle family
residen ces and

s everal si ngle

- s tory, multi -tenant r esidential bu ildings. Indoor a ir s ampling ha s i dentifie d VOC’s
within the l iving s pace of n umerous r esidential s tructures, including s everal c ontaminant c oncentrations exceeding can cer/non

- cancer be nchmarks. In addition, s ubslab sampling ha s be e n conducte d a nd elevated
concentrations ha ve been d ocumented t o be underlying sev eral of residential s tructures.

Test S ite N

o. 011
Test site 01 1 is l ocated in southeastern M innesota w ithin a p rimarily r esidential neighborhood. In 2009, the
Minnesota P ollution Control A gency ( MPCA) c onducted an investigati on to identify pot ential s ources f or a large
contaminant g round water plume. Part of on e s tudy a rea, Area A , was d ocumented to have e levated
concentrations of chlorinated solvents i n ground water surrounding a s ingle-family r esidential structure. Although
this residential a

rea with elevated

co ntaminant g round wate r c oncentrations w as d etermined not t o be a likely

source of t he c ontamination, subsequent s oil g as a nd indoor a ir sampling e vents d etecte d contaminants w it hi n 11
of t he s urrounding r esidences t hat w ere sampled. Sufficient i nformati on t o identify t he or ig i nating sour ce of
contamination f

or t hi s ar ea is not av ailable.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 coun try 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. F igure 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 l isting 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

reg ional 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 Distribution132 

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

Seve ral 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 upd ated to reflect this evolution. These updates include:

• Iden tification 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.

Thes e terminolog y update s ar e explained bel ow and do not resu lt in any substantiv e c h ang e i n th e a pplication of
the HRS.

a. Identificati on of Ambient W ater Quality C riteria:

Amb ient Water Quality Criteri a (AWQC) ar e used in the HRS for two purpo ses in the environmental threat of the
Surfa ce Water Migration Pathway. The first us e i s in assigning an enviro nmental toxicity factor value for
individua l hazardou s substances

. (Se e HR S section 4.1.4.2.1.1, Ecosystem toxicity ). T h e s econd u se i s in assigning weighting for aquati c s ensitiv e e nvironment s exposed to relea ses (i.e., considered “actually contaminated”)

. (Se e HR S section s 2.5.2, Comparis on to Benchmarks , and 4.1.4.3., Environment al threat
targets ).

The se criteri a a r e l isted in the tab l e identified as N ational Reco mmended Water Quality Criteri a (NRWQC) on an EPA websit e ( S ee E PA web page http://www.epa.gov/wqc/national-r ecommended -w ater -quality-c riteria ). T o

ensur e clarity, t he definition f or Ambien t Wate r Qualit

y Criteri a in HR S section 1.1, Definitions

, i s proposed to be
revised to reflect thi s t erminology.

In addition, the term s “ acut e” a n d “chroni c” ar e also referred to at tim es u sin g differen t terminolog y tha n in the

19 90 HRS definition section. Th e t erm “acut e” i s also referred to as th e C riteri a Maximum Concentration (CMC).
The CMC i s a n estimat e o f th e highest concentration of a m aterial in surf ace w ater to which an aquatic
communi ty can be exposed briefly without resulting in an unacceptabl e effect. The “chroni c” c riteri a ar e a lso
referred to as th e C riterion Continuou s Concentration (CCC). Th e C CC i s an estimat e o f t h e h ighest concentration
of a m aterial in su rface w ater to wh ich an aquatic community can be exposed indefinitely without resulting in an

un acceptable e ffect. (See E PA web p ag e http://www.epa.gov/wqc/national-re commend ed -w ater -q uality -c riteria

aquatic-l ife -c riteria -t able ). The definition of Ambient Water Quality Criteria in the HRS is also proposed to be
revised to reflect thi s a lternativ e n omenclature.

b. Use of R eference C oncentrati on (RfC) Criteria

The 19 90 H R S use s t he Referenc e Dos e (RfD ) fo r determini ng t he human toxicit y fact or val ue and th e human
health benchmar k f

or non- can cer effect s for all exposur e r o ut es in all four HRS pathways. (S ee 1 990 HRS
section s 2.4.1.1, To

xicit y F actor, a nd 2.5.2 , Comparison to benchmarks. ) Sinc e 1990, EP A ha s adopte d t he us e of133

Reference Con centrations (RfCs) instead of Referen c e Doses (RfDs) when determining non

- cancer related risk
level s for inhalatio n exposures.

The term RfD is defined by EPA in the 1990 HRS in HRS section 1.1 Definitions and on EPA web page : http://o fmpub.epa.gov/sor_internet/registry /termreg/searchandretrieve/glossariesandkeywordlists/search.do?detail s=&glossaryName=IRIS%2 0Glo ssary#formTop

. An RfD is an estimate (with uncertainty spanning perhaps an orde r of magnitude ) o f a dail y ora l exposur e to th e human populati on (includi ng sensitiv e subgroups ) tha t i s likely
to be with out an appreciabl e ris k of deleteri ous effect s duri ng a lifetime. Th e ter m Rf C i s also defined by EP A on the following EPA web page : http://o fmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/g lossariesandkeywordlists/search.d o?detail s=&glossaryName=IRIS%20Glossary #formTop . It is defined there, as an estimate (with uncertainty spanning perha ps a n orde r o f magnitude ) o f a continuo us inhalation exposur e to t he huma n population (includi ng sensitive subgroups ) tha t i s likel y t o be wit hout an appreciabl e ris k of deleteri ous effect s duri ng a lifetime. A s illustrated in
the se definitions, th e two value s ar e roughl y equivalen t i n thei r purpose , a nd t he addition o f t he us e o f an Rf C is

not a chang e in concep t or function, onl y nomenclature.

In t he 1990 HRS, when assigni ng a human toxicit y t o hazar dous substance s base d on an RfD

, t he facto r val ue is directly related to the magnit ud e of t he Rf D (se e 1 990 HR S Tabl e 2-4, Toxicit y Facto r Evaluation). A n additional
tabl e ha s been develope d t o direc t t he assignme nt of a human toxicit y facto r val ue base d on RfCs

. T he proces s for developi ng th e 1990 tabl e i s discusse d in secti on VI- 2 of th e 1988 Revised HR S TS D f or t he 1988 propose d HRS.

T he Rf D of 229 hazar dous substance s wer e examine d and found to be l og norma l in distributi on. Th e value s were binned into 6 catego ries corresp onding to the 6 p ossible factor values. This same process w as d uplicated for assigni ng fact or value s based on 81 RfC s obtaine d fro m EP A IRI S database

. Th e boundarie s of each bin were
established s o tha t the y resulted i n roughl y t he sam e proportiona l distribution o f facto r value s a s in 1988 using
RfDs.

Thi s wa s done by firs t calculati ng t he Standard Norma l cumulativ e probabilitie s o f th e log- transformed RfCs.
Because th ese cumulative probabilities estimat e the proportio n of analytes with RfCs below a given
concentrati on, the y could then be use d to determi ne Rf C boundarie s fo r each bi n tha t would assign analyte s to each categor y a t frequencie s approximatel y equa l t o th e 1990 table

. Th e distribution wa s also verifie d by
examini ng t he substance s tha t fel l in each categor y usi ng both procedures

. T he frequenc y of values and the suggested assign ed HRS factor value s are presented in the table below.134 Table 9-1 . Assignment of Human Toxicity Factor Values using RfCs

Reference Concentration (RfC) (mg/m 3 ) 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 hazardou s s ubstanc es and the R fCs us ed i n developing t his table is c ontaine d in Appendix B of this
document.

c. Use of I nhalation U nit Risk (IUR)

The 1990 HRS u ses the C ancer S lope f actor f or d etermining t he hum an toxicity f actor v alue a nd the hum an health
benchmark f

or c ancer e ffects f

or a ll e xposure r outes i n all four H RS pa thways. (See 1990 HRS s ections 2.4.1. 1, Toxicity F

actor, and 2.5.2

, Comparison to be nchmarks.) S ince 19 90, EPA ha s a dopte d the u se o f a n Inhalation
Unit Risk (IUR) instead of the C ancer Slop e F actor when de termining cancer

- related risk level s for inhalation

exposures.

The term Slope factor is defined by EPA on EPA web page http ://ofmpub. epa.gov/so r_internet/registry/termreg/searchandretrieve/glossariesandkeyw ordlists/search.do?detail s=&glossaryName=IRIS%20Glossary#formTop as an upper bound, approximating a 95 percent confidence limit,

on the increased cancer risk f rom a l ifetim e e xposur e to an agent. Thi s estimate, usually expressed in unit s o f proporti on (of a popu lation) a ffected pe r m g/kg- day, i s g enerally reserved for us e i n the low -dose r egion of t he dose-response r elationshi p, that i s, for e xposures corresponding t o risks l ess t han 1 i n 100. A n Inhalation Unit
Risk (IUR) i s t he up per -bound excess lifetime c ancer risk e stimated to result from c ontinuous e xposure to an agent at a c oncentration of 1 μg/m 3 in air

. The i nterpretation of I UR w ould be a s follows: if u nit r isk = 2 × 10 - 6 pe r μg/m 3 , 2 excess c ancer c ases ( upper b ound estimate) are e xpected to de vel op per 1,000,000 people i f e xposed daily for a lifetim e t o 1 μg of th e c hemical per m 3 of air. The typica l un its for the IUR is (μg/m 3 ) -1

. As illustrated
in the definitions , the two values a re r oughly e quivalent i n their purpose

, a nd the addition of the u se o f a n IUR i s

not a change i n concept or function.

In the 1990 HRS w hen assigning a hum an toxicity t o hazardous substances b ase d on an Cancer S lope f actor

, the
factor value i s d irectly r elated to the m agnitude of the factor and t h e W eight -of- Evidenc e t hat a s ubstance i s a carcinogen( see 1990 HRS Tabl e 2 -4, Toxicity F actor Evaluation). A n additional table has b een developed to
direc t the assignment

of a human toxicity facto r value based on th e I UR and the Weight -of-Evidence

. The binning
of th e I UR s i nto ca tegori es w as pe rformed in a pa rallel manner as that for th e b inning of RfC s d iscussed in the
abov e s ection of thi s do cument, except that IUR s w er e a lso subcategorized based on their W eight-of- Evidence
criteria a s w ell a s their m agnitude

. The bi ns w ere s ized again s o that t hey ha d the s ame pr oportional d istribution

as pr eviously w hen the binning w as ba sed on Cancer Slope F actors. B ecause o f t he l imited number o f a nalytes i n135 W eight -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 t hese g roupings a

re c ombined wit h Weight-of-Evidence g roupings i s di scussed i n the following s e c tion of
this document. The list of hazardous s ubstances, their W eight -of- Evidence and t heir I UR i s contained in Appe ndix B of this doc ument.

d. Weight

- of

- Evidence Groupings:

The 1990 HRS a

nd the proposed HRS a ddition utilize EPA W eight-of-Evidence ( WOE) g roupings f or categorizing t he information on the c arcinogenicity of a s ubstance. As defined i n the 1990 HRS s ec ti on 1.1, Definitions, a nd in IRIS, the W OE g roupings a re a s ystem us ed by E PA to c ha racteriz e t he extent t o w hich the available data support t he hypothesis that an ag ent causes cancer i n humans. U nder

E PA's 1986 r isk

asse ssment guidelines, the WOE w as described by cat egories " A through E ," G roup A f or k nown human carcinogens through Group E f or a gents w ith evidence of no ncarcinogenicity.

In 2005 EPA r evised the g roupings

. The approach out line d in EPA's g uidelines f or c arcinogen risk a ssessment (2005) c onsiders a ll s cientific i nformati on in determining w hether and under w hat conditions a n a gent m ay caus e

cancer i n hum ans, and prov ide s a narrative approach to cha racterize carcinogenicity r at her t han ca tegories. F ive
standard WO E d escriptor s are used as part of the narrative. The designation o f a substan ce as a human

car cinogen has been incor porated in to the H RS a lgorithm f or d etermining th e to xicity f actor v alue. (See U.S.

EPA. 2005. Guidelines for Carcinogen Risk Assessment

. ) The table b elow c ompares th e two se ts of g uidelines.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 toxi city 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 ≤ SF b 5 ≤ SF 50 ≤ SF 10,000

0.05 ≤ SF < 0.5 0.5 ≤ SF < 5 5 ≤ SF < 50 1,000

SF < 0.05 0.05 ≤ SF < 0.5 0.5 ≤ SF < 5 100

_ _ _ SF < 0.05 SF < 0.5 10

Slope factor not available. Slope factor not available. Slope factor not available. 0

Weight

- of

- Evidence a /Inhalation unit risk (μg/m 3 )

A or Carcinogenic to
humans B or Likely to be carcinogenic to humans C or Suggestive
evidence of carcinogenic potential Assigned
value

0.00004 ≤ IUR c 0.0004 ≤ IUR 0.004 ≤ IUR 10,000

0.00001 ≤ IUR < 0.00004 0.0001 ≤ IUR < 0.0004 0.001 ≤ IUR < 0.004 1,000

IUR < 0.00001 0.00001 ≤ IUR < 0.0001 0.0001 ≤ IUR < 0.001 100

_ _ _ < 0.00001 IUR < 0.0001 10

Inhalation unit risk not
available. Inhalation unit risk not
available. Inhalation unit risk not
available. 0137 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 testin g 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: S cience Advisory Board
Review of Hazard Ranking System. Office of Emergency and Remedial Response. December 9, 1986.

3. United

S tates En vironmental Protection Agency. (1987) Analysis o f the A ir T arget Distance L imi t
in the Hazard Ranking System . Office of Solid Waste and Emergency Response. July 15, 1987.

4. United States Environmental Protection Agency. (1987). Chemicals C ontributing

to E st imated R isks

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. Ma y 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 H RS T echnical S upport
Document.

8. United States Environmental Protection Agency. (1988). Review of the Superfund Hazard Ranking system. (SAB -E C -88-008)

. Review b y t he Hazard R anking S ystem R eview S ubcommittee of the Science Advisory B oard. O ffice of t he Administrator

S cience Advisory B oard.

9. United States Environmental Protection Agency. (1989). Risk Assessment G uidance for Superfund (RAGS) Pa rt

A. Office of

S olid Wast e and

E mergency R esponse (OSWER). Available on -l ine a t:
h ttp://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 o f R ecent R esearch o n V

apor In trusion. 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 Intru sion D atabase: P reliminary Evaluation and Attenuation Factors. Office of S olid Waste.

15. United States Environmental Protection Agency. (2009). Vertical Distribution o f VOCs in S

oils fr om
Groundwater

t o the Sur

face/Subslab. Office of

R esearch a nd

D evelopment N ational Exposure Research Laboratory. Available on-l ine a t: https://clu

in.org/download/contaminantfocus/vi/Vertical%20Distribution%20of%20VOCs.pdf

16. United States Environmental Protection Agency. (2008). Quality System a nd T echnical P rocedures for SESD Field Branches. EPA Region 4. Available on-l ine a t: http://www.epa.gov/quality/quality

system -and-t echnical -procedures-s esd -f ield -branches

17. United States Environmental Protection Agency. (2009). Vapor Intrusion Framework (Interim Final). Region 3. Hazardous Site Cleanup Division.

18. United S tates E nvironmen tal Protection Agency. (2010). EPA Region 9 's "RARE" O pportunity 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 S tates E nvi ronmental Protection Agency. (2010). Risk -Based Screening Table – G eneric
Tables. A vailable on -l ine a t: http://www.epa.gov/risk/risk -b ased -s creening -t able -generic-t ables

20. United States Environmental Protection Agency. (2010). Vapor I ntrusion G uidebook

. Re gion 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. (E PA 5 30 -R-10-001). Office of Solid Waste and Emergency Response.

22. United States Environmental Protection Agency. (2011). Beneficial E ffects of t h e Superfund Program. (EPA C ontract E P W -07-037). Office of Superfund Remediation and Technology Innovation.

23. United States Environmental Protection Agency. (2011). Exposure Factors Handbook: 2011 Edition. (EPA 6 00 -R-090-052F). Office of Research and Development.

24. United States Environmental Protection Agency. (2011). Intensive Study of COVs and Radon in a Historical

D uplex - 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 S tates E nvironmental Pro tection Agency. (2011). Potential Addition of Vapor Intrusion Component to the Hazard Ranking System. (EPA -H Q -S FUND -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 Intrusio n ( VI): E PA's Experiences with Analogous Gases. Proceedings to U SEPA Wo rkshop: A ddressing R egulatory C hallenges. A EHS
West Coast Conference. Presented by Schuver, H. March 15, 2011. San Diego CA.

28. United States Environmental Protection Agency. (2011). VI + H RS + EXPO SURE = S

HIVER. Region 2. Presentation by Sivak, M.

29. United States Environmental Protection Agency. (2011). VI A ssessment

& M itigation D ecisions. 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. Proceeding s to U SEPA Wo rk shop: A ddressing R egulatory C hallenges. A EHS West Coast Conference. Presented by Dawson, H. March 15, 2011. San Diego CA.

31. United States Environmental Protection Agency. (2012). Agen cy for Toxic Substances and D isease Reg istry National Priorities L ist (NPL) S ites. Accessed on May 2013, at: http://www.epa.gov/superfund/agency-toxic-substances-and-d isease -r egistry -a tsdr -national-p riorities

list -npl-s ites

32. United States Environmental Protection Agency. (2012). Assessing P rotectiveness at

S it es for Vapor Intrusion: Supplement to the “Comprehensive Five-Year Review Guidance.” Accessed at: http://www.epa.gov/superfund/superfund-f ive -y ear -r eviews -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 . D r. 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 O SRIT, a s c oauthor.

34. United States Environmental Protection Agency. (2012). Screening L evel I mplementation o

f the Johnson and Ettinger Vapor Intrusion Model. EPA o n -l in e t ools for site assessment and ca lculation. Available on -l ine a t: http://www.epa.gov/athens/learn2model/part-t wo/onsite/JnE_lite.html

35. United S tates E nvironmen tal Protection Agency. (2012). State-Designated Top-Priority National
Priorities List

( NPL) S ites. 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 S ites. A EHS Wes t C oast 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 U SEPA Wo rk shop: V ap or 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 t o U SEPA Workshop: Vapor Intrusion Stakeholder -Involvement Forum: Improving Communicati on, O utreach, and Decisions at Vapor Intrusion Sites. Presented by Horton, J. March 18, 2013. San Dieg o C A.

39. United States Environm ental 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. U nited 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 i n V ap or Intrusion: Understanding, Controlling, and Addressing Site Variables for Improved Practices and Effective Protection Strategies. AEHS West Coast Conference. Presented by Kurtz, J. Marc h 19,
2013. S an Diego CA

.

41. U nited 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 Ef fective Protection Strategies. AEHS West Coast Conference. Presented by Dawson,
H . M arch 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, 2 013. S an 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, 2 013. S an Diego CA.

44. United States Environmental Protection Agency. (2013). Living with Variability: Development o f
E

ffective and Protective Sampling Strategies

. Proceedings to USEPA Workshop: Looking Bey ond
N atural 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. Mar ch 19, 2013. San Diego CA

.

45. U nited States Environmental Protection Agency. (2013). Multi -Year Monitoring of a House Over a D ilute CHC Plume: Implications for Pathway Assessment Using Indoor Air Sampling and Forced Under- Pressurization Tests

. Proceedings to USEPA Workshop: Looking Beyond Natural Variation i n V apor Intrusion: Understanding, Controlling, and Addressing Site Variables for Improved Practices and Effective Protection Strategies. AEHS West Coast Conference. Presented by Holton, C. Marc h 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 and142 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
Statisti cs fo r Indoor Air Concen trations from 2 S ites with L arge Data S ets. Proceedings t o U SEPA Workshop: Looking Beyond Natural Variation in Vapor Intrusion: Understanding, Controlling, and Addressing S it e Variables for I mproved

P ractices and Effective Protection S trategies. A EHS W est 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 S trategies. A EHS W est C oast C onference. P resented b y S teck, D

. Mar ch 1 9, 2013. San Diego CA.

49. United S tates E nvi ronmental Protection Agency. (2013). V apo r Intrusion A cro ss Space and T ime: A Pilot Community Study in San Antonio. Proceedings to USEPA Workshop: Looking Beyond Natural Variation in Vapor Intrusion: Understanding, Controlling, and Addressing Site Variables fo r Improved P ractices and

E ffective Protection S trategies. A EHS West C oast C onference. P resented b y 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

S olid Waste and

E mergency R esponse (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. O ffice 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 Petroleu m Vapor Intrusion at Leaking Underground Storage Tank Sites. O ffice 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)

. A ccessed

f rom: 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. A ccessed

f r om: http://www.epa.gov/iris/help_ques.htm

.143 10.2 Other Federal Agency Resources

57. Agency for Toxic Substances and Disease Registry. (2011). Public Health A ssessmen t f or W.R. G ra ce Superfund Site. (CERCLIS NO: MAD001002252)

. U.S. D epartment o f H ealth an d

H uman S ervices Public Health S ervice.

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 A irforce Ba se VI P rogram.

60. Naval F acilities Engineering C ommand. ( 2011). Guidance for Environmental background Analysis. Volume IV: Vapor Intrusion Pathway. A Users Guide.

61. Naval F acilities Engineering C ommand. ( 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 Secur ity T echnology Certification Program ( ESTCP). A vailable o n -l ine a t : http://www.frtr.gov/pdf/FRTR-2010-f actsheet.pdf

63. United States Department of Defense (DoD). (2009). Vapor Intrusion Handbook. Prepared by the TriService Environmental R isk A ssessment Wo rkgroup.

64. United States Government Accountability Office. (2010). Superfund, EPA's Estimated Costs to Remediate Existing

S ites Exceed

C urrent F unding L evels, a nd Mo re Sit es are Expected t o be Added

to th e N ational Priorities L ist. Report to Congressional Requesters. (GAO -10-380).

10. 3 Other Published References

65. Associa tion 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 -55.144

71. Brown, R. D. (1986). Hazard Ranking System Issue Analysis: Use of Significance in Determining Ob served R elease. (Contract

NO: E PA -68-01-7054.)

72. Burger, C.K., and Kushner, L.M. (1986). Hazard Ranking System Issue Analysis: Target Distance
Lim it for S urface W ater. (Contract

NO: E PA -68-01-7054.)

73. Carr, D.B., Levy, L.C., Horneman, A.H., (2011). Stylistic Modeling of Vadose Zone Transport. Insight into V

a po r Intrusion p rocesses. 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 Prioriti es List Technical Data Base. (Contract No: 68-01-6278.)

75. Chrostowski, P. (1986). Risk Assessment a nd the H

azard R anking S ystem.

76. Clark, J. N., Goodwin, D., O'Neill, H. and Odencrantz. J.E. (2008). Application of Passive Soil Gas Technology to Determine the Source and Extent of a PCE Groundwater Plume in an Urban
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 Su bsurface Diesel

P lume Using Multiple Lines of E vidence. 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

. A vailable online a t: http://airsampling.com/instructions/1786.pdf

79. Crook, A. and Morrison, B. (1987). Overview o f the R emedial

A ction P riority 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 V apor I ntrusion from S

oil or Groundwater to Indoor Air: Significance of Unsaturated Zone Biodegredation of Aromatic Hydrocarbons. Soil and Sediment Contamination, 11(4): 625-641.

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-O rder Rate
Constants. Proceedings of the Petroleum Hydrocarbons and Organic Chemicals in Groundwater Conference: Prevention, Detection and Remediation. November 12-14, 1997, Houston, TX. Ground
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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: First

Order 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 select ed a reas of t he temperate zone. Tellus B, 39B: 114–121.145 85. E lliot, 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. Proceedi ngs of the Air and Waste Management Association Specialty Conference: Vapor Intrusion. January 27

- 30, 2009.

86. E nvironmental security Technology Certification Program. (2008). Final Report: Detailed Fiel d
I

nvestigation of Vapor Intrusion Processes. (ESTCP P roject 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. F itzpatrick, 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. F olkes, 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. Present ation from National Environmental Monitoring Conference (NEMC)

, A ugust 10-16, 2008, Washington, DC

.

93. G erstein, R. E. (1986). Hazard Ranking System Issue Analysis: Geohydrology

.

94. G oldfarb, A.S., Malone, G.A. and Vogel, G.A. (1992). Remedial Technology Design, Performance

, and C ost Study.

95. Gossett, J.M. 2010. Sustained Aerobic Oxidation of Vinyl Chloride at Low Oxygen Concentrations

. E

nvironmental Science & Technology 44(4): 1405

- 1411.

96. H aus, S.A. (1987). Hazard Ranking System Issue Analysis: Containment Rating Factor

.

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