1 

## Technical Support Document for U.S. EPA’s Proposed Rule for Addition of a Subsurface Intrusion Component to the Hazard Ranking System 

# Appendix E: Summary of Peer Review Comments and Responses2 

## Summary of Peer Review Comments and Response s on t he Addition of a Subsurface Intrusion Component to the HRS

Table of Contents

Terms and Abbreviations ........................................................................................................................................5

Executive Summary

................................................................................................ ..................................................6

1.0 Introduction ..................................................................................................................................................7

Hazard Ranking System .........................................................................................................................................7
Subsurface Intrusion Component ...........................................................................................................................8
Organization of Comments and Responses ............................................................................................................9

2.0 Peer Review Comments on Charge Questions and EPA’s Response .....................................................10

2.1 Charge Question #1: .................................................................................................................................10 2.1.a Equation for Potential for Exposure

..................................................................................................................... 11 2.1.b Structure Containment

................................................................................................ .......................................... 12

2.1.b.1 Structure Containment Factors

.................................................................................................................... 12 2.1.b.2 Scientific Basis for Structure Containment Categories

............................................................................... 13 2.1.b.3 Consideration of Biogases and Inert Gases in Determining Structure Containment

................................... 14 2.1.c Depth to Contamination

................................................................................................ ....................................... 14

2.1.c.1 Definition of Depth to Contamination

......................................................................................................... 14 2.1.c.2 Scientific Basis for Depth to Contamination Factor

.................................................................................... 15 2.1.c.3 Depth and Probability of Intrusion

.............................................................................................................. 16 2.1.c.4 Method Used for Assigning Depth to Contamination Weighting Values

................................................... 17 2.1.d Vertical Migration

................................................................................................ ................................................ 17

2.1.d.1 Use of Term “Effective Porosity/ Permeability”

......................................................................................... 17 2.1.d.2 Effective Porosity/ Permeability Categories

............................................................................................... 18 2.1.d.3 Scientific Basis for Vertical Migration Weightings

.................................................................................... 18 2.1.d.4 Effect of Soil Type and Moisture Con tent on Vertical Migration Values

................................................... 19 2.1.d.5 Hydraulic Pumping

................................................................................................ ..................................... 19 2.1.e Vapor Migration Potential

................................................................................................ .................................... 20 2.1.f Biodegradation/Bioattenuation

............................................................................................................................. 20 2.1.g Consideration of Volatile Substances

................................................................................................................... 21

2.2 Charge Question #2: .................................................................................................................................22 2.2.a Support for Method

................................................................................................ .............................................. 22 2.2. b Considering Waste Quantity above Areas of Subsurface Contamination

............................................................ 22 2.2.c Considering Waste Quantity of Multiple Structures

............................................................................................ 22 2.2.d Including Subsurface Contamination in the Waste Quantity

................................................................................ 23 2.2.e Minimum Waste Quantity

................................................................................................ .................................... 23 2.2.f Relevance of Waste Quantit y

............................................................................................................................... 23 2.2.g Consideration of Source Concentrations in Estimating Waste Quantity

.............................................................. 24 2.2.h Relative Risk of Airborne Contamination

............................................................................................................ 25 2.2.i Biodegradation and Waste Quantity

..................................................................................................................... 253 2.2.j Impact of Indoor Sources on Waste Quantity

...................................................................................................... 26 2.2.k Applicability of the Hazardous Waste Quantity Calculation

................................................................................ 26 2.2.l Consider Future Structures

................................................................................................ ................................... 26 2.2.m Adequacy of Site Ins pection Data to Identify Subsurface Contamination

........................................................... 27 2.2.n Need for Discussion and Reference Support in TSD

........................................................................................... 27

2.3 Charge Question #3: .................................................................................................................................28 2.3.a Support for Weighting Targets

............................................................................................................................. 28 2.3.b Appropriate Benchmark Risk (Cancer and Non

- Cancer)

..................................................................................... 28

2.3.b.1 Number of Eligible Sites

............................................................................................................................. 29 2.3.b.2 Benchmark and Background (Understanding Overall HRS Process)

......................................................... 29 2.3.b.3 Cumulative Eff ects and Benchmarks

.......................................................................................................... 30 2.3.b.4 Weighting Based on Exceeding Benchmark

............................................................................................... 30 2.3.b.5 Benchmark Levels in Crawl Spaces

............................................................................................................ 31 2.3.c Target Weighting

................................................................................................ .................................................. 32

2.3.c.1 Strength of Weighting Assumptions

........................................................................................................... 33 2.3.c.2 Weighting Based on Benchmark Comparison

............................................................................................ 34 2.3.c.3 Weighting by Subsurface Conditions

.......................................................................................................... 34 2.3.c.4 Weighting by Sample Type

......................................................................................................................... 35 2.3.c.5 Weighting Factor When Not Ten Times Benchmark

.................................................................................. 35 2.3.c.6 Weighting Based on Subslab Samples and Ground Water Samples

........................................................... 36 2.3.c.7 Weighting Factors for Crawl Space Samples and Subslab Samples

........................................................... 36 2.3.d Cons istency of Weighting Factors with Database/Guidance

................................................................................ 36

2.3.d.1 Inconsistency of Attenuation Factors

.......................................................................................................... 37 2.3.d.2 Analyzing Raw Data to Determine Weighting Factors

............................................................................... 37 2.3.e Consideration of Biodegradation of Hazardous Contaminants in Weighting Targets

.......................................... 37 2.3.f Eval uating Vacant/Non

- occupied Structures

........................................................................................................ 38 2.3.g Evaluating Future Targets

................................................................................................ .................................... 38

2.4 Charge Question #4 ..................................................................................................................................39 2.4.a General Support for the Approach

....................................................................................................................... 39 2.4.b Consider Preferential Pathways Between Floors

.................................................................................................. 39 2.4.c Inferring Level I Contamination

........................................................................................................................... 40 2.4.d Consider Discontinuous Contamination

............................................................................................................... 40 2.4.e Consideration of Lower Floors

............................................................................................................................. 40 2.4.f Weighting Based on Basement Samples

.............................................................................................................. 41 2.4.g Evaluating Worker Exposure in Multi

- Story Buildings

....................................................................................... 41

2.5 Charge Question #5 ..................................................................................................................................43 2.5.a Support for Worker Factor Value

......................................................................................................................... 43 2.5.b Indoor Air Exposure and Worker Risk

................................................................................................................. 43 2.5.c Rationale for Worker Factor Value

...................................................................................................................... 44 2.5.d Worker Factor Value Correlation

......................................................................................................................... 44 2.5.e Worker Benchmark Values

................................................................................................ .................................. 44

2.6 General Comments Affecting All Charge Questions ...............................................................................45 2.6.a Definitions

................................................................................................ ............................................................ 45 2.6.b Consistency with Vapor Intrusion Guidance

........................................................................................................ 45

3.0 Comments Outside the Scope of the Charge ............................................................................................46

3.1 Technical Comments Outside the Scope of the Charge ...........................................................................46 3.1.a General Comments

................................................................................................ ............................................... 46 3.1.b Likelihood of Exposure/Release

........................................................................................................................... 46 3.1.c Waste Characteristics:

................................................................................................ .......................................... 474 3.1.d Targets

................................................................................................ .................................................................. 47

3.2 Policy Comments Outside the Scope of the Charge .................................................................................47

4.0 Editorial Comments ..................................................................................................................................48

5.0 List of Commenters ....................................................................................................................................485 

## Te rms and Abbreviations

The following acronyms and abbreviations are used throughout the text:

AO E Area of Observed Exposure ASC Area of Subsurface Contamination CERCLA Comprehensive Environmental Response, Compensation, and Liability Act of 1980, 42 U.S.C. Sections 9601 et seq ., also known as Superfund EPA U.S. Environmental Protection Agency , also USEPA FR Federal Register HI Hazard Index HRS Hazard Ranking System, Appendix A of the NCP HVAC Heating, Ventilation, and Air-Conditioning System NCP National Oil and Hazardous Substances Pollution Contingency Plan, 40 C.F.R. Part 300 NPL National Priorities List, Appendix B of the NCP OSWER Office of Solid Waste and Emergency Response OLEM Office of Land and Emergency Management OUST Office of Underground Storage Tanks RfC Reference Concentration SAB Science Advisory Board SsI Subsurface Intrusion TSD Technical Support Document VOC Volatile Organic Compounds6 

## Executive Summary
The U.S. Environmental Protection Agency (EPA) is proposing to add a subsurface intrusion (SsI) component to the Hazard Ranking System (HRS) . The HRS is the princip al mechanism that EPA uses to evaluate sites for placement on the National Priorities List (NPL)

. This addition will a dd to the HRS the consideration of the threat posed by exposure to hazardous substances, pollutants or contaminants 1 that enter regularly occupied structures through subsurface intrusion

. The agency is not considering changes to the remainder of the HRS except for minor updates reflecting changes in terminology.

D uring development of this proposed HRS update, external independent scientific peer review of proposed approaches for assigning HRS scores to specific situations at sites being evaluated was performed . For each of these proposed approach es the agency developed a specific charge question for consideration by the peer reviewers:

• Charge question 1 addressed the calculation of the potential for exposure factor value.

• Charge question 2 addressed the calculation of the hazardous waste quantity factor value.

• Charge question 3 addressed the evaluation of targets in areas of subsurface contamination.

• Charge question 4 addressed the evaluation of targets and hazardous waste quantity in multitenant buildings.

• Charge question 5 addressed the evaluation of worker targets.

T his document explains the results of this external independent scientific peer review , including summaries of the peer reviewers’ comments, grouped according to the relevant charge question, and the responses to those comments. Many of the peer review comments were supportive of the methods used in the proposed addition, but requested modifications to specific aspects of the evaluation methodologies. In particular, changes were made in the evaluation of structure containment used in calculating the potenti al for exposure factor value, in the calculation of hazardous waste quantity to include occupied structures in areas of subsurface contamination, and in the weighting of targets in areas of subsurface contamination to reflect the type and depth of contamin ation instead of comparing concentrations to benchmarks. In addition, consideration of preferential pathways was emphasized in identifying targets in multi

- tenant 2 buildings. Furthermore, additional changes were made in response to peer review comments to clarify the intended approaches of the proposed addition. These changes are also described in the following sections discussing the comments on the aforementioned charge question s.

1 For the purpose of this document, the term “hazardous substances, pollutants or contaminants” will be referred to simply as “hazardous substances.” See section 1.1, of the 1990 HRS for the definition of a hazardous substance. 2 The proposed subsurface int rusion SsI addition to the HRS has undergone revision since it was submitted for peer review in 2011. The current version of the proposed SsI addition has replaced the term “multi

- tenant” with “multi

- subunit”. This appendix reflects the use of “multi

- tenan t” in charge questions and peer review comments since that was the language used at the time of the peer review. In responses the new, “multi

- subunit” term is used for consistency with the proposed addition. This change in terminology has no effect on the responses.7 

## 1.0 Introduction

The U.S. Environmental Protection Agency ( EPA ) is proposing to add a subsurface intrusion ( SsI ) component to the Hazard Ranking System ( HRS ), which is the princip al mechanism that EPA uses to evaluate sites for placement on the National Priorities List ( NPL ). This addition will a dd to the HRS the cons ideration of the threat posed by exposure to hazardous substances, pollutants or contaminants that enter regularly occupied structures through subsurface intrusion

. The agency is not considering changes to the remainder of the HRS except for minor updates reflecting changes in terminology.

T his proposed rule consists of narrow technical modifications to, and an expansion of the scope of, the 1990 HRS. The 1990 HRS was previously peer reviewed by the agency’s Science Advisory Board (SAB). The 1988 SAB revie w was comprehensive in scope and addressed the basic structure and concepts of the HRS. This proposed addition adheres to the basic structure and concepts of the 1990 HRS, and thus, is consistent with the recommendations of the SAB at that time

. The 1988 S AB report ( Review of the Superfund Hazard Ranking System ) is available in the public docket for this proposed rulemaking.

D uring development of the proposed addition of an SsI component to the HRS, several of the proposed approaches for evaluating and as signing HRS factor values warranted external independent scientific peer review. These approaches selected for review either represent the application of scientific concepts not addressed in the 1990 HRS or approaches for which the scientific understanding of has improved since 1988. The review has been utilized to ensure the proposed addition of an SsI component to the HRS has a sound and credible basis.

A peer review panel consisting of six individuals from the academic, government, and private sectors representing a range of expertise in public health risk assessments, contaminated soil and ground water investigations and the topic of vapor intrusion was assembled to evaluate these questions and provide comments to the EPA. The charge questions dealt with the following topics:

• C harge question 1 addressed the calculation of the potential for exposure factor value.

• Charge question 2 addressed the calculation of the hazardous waste quantity factor value.

• Charge question 3 addressed the evaluation of targe ts in areas of subsurface contamination.

• Charge question 4 addressed the evaluation of targets and hazardous waste quantity in multi-tenant buildings.

• Charge question 5 addressed the evaluation of worker targets.

F or each element above, a specific charge question was developed for consideration by peer reviewers (these charge questions are detailed in section 2, Peer Review Comments on Charge Questions and EPA ’s Response, of this response document). The proposed addition reflects modifications made as a re sult of EPA ’s peer review process. Furthermore, the agency notes that the development of this proposed addition to the HRS underwent additional internal review by workgroup participants engaged in developing the proposed addition, EPA ’s Office of Research and Development , agency senior management and the Department of Defense. 

### Hazard Ranking System

The HRS is the princip al mechanism used to place uncon trolled waste sites on the NPL. It is a numerically based screening system that uses information from init ial, limited investigations (e.g., preliminary assessment s and site inspection s) to assess the relative potential of sites to pose a threat to human health or the environment. Placing a site on the NPL does not mean the site will require remediation only t hat based on the limited data available, the site poses sufficient risk to warrant further investigation . For further information see the preamble to the proposed addition to the HRS. Under the 1990 HRS, four pathways can be scored , which include ground water migration, surface water migration, soil exposure, and air migration. The HRS uses a structured value analy sis approach to8 sco ring sites. This approach assigns numerical values to factors that relate to or indicate risk, based on conditions at the site. The factors are grouped into three categories:

• L ik elihood of Release: likelihood that hazardous substances have been released or ha ve the potential to be release d into the environment;

• Waste Characteristics: characteristics of the waste ( e.g., toxicity and waste quantity) that reflect toxicity and mobility of the released substances and the magnitude of the release; and

• Targets : people or sensitive environments (receptors ) actually or potentially affected by the release.

T he to tal score for a site is the sum of all pathway scores. For further detail on the HRS scoring algorithm see Section 2 of the 1990 HRS (55 FR 51587, December 14, 1990 ). 

### Subsurface Intrusion Component
The current approach for scoring the ground water, surfa ce water or air migration pathways is not being altered by the proposed addition of a subsurface intrusion component. EPA is proposing to add the subsurface intrusion threat to the present (1990) soil exposure pathway, as this is the pathway that reflects the risk posed by direct exposure to targets. This pathway is proposed to be restructured and renamed the soil exposure and subsurface intrusion pathway. The restructured pathway will retain the existing two soil exposure threats (resident population and nearby population) in the pathway as one component. The threat posed by subsurface intrusion is proposed to be added as a new component (a s Section 5.2 of the HRS).

T he present soil exposure pathway addresses direct contact with contamination outside of structures. The ne w subsurface intrusion component also addresses direct contact with contamination that has already been demonstrated to have entered into regularly occupied structures or where the contamination is present beneath the regularly occupied structures and is l ikely to enter or likely has already entered

.
T he proposed subsurface intrusion component will be added as a new component of the restructured soil exposure and subsurface intrusion pathway. The scoring algorithm of the soil exposure component will remai n the same as for the present soil exposure pathway , but the score will be assigned as a soil exposure component score, not a pathway score. The score assigned to the two components will be summed to produce the pathway score.

T he draft subsurface intrus ion component , at the time of the peer review process, was proposed to be evaluated using the three factor categories that are also used in the 1990 HRS pathways: likelihood of exposure (also called likelihood of release for migration pathways), waste char acteristics, and targets. These three factor categories , as incorporated in the current proposed HRS addition, include factor values, many of which are similar to those used in the 1990 HRS pathways and are outlined below:

• L ikelihood of exposure

o Observed exposure : O bserved exposure may be established by “direct observation” based on a solid, liquid or gaseous hazardous substance attributable to the site being observed or known t o h ave entered a regularly occupied structure from the subsurface (e.g., documented by sampling of the medium as it intrudes). Or, observed exposure may be established by “chemical analysis ” b ased on a significant increase in a hazardous substance (relative to the background level) measured in an indoor air sample taken from a regularly occupied structure
, and that at least part of the significant increase can be shown to be attributable to a release from the site being evaluated

.

o Potential for exposure : Potential for exposure may be evaluated for areas in which subsurfa ce contamination underlying regularly occupied structures (such as in surficial ground water or soil9 v apor) has been documented, but at which either sampling has not documented that subsurface contamination has entered an occupied structure or no sampling of indoor air has been undertaken . This evaluation assess es the probability of exposure based on factors such as the structural containment features of the occupied structure and the route characteristics within the subsurface.

• Was te characteristics

o Toxicity: The toxicity factor reflects the toxicity of a hazardous substance associated with a release or exposure , and considers the magnitude of each hazardous substance’s acute and chronic toxicity to humans.

o Degradation: The degradat ion factor reflects the potential for a substance to degrade within the sub surface prior to intruding into an overlying structure.

o Hazardous w aste q uantity : The hazardous waste quantity factor reflect s the amount of hazardous substances that people are currently or may be exposed to (i.e., the amount in regularly occupie d

s tructures ).

• T argets
o Exposed Individual : The reasonably maximally exposed individual factor reflects a person living
, attending school or day care , or working on a re gular basis in a structure with observed exposure
, or living in a structure within an area of observed exposure (AOE) and/ or area of subsurfac e
c ontamination (ASC) . The assigned value reflects individuals in the eligible target population tha t
ar e expected to be exposed to the highest hazardous substance concentration of the released hazardous substance (s)

.

o Population: The population factor reflects the number of people qualifying as an expose d individual or a person living
, working, or attending school or day care in a st ructure within an AOE and/ or ASC ; this factor assigns a higher weight to a population exposed to hazardous substances present , or likely to be present, at higher concentrations

.

o Resources : The resources factor reflects the threat posed to populations regularly using structures
, but not accounted for in other target categories. (e.g., populations using librar ies , church es , triba l f acilit ies, recreational structures ), located in an AOE or ASC

. 

### Organization of Comments and Responses

The remainder of this summary document is organized into four sections.

• Section 2: Peer Review Comments on Charge Questions and EPA’s Response. This section contains EPA responses to comments on subsurface intrusion

- specific issues generated by the external independent scientific peer review ; these comments and responses are grouped into six subsections (2.1 through 2. 6)

.
• S ection 3: Comments Outside the Scope of the Charge . This section identifies those comments on topics not included in the charge questions (e.g., comments on the impact of the proposed addition an d

c omments on changes to HRS Sections 2 and 5).

• Section 4: Editorial Comments.

• Section 5: List of Commenters. This section li sts the peer reviewers and their credentials.

T he actual charge question submission and accompanying documents and the submissions with the peer reviewers’ annotated comments are included in the public docket for the proposed SsI addition to the HRS.10 

## 2. 0 Peer Review Comments on Charge Questions and EPA’s Response
The following discussion is a summary of the peer review comments and the agency’s responses to each of the five charge questions submitted for peer review. Comments received that were applicabl e to each individual charge question are addressed in the subsections below . In cases where one comment could be considered applicable to more than one charge question, the comment has been addressed in the section where it was raised by the peer reviewer, and the other applicable subsections have been noted.


### 2.1 Charge Question #1:

Is the Environmental Protection Agency’s (EPA’s ) proposed approach scientifically reasonable for evaluating and assigning proportional weightings for the following when evaluating potential for exposure? If no, what alternative would you recommend?

a. Cons ideration of the construction/structure type for which sufficient site -specific informati on
c ould be collected during a site inspection for the occupied structure;

b. D istance between occupied structure and depth of contamination;

c . E valuation of the vertical migration factor based on the thickness and porosity/permeability o f ge ologic materials in the interval between the lowest point of an occupied structure and the highest known point of hazardous substances in the subsurface; and

d. A lternative approaches.

C harge question #1 is on the procedure for assigning the potential for exposure factor of the proposed subsurface intrusion component addition to the HRS. The potential for exposure factor is one of two ways to assign a value for the likelihood of exposure category. The likelihood of exposure refle cts the probability that an individual may be exposed to hazardous substances via subsurface intrusion into regularly occupied structures. If such an exposure has been documented (observed exposure) a value of 550 is assigned. If an observed exposure is no t documented, the potential for exposure is evaluated. Potential for exposure is based on the physical features of the structures and the subsurface materials as well as the characteristics of the volatile hazardous substances at the site being evaluated. More specifically, the p otential for exposure at a site is evaluated at each area of subsurface contamination (ASC), which are areas in which subsurface contamination underlying regularly occupied structures (such as in surficial ground water or soil vapor ) has been documented, but no observed exposure has been documented.

T his evaluation includes assessing the probability of exposure in the ASC based on factors such as structural containment features of the occupied structure and the route characteristics within the subsurface. The specific factors evaluated for potential for exposure include: structure containment, depth to contamination, vertical migration, and vapor migration potential. The equation for combining these factors to produce a potential for exposure factor value (subject to a maximum of 500) is:

a x (b+c+d)

a = Structure containment factor value (Subject to a maximum of 10) b = Depth to contamination factor value (Subject to a maximum of 10)

c = Vertical migration factor value (Subject to a maximum of 15) d = Vapor migration potential factor value (Subject to a maximum of 25)11 T his approach includes several factors for which sufficient site -specific information could be collected during a site inspection representing the g reatest impact on the potential for exposure: structure containment, depth to contamination, vertical migration, and vapor migration potential. T he 1990 HRS evaluates the potential for a release to occur within the ground water, surface water and air migra tion pathways. The potential for a release is determined in th ese pathway s by the consideration of pathway specific factors that aid the site evaluator in predicting whether a release is likely to occur.

S pecific comments regarding the factors selected f or use in assigning a potential for exposure factor value are addressed in the following sections:

• 2.1.a Equation for Potential for Exposure

• 2.1.b Structure Containment

• 2.1.c Depth to Contaminati on

• 2.1.d V ertical Migrati on

• 2.1.e Vapor Migration Potent ial.

2. 1.a Equation for Potential for Exposure
Comment(s) : Peer reviewers commented that the draft subsurface intrusion ( SsI ) addition to the HRS uses incorrect or inconsistent factors when evaluating potential for exposure

. Peer reviewers questioned:

• Wh y the factors used to evaluate the potential for exposure factor were selected. For example, th e
r eviewers also commented that the vapor migration potential should consider more factors than just vapor pressure and Henry’s constant to better evaluate the greatest health threat at the site.

• The rationale for the relative weightings amongst the different factors.

• Why the potential for exposure equation was selected.
Response : The factors selected for 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 a Hazard Ranking System ( HRS ) evaluation is likely to be performed. The potential for exposure facto r value reflects the probability that vapor intrusion into a regularly occupied structure will occur, but it does not directly reflect the concentration of the hazardous substances that may intrude . The projection of actual indoor concentrations cannot be accurately predicted based on information typically available at sites being evaluated using the HRS ( i.e., the site inspection stage ).

T he four factors included in the potential for exposure assessment are : a structure containment factor, a depth to cont amination factor, a vertical migration factor and a vapor migration potential factor. The structure containment factor reflects whether a volatile substance can enter a regularly occupied structure after t he substance has migrated from the subsurface to th e base of the structure. The assigned containment factor depends primarily on the presence of structural features (e.g., preferential pathways such as cracks in a foundation) and mitigation actions that have been taken (e.g., active or passive ventilation systems)

. Two of t he other three factors (depth to contamination and vertical migration) reflect the possibility of migration of the subsurface contamination to the base of the regularly occupied structure and are mainly dependent on the physical environment. T he depth to contamination factor reflects the distance a hazardous substance has to travel to reach the regularly occupied structure ; the vertical migration factor reflects the presence of vapor migration routes in the subsurface and reflects the effective porosity of the lowest conductive layer between the contamination and the structure
, and the thickness of this layer

. The fourth fac tor, the vapor migration potential factor, reflects the volatility of the hazardous substances and thus their ability to migrate as a vapor or gas.

R egarding the relative weightings of the different factors, these weightings were assigned based on a sens itivity analysis evaluating the importance of these factors in modeling vapor migration through the vadose zone using1 2 t he Johnson and Ettinger ( J&E ) Vapor Intrusion Model. This model was selected because of its general acceptance as a reasonable predictor for subsurface vapor migration, although it may not accurately reflect sitespecific vapor intrusion rates. The accuracy of the projection may not be high on a site- spec i fic basis but the model results are sufficient to reflect the relative likelihood of i ntrusion occurring at different locations

.
R egarding the structure of the formula for combining the factors into a single value, the structure is that of a first order transport equation and is used in all of the HRS pathways. The general form of the form ula is used to determine potential for a hazardous substance to migrate to targets ( and to the receptors associated with the target ) through various media. The formula was reviewed and found acceptable for this purpose by the Science Advisory Board ( SAB ) when the HRS was revised in 1990; it has proven to be an effective and reasonable approach.

T hese comments result in no change to the proposed addition to the HRS.

2. 1.b Structure Containment
A structure containment value is assigned to a regularly occup ied structure to indicate the likelihood for subsurface vapors to enter a structure once they have migrated through the subsurface to the base of the structure

. The lower the assigned value, the less likely subsurface intrusion will occur. The draft SsI ad dition provided to the peer reviewers included a table with structures having different levels of containment. The table identifies containment factor values ranging from 0 to 10, with 0 corresponding to total containment ( no intrusion likely)

. For example, a regularly occupied structure with an engineered, active vapor mitigation system with documented institutional controls or evidence of regular maintenance and inspection will be assigned a lower structure containment value than a regularly occupied structure with open preferential pathways from the subsurface.

T his section covers the following topics:

• 2.1.b.1 Structure Containment Factors

• 2.1.b.2 Scientific Basis for Structure Containment Categories

• 2.1.b.3 Consideration of Biogases and I nert Gases in Determining Structure Containment

2.1.b .1 Structure Containment Factors

Comment (s) : Several peer reviewers requested that Table 5 -12, Structure Containment, of the draft SsI addition to the HRS be revised to include additional information related to factors that can influence the evidence of structure containment and the related assigned value. Review ers also suggested that structure containment categories in Table 5

- 12 be revised to represent a broader range of structures that could be enc ountered, as typical home construction includes containment structure values that fall between the structure containment values represented in the Table.
Response: In response to peer review comments
, EPA has modified Table 5 -12, Structure Containment, in the proposed addition to the HRS

. While the structure containment table has been revised to reflect peer review comments, given the wide variety of possible building structures encountered
, site

- specific conditions will always have to be considered and addressed as they arise . However, a new category has been added to the table and other categories were modified. A new C ategory No. 8 has been added to include regularly occupied structures with unknown containment features and is assigned a value of 2 out of a possible 10. This value was assigned because,

as noted in peer reviewer comments , recent studies show that most structures located over a significant subsurface contaminant plume are likely to experience subsurface intrusion unless they have specific design features that prevent it. Additionally structure containment categories in Table 5 -12 have been modified to not consider differences in construction type ; the peer reviewers’ indicated that s tudies have not found construction type to be a discrimin ating factor in whether subsurface intrusion can occur. However, regularly occupied structures with a foundation raised greater than six feet above ground still receive a structure containment factor value of 0.13 F urther, given the large number of possible combinations of containment features, it is impossible to include a single category in the table for each possible combination type. Therefore, interpretation of the table will be based on site- specific conditions. If a structure appears to fit multiple ca tegories, the highest applicable structure containment factor value is assigned.

2.1.b .2 Scientific Basis for Structure Containment Categories

Comment (s) : One peer reviewer suggested that containment features in Table 5 -12, Structure Containment, of the draft SsI addition to the HRS may not be as protective as the assigned values suggest and commented that, in the absence of definitive studies that show the effectiveness of containment features, Table 5 -12 should be reduced to three structure containment values. The reviewer specifically commented that EPA ’s Empirical Database does not show any significant impact of foundation type on indoor air quality.

A nother peer reviewer commented that the structure containment value assigned in Table 5 -12 to vented crawl spaces, which have fairly small attenuation factors, is too high and should be lowered.
Response : In developing the proposed structure containment table, after a review was done of available literature, EPA identified multiple structural and mitigation features as possibly influencing the likelihood that volatile hazardous substances present at the base of a regularly occupied structure could enter into the structure. These features include: documented evidence that gases or vapors have been enteri ng the structure; construction characteristics such as the presence of a basement, a slab foundation, or an enclosed crawl space; 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 the systems are operating.

T he structure containment table submitted to the peer reviewers was developed using an approach that reflected the physical presence of features that theoretically could affect the occurrence of vapor intrusion. The higher the containment value assigned, the greater the likelihood that vapor intrusion would occur. Regular ly occupied

s tructures with evidence of intrusion or evidence of open preferential pathways a re assigned the highest value, and structures with no actual migration route from the subsurface to the structure or structures containing effective active mitigat ion systems are assigned a lower value indicating the low likelihood of intrusion. As stated by a peer reviewer, no studies that focused on documenting the impacts of the features individually or in combination were identified or reviewed in the creation o f Table 5 -12, and for that reason, among others, this table was submitted for peer review input.

EPA has modified the assigned values in Table 5 -12 of the proposed addition to the HRS regarding passive vapor mitigation systems . While there may be little published evidence on the effectiveness of passive systems, as these systems at least provide a possibly less resistant route for vapors to migrate, passive systems remain as a factor in the structure containment evaluation, and the agency considers their inclusion reasonable . The table has been modified to assign different values to regularly occupied structures depending on whether information is available to document that the passive systems are maintained, functioning or effective. The presence of a passive system alone does n ot result in the minimum containment value being assigned (i.e., that intrusion will not occur).

A dditionally, EPA has modified Table 5 -12 to not consider foundation type, except for those structures with foundations raised greater than 6 feet from the land surface or that have been built and maintained in a manner to prevent subsurface intrusion , when assigning a structure containment factor value. The agency also removed classifications based on structure type (e.g., structure with enclosed crawl space) . T his reflects that , as pointed out by the peer reviewers, there has been little consistent correlation between construction type (e.g., structure with a crawl space) and subsurface intrusion in existing studies

. Crawl spaces were considered as a possible special case, but the construction of crawl spaces and partially enclosed spaces and the efficiency of passive venting systems14 can be highly variable, therefore crawl spaces did not receive spe cific consideration in Table 5 -12 of the proposed addition to the HRS.

I nstead of structure type , consideration is given to the presence of passive/active vapor mitigation/vapor migration barrier systems and open preferential pathways from the subsurface

. (Not e that crawl spaces are often accompanied by openings to the structure by hatches, vents, etc., that would be considered preferential pathways.) As mentioned previously, s ite

- specific conditions will be addressed as they arise during an evaluation of the

s tructure containment factor value

.
Wh ile Table 5

- 12 has been modified based on peer review comments, certain features of the table have been retained and the number of structure containment categories was not reduced as suggested by one peer reviewer.

2.1.b .3 Consideration of Biogases and Inert Gases in Determining Structure Containment

Comment (s) : One peer reviewer questioned the usefulness of considering evidence of intrusion of biogases and inert gases, as referenced in Table 5 -12 of the draft SsI ad dition to the HRS, to document structure containment. The reviewer added that evidence of these intrusions is not typically gathered during short inspections.
Response : T he text associated with Category No.1 in Table 5 -12, Structure Containment, was modi fied to clarify that evidence of intrusion, including by bio or inert gases, provides support for evaluating a regularly occupied structure's containment value. EPA notes that evidence of intrusion by these gases (e.g., methane, radon) is often monitored in various regions of the country , and can be measured fairly rapidly using existing field equipment

. While this information may not always be readily available during a typical site inspection
, the agency considers it reasonable to use such information wh en it is available. When it is not available, the table identifies other features that can be used to assign a structure containment factor value.

2. 1.c Depth to Contamination
The depth to contamination factor represents the vertical distance between contamination ( e.g., in soil gas or surficial ground water) and the lowest horizontal point of a regularly occupied structure (e.g., a basement floor). This distance represents how far a hazardous substance would have to travel through the subsurface to in trude into that structure . This distance is proportional to the time during which the concentration of the vapors can be reduced through diffusion

. For example, a contaminated crawl space sample would be assigned the maximum factor value of 10, while a con taminated soil gas sample at a depth of 12 feet could receive a factor value of 8 (depending on the presence (or absence) of channelized flow features). Hence, the probability of significant exposure decreases as this depth increases. The proposed d epth to con tamination factor values range from zero to ten, where increasing depth results in a lower factor value. A value for depth to contamination is assigned based on the least depth to contamination underlying a regularly occupied structure.

P eer reviewers’ specific c omments on depth to contamination are addressed in the following subsections :

• 2.1.c .1 Definition of Depth to Contamination

• 2.1.c.2 Scientific B asis for D epth to Contamination F actor

• 2.1.c.3 Depth and Probability of Intrusion

• 2.1.c.4 Method U sed for Assigning Depth to Contamination Weighting Values

2.1.c .1 Definition of Depth to Contamination
Comment(s) : One peer reviewer requested that a clarification of the meaning of “depth to contamination” be provided in the draft SsI addition t o the HRS.15 Response: EPA has incorporated this suggestion by clarifying the meaning of "depth to contamination" in the proposed addition to the HRS . Depth to contamination is measured based on the distance between the lowest point of a regularly occupied structure to the highest known point of an eligible hazardous substance (see Section 5.2.1.1.2.2 of the proposed addition to the HRS ). Additionally, if the evaluation is based on semi- enclosed or enclosed crawl space samples, a depth to contamination fact or value of 10 is assigned. If more than one depth to contamination distance can be estimated, the lowest depth measurement is used in the HRS evaluation

.
2.1.c .2 Scientific Basis for Depth to Contamination Factor
Comment(s ): Several peer reviewers raised questions about the scientific basis for including a depth to contamination factor and questioned the associated range of assigned values . One peer reviewer commented that, due to the information likely to be available at most sites, the depth to co ntamination factor will likely be based upon depth to ground water; the reviewer asserted that modeling and empirical data ha ve not shown a correlation between the depth to ground water and the impact to vapor intrusion rates as it is suggested in Table 5 -13, Depth to Contamination, of the draft SsI addition to the HRS. Reviewers specifically commented on the values assigned to the depths to contamination in Table 5-13 and included the following comments:

• D epths to at least 50 feet in Table 5 -13 should all be assigned the same value as there is no evidence that there is a reduction in exposure potential below 20 feet

.

• T he maximum depth in Table 5-13 should be 100 feet and the assigned value should be 0 for contamination below 100 feet

. A dditionally, one p eer reviewer commented that the depth to contamination is not as important a factor in predicting subsurface intrusion as whether or not a chemical is aerobically biodegradable.
Response: The depth ranges selected for Table 5 -13, Depth to Contamination, of the proposed addition, were based on projecting indoor air contaminant levels using the J&E Vapor Intrusion Model 3 and data from EPA’s Vapor Intrusion Database. T he J&E Vapor Intrusion Model projects that depth to contamination is a significant factor in predicting vapor intrusion into an occupied structure. Furthermore, an EPA evaluation of EPA’s Vapor Intrusion Database 4 , as of 2012, documents that depth to contamination is a significant factor . Therefore, depth to contamination is being retained as a factor in the potential for exposure evaluation in the proposed addition to the HRS. EPA notes however, that the likelihood of exposure factor is only one of many factors used in the HRS evaluation
, but because it a multiplying factor in the HRS formula, even a small drop in this depth to

c ontamination factor value requires a significant increase in other factors for a site to qualify for the NPL. Regarding biodegradation
, EPA has incorporated a degradation factor into the proposed addition, but not in the evaluation of the potential for exposure factor . This degradation factor takes into account the depth to contamination, the half- life of the substance and the parent -daughter relationship of breakdown/decay products. Because degradation is both substa nce

- specific and site-specific, it was determined that it would be appropriate to evaluate the degradation potential in the HRS consideration of contaminant characteristics (i.e., the waste characteristics factor category value). This addition of a degrada tion factor is consistent with EPA 's general

3 EPA On

- line Tools for Site Assessment Calculations. Screening Level Implementation of the Johnson and Ettinger Vapor Intrusion Model. http:// www.epa.gov/vaporintrusion/epa

- spreadsheet

- modeling

- subsurface

- vapor

- intrusion . U.S. Environmental Protection Agency, Ecosystems Research, Athens, Georgia. 4 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 Te chnology Innovation (OSRTI), served as the principal author, with Dr. Richard B. Kapuscinski, USEPA

. OSRTI, as coauthor. March 16, 2012.16 a pproach for addressing vapor intrusion in both EPA 's Office of Land and Emergency Management (OLEM) 5 and Office of Underground Storage Tanks ( OUST ) 6 programs.

R egarding the peer reviewers’ comments that the depth to contamination assigned values in Table 5 -13 of the proposed HRS should be revised, sufficient evidence exists to predict that there is a difference in contamina nt migration potential between the depth categories listed in Table 5 -13.

R egarding the reviewers’ concerns on the correlation of depth to ground water and the impact on vapor intrusion rates, t he HRS evaluation must be based on data available at the time of a site inspection . At that time indoor air measurements , soil gas and ground wate r analytical data may not be available at all possibly threatened structures

. While contaminated ground water may or may not be as representative of the vapor intrusion risk as soil gas, the presence of volatile substances in ground water, at a minimum, certainly represents a higher risk than sites with no such information

. Therefore , both the depth to contamination and degradation factors have been retained.

2.1.c .3 Depth and Probability of Intrusion
Comment(s) : Several peer reviewers commented that t he probabil it y of intrusion will depend more on the media and the lithology of the subsurface (including whether preferential manmade and natural pathways are present) underlying a site than the actual depth to contamination. A reviewer recommended including the consideration of manmade preferential pathways in the draft SsI addition. One r eviewer pointed out that if a ground water plume was present at the water table, the corresponding soil gas contamination measurements would always be shallower than ground water samples . The reviewer stated that this would affect the accuracy of values presented in Table 5 -13. Additionally, one reviewer commented that the depth to contamination factor (specifically Table 5

- 13) s hould prioritize the intrusion of liquids in to structures over structures that do not have a documente d i ntrusi on.

R esponse: T he proposed depth to contamination factor reflects how much farther the contaminant has to travel to enter a regularly occupied structure, and does not reflect the sample media type. It is not used if there is evidence of intrusion into regularly occupied structures already collected at a site. The impact of lithology and me dia is considered in the assignment of a different HRS factor value (i.e., vertical migration factor ) used in reflecting the probability of intrusion . T he probability of intrusion is certainly relatively higher if
, for example, there is evidence of contamination at a depth of 15 feet rather than 20 feet, regardless of sample media type , given there is less distance for the contamination to travel. If other sampling results become available
, the site could be rescored. Regarding preferential pathways, EPA has incorporated th e suggestion to include manmade preferential pathways in the consideration of channe lized flow ’s impact on the potential for vapor intrusion ( see Section 1.1 of the proposed addition to the HRS ). Regarding lithology, its impact on vapor migration is represented by a separate factor; the vertical migration factor.

R egarding the prioritization of the intrusion of liquids into occupied structures, EPA notes that if a regularly occupied structure under the commenter's scenario is documented to have an eligible hazardous substance(s) intruding via surficial ground water, the likelihood of exposure factor category would be scored based on the presence of an observed exposure. This would result in a higher likelihood of exposure factor category value than if it was scored using the potential for exposure factor category ev aluation.

5 United States Environmental Protection Agency. (2015). OSWER Technical Guide for Assessing and Mitigating the Vapor I ntrusion Pathway from Subsurface Vapor Sources to Indoor Air. Office of Solid Waste and Emergency Response. June 2015. (EPA Publication No. 9200.2

- 154). 6 United States Environmental Protection Agency. (2015).Technical Guide for Addressing Petroleum Vapo r Intrusion at Leaking Underground Storage Tank Sites. Office of Underground Storage Tanks (OUST).17 2.1.c .4 Method Used for Assigning Depth to Contamination Weighting Values
Comment(s) : One peer reviewer commented that the method for assigning weighting values to the different depths to contamination in Table 5 -13 of the draft SsI addition to the HRS should be related to changes in diffusive flux based on Fick’s Law.

R esponse: EPA considers the J&E Vapor Intrusion Model 7 to be sufficient to provide a relative projection of the importance of different depths to contamination in determining the potential for vapor intrusion into a regularly occupied structure . Furthermore, EPA has contrasted the effects of weighting depth to contamination factors using Fick's Law and the J&E Vapor Intrusion Model and compared the resulting predictions. The use of either model would result in the assignment of similar factor values
, as both result in similar proportional values (e.g., both vary in the same direction with changes in common factors) and would result in similar relative rankings amongst sites . T herefore, the agency decided not to alter the basis for assigning weighting values to the range of depths presented in Table 5 -13 of the proposed addition based on Fick’s Law

.
T hese comments result in no change to the proposed addition to the HRS

.
2. 1.d Vertical Migration
The vertical migration factor considers the geologic makeup of materials between the base of a regularly occupied structure and the hazardous substance plume and the rate in which substances are likely to have moved through the geologic materials. The agency proposes to evaluate vertical migration based on two factors: effective porosity (or equivalently, permeability) of geologic materials and the thickness of the lowest porosity layer.

T he range of possible factor values for the effective porosity of geologic materials is based solely on the typical porosity of subsurface materials (e.g., gravel, sand, silt and clay). These factor values are used in conjunction with the thickness of the applicable lowest porosity layer to establish a vertical migration factor value.
Specific peer reviewer comments on vertical migration are addressed in the following sections:

• 2.1.d.1 Use of Term “ Effective Porosity/ Permeability ”

• 2.1.d.2 Effective Porosity/ Permeability Categories

• 2.1.d.3 Scientific Basis for Vertical Migration Weightings

• 2.1.d.4 Effect of Soil Type and Moisture Content on Vertical Migration Values

• 2.1.d.5 Hydraulic Pumpi ng

2.1.d .1 Use of Term “Effective Porosity/ Permeability”
Comment(s) : One peer reviewer commented th at the use of the term “ effective porosity/permeability ” to evaluate vertical vapor migration is unnecessary and the term “soil type” should be used in stead.
Response : Studies have demonstrated that effective porosity, which largely reflects moisture con tent, is an important factor in projecting vapor intrusion. However, long- term measurements of effective porosity are beyond the scope of a typical time

- limited site inspection in that they vary with time, climate and depth. The HRS uses soil type as a surrogate for effective porosity, in that soil type reflects at least in part long- term moisture porosity

7 A description of the basic model can be accessed at h ttp://www.epa.gov/vaporintrusion/epa

- spreadsheet

- modeling

subsurface

- vapor

- intrusion18 condit ions. The term “ soil type ” was not adopted as a substitute name to reflect that it is only a surrogate for effective porosity.

Thi s comment results in n o change to the proposed addition to the HRS

.
2.1.d.2 E ffective Porosity/ Permeability Categories
Comment(s) : One peer reviewer commented that Table 5

- 14, Effective Porosity/Permeability of Geologic Materials, of the draft SsI addition to the HRS contain s two categories that are better categorized as one material type . The reviewer stated that both the silt and sand material categories “show little difference in vapor migration across the soil types.”
Response: The silt and sand categories h ave been retained. Calculations using an EPA version of the J&E Vapor Intrusion Model 8 indicate there are differences in vapor migration rates due to effective porosity differences in silt and sand soil types. Furthermore, insufficient real -world data coul d be found to be confident that there is no difference in vapor intrusion rates due to the two soil types. Moreover , t he soil type classification system in the proposed addition to the HRS has been used in HRS evaluations for the ground water migratio n pat hway and the data needed to classify soil type at a location ha ve been found to be readily available.

Thi s comment results in n o change to the proposed addition to the HRS

.
2.1.d.3 S cientific Basis for Vertical Migration Weightings
Comment(s) : One peer reviewer commented that the vertical migration factor value associated with clays is too high relative to the values used to weight other geologic materials. The reviewer commented that “true clays and unfractured rocks” have extremely low porosities and are effective barriers to vapor transport , and recommend ed lowering the migration factor values for clay materials in Table 5 -15, Vertical Migration Factor Values
, of the draft SsI addition to the HRS.
Response: The difference in the present vertical migration weightings between typical clays and rock layers and other more porous geologic material layers is sufficient to reflect a relative difference in vertical migration rates and increases with the thickness of the layers. This difference between layer types has not been increased because not all clay and rock layers a re competent horizontally under an entire site. C ompetent clay layers and other low porosity materials can be very effective vapor barriers, especially when they are th ick and continuous ov er a large

area . However , the horizontal continuity of the layers can often be incomplete ( e.g., differential depositions during formation, erosion after deposition, or manmade preferential pathways limit the horizontal continuity) and allow for vapor migration through the resulting gaps. Similarly, bedrock often contains cracks and fractures that function essentially as open channels for vapor migration. This phenomenon is especially important when these layers are relatively thin; the refore, Table 5 -15, Vertical Migration Factor Values , of the proposed addition to the HRS assigns a high value in these situations. However, at greater thicknesses these layers tend to be more competent over a wider area , and thereby hinder vap or migration more effectively, and are assign ed a lower value in these situations. Because the competence of the layers is not always known at the time of an HRS evaluation, the factor values were designed to consider this possibility. Regarding the values relative to those assigned to other geologic materials, these values were based on simulations run using the J&E Vapor Intrusion Model. While this model may not be site- specifically accurate, the predictions based on it are considered sufficiently accurate for relati ve comparisons between sites

.

8 Johnson and Ettinger (1991) Model for Subsurface Vapor Intrusion into Buildings . http://www.epa.gov/vaporintrusion/epa

spreadsheet

- modeling

- subsurface

- vapor

- intrusion . U.S. Environmental Protection Agency19 F urthermore, competent clay layers are likely to also influence the depth to contamination factor value as well as the vertical migration factor value

. The presence of any low porosity layer would likely also correspond with a greater depth to contamination factor value and both together will further lower the potential for exposure factor value.

T hese comments result in n o change to the proposed addition to the HRS

.
2.1.d .4 Effect of Soil Type and Moisture Content on Vertical Migration Values
Comment(s) : Peer reviewers commented that differences in soil type (effective porosity/permeability) would not result in significant impacts on migration factor values. However, reviewers commented that soil moisture content

or the presence of perched water should be accounted for in the vertical migration factor values as it can have a significant impact on the diffusive/advective flux.
Response: EPA used a version of the J& E Vapor Intrusion Model to weight the assignment of vertical migration factor values based on effective porosity and thickness of the lowest porosity layer. Additionally, EPA’s Vapor Intrusion Database , as of 2012, shows that soil type is a significant fac tor in datasets that include this parameter.

E PA notes that soil moisture content can be a significant factor, but after discussion with subject matter experts, determined that soil moisture content over time could not be effectively represented by the li mited data available from a typical short- term site inspection during the site assessment phase of the CERCLA process. It was decided that since soil type is a function of climate and is available for most locations, it is a practical (although imperfect) relative indicator of effective porosity and soil moisture over time. Additional consideration of how soil moisture can impact the potential for subsurface intrusion is discussed in the proposed HRS addition's Technical Support Document ( TSD ). Regarding pe rched water, which can also be a significant factor, given the short duration of sampling events for HRS studies; it will not be possible to predict the transient nature of perched water or its effects. Furthermore the possible effects of the presence of p erched water on vapor migration are reflected in the potential for exposure equation in that the cause of the perched water, such as a clay layer, is reflected in the evaluation.

T hese comments result in n o change to the proposed addition to the HRS

.
2.1.d .5 Hydraulic Pumping

C omment(s) : Peer reviewers commented that hydraulic pumping of vapors into structures should be considered in determining the vertical migration factor value

. Response: It is likely that if significant hydraulic pumping is occurring in an area, the potential for intrusion would be greater than in other areas. However, at this time the relative importance of this phenomenon and factors that would influence its occurrence are not well documented, and so a special consideration for this occurrence has not been incorporated into the proposed addition to the HRS. The pr oposed potential for exposure formula would to some extent account for this situation , as hydraulic pumping impacts of vapor intrusion rates would likely be occurri ng in areas with shallow ground water (and thus shallow depths to contamination) and high porosity soils . Shallow ground water and high porosity soils are both factors corresponding to a high potential for exposure factor.

T hese comments result in no change to the proposed addition to the HRS.20 2. 1. e Vapor Migration Potential
Comment(s) : Several peer reviewers questioned the use of an average of vapor migration potential values in Table 5-18, Vapor Migration Potential Factor Values, of the draft SsI addition to the HRS, to determine the assigned value for the vapor migration potential factor value.
Response: The vapor migration potential factor is meant to reflect the volatility of a substance in the subsurface at a site. EPA has revised Section 5.2.1.1.2.4 of the proposed addition to the HRS by assigning the vapor migration potential factor value based on the most volatile substance associated with each ASC being evaluated

. However, this value will be overridden with a higher value if the depth to contamination associated with the ASC being evaluated is documented to be 10 feet or less. The ASC would be assigned a vapor migration potential factor value of 25 (the maximum value). This value is assigned if the hazardous subst ance is both associated with an ASC (i.e., meets observed release criteria) and i s presen t at a depth of 10 feet or less beneath a regularly occupied structure (i.e., at a depth where preferential pathways are likely present). This assignment reflects that the substance has migrated through the subsurface environment and is extremely likely to enter a regularly occupied structure.

2. 1. f Biodegradation/Bioattenuation
Comment(s) : One peer reviewer identified that biodegradation can be a significant factor in determining the potential for vapor intrusion and commented that biodegradation should be accounted for in the draft SsI addition to the HRS, either as a separate factor or as part of other potential for exposure factors. Specifically, it was commented that there should be a distinction between chemicals that readily bioattenuate (i.e., biodegrade) and chemicals that do not bioattenuate. One reviewer suggested including a bioattenua tion factor.
Response: B iodegradation can play a maj or role in subsurface intrusion and, in response to peer review comments, EPA has added a degradation factor in the determination of the waste characteristics factor category value , but not in the potential for exposure evaluation. This addition of a degradation factor is consistent with EPA’s approach for addressing vapor intrusion in both EPA’s OLEM 9 and OUST 10 programs. Whether a substance will degrade depends on both the substance and site

- specific co nditions, e.g., while the ability of a substance to degrade in the environment can be predicted based on its characteristics, whether it actually degrades is dependent on site-specific conditions. The site

- specific conditions that can affect the rate of de gradation can be extremely variable in time and location (e.g., oxygen content, moisture , soil matrix ) and it is beyond the scope of a short

- term site inspection to measure or predict these conditions with accuracy . Thus, a degradation factor was added to the waste characteristics category, which reflects the chemical properties of the substances at a site.

T he degradation factor is based on the substance being evaluated , whether there is sufficient depth to contamination at the site to ensure degradation of the substance , the half-life of the substance, and the parentdau ghter relationship of breakdown and decay products. Th e degradation factor also considers the difference in half- lives of different hazardous substances, e.g., between petroleum or petroleum- like hazardous substances, which have the greatest potential to biodegrade , the presence of biologically active soil, and the depth to contamination . (See Section 5.2.1.2 and applicable subsections of the proposed addition to the HRS for further detail.) The inclusion of a degradation factor will result in a lower overall HRS site score for substances in an area of subsurface contamination (ASC) that are known to readily biodegrade at sites where the contamination is likely to be in a biologically active subsurface layer of sufficient depth to support degradation of the substance, or if a parent substance is present and is not constantly regenerating the daughter substance

.

9 See footnote 4.

10 See footnote 5.21 2 .1.g C onsideration of Volatile Substances
Comment(s) : One peer reviewer suggested that the draft SsI addition to the HRS be modified to consider the absence of vapor intrusion risks associated with high molecular weight semi -volatile compounds.
Response: The recommendation to have a potential for exposure value of zero if no volatile substance is present has been implemented by adding the instruction to not assign a potential for exposure factor value if no volatile hazardous substance s are present

. With this change, only those substances with a vapor press ure greater than or equal to 1 torr or a Henry’s Law constant of 10 -5 atmosphere-meter cubed per mole may be considered when evaluating the potential for exposure factor value ; however, substances meeting observed exposure criteria will continue to be used for delineating an area of subsurface contamination. This achieves the recommendation but maintains the potential for exposure mathematical structure which is consistent with that used for equivalent calculations in other parts of the HRS (e.g., potential to release to ground water) . T hus, the change also maintains the appropriate balance between the pathways and components.22 

### 2.2 Charge Question #2 : Is EPA’s approach of basing the hazardous waste quantity on the amount of contaminants that could be found in the intruded structures appropriate? If no, what alternative approach would you recommend?

The hazardous waste quantity facto r value in the proposed subsurface intrusion component reflects the amount of hazardous substances to which targets are exposed . The hazardous waste quantity is based on the amount of contaminants that could be found in the intruded structures, reflecting the subsurface intrusion component’s focus on exposure. This follows a similar approach to calculating hazardous waste quantity as the existing soil exposure pathway. In the 1990 HRS soil exposure pathway, the hazardous waste quantity does not include the total amount of hazardous substances in all site sources (as would be the case for the ground water, surface water, and air migration pathways), but rather is based on the amount of hazardous substances to which a target can be exposed where actual contamination has been documented.

2. 2.a Support for Method
Comment(s) : Peer reviewers agreed with the approach for calculating the hazardous waste quantity and agreed that it was a reasonably conservative approach.
Response : The agency has retained the proposed approach that bases the hazardous waste quantity on the amount of contaminants that can be found in the intruded str uctures. However, in response to peer review comments, EPA has included the consideration of the probable exposures within regularly occupied structures located within ASC s in estimating the site hazardous waste quantity. The procedure for estimating the hazardous waste quantity is the same as used in the other HRS pathways and components for estimating hazardous waste quantity.

2. 2. b Considering Waste Quantity above Areas of Subsurface Contamination
Comment(s) : One peer reviewer commented that only the hazardous substances in areas of observed exposure are included in the calculation of the hazardous waste quantity and suggested that hazardous substances in area s of subsurface contamination be included in the calculation of the hazardous waste quantity.
Response : The method for estimating the hazardous waste quantity factor value has been modified to reflect the amount of hazardous substances that may be entering regularly occupied structures within an ASC . The hazardous waste quantity factor value for the subsurface int rusion component is meant to reflect the amount of hazardous substances that populations are likely to be exposed to via subsurface intrusion.

EPA agrees with the peer reviewer that populations are likely to come into contact with intru ded hazardous substances in an ASC

. Areas of subsurface contamination are delineated by sample locations meeting observed release criteria for subsurface intrusion , which document the presence of volatile hazardous substances, and EPA studies have shown it is possible, if not probable, that these substances will enter the structures in these areas

. Therefore, EPA has include d consideration of the probable exposures within these structures within an ASC in estimating the site hazardous waste quantity . N ot in cluding these areas may significantly underestimate the amount of hazardous substances that populations are exposed to from the subsurface.

2. 2. c Considering Waste Quantity of Multiple Structures
Comment(s) : One peer reviewer questioned the method for determining a source hazardous waste quantity value in the case where multiple homes were located above a large plume. The reviewer questioned how the structures above an ASC would be evaluated to determine the hazardous waste qu antity.23 Response : All homes (and/or other regularly occupied structures) located in area s of observed exposure (AOE) (and as modified to reflect peer review comments, in ASCs ) are considered in estimating the hazardous waste quantity. The waste quantity assigned to each structure within all AOE s or ASC s is summed and the resulting value is used to determine the component hazardous waste quantity factor value

.
2. 2. d Including Subsurface Contamination in the Waste Quantity
Comment(s ): One peer reviewer com mented that the hazardous waste quantity value calculation should reflect the quantity of hazardous substances in the subsurface as well as in buildings

. Response : The hazardous waste quantity value for this proposed component reflects the amount of intruded hazardous substances that people may be exposed to when inside regularly occupied structures. Not all of the contamination in the subsurface will enter occupied structures, as most will be vented directly to the atmosphere. Thus, including the qu antity of all hazardous substances in the subsurface could greatly overestimate the amount that will result in indoor exposure. Although the entire quantity of hazardous substances in the subsurface is not included in the estimation of hazardous waste quan tity, the consideration of regularly occupied structures in areas of subsurface contamination in this calculation would reflect that some of the contamination would enter these structures. ( EPA notes , however, that if the amount venting to the atmosphere is sufficient to pose a significant threat
, this threat can be considered in a different HRS pathway eval uation, the a ir migration pathway, as it is a release directly to the atmosphere. )

T his comment results in no change to the proposed addition to the H RS

.
2. 2. e Minimum Waste Quantity
Comment(s) : Peer reviewers questioned the relevance of using the ratio of subs lab sample concentrations to health

- based benchmarks in the calculation of a minimum hazardous waste quantity factor value . One reviewer suggested that subslab samples may be influenced by indoor air sources and are not necessarily a reliable indicator of vapor intrusion potential. Another reviewer suggested modifying the criteria for crawl spaces in the calculation of a minimum hazardous w aste quantity factor value based upon EPA ’s attenuation factor for crawl spaces.
Response: EPA has remov ed the consideration of subslab and crawl space sample concentration s when determining a minimum hazardous waste quantity. Research has shown that the rate of vapor intrusion is extremely variable over time, and the variability of values f ro m crawl space and subslab samples even from a single structure during a single sampling event is significant. Therefore the actual quantity of contamination via vapo r intrusion into a structure may not be accurately represented using crawl space or subslab samples from a short- term sampling event such as those used for HRS evaluation purposes

.
R egarding the possibility of indoor air sources impacting subsurface cont amination, this possibility is reflected in another part of the HRS evaluation: when identifying an actual observed exposure due to subsurface intrusion. This identification requires attributing the release to subsurface contamination, and not to indoor so urces.

2. 2. f Relevance of Waste Quantity
Comment(s) : One peer reviewer questioned the relevance of the hazardous waste quantity in determining the relative risk associated with hazardous substances migrating via the subsurface intrusion component. The

re viewer commented that the hazardous waste quantity has little relevance to vapor intrusion impacts, and suggested that t he HRS use the duration of time that a vapor source will produce vapor concentrations.24 Response : The hazardous waste quantity factor serves as a surrogate for contaminant concentration and the extent of contamination. The decision to include waste quantity in all pathways and components in the HRS algorithm was made when the HRS was last revised in 1990 (see Section V.3 of the proposed 1988 HRS, 53 FR 51692, December 23, 1988; Section III.C of the 1990 HRS, 55 FR 51542, December 14, 1990). The decision was based on the concept that determining an accurate dose and length of exposure for the waste quantity estimate was beyond the scope of information available after a site inspection. Instead, hazardous waste quantity is used as a surrogate for these two factors. The HRS is only predicting that with a high hazardous waste quantity, it is more likely an exposure will occur and that people will be exposed to a high dose.

T h e inclusion of hazardous waste quantity in the subsurface intrusion component is consistent with the other existing HRS p athway evaluations, allowing the ranking of the relative risk posed among different sites with different migration and exposure pathways (and thus consistent with the goal that the new component not affect the overall HRS site scoring algorithm).

F urther more, regarding the reviewer’s suggestion, it is not possible to predict with confidence the duration of a vapor intrusion episode based on a typical time- limited site inspection. However if an estimate with reasonable confidence can be developed for how m uch vapor intrusion has occurred over time, it can be used to estimate a wastestream quantity (s ee discussion of Tier B hazardous waste quantity estimate in Sec ti ons 2.4.2.1.2 and 5.2.1.2.2 of the proposed addition to the HRS) that can in turn be used to a ssign the hazardous waste quantity factor value for the component.

T hese comments result in no change to the proposed addition to the HRS

. 2. 2. g Consideration of Source Concentrations in Estimating Waste Quantity
Comment(s) : One p eer reviewer commented that the calculation of the hazardous waste quantity should include consideration of source concentrations

. Response : While it would result in a better estimate of risk to have the HRS algorithm reflect the concentration of hazardo us substances at a site, particularly for subsurface intrusion, it is unlikely that representative site contaminant concentrations could be obtained using data collected during a typical time

- limited site inspection. Studies have shown that the rate of vap or intrusion varies significantly over time and this range cannot be estimated from a limited site inspection. Therefore, it is not likely that a measurement taken during a short time period would be accurate in predicting dose or actual waste quantity.

H owever, if the concentration information necessary to calculate the actual mass of hazardous substances that individuals are exposed to is available, the HRS has a procedure to use this information to estimate the actual

mass (see discussion of Tier A hazardous waste quantity estimate in Section s 2.4.2.1, 2.4.2.1.1 and 5.2.1.2.2 of the proposed addition to the HRS ).

EPA not es, however, that in another part of the HRS algorithm, the evaluation of targets, a higher weighting is assigned to targets (and
, co rrespondingly, a higher site score) if the concentration of one or more hazardous substances found during a sampling event exceeds an appropriate health

- based benchmark (see Sections 2.5 and 5.2.1.3 [ and all applicable subsections ] of the proposed addition regarding Level I actually exposed targets).

R egarding considering contamination in both the subsurface and in regularly occupied structures, t he proposed addition is an exposure component ( i.e.
, the threat of exposure to contamination wher e it is presently located is considered, as contrasted with the threat resulting from migration of the contamination, a migration component) ; therefore, the hazardous waste quantity factor value reflects the amount of contamination a target(s) may be exposed too. Including an estimat ion of subsurface contaminant sources in the calculation of hazardous waste quantity could overestimate the amount of hazardous substances a population is exposed to, as not all of the25 c ontamination will enter occupied structures (e.g., venting directly to the atmosphere instead). Although the quantity of hazardous substances in the subsurface is not included in the estimation of hazardous waste quantity, the consideration of regularly occupied structures in areas of subsurface co ntamination in this calculation would reflect that some of the contamination could enter these structures.

T his comment result s in n o change to the proposed addition.

2. 2. h Relative Risk of Airborne Contamination
Comment(s) : One peer reviewer questioned the method for calculating hazardous waste quant ity , stating that it would underestimate the risk associated with the subsurface intrusion component relative to other pathways. The reviewer commented that it does not reflect that airborne contaminants pose a higher risk at lower concentrations than contaminants in other media

. Response : T he role of waste quantity in an HRS evaluation is not designed to reflect by itself the level of risk to targets. In an HRS analysis
, waste quantity serves as a surrogate for the amount of a contaminant(s) that the population associated with a site might come in contact with . This factor is a surrogate for a measured dose used in a risk analysis . The higher the hazardous waste quantity value the more likely a population will be exposed and the higher the amount they will likely be exposed to. Hence the higher the hazardous waste quantity value the higher the HRS site score.

T he effect of the exposure , including the concentration, is considered in another part of the HRS algorithm, when assigning the target weightings . The lower the health

- based benchmark associated with air exposure
, the higher the HRS target factor category score will likely be. A higher weighting is given to target populations who can be documen ted to be exposed to hazardous substance levels at or above health

- based criteria

. The health

- based benchmarks EPA uses to assign the target weightings reflect the higher risk associated with airborne contaminants tha n that for other media.

T herefore, the combination of hazardous waste quantity and weighting of targets based on benchmarks together accounts for the higher risk associated with exposure to airborne contaminants than to contamination in other media.

T his comment results in no change to the p roposed addition to the HRS.

2. 2. i Biodegradation and Waste Quantity
Comment(s) : Peer reviewers commented that the sampling for subsurface intrusion and subsequent calculation of the hazardous waste quantity should differentiate between hydrocarbon volati le organic compounds ( VOCs ) and halogenated VOCs to consider biodegradation. The reviewer commented that paired subslab and indoor air samples showing similar contamination 80% of the time is incorrect because it does not differentiate between types of VOCs.
Response: In response to peer reviewers’ comments, EPA has added a degradation factor to the evaluation of the waste characteristics factor category value for the subsurface intrusion component of the HRS. EPA determined that it would be more appropriate to evaluate a substance's degradation potential as a separate factor value and not within the hazardous waste quantity calculation. This is because many of the site

- specific characteristics impacting a substance's degradation rate over time are beyond the scope of a typical site inspection to measure ; therefore, it could r esult in a significant error in estimating the hazardous waste quantity for substances that might or might not biodegrade based on site

- specific conditions

.26 T his degradation factor reflects the difference in degradation rates e.g., between readily degrada ble hydrocarbons such as st raight chain or simply ring structures found in petroleum or petroleum- like hazardous substances
, the presence of biologically active soil, and the depth to contamination

. The agency notes , however, that in the HRS algorithm, the degradation factor is multiplied by the toxicity factor value, with the resulting factor value then multiplied by the hazardous waste quantity factor. The overall result is that if a n ASC at a site is associated with substances that are likely to degrade, this factor will reduce the overall score for that site relative to other sites without degradable substances

.
2. 2. j Impact of Indoor Sources on Waste Quantity
Comment(s) : One peer reviewer commented that, in some cases, indoor air contaminant concentrations have been shown to result from an indoor source instead of subsurface intrusion.
Response : The hazardous waste quantity estimate is designed to reflect the amount of hazardous substance intrusion from subsurface sources. In estimating hazardous waste quantity using the HRS tiers, the Tier A estimate
, called hazardous constituent quantity, actually represents an exact calculation of the mass of hazardous substances that has entered into a structure. This calculation would require subtracting out any indoor sources and thus would not include the quantity of contaminants originating from indoor sources.

T his comment results in no change to the proposed addition to the HRS.

2. 2. k Applicabil ity of the Hazardous Waste Quantity Calculation
Comment(s) : One peer reviewer questioned the applicability of the subsurface intrusion hazardous waste quantity calculation in the case where only an ASC exists. The reviewer questioned whether the waste quantity in this scenario would be better related to ground water or soil gas concentrations to more accurately estimate the quantity.
Response : The hazardous waste quantity factor value represents the amount of hazardous waste a target may be exposed to, which is not the same as the amount located in the subsurface. Much of the contamination in the subsurface will vent into the atmosphere and not enter into an overlying structure. Therefore, a minimum hazardous waste quantity value based on soil gas or g round water concentrations could significantly overestimate the amount a target could be exposed to inside a structure. However, the agency notes that regularly occupied structures in an area of subsurface contamination are now considered in determining hazardous waste quantity in the proposed addition.

T his comment results in n o change to the proposed addition to the HRS

.
2. 2. l Consider Future Structures
Comment(s) : One peer reviewer commented that the pro posed structure of the draft SsI addition does n ot allow for the determination of a hazardous waste quantity for locations that exhibit significant contamination and currently have no structures, but where structures would or will be constructed. The reviewer suggested that an approach be created to estimate the air concentrations if a structure were present in a location with VOC contamination based upon data collected from other sampled media

. Response : The HRS scoring of the SsI component is based on current conditions , as demonstrated in the wording of the text in the proposed addition to the HRS.27 T he agency also considers that the priority of a site without eligible structure s would be low er than sites with eligible structures . However, if structures were later built on the site or the subsurface contamination migrates beneath existing structures, the HRS evaluation could be revisited.

T his comment results in no change to the proposed addition to the HRS.

2. 2. m Adequacy of Site Inspection Data to Identify Subsurface Contamination
Comment(s) : One peer reviewer questioned the ability to complete the necessary sampling within the time frame of a typical site inspection to estimate the hazardous waste quantity. It was suggested that prior sampling may be required to identify hazardous substances associated with subsurface contamination that should be targeted for analysis in indoor air and to gather the necessary information to estimate the hazardous waste quantity.
Response : A phased approach may be necessary to collect the data required to score this component, if indoor air sampling is deemed necessary or has not been previously undertaken. In the first phase, data on ground water and soil gas samples and informati on on site- specific conditions are likely to be collected. If the information from the first phase investigation indicates a sufficient possibility of risk to populations, a second phase may occur. This second phase will likely focus on sampling indoor air and collecting additional information on occupied structures , which could be used to estimate the hazardous waste quantity. Further, if subsequent sampling shows further contamination, the site can be re

- evaluated for the NPL.

T his comment results in no change to the proposed addition to the HRS.

2. 2. n Need for Discussion and Reference Support in TSD
Comment(s) : One peer reviewer commented that there is too much uncertainty surrounding the assumptions used in evaluating the waste quantity in multi -story buildings and stated that there should be more discussion in the proposed addition’s TSD.
Response: Regarding the evaluation of the waste quantity, t he general approach for evaluating multi-story /multi

tenant buildings has been revised and is discussed in greater detail in the proposed addition to the HRS and its TSD. Furthermore, while this comment was received on charge question 4, which dealt with the evaluation of multi

- story and multi- tenant structures , it was decided to group it with the other comm ents related to hazardous waste quantity.28 

### 2.3 Charge Question #3:
Is it appropriate to consider individuals/populations located within the area of subsurface contamination (ASC) to be at a higher relative risk in situations when subslab contaminant concentrations are at least ten times greater than a health-based benchmark? If no, please explain.

I n the subsurface intrusion component, t argets subject to actual contamination receive greater weighting in scoring if the hazardous substance is present at o r above a health

- based benchmark. Targets subject to potential contamination are weighted less than those subject to actual contamination, and also receive lesser weighting s based on the distance from a source of hazardous substances or possible dilution of the hazardous substances during migration from a source to a target. The subsurface intrusion component generally followed a similar target weighting structure as the one used in the 1990 HRS. In the 1990 HRS, all targets in all pathways are evaluated using two main categories: whether they are exposed to 1) actual contamination; or 2) potential contamination. If targets are exposed to hazardous substance levels meeting observed release criteria, they are identified as actually contaminated; however, if the targets are not exposed to a hazardous substance that meet the observed release criteria but are within the target area being evaluated, they are considered potentially contaminated.

2.3.a Support for Weighting Targets
Comment(s) : Several peer reviewe rs commented that it was appropriate to consider individuals to be at a higher relative risk when subslab contaminant concentrations are at least ten times greater than a health

- based benchmark. One peer reviewer agreed that it was reasonable to consider t hose populations in occupied structures that have not been sampled but are surrounded by occupied structures with documented contamination (i.e., in an ASC), as being contaminated unless evidence indicates otherwise. Additionally, another peer reviewer commented that the proposed approach i s consistent with the California vapor intrusion guidance.
Response: The agency acknowledges that the reviewers generally agree with the differential weightings of targets based on projected indoor contaminant levels. H owever , based on peer review responses, the weighting of targets in the ASC (delineated in most situations based on subsurface samples) has been changed in the proposed addition to the HRS to not reflect a comparison to benchmark levels . The weighting of t he populations based on the hazardous substance concentration in subslab and crawl space samples relative to benchmarks was changed so that it does not compare subsurface contaminant concentrations to health-based benchmarks when evaluating potential targe ts. This change was made because of studies that documented the unpredictability of vapor intrusion rates based on time- limited sampling events. However, the weighting based on sample type has been retained and expanded to also consider other sample media (both above and below ground) and the depth or distance to contamination (see Table 5 -20, Weighting Factor Values for Populations within an Area of Subsurface Contamination, of the proposed addition to the HRS for further information).

2. 3.b Appropriate Benchmark Risk (Cancer and Non

- Cancer)
Comment(s) : One peer reviewer recommended increasing the risk level used to establish benchmarks for cancer and non- cancer risks. The reviewer suggested raising the risk level for benchmarks from greater than a one i n a million risk (1 x 10 -6 ) to one in ten thousand (1 x 10 -4 ) for cancer effects and raising the non

- cancer hazard index (HI) from 1 to 10.
Response: The one in a million risk level for benchmarks has been maintained

. Comments on this issue were solicited at the time the HRS was last revised (see Section III .H of the 1990 HRS, 55 FR 51547 , Dec ember 14, 1990). T he cancer risk of 10 -6 for all HRS pathway evaluations was chosen because it is the lower end of the cancer risk range (i.e., 10 -4 to 10 -6 ) identified in the National Oil and Hazardous Substances Pollution Contingency Plan (NCP) and used by other EPA programs . A one in a million ( 1 x 10 -6 ) risk level for cancer effects and a HI of29 1 for non

- cancer effects are built into the overall HRS structure and used for all benchmarks in all HRS pathways and components. These risk levels are also the basis for EPA Regional Screening Levels which are also used as a screening tool to determine co ntaminants of concern at Comprehensive Environmental Response, Compensation, and Liability Act ( CERCLA ) sites.

I n addition, changing the benchmark risk levels for just the subsurface intrusion pathway would imbalance the HRS evaluation among pathways and components; using one benchmark ri sk level for subsurface intrusion and another for other HRS pathways and components would result in a lower qualifying risk level for sites scored on subsurface intrusion compared to sites scored using other pathways and components. Furthermore, given the large variability in vapor intrusion rates over time , assigning only those populations documented as exposed to a 10 -4 based benchmark a higher weighting could significantly underestimate the contaminant concentration and the actual risk at a site. Existing studies indicate it is likely that short duration sampling, such as those used to support HRS evaluations, could underestimate the average exposure concentration and a higher risk

- based benchmark would only magnify this underestimation of risk at the site.

T hese comments result in no change to the proposed addition to the HRS

.
Mo re specific comments on benchmarks are addressed in the following subsections:

• 2.3.b.1 Number of Eligible Sites

• 2.3.b.2 Benchmark and Background (Understanding Overall HRS Process)

• 2.3.b.3 Cumulative Effects and Benchmarks

• 2.3.b.4 Weighting Based on Exceeding Benchmark

• 2.3.b.5 Benchmark Levels in Crawl Spaces

2.3. b.1 Number of Eligible Sites
Comment(s) : One peer reviewer commented that assigning benchmarks based on one in a million (1 x 10 -6 ) risk level is far too low and suggested increasing the benchmark to 1 in 10,000 (1 x 10 -4 ) to avoid listing too many sites.
Response: The number of sites on the NPL is not a factor considered in setting the risk level for HRS benchmarks; rather, the goal is to protect human health and the environment. A one in a million (10 -6 ) risk level for cancer was chosen because it is the lower end of the cancer risk range (i.e., 10 -4 to 10 -6 ) identified in the NCP and used by other EPA pr ograms. As a result, it is built into the overall HRS structure and used for all benchmarks in all HRS pathways and components. Comments on this issue were solicited at the time the HRS was last revised (see S ection III .H of the 1990 HRS, 55 FR 51547 , Dec e mber 14, 1990). These risk levels are also the basis for EPA Regional Screening Levels which are also used as a screening tool to determine co ntaminants of concern at CERCLA sites

.
T hese comments result in no change to the proposed addition to the HRS

.
2.3.b .2 Benchmark and Background (Understanding Overall HRS Process)
Comment(s) : One peer reviewer suggested changing benchmarks for cancer and non- cancer risk s and stated that if the benchmarks were raised, the issue of background samples containing contamination from indoor sources would be resolved.
Response: T he benchmark risk level is not used in any comparison to a background level in the HRS

. In the HRS algorithm, benchmarks are used to increase the weighting of target populations that have been determined to be30 e xposed to hazardous substances (i.e., if a target population is determined to be exposed to a hazardous substance at concentrations a bove an appropriate benchmark, then the target weighting will be higher than another target population that has been determined to be exposed to concentrations below an appropriate benchmark value). (See Section 5.2.1.3 and all applicable subsections of th e proposed addition to the HRS .)

A s explained in section 2.3.b, Appropriate Benchmark Risk (Cancer and Non- Cancer), of this support document,

a one in a million (10 -6 ) risk level for cancer effects and a HI of 1 for non

- cancer effects are built into the overall HRS structure and are considered appropriate for use in a screening tool. Assigning only those populations documented as exposed to a 10 -4 benchmark in a higher weighting based on a limited site inspection sampling event could significantly underest imate the contaminant concentration and the actual risk at a site, given the large variability in vapor intrusion rates over time.

T his comment results in n o change to the proposed addition to the HRS

.
2.3.b .3 Cumulative Effects and Benchmarks
Comment( s): One peer reviewer questioned whether the additive effects of contaminants would be considered in the draft SsI addition to the HRS.
Response: EPA has incorporated the suggestion of evaluating the additive effects of hazardous substances in the proposed addition to the HRS. With this change, populations within an AOE that are not exposed to hazardous substances above their individual health-based be nchmarks can be weighted the same in certain situations as those exposed to hazardous substances documented to have exceeded their benchmark. This occurs if the ratios of the concentrations of multiple hazardous substances to their applicable benchmarks su m to greater than 1 for either the cancer or the non-cancer benchmarks. This is not done for weighting populations within an ASC because
, given the large variability in vapor intrusion rates
, it is unlikely that an actual exposure concentration can be proj ected based on subsurface samples with sufficient accuracy to justify the increased weighting . (See Section 2.5.2 of the proposed addition to the HRS regarding I and J indices.)

2.3.b .4 Weighting Based on Exceeding Benchmark
Comment(s) : Several peer reviewers commented that the current weighting of targets, which is based on comparing contaminant levels to health

- based benchmark levels, should be adjusted. Reviewers suggested that the requirement of being ten times above a health

- based benchmark be revise d to 50 times above the health -based benchmark when determining the population above an ASC.
Response: Based on peer review comments, the weighting of targets in the ASC has been changed in the proposed addition to the HRS to not include comparisons of subslab or enclosed crawl space samples to benchmark levels

. The weighting of the populations based on the hazardous substance concentrations in subslab and crawl space samples relative to benchmarks was changed so that it does not compare subsurface co ntaminant concentrations to health

- based benchmarks when evaluating potential targets. However, the weighting based on sample type has been retained and expanded to also consider other sample media (both above and below ground) and the depth or distance to contamination (see Table 5 -20, Weighting Factor Values for Populations within an Area of Subsurface Contamination, of the proposed addition to the HRS for further information).

T he comparison to benchmarks has been revised because recent studies have sh own that the variability in vapor intrusion rates over time demonstrate s that a simple arithmetic average exposure level cannot be projected with31 c onfidence based on limited sampling events. However, use of the 95 th percentile alpha 11 (i.e., attenuation) fa ctor has been retained in that it was used to set the relative weightings for targets in the ASC and reflects a reasonabl e worst case scenario for projecting indoor concentrations due to vapor intrusion. A ttenuation factors project that the higher the fact or, the more likely it is that a greater portion of the contamination found in the subsurface and crawl space samples will actually enter structures above the contamination. Given that the HRS evaluation is designed to be performed when limited sampling ha s occurred , and studies have shown that contaminant levels from a time

- limited sampling event are likely to underestimate the actual exposure levels, it is appropriate to consider the 95 th percentile attenuation factors in weighting populations in an ASC

.
Wh ile EPA attenuation factors may not be sufficient to predict concentrations relative to a benchmark on a structure

- specific basis, an analysis of the data in EPA ’s Vapor Intrusion D atabase 12 shows that there are statistically significant correlations by sample type and depth in the ratio of the concentration in subsurface samples and in indoor air samples. These findings have been used to develop a revised weighting system (see Table 5 -20, Weightin g Factor Values for Populations within an Area of Subsurface Contamination, of the proposed addition to the HRS for further information). Th is analysis shows the highest correlation in concentrations (i.e. , the attenuation factor is closest to 1) is betwee n crawl space and indoor air sample concentrations . These findings also show decreasing 95 th percentile attenuation factors between indoor air and subslab samples, indoor air and subs urface soil or soil gas samples, and indoor air and ground water samples. Similarly, studies show that there is a significant correlation between indoor air concentrations and depth to contamination

.
T herefore, for example, in the new proposed weighting system, the high est target weighting of 0.9 will be assigned to those pop ulations in regularly occupied structures associated with contaminated crawl space samples meeting observed release criteria . The next highest target weighting of 0.4 will be assigned to populations associated with contaminated samples (meeting observed release criteria) in any media located within a distance of 5 feet of a regularly occupied structure’s foundation (including subslab samples). The weightings continue to decrease for other contaminated sample media up to 30 feet in depth. A minimum target weighting (i.e., 0.1) is assigned for contaminated samples at greater than 30 feet in depth. (A minimum weighting is used to reflect that at the time of an HRS evaluation, a profile of concentration with depth may not be available and onl y ground water samples may be available.) For further categories regarding the weighting of targets see Table 5 -20, Weighting Factor Values for Population within an Area of Subsurface Contamination, of the proposed addition

.
2.3. b. 5 Benchmark Levels in Cr awl Spaces
Comment(s) : One peer reviewer suggested modifying the criteria for determining the population value above crawl spaces. The reviewer recommended that since the attenuation factor for crawl spaces is 1, if a crawl space sample has contamination above a health

- based benchmark (in place of the requirement to be ten times above a health

- based benchmark) the residents of the s tructure should be assigned the weighting factor reflecting exposure above the benchmark in the evaluation of targets.

11 The term “ alpha ” is used interchangeably with the term “attenuation” and both are referred to in this document as factors. This factor refers to the reduction in concentration of volatile substances that occurs during vapor migration in the subsurf ace (as a result of diffusion, advection, sorption, transformation reactions and other processes), coupled with the dilution that can occur when the vapors enter a building and mix with indoor air. The quantification of the value can be defined as the ratio o f the indoor air concentration arising from vapor intrusion to the subsurface vapor concentration at a point or depth of interest in the vapor migration pathway. 12 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, 2012.32 Response: Based on the peer review comments and a 2012 EPA evaluation of EPA’s Vapor Intrusion Database, the methodology for evaluating the populations within an ASC has been modified so as to not compare a contaminated sample concentration in any non- indoor sample to a benchmark. The weighting assigned based on the ratio of the contaminant concentration in subs lab and crawlspace samp les to the applicable benchmark was removed because, as pointed out by the peer reviewers, the EPA’s Vapor Intrusion Database as of 2012 13 shows that there is inadequate correlation , due to high variability, to predict with confidence whether a benchmark is likely to be exceeded even when extensive site

- specific data are available. This correlation would be even more tenuous if the projection w ere based on the limited sampling data collected during a typical short duration site inspection (i.e., the data normally available during an HRS evaluation).

I n addition, t he weighting system for targets in the ASC has been revised based on peer review comments and is now also based on sample typ e (e.g., crawl space, soil gas) and the depth or distance to contamination

. The revised weighting system is included in the proposed addition to the HRS (see Table 5 -20, Weighting Factor Values for Population within an Area of Subsurface Contamination , of the proposed addition). See section 2.3.c.7, Weighting Factors for Crawl Space Samples and Subslab Samples, of this support document for further explanation of the specific weighting values associated with crawl spaces.

2. 3.c Target Weighting
Comment(s) : Peer reviewers commented that the weighting of individual targets in an ASC does not appear consistent with EPA ’s compilation of paired subslab and indoor data. Reviewers commented that rationales for using specific weighting values were not provided and that these inconsistencies in the weighting factors could underestimate the target score at a site. Additionally, one reviewer commented that the paired subslab and indoor air data may allow for an observed exposure to be predicted based on subslab samples.
Response: While multiple weighting scena rios were originally considered, the assigned weighting values were determined to most appropriately reflect the relative threat posed to target indiv iduals in a regularly occupied structure based on the paired subslab/indoor air data reviewed. However, ba sed on the peer review comments and an analysis of EPA ’s Vapor Intrusion Database as of 2012, the methodology for evaluating the populations within

an ASC has been modified. The weighting assigned based on the ratio of the contaminant concentration in subslab and crawlspace samp les to the applicable benchmark was removed because, as pointed out by the peer reviewers, this analysis shows that there is inadequate correlation to predict with confidence whether a benchmark is likely to be exceeded even when significant site

- specific data are available.

A dditionally, using the paired subslab and indoor data to predict an observed exposure has not been adopted b ecause new data suggest extreme variability of vapor intrusion rates, and representative site contaminant concentrations are unlikely to be predicted with any precision based on data collected during a typical short duration site inspection (i.e., the data normally available during an HRS evaluation ).

Wh ile EPA attenuation factors may not be sufficient to predict concentrations relative to a benchmark on a structure

– specific basis, the 2012 analysis of the data in EPA ’s Vapor Intrusion D atabase shows that there are statistically significant correlations by sample type and depth in the ratio of the concentration in subsurface samples and in indoor air samples. These findings have been used to develop a revised weighting system (see Table 5 -20, Weighting Fact or Values for Population within an Area of Subsurface Contamination, of the proposed addition to the HRS for further information). Th is analysis shows the highest correlation in concentrations (i.e
., the attenuation factor is closest to 1) between crawl sp ace and indoor air sample concentrations . It also shows decreasing 95 th percentile attenuation factors between indoor air and subslab samples, indoor air and subs urface

13 U.S. EPA Vapor Intrusion Databas e: Attenuation Factor Analysis. Accessible at: http://www.epa.gov/vaporintrusion33 so il or soil gas samples, and indoor air and ground water samples. Similarly, studies show that there is a significant correlation between indoor air concentrations and depth to contamination

.
T herefore, for example, in the new proposed weighting system, the high est target weighting of 0.9 will be assigned to those populations in regularl y occupied structures associated with contaminated crawl space samples meeting observed release criteria. The next highest target weighting of 0.4 will be assigned to populations associated with contaminated samples (meeting observed release criteria) in a ny media located within a distance of 5 feet of a regularly occupied structure’s foundation (including subslab samples). The weightings continue to decrease for other contaminated sample media up to 30 feet in depth. A minimum target weighting (i.e., 0.1) is assigned for contaminated samples at greater than 30 feet in depth. (A minimum weighting is used to reflect that at the time of an HRS evaluation, a profile of concentration with depth may not be available and only ground water samples may be available. ) For further categories for the weighting of targets see Table 5 -20, Weighting Factor Values for Population within an Area of Subsurface Contamination , of the proposed addition.

M ore specific weighting value comments are addressed in the following subsec tions:

• 2.3.c .1 Strength of Weighting Assumptions

• 2.3.c.2 Weighting Based on Benchmark Comparison

• 2.3.c.3 Weighting by Subsurface Conditions

• 2.3.c.4 Weighting by Sample Type

• 2.3.c.5 Weighting Factor When Not Ten Times Benchmark

• 2.3.c.6 Weighting Based on Subslab Samples and Ground Water Samples

• 2.3.c.7 Weighting Factors for Crawl Space Samples and Subslab Samples

2.3.c .1 Strength of Weighting Assumptions
Comment(s) : One peer reviewer questioned the assumptions underlying weighting of potential targets (i.e., those populations in an ASC). The reviewer commented that the approach for weighting potential targets assumes that areas of subsurface contamination have a ten times lower relative impact than areas with observed exposure. The reviewer questioned the assumption that subslab soil gas samples are reliable indicators of vapor intrusion. Additionally, the reviewer commented that without actual samples there is no way to confidently predict which occupied structures within an ASC might be contaminated.
Response: The HRS weighting of targets in an ASC does not predict contamination concentrations in structures within the ASC
, but instead the HRS site score reflects the possibility of vapor intrusion occurring

. The HRS score is a relative ranking between sites and the quantitative impact on potential targets (i.e., those in the ASC) is not assumed to be ten times lower; a score does not correspond to actual risk levels at a site. The weightings are assigned to allow differentiation in site scores to reflect the relative risk among sites . The ten fold difference in weighting populations potentially exposed and those documented to be exposed (actually exposed) is used in all HRS pathways to emphasize the presence of actually exposed targets at sites

.
C onsistent with the NCP, a n HRS evaluation of a site is based on the data available at the time of the evaluation, and is designed to be performed after a n initial t ime

- limited site inspection has been completed

. Since the HRS is used to prioritize sites for further investigation and was directed in CERCLA to put emphasis on actually exposed populations, it is reasonable to assign a higher score to a site where populations are known to be exposed to indoor contamination, than to a site with no documented exposures. While the difference may be simply that one site has not been as thoroughly investigated as another site or different samples were collected , this situation is beyond the scope of the HRS regulation (specifying sampling requirements in a regulation could easily result in the regulation being outdated when new or improved sampling procedures are adopted). Any ranking system is34 s ubject to the quality of the inf ormation available ; h owever , based on peer review comments, the weighting of populations in areas of subsurface contamination has been revised to consider other sample types.

F urthermore, the weighting of populations located within an ASC has been modified to also consider the distance or depth to contamination from regularly occupied structures. (Sampling procedures for monitoring vapor intrusion in general is a topic being addressed in other EPA efforts.)

R egarding the assumption that subslab soil gas samples are reliable indicators of vapor intrusion and not the result of indoor sources , this is assumed only in situations where the subsurface contamination m eets observed release criteria. Observed release criteria include both that the subslab hazardous substance concentrations are significantly above background and that the increase can be attributed at least in part to the release from the site being evaluated and not entirely from anthropogenic indoor sources.

T his comment results in no change to the proposed addition to the HRS.

2.3.c .2 Weighting Based on Benchmark Comparison
Comment(s) : One peer reviewer suggested an alternative scheme for weighting targets within an ASC, based on both contaminant concentrations and health-based benchmarks. The reviewer recommended assigning a weighting factor of 0.1 to occupants with contaminant concentrations less than 50 times the health- based benchmark; a weighting factor of 1 to occupants with c ontaminant concentrations between 50 times and 500 times the health

based benchmark; and a weighting factor of 10 to occupants with contaminant concentrations greater than 500 times the health -based benchmark.
Response : This alternative weighting approac h has been considered, but because of the peer reviewers ’ comments on the large variability in vapor intrusion rates, consideration of this level of quantitative detail on projecting indoor contaminant concentrations is not recommended. R epresentative site contaminant concentrations may not be available based on the data collected during a typical short duration site inspection ; therefore, this approach to weighting potential targets has not been implemented . However, based on additional peer review responses, the weighting of potential targets has been updated in the proposed HRS addition to reflect the contaminant media both above (e.g., crawl space) and below ground (e.g., soil gas vs. ground water) and the depth or distance to contamination

. (Please see section 2.3.c, Target Weighting, of this response document regarding the proposed alternative weighting system.)

2.3.c .3 Weighting by Subsurface Conditions
Comment(s) : One peer reviewer suggested that th e potential for exposure in one part of a site should be considered the same as in another part of the same site, in areas of similar subsurface conditions, unless extensive data collected ha ve indicated otherwise. The reviewer stated that there is no basis for assigning a lower risk to an occupied structure with similar subsurface conditions to nearby structures that have observed exposure, just because impacts have not been observed.
Response: The agency considers it reasonable to assign different weighting factors to target populations depending on whether the populations are documented to be actually exposed to subsurface intrusion

. Documented exposure to a hazardous substance based on a limited sampling event indicates that a site poses higher re lative risk , and warrants further investigation, than sites w it h no documented exposure to a hazardous substance. EPA acknowledges that a potentially exposed target may be subject to exposure at different time periods than that during a site inspection . Ho wever, an HRS evaluation is based on currently available data and a site may be re

- evaluated if additional data are obtained in the future. The proposed HRS addition contain s a provision for inferring that contamination is entering structures surrounded by other contaminated structures , and35 f or assigning a weighting of targets in the structures with inferred contamination unless information indicates otherwise.

T his comment results in no change to the proposed addition to the HRS.

2.3.c .4 Weighting by Sample Type
Comment(s) : Several peer reviewers commented that the method of weighting targets may result in a low bias to population risks. The reviewer commented that the method is dependent on the type of samples collected, and noted that low bias might particularly happen at sites where subslab samples are taken but no indoor samples are collected. Additionally, reviewers commented that the method for weighting targets did not fully consider subslab data and undervalued subslab data relative to indoor da ta.
Response: A site with limited subsurface and indoor sampling might receive a lower HRS score tha n a site with more extensive sampling, especially if a more extensive sampling event documents indoor exposures or contamination at shallower depths. It h as also been well documented that due to the variability in vapor intrusion rates with time, it is possible that short duration sampling events may misidentify whether subsurface intrusion is occurring or will occur under different conditions . Therefore, w hen an ASC has been identified (i.e., no actual exposures documented in a specific area but sufficient sampling has been performed to demonstrate subsurface contamination is present ), EPA is proposing to evaluate target populations in this area as potentia lly exposed. In the case of an ASC delineated at least in part by indoor air samples meeting observed release or observed exposure criteria, target populations with in the ASC would also be evaluated as potentially exposed.

Re g arding the possibility of co nservative bias, t hese potentially exposed populations are being proposed to be weighted at a higher value than potential targets in other HRS pathways and components, due to the high variability in vapor intrusion rates and the difficulty in obtaining rep resentative samples in a short duration sampling event . In other HRS pathways and components, the maximum weighting factor is 0.1 for potential targets. In the subsurface intrusion component, the maximum weighting proposed is 0.9 and the minimum factor is 0.1.

F urthermore
, if fu rther sampling performed at a later time shows a greater degree of contamination, the HRS site score can be re

- evaluated and the decision regarding whether to place the site on the NPL can be reconside red

.
2.3.c .5 Weighting Factor When Not Ten Times Benchmark
Comment(s) : One peer reviewer commented that in weighting potential targets in structures where soil vapor or shallow ground water samples contain contaminants but subslab or enclosed crawl space samples do not exceed ten times the health -based benchmark, the residents should be multiplied by 0 in place of 0.1.
Response: The proposed addition has been modified to not compare subsurface contaminant concentrations to health

- based benchmarks when e valuating potential targets ( s ee section 2.3.b.4, Weighting Based on Exceeding Benchmark, of this response document for additional information). However, a ssig ning a weighting greater than

zero to potential targets is consistent with the overall HRS structure reflecting the limited sampling events prior to an HRS eval ua tion . Using a non- zero value reflects the potential for intrusion into a regularly occupied structure to have occurred or to occur in the future. Furthermore, a site with potential targ ets poses a greater relative risk than a site containing no potential targets ; therefore, a factor value of zero is not appropriate because it would not account for differences among sites that either do or do not have potential targets.36 2.3.c .6 Weighting Based on Subslab Samples and Ground Water Samples
Comment(s) : One peer reviewer commented that when determining the population in an ASC, applying a higher weight to subslab samples than to ground water samples assumes a higher confidence in subslab sample data.
Response: The a gency does have more confidence that populations located in an ASC will be ex posed to subsurface intrusion of a hazardous substance if a subslab sample documents contamination compared to only if ground water contamination is documented. I f the contamination is in subslab samples in an ASC , volatiliz ation from the ground water has already occurred and contamination has moved through the subsurface media under neath a structure. Additionally, the 95 th percentile attenuation facto r from the 2012 EPA evaluation of EPA ’s Vapor Intrusion Database is higher for subslab samples than it is for ground water samples, indicat ing that more of the contamination in subslab samples is more likely to enter structure s than that in ground water sa mples.

T his comment results in no change to the proposed addition to the HRS.

2.3.c .7 Weighting Factors for Crawl Space Samples and Subslab Samples
Comment(s) : Peer reviewers commented that the proposed addition to the HRS evaluates and weights crawl space data and subslab data similarly, but notes that EPA generally considers the attenuation factor between crawl spaces and indoor spaces to be 1. Reviewers commented that crawl spaces and subslab samples should receive different weighting factors based on the most recent EPA guidance.
Response : Based on the peer review comments and a 2012 EPA evaluation of EPA ’s Vapor Intrusion Database
, the methodology for evaluating the populations within an ASC has been modified. These changes reflect the most recent EPA vapor intrusion guidance. As mentioned in section 2.3.c, Target Weighting, of this response document, t he analysis of EPA ’s Vapor Intrusion Database as of 2012 shows that there are statistically significant correla tions by sa mple type and depth in the ratio of the concentration in subsurface samples and in indoor air samples

. These findings have been used to develop a revised weighting system (see Table 5 -20, Weighting Factor Values for Populations within an Area of Subsurface Contamination , of the proposed addition to the HRS for further information). In this revised weighting system, crawl space and subslab samples are associated with different weightings for targets in an ASC. Populations associated with regularly occupied structures containing semi

- enclosed or enclosed crawl space samples meeting observed release criteria would be assigned a target weighting of 0.9.

2. 3.d Consistency of Weighting Factor s with Database/Guidance
Comment(s) : One peer reviewer comm ented that in estimating a population in an ASC, the requirement of being ten times above a health -based benchmark is inconsistent with EPA vapor intrusion guidance.
Response: Based on additional peer review responses, the weighting of potential targets has been updated in the proposed addition to reflect the contaminant media both above (e.g., crawl space) and below ground (e.g., soil gas

vs . ground water) and the depth or dist ance to contamination . In addition, the proposed addition to the HRS was modified to not compare subsurface contaminant concentrations to health-based benchmarks when evaluating potential targets (see section 2.3.b.4, Weighting Based on Exceeding Benchmark, of this response document for additional information ).

EPA a lso notes that the workgroup developing this proposed addition to the HRS has worked with EPA ’s vapor intrusion guidance workgroup to ensure consistency between the two documents. However, in s ome cases , because the HRS is applied at a different stage in the Superfund process when only limited data are available , the HRS methodology may be different than that generically discussed in other guidance documents designed for different stages in the risk assessment and remediation process.37 Mo re specific comments on the relationship between weighting factors and existing data are addressed in the following subsections:

• 2.3.d.1 I nconsistency of Attenuation Factors

• 2.3.d.2 Analyzing Raw Data to Determine Weighting Factors

2.3. d.1 Inconsistency of Attenuation Factors
Comment(s) : One peer reviewer commented that the current attenuations factors in the draft SsI addition for weighting targets are inconsistent with previous analysis of vapor intrus ion data.
Response: The attenuation factors being used to develop the proposed addition have been updated to the 2012 EPA values 14 . These 2012 values have been used to revise the weighting system being proposed. While recent studies have shown that the va riability in vapor intrusion rates over time demonstrates that a simple arithmetic average exposure level cannot be projected with confidence, the 95 th percentile attenuation factor does reflect a reasonably worst case scenario for projecting indoor concen trations due to vapor intrusion. Given the HRS is a screening tool used to identify sites for further investigation, the large variation in intrusion rates and that the HRS is not predicting actual intrusion rates but rather only a relative dif f erence in r ates under different circumstances, it is reasonable to use the 95 th percentile factor when identifying sites for further investigation.

2.3. d.2 Analyzing Raw Data to Determine Weighting Factors
Comment(s) : One peer reviewer recommended that the raw dat a from EPA ’s Empirical Database [Vapor Intrusion Database] should be reviewed when determining the weighting factors to be applied to potential targets.
Response: The agency has reexamined the raw data from EPA ’s Vapor Int rus ion Database (as of 2012 ) in response to the peer review comments. Based on additional peer review comments, the weighting of potential targets has been updated in the proposed addition to reflect the contaminant media both above (e.g., crawl space) and below ground (e.g., soil gas vs. ground water) and the depth or distance to contamination. EPA 's Vapor Intrusion Database was also re evaluated and used to determine the relative weightings used in the revised target weighting system ( see Table 5 -20, Population Weighting Factor Values for Populations within an Area of Subsurface Contamination, of the proposed addition to the HRS for further information).

2. 3.e Consider ation of Biodegradation of Hazardous Contaminants in Weighting Targets
Comment(s) : One peer reviewer commented that the study serving as the basis for the tenfold difference used to assign a higher potential target weighting was based only on halogenated VOCs and therefore should not necessarily be used for hydrocarbon VOCs.
Response: The agency agrees that hydrocarbon VOCs may have a different fate in the subsurface than halogenated VOCs. Biodegradation of hydrocarbon VOCs can play a maj or role in subsurface intrusion and, in response to peer review comments
, EPA has added a degradation factor in the proposed addition to the HRS

. Th e degradation factor considers the difference in half- lives of different hazardous substances, e.g., between petroleum or petroleum- like hazardous substances, which have the greatest potential to biodegrade
, the presence of biologically active soil, and the depth to contamination. However, because a substance will degrade at a

14 Please see EPA ’s 2012 evaluation of its Vapor Intrusion Database written by Dr. Helen E. Dawson and Dr. Richard B. Kapuscinski

. March 16, 2012.38 sp ecific site depends on both the substance and site- specific conditions, it was determined that it would be more appropriate to include this factor in the evaluation of the HRS waste characteristics factor category value and not in the weighting of targets . Please see Section 5.2.1.2 (and all applicable subsections) of the proposed addition to the HRS and the proposed addition ’s TSD for additional information.

EPA also no tes that hydrocarbon VOCs associated strictly with petroleum releases are excluded fr om consideration under CERCLA

.
2. 3.f Evaluating Vacant/Non

- occupied Structures
Comment(s ): One peer reviewer commented that regularly occupied structures that have been vacated due to site

related contamination, be evaluated based on the probability that the structure will be occupied again.
Response : The definition in the HRS of "occupied str uctures" contains the phrase, "present on a regular basis ; " therefore for a structure to be eligible for evaluation , people must be regularly present in the structure. Specific criteria for meeting th e “regular basis” requirement are not set in the HRS and are evaluated on a site

- specific basis. The criteria or guidelines used in the evaluation of “regular basis” will be consistent with th ose for identifying targets in other HRS pathways (e.g., the criteria for identifying the number of people served by a

r esidential well ) . These criteria have included considerations that relate to the probability of exposure and the duration of exposure to contamination, such as the frequency and duration of occupancy, seasonality of occupancy and county occupancy averages obtained from the U.S. Census Bureau . However, because of the large number of possible site- specific situations
, it is unrealistic to develop specific requirements that will address all possible situations.

T his comment results in no change to the proposed addition to the HRS

.
2. 3.g Evaluating Future Targets
Comment(s) : A peer reviewer commented that the current structure of the draft SsI addition to the HRS does not consider future target structures or populations above an ASC where structures ar e not currently present ; the reviewer also suggested adding default structure properties that could be scored in the absence of actual structures

. Response : Consistent with the HRS being used as a tool to identify sites that most warrant further investig ation
, an HRS evaluation is focused on current conditions that exist at a site. Sites with regularly occupied structures above subsurface contamination are a higher priority for further investigation than sites where future structures could be built above subsurface contamination or to which subsurface contamination might migrate in the future

. The agency did consider reflecting the possible migration of hazardous substances through the subsurface in an HRS evaluation, but based on the current state of the science of horizontal subsurface vapor migration modeling, the agency considered the available scientific knowledge in sufficient to support such an evaluation . Similarly, to predict the construction of future structures is speculative and not defensible. However, if situation s change at a particular site in the future , the site can be re- evaluated using any new information.

T his comment results in no change to the proposed addition to the HRS

.39 

### 2.4 Charge Question #4

W

hen calculating waste quantity and population, is it scientifically reasonable for populations in multi

- story or multi -tenant structures to be evaluated based on using only the population inhabiting the intruded space and the population on the floor above and below the intruded space (For example, if contamination is found on the first floor, population from the basement and second floor would be used in the calculation. If contamination is found on the fifth floor, population from the fourth floor and sixth floor would be used in the cal culation .)? If no, what alternative approach would you recommend?

T he SsI component, at the time of peer review
, generally evaluate d targets and hazardous waste quantity in multi

story/multi -tenant buildings based on those areas and individuals that reasonably can be exposed to subsurface intrusion. That is, f or the purposes of calculating hazardous waste quantity and the target population for the subsurface component , targets are counted as follows: i f there is no observed exposure w ithin the structure, only th ose targets in the lowest story would be counted; or, if there is observed exposure in any story, those targets in that story, one story above and all stories below the story would be counted. This is similar to the approach for identifying targets and hazardous waste quantity in the 1990 soil exposure pathway, in that in the 1990 HRS soil exposure pathway, targets must be able to come into contact with areas of contamination for both them and the contamination to be considered e ligible for consideration in the soil exposure evaluation.

*N ote: The proposed HRS addition has undergone revision since it was submitted for peer review in 2011. The current version of the proposed HRS addition has replaced the term “multi -tenant” with “ multi -subunit”. This appendix reflects the use of “multi -tenant” in charge questions and peer review comments since that was th e l anguage used at the time of the peer review. In responses , the new “multi- subunit” term is used for consisten cy
w ith the propo sed addition. This change in terminology was made to reflect that tenant spaces within a structure may not actually have different features such as dividing walls or different heating, ventilation and air conditioning ( HVAC ) systems, but has no effect on the responses to the peer review comments

.
2. 4.a General Support for the Approach
Comment(s) : Peer reviewers commented that the overall approach for evaluating the waste quantity and population in multi-story or multi- tenant structures is reasonable and appropriate.
Response: While the reviewers generally agree d with the approach for evaluating multi

- story/multi -subunit regularly occupied structures, the consideration of other peer review comments resulted in modifications of the proposed addition.

2. 4. b Consider P referential P athways B etween F loors
Comment(s) : Peer reviewers commented that the draft SsI addition should consider preferential pathways (e.g., pipe/electrical chases, elevators) between floors when evaluating multi -story and multi- tenant structures. The reviewers commented that if a preferential pathway to a higher floor exists and there is a possibility of contaminant transport, then tho se targets on the affected floors should be considered in the scoring unless information exists to indicate otherwise.
Response: To allow for consideration of situations on a site

- specific basis
, such as where contaminant transport within a structure mig ht be impacted by specific building features (i.e., preferential pathways) , t he phrase "unless available information indicates otherwise" has been incorporated into the text of the proposed addition (see Section 5.2.0 of the proposed addition to the HRS). Therefore, if site

- specific conditions document the presence of effective preferential pathways, such as elevator shafts and centralized HVAC systems that enhance migration40 of

contamination throughout the structure, then the weighting of the populations in different parts of the structure can reflect the site

- specific situation and be considered in the HRS evaluation.

E PA acknowledges that elevators and/or conduits have been demonstrated to represent a preferential pathway for intruded vapors in multi- story (multi

- level) buildings. However, depending on the design of the structures and the location of the preferential pathways, these pathways may not allow even distribution of contamination throughout the entire building; therefore, consideration of these s ite

- specific conditions will be addressed as they arise.

2. 4.c Inferring Level I Contamination
Comment(s) : One peer reviewer commented that in the case where multi

- tenant residences contained confirmed Level I targets in either multiple basement residences or in residences directly above the basements, then all basement residences should be considered L evel I targets due to indoor air quality variability.
Response : Target populations in multi-story or multi-subunit structures will not be inferred to be exposed to levels at or above the appropriate benchmarks (Level I) unless site

- specific information indicates otherwise. Due to the documented potential for spatial and temporal variability in contaminant concentrations in the subsurface environment and th e variation of the movement of contamination from the subsurface environment into occupied multi

- story/multi -subunit structure (as well as within the structure), Level I concentrations are only assigned to units or subunits with indoor air samples confirming that targets are exposed to contamination above appropriate benchmarks. However, target populations may be inferred to be exposed to Level II concentrations if they are located in an area of observed exposure. This approach is consistent with how target populations are evaluated in the residential threat of the current soil exposure pathway in the 1990 HRS.

T his comment results in no change to the proposed addition to the HRS

.
2. 4.d Consider Discontinuous Contamination
Comment(s) : One peer reviewer commented that the example exposure scenario presented in charge question #4, where contamination is measured only on the 5 th floor of a multi-story structure, is highly unusual and inconsistent with the proposed addition to the HRS. The reviewer commented that all residences in a multi

story/multi -tenant building beneath a floor where contamination has been found should be factored into the waste quantity and target population calculation, not just the one floor above and below the observed exposure.

Res ponse : The reviewer is correct that the example exposure scenario presented in the charge question is inconsistent with the method described in the proposed addition . The method presented in the proposed addition to the HRS, not in the charge question, is the intended approach. All populations on floors below a documented actual exposure will be considered actually exposed (within an AOE) and the population one floor above will be considered potentially exposed (within an ASC) unless information indicates otherwise. Furthermore, Sections 5.2.0 and 5.2.1.3.2.3 of the proposed addition to the HRS have been modified to clarify how multi-story /multi

subunit buildings are evaluated. This example of an observed exposure on the 5 th flo or of a structure may not be common, but this example was provided to illustrate a generic approach.

2. 4.e Consideration of Lower Floors
Comment(s) : Peer reviewers questioned why a residence ( or floor ), which has not been sampled, in a multistory/multi -tenant structure would ever be discounted if the residence ( or floor ) is located beside or beneath a documented observed exposure. Reviewers commented that all residences (or floors) in a multi- story/multi -tenant building beneath a residence where an observed exposure has been documented should be considered as actually exposed and not discounted.41 Response : The proposed addition to the HRS has been clarified to identify that all targets on and below floors (or levels) with a documented actual exposure are considered to be actually exposed. However, due to the documented potential for spatial and temporal variability in contaminant concentrations in the subsurface environment, and the variation of the movement of contamination from the subsurface environmen t into an occupied multi-subunit structure (as well as within the structure), Level I concentrations are only assigned to units with indoor air samples confirm ing that targets are exposed to actual contamination above appropriate benchmarks.

I n the proposed addition, unless available information indicates otherwise, target populations residing on lower floors within a regularly occupied structure where subsurface intrusion is found are also evaluated as actually exposed to contamination (at Le vel II concentration levels). The population residing one floor above will be considered potentially exposed unless information indicates otherwise.

T he proposed addition's instructions have also been amended to include the phrase "unless information ind icates otherwise" to allow site

- specific conditions to be considered in assigning levels of exposure. Furthermore, Sections 5.2.0 and 5.2.1.3.2.3 of the proposed addition ha ve been modified to clarify how multi- level /multi

subunit buildings are evaluated.

2. 4.f Weighting Based on Basement Samples
Comment(s) : One peer reviewer commented that in a multi

- story/multi -tenant structure where only an unoccupied basement has been sampled and found to be contaminated, the weighting of the occupants on the first le vel of the building should be based on the concentration found in the basement.
Response : The suggested weighting system provided by the peer reviewer has not been adopted for use in the proposed addition. Contamination in a multi

- story/multi -subunit structure cannot be assumed to move from the lower floor, including a basement, to an upper floor without some amount of dispersion or dilution. Thus, intruded vapors cannot be assumed to reach the occupied units in an above floor and spread evenly into ea ch unit or at comparable rates, unless site

- specific information indicates otherwise.

H owever, the modification in the proposed addition to the target weightings for populations in an area of subsurface contamination allow for the population residing on the floor above an observed exposure to now be weighted at a 0.9 value (as compared to a weighting of 1.0 for actually exposed targets); this value reflects attenuation factors presented in an EPA evaluation of EPA 's Vapor Intrusion Database as of 2012. Th e contaminated sample, collected in the basement in this scenario, is considered conceptually to be equivalent to a crawl space sample collected from a single family residence, and is therefore weighted the same as a crawl space sample in an HRS evaluation . Furthermore, populations residing within regularly occupied tenant spaces located above a level with a gaseous indoor air sample meeting observed release criteria but no residential tenants on this lower level would also be assigned a weighting factor value of 0.9.

2. 4. g Evaluating Worker Exposure in Multi

- Story Buildings
Comment(s) : One peer reviewer commented that the proposed addition should consider preferential pathways between floors when evaluating the workers factor value.
Response: In response to this comment, and other comments, EPA has removed the separate factor the worker factor value from the proposed addition to the HRS . Workers are now included in the same category as other exposed individuals, and are therefore evaluated in a si milar manner as an exposed individual or population within a multi -story building . The evaluation still reflects that workers are exposed to indo or air at the workplace42 f or a different time period than residents at home. The contribution of workers to the population factor value also varies depending on whether they are full - or part- time employees.43 

### 2.5 Charge Question #5
Because of longer exposure times indoors, is it reasonable to assume that the relative risk is three times greater for worker s exposed to subsurface intrusion within the workplace than it is for workers exposed to outdoor surface contamination at the workplace? If no, please explain.

The draft SsI addition to the HRS, at the time of peer review
, assigned a target factor value t o workers based on the number of workers actually exposed or inferred to be exposed to contamination, as is performed in an evaluation using the soil exposure pathway in the 1990 HRS. In the 1990 HRS, for the resident population threat of the soil exposure pathway, a factor value for workers is assigned based on the total number of workers present within the area of observed contamination. These factor values range from 0 to15, depending on the number of workers exposed to outdoor contamination. However, it is anticipated that workers spend at minimum a larger percentage of their time indoors . T herefore , these workers may be exposed longer to contamination resulting from subsurface intrusion than would workers exposed to outdoor soil (or other surface) conta mination

. EPA has changed the method of evaluating workers in the proposed addition to the HRS based upon comments received. The previous version of the proposed HRS addition assigned a factor value to workers in a similar method to the existing soil exposure pathway which included a maximum factor value . The proposed addition to the HRS now includes workers in the population target factor value ; this change enables workers to be evaluated by the proposed addition to the HRS using health- based benchmarks, and removes the maximum limit that workers can contribute to the site score.

2. 5.a Support for Worker Factor Value
Comment(s) : Peer reviewers agreed that it is reasonable to weight workers exposed to subsurface intrusion in regularly o ccupied structures at three times the relative risk to workers exposed to outdoor contamination in an area of observed contamination because of longer exposure times indoors.
Response: As noted above, EPA has changed the method in which workers are evalu ated in the proposed addition to the HRS since the peer review. The comment was comparing the method of evaluating workers in a previous version of the proposed addition to the HRS to the method of evaluating workers in the existing soil exposure pathway. The current proposed addition to the HRS no longer evaluates workers in manner similar to the existing soil exposure pathway.

2. 5.b Indoor Air Exposure and Worker Risk
Comment(s) : One peer reviewer commented that the current method for assigning factor values to weight indoor versus outdoor worker exposure would underestimate the relative risk of exposure for indoor workers. The peer reviewer commented that an indoor worker is expos ed to contaminated air in a limited volume compared to outdoor air that is not constrained and is diluted by atmospheric air. The peer reviewer suggested that the draft SsI addition’s TSD contain support to justify the weighting factors used and the increa sed relative risk for indoor workers.
Response: As discussed above , workers are now included in the population factor value, and are therefore evaluated against health -based benchmarks in areas of observed exposure ( AOEs ). This method will help to more

a ccurately assess the relative

- risk to workers, whereas the previous method of using a separate worker target factor value, which did not consider if the exposure was above health- based benchmarks , could potentially have underestimated the risk to workers.44 2. 5.c Rationale for Worker Factor Value
Comment(s) : One peer reviewer commented that, while it is reasonable to assume that the risk is greater for indoor workers , the basis for using a factor of three to score indoor workers higher has not been explained and should be included in the proposed addition.
Response : The basis for the threefold increase in the weighting of worker targets, as discussed in the proposed addition's companion TSD, was based on the projected length of exposure to indoor air contaminants being at least three times greater for those working in an indoor area compared to workers exposed to contaminated soil in general. Given the diversity of possible exposure scenarios, it was determined that a threefold increase in the assigned values was a conservative, defensible projection. The rationale for this threefold difference has been reconsidered, and instead EPA has weighted workers based on the difference in exposure time for full

- time workers of 8 hours a day and part- time workers of 4 hours a day versus the typical exposure scenario for residents of as high as 24 hours a day. The agency recognizes that this ratio is not exact for all situations, and that to determine the most appropriate ratio of exposure would require site- specif ic information beyond that collectable during a typical site inspection

. If a more universally

- accepted ratio is identified during public comment on this issue, it will be considered at that time.

I n addition, a s discussed above, workers are now included in the population target factor value, and are therefore evaluated against health -based benchmarks in AOEs. The comment provided was based upon the previous version of the proposed HRS addition and is no longer applicable to the current version.

2. 5. d Wo rker Factor Value Correlation
Comment(s) : One peer reviewer questioned the comparison of indoor vapor inhalation to outdoor soil exposure for workers and stated that a fixed correlation between the two is inappropriate.
Response : As noted above in sectio n 2.5.c, EPA has changed the method of evaluating workers in the proposed HRS addition based upon comments received. The method for evaluating workers in the current version of the proposed HRS addition has changed to reflect th at the length of exposure to a worker in a workplace as compared to exposure scenarios for residents at home is considerably less. This comment results in no change to the proposed addition to the HRS.

2. 5. e Worker Benchmark Values
Comment(s) : One peer reviewer commented that the benchmarks for workers should be based on industrial or commercial screening values and not residential values.
Response : As noted above, EPA has changed the method of evaluating workers in the proposed HRS SsI addition based upon comments received. In the current version of the proposed HRS addition workers are included in population. The suggested HRS factor value for workers in the version of the draft version of the SsI addition presented for peer review was not we ighted based on a comparison of the level of contamination to a health

- based benchmark, calculated based on residential exposure scenarios. The benchmarks for comparison for populations in the proposed HRS addition are the oral cancer slope factor and the reference concentration for inhalation exposures. However the benchmarks to be used are for the worst case exposure scenario regardless of land use , as is done in all other pathways and components of the HRS. It would be beyond a screening evaluation to p recisely establish a site

- specific benchmark reflecting different exposure sce narios (Note : T he relative weighting for full and part- time workers is 1/3 and 1/6, respectively , which reflect s a difference in a typical daily exposure length for workers and residents.)

T his comment results in no change to the proposed addition to the HRS.45 

### 2.6 General Comments Affecting All Charge Questions
The following comments apply generally to the draft SsI addition to the HRS as well as each of the 5 charge questions p resented above

.
2. 6.a Definitions
Comment(s) : One peer reviewer questioned whether the terms “gas” and “vapor” have the same meaning or if the proposed addition to the HRS intends to define them differently than one another.
Response: The terms “gas” a nd “vapor” are used as synonyms in many EPA guidance documents; however, technically they can hold different meanings. Scientifically, gases refer to a substance in a gas phase while a vapor is a suspension in a gas. Because of this issue, definitions of t hese terms have been included in the proposed addition's TSD. The agency also notes that for the purposes of the proposed addition, the term "vapor intrusion," includes the intrusion of both vapors and gases and that the two terms are used interchangeably for this purpose.

2. 6.b Consistency with Vapor Intrusion Guidance
Comment(s) : One peer reviewer commented that the draft SsI addition to the HRS is in general inconsistent with the current EPA - Office of Solid Waste and Emergency Response ( OSWER ) 15 and EPA

- OUST vapor intrusion guidance documents.
Response : EPA workgroup developing this proposed addition to the HRS has worked with the vapor intrusion guidance workgroups to ensure consistency among the t hree documents. However, in some cases
, because the HRS is applied at a different stage in the Superfund process when only limited data are available
, the HRS methodology may be different than the methodologies in other guidance documents that are designed for different stages in the risk assessment and remediation processes

.

15 During the peer review period, EPA’s OLEM was known as the Office of Solid Waste and Emergency Response.46 

## 3.0 Comments Outside the Scope of the Charge
In addition to comments directly related to the five charge questions, peer reviewers submitted comments on topics not included in the charge questions. These comments are acknowledged in the following subsections, and have been divided into technical and policy comments. While these comments are not responded to in this document, they were considered in developing the proposed addition.


### 3.1 Technical Comments Outside the S cope of the Charge
Of the p eer reviewers ’ comments on topics outside the scope of the charge questions
, some were technical in nature —that is, they are comments on the rationale for the approaches used in the Hazard Ranking System ( HRS ) or in the implementation of the HRS . The U.S. Environmental Protection Agency ( EPA ) reviewed these comments and, where applicable, made changes to the draft subsurface intrusion (SsI) addition. An individual response is not provided for these comments in this document ; as th ey were related to sections of the 1990 HRS that are not being modified or were beyond the original charge put to the peer reviewers. However, because the comments involve the technical content of the proposed addition, they are summarized below.

3. 1. a Ge neral Comments

General c omments on HRS

- related terminology and the equation used to derive an HRS site score were provided and are summarized below:

• S uggested modifying the terminology used in the draft SsI component (HRS Section 2.1.3) to identify sources or areas of contamination.

• Agreed with the form of the HRS site score equation and how the proposed subsurface intrusi on
c omponent was incorporated into it

.
• Q uestioned the definition of the draft SsI component as a single threat evaluation if def ined to include intrusion by liquids (direct contact and inhalation) and vapors (inhalation). Additionally, it was noted that subsurface intrusion threats could include the potential for explosions

.
3. 1. b Likelihood of Exposure /Release
Several comments were submitted on aspects of HRS observed release criteria and the establishment of an observed exposure as described in the draft SsI addition at the time of peer review

. These comments are summarized below:

• Q uestioned the identification and use of solid or liquid substances as a means to identify an observe d

e xposure by direct observati on.

• I nquired if there was a criterion for establishing an observed exposure in the draft SsI addition to the HRS

.
• Q uestioned the suitability of the criteria used to estab lish an observed release of a hazardous substan ce unde r the HRS for evaluating the “vapor intrusion pathway.”

• Suggested restricting observed release evaluations to using ground water and soil gas data and not indoor air data

.
• Q uestioned how conflicting l ines of evidence (e.g., detections in ground water but not soil gas) would be evaluated

.
• S uggested that if the background level is elevated above an applicable benchmark, any indoor air result greater than that level should be considered elevated under the draft SsI component

.47 • I nquired what the evaluation procedures would be for a situation where “health levels of concern are at or below background levels.”

• Recommended the use of reference values instead of nearby sampling data to establish background levels.

• Recommended either including additional discussion on the appropriate use of background levels in the draft SsI component or references to supplementary discussions o f this topic in other rulemaking documents

.
• N oted that the rulemaking documents fo r the draft SsI component did not discuss the type of values to be used in setting background levels (e.g., average concentration, maximum concentration). If a discussi on

w as not provided, it was suggested that the agency provid e a reference to an appropriate guidance document

.
• No ted that as indoor air can enter the subsurface, the identification of a hazardous substance in both the subsurface and indoor air does not “conclusively prove that indoor concentrations are resulting from SsI.”

3. 1. c Wa ste Characteristics:

Peer reviewers submitted comments regarding the evaluation of waste characteristics as described in the draft SsI addition at the time of peer review

. These comments are summarized below:

• Q uestioned the use of categories A, B and C as weight -of- evidence classifications for carcinogenicity

.
• Q uestioned why the changes in reference concentration ( RfC ) values in HRS Table 2 -4 of the draft SsI addition to the HRS do not increase at the same rate as the assigned toxicity values.

• Recommended that to “capture true risk” to not assign the toxicity factor value based on only one substance

.
3. 1. d Targets

P eer reviewers submitted comments on the evaluation of targets as described in the draft SsI addition at the time of peer review . T hese comments are summarized below :

• Q uestioned assigning a value of 20 for the exposed individual factor value if at least one eligible receptor is located in an occupied structure within an area of subsurface contamination (ASC)
, as there is no
ev idence that this individual is expose d.

• P roposed an alternative weighting system for actually exposed targets that assign ed one of three potential weightings based on comparison to a benchmark

.
• S uggested using long duration or multiple targeted shorter

- term ambient air sampling events to better represent long

- term indoor air exposure levels

.
• A greed with the agency’s rationale for not including sensitive environments as an eligible target category.



### 3.2 Policy Comments Outside the Scope of the Charge

Peer reviewers submitted several comments on policy related to the 1990 HRS and the HRS evaluation described in the draft SsI addition. Such comments are beyond the scope of the charge, and therefore neither summaries of these comments nor responses are provided. However, where applicable, modifications in response to the se suggestions were incorporated into the proposed addition.48 

### 4.0 Editorial Comments

Peer reviewers submitted several comments on the 1990 Hazard Ranking System ( HRS ) and the draft s ubsurface intrusion ( SsI ) addition that were editorial in nature. Such comments are beyond the scope of the charge, and therefore neither summaries of these comments nor responses are provided. However, where applicable, modifications in response to the se suggestions were incorporated in to the proposed addition.


## 5.0 List of Commenters
The peer review panel for the U.S. Environmental Protection Agency’s ( EPA’s ) Proposed Rule: Addition of a Subsurface Intrusion Component to the Hazard Ranking System included six individuals from the academic, government (both federal and state) , and private sectors representing a range of expertise in public health risk assessments, contaminated soil and ground water investigations and the topic of vapor intrusion . T he six members of the peer review panel are listed below:

T onia R. Burk, Ph.D. , Environmental Health Scientist, U.S. Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, Atlanta, Georgia. Dr. Burk’s current responsibi lities include serving as the agency’s expert on vapor intrusion and making recommendations on actions that serve to protect the public health.

B layne Hartman, Ph.D. , Vapor Intrusion Assessments of methane and volatile organic compounds and Soil Gas Support, Hartman Environmental Geoscience, California. Dr. Hartman specializes in vapor intrusion support services, with many years of experience conducting vapor intrusion assessments and soil gas sampling events. Dr. Hartman has served as a contributing auth or and editor to vapor intrusion and soil gas related guidance documents for federal and state agencies and private organizations. Dr. Hartman has also participated in multiple state, federal and private workgroups related to the field of soil vapor invest igation methods.

P aul C. Johnson, Ph.D. , Dean & Professor, Fulton Schools of Engineering, Arizona State University, Tempe, Arizona. Dr. Johnson is recognized for his contributions to the fields of soil and ground water remediation and risk assessment. Dr. Johnson has also authored multiple professional journal articles related to the sciences of vapor intrusion and subsurface contaminant monitoring and remediation.

K imiko A. Klein, Ph.D., Staff Toxicologist Emerita, California Department of Toxic Substanc es Control, Berkeley, California. Dr. Klein was a part of the Department of Toxic Substances Control’s working group to develop guidance for evaluating the intrusion of subsurface vapors into indoor air pathway.

D ot Lofstrom , M.S. , Section Chief, Geological Services Unit, California Environmental Protection Agency – Department of Toxic Substances Control, Sacramento, California. Ms. Lofstrom’s current responsibilities include serving as a supervisor of geologists working on investigations of and remediatio n activities of soil, soil gas and ground water contamination within the State of California. Ms. Lofstrom also serves as the vapor intrusion/soil gas collection specialist within the Department of Toxic Substances Control and was the lead author of the department’s Vapor Intrusion Mitigation Advisory and technical editor for the department’s Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (Vapor Intrusion Guidance)

.
H enning Larsen, R.G. and , M.S. , Senior Hydrogeologist, Oregon Department of Environmental Quality, Portland, Oregon. Mr. Larsen’s current responsibilities include serving as an agency expert on contaminant fate and transport modeling, developing and providing senior level review of hyd rogeological investigations, developing statewide policy, guidance and rule recommendations for the State of Oregon Cleanup Program. Mr. Larsen also was the coauthor/primary author of the State of Oregon’s Guidance for Assessing and Remediating Vapor Intrusion in Buildings

.