ontario soil vapour guidance_final draft_jan_12_11

8/7/2019 Ontario Soil Vapour Guidance_Final Draft_Jan_12_11

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ISBN ###-#-####-####-#

Draft Technical Guidance:

Soil Vapour Intrusion Assessment

November 2010

Ministry of the Environment

PIBS ####


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Table of Contents iii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS .............................................................................................. iTABLE OF CONTENTS ................................................................................................ iii

CHAPTER 1. INTRODUCTION1.1 Background................................................................................................................1-2

1.2 Objectives ..................................................................................................................1-2

1.3 Relationship to Other Guidance Documents..............................................................1-31.3.1 Existing Soil Vapour Intrusion Guidance..................................................................1-4

1.3.2 Relation of this Soil Vapour Intrusion Guidance Document and the Record of Site

Conditions (RSC), Ontario Regulation 153/04..........................................................1-61.4 Guidance Structure.....................................................................................................1-6

CHAPTER 2. OVERVIEW OF THE SOIL VAPOUR INTRUSION ASSESSMENT

PROCESS

2.1 Screening process.......................................................................................................2-22.1.1 Development of Conceptual Site Model....................................................................2-3

2.1.2 Preliminary Screening................................................................................................2-42.1.2 Comparison to the Generic Site Condition Standards ...............................................2-4

2.1.3 Screening Level vapour intrusion Assessment ..........................................................2-4

2.1.4 Detailed Vapour Intrusion Assessment......................................................................2-52.2 Guidance Application and Implementation Process..................................................2-5

2.3 Vapour Intrusion Mitigation......................................................................................2-7

2.4 Community Outreach.................................................................................................2-7

CHAPTER 3. CONCEPTUAL SITE MODEL

3.1 What is a Conceptual Site Model?.............................................................................3-23.2 Information Needed to Build the Conceptual Site Model .........................................3-2

3.3 Fate and Transport Processes for Vapour Intrusion...................................................3-3

3.3.1 Generation of Vapours and Gases..............................................................................3-4

3.3.2 Fate and Transport in the Vadose Zone.....................................................................3-53.3.3 Soil Vapour Intrusion through the Building Envelope ..............................................3-5

3.3.4 Mixing of Vapours inside the Building .....................................................................3-6

CHAPTER 4. SITE SCREENING PROCESS

4.1 Introduction................................................................................................................4-2

4.2 Preliminary Screening................................................................................................4-34.2.1 Site Characterization..................................................................................................4-3

4.2.2 Are There Chemicals of Potential Concern for Vapour Intrusion? ...........................4-4

4.2.3 Does the Site Represent a Safety or Acute Health Risk Concern?............................4-44.2.4 Are Buildings Located in Sufficiently Close Proximity to Contamination? .............4-5

4.2.5 Comparison to the Generic Site Condition Standards (SCS).....................................4-6

4.2.5.1 Precluding Conditions................................................................................................4-6

4.2.5.2 Additional Considerations .........................................................................................4-7

MOE Technical Guidance: Soil Vapour Intrusion Assessment 2010


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Table of Contents iv

4.3 Screening Level Assessment......................................................................................4-84.3.1 Site Characterization..................................................................................................4-8

4.3.2 Screening Level Vapour Intrusion Assessment .........................................................4-15

4.3.2.1 Determine Soil Textural Type ...................................................................................4-164.3.2.2 Select Land Use .........................................................................................................4-16

4.3.2.3 Estimate Distance to Vapour Contamination Source.................................................4-174.3.2.4 Derivation of Vapour Attenuation Factor..................................................................4-174.3.2.5 Adjusted Attenuation Factor Based on Building Mixing Height...............................4-21

4.3.2.6 Adjusted Attenuation Factor Based on Building Air Exchange Rate........................4-21

4.3.2.7 Adjusted Attenuation Factor Based on Biodegradation.............................................4-21

4.3.2.8 Back-calculate site specific screening levels for soil vapour and groundwater.........4-244.3.2.9 Data Evaluation and Next Steps ................................................................................4-24

4.3.3 Soil Vapour Screening Using the Modified Generic Risk Assessment Model .........4-26

4.4 Detailed Vapour Intrusion Assessment......................................................................4-264.4.1 Estimation of Input Parameters..................................................................................4-27

4.4.2 Indoor Air Quality (IAQ) Testing..............................................................................4-31

4.4.2.1 IAQ Study Design......................................................................................................4-314.4.2.2 Background Issues .....................................................................................................4-32

4.4.3 Data Evaluation and Next Steps ................................................................................4-33

4.4.4 Exposure Controls......................................................................................................4-33

CHAPTER 5. SOIL VAPOUR CHARACTERIZATION

5.1 Context, Purpose and Scope ......................................................................................5-2

5.2 Study Objectives........................................................................................................5-35.3 Soil Vapour Sampling Approach and Design............................................................5-3

5.3.1 Overview of Sampling Strategy.................................................................................5-3

5.3.2 Considerations for Sampling Locations.....................................................................5-4

5.3.3 When to Sample and Sampling Frequency................................................................5-125.3.4 Biodegradation Assessment .......................................................................................5-135.4 Soil Gas Probe Construction and Installation ............................................................5-15

5.4.1 Probes Installed in Boreholes.....................................................................................5-16

5.4.2 Direct Push Technology.............................................................................................5-175.4.3 Use of Water Table Monitoring Wells as Soil Gas Probes........................................5-18

5.4.4 Subslab Soil Gas Probes ............................................................................................5-18

5.4.5 Probe Materials..........................................................................................................5-205.4.6 Short-Circuiting Considerations and Shallow Probes................................................5-21

5.5 Soil Gas Sampling Procedures...................................................................................5-21

5.5.1 Probe Development and Soil Gas Equilibration ........................................................5-21

5.5.2 Flow and Vacuum (Probe Performance) Check ........................................................5-225.5.3 Sampling Container or Device...................................................................................5-22

5.5.4 Decontamination of Sampling Equipment.................................................................5-24

5.5.5 Testing of Equipment for Leaks and Short Circuiting...............................................5-255.5.6 Sample Probe Purging and Sampling ........................................................................5-26

5.6 Soil Gas Analysis.......................................................................................................5-28

5.6.1 Selection of Method...................................................................................................5-285.6.2 Field Detectors...........................................................................................................5-29



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Table of Contents v

5.6.3 Field Laboratory Analysis..........................................................................................5-315.6.4 Fixed Laboratory Analysis.........................................................................................5-32

5.6.5 Quality Assurance / Quality Control Considerations.................................................5-37

5.7 Ancillary Data............................................................................................................5-395.8 Data Interpretation and Analysis ...............................................................................5-42

5.8.1 Data Organization and Reporting ..............................................................................5-425.8.2 Data Quality Analysis................................................................................................5-425.8.3 Data Consistency Analysis ........................................................................................5-43

5.8.4 Further Evaluation ....................................................................................................5-44

CHAPTER 6. INDOOR AIR QUALITY TESTING FOR EVALUATION OF SOIL

VAPOUR INTRUSION

6.1 Context, Purpose and Scope ......................................................................................6-2

6.2 Conceptual Site Model for Indoor Air.......................................................................6-46.2.1 Background Indoor Air Concentrations.....................................................................6-4

6.2.2 Building Foundation Construction.............................................................................6-9

6.2.3 Building Ventilation...................................................................................................6-96.2.4 Building Depressurization and Weather Conditions..................................................6-11

6.2.5 Mixing of Vapours Inside Building...........................................................................6-11

6.2.6 Vapour Depletion Mechanisms..................................................................................6-126.3 Development of Indoor Air Quality Study Approach and Design ............................6-12

6.3.1 Define Study Objectives ............................................................................................6-12

6.3.2 Identify Target Compounds .......................................................................................6-13

6.3.3 Develop Communications Program...........................................................................6-136.3.4 Conduct Pre-Sampling Building Survey....................................................................6-14

6.3.5 Conduct Preliminary Screening .................................................................................6-14

6.3.6 Identify Immediate Health or Safety Concerns .........................................................6-14

6.3.7 Define Number and Locations of Indoor and Outdoor Air Samples.........................6-156.3.8 Define Sampling Duration.........................................................................................6-156.3.9 Define Sampling Frequency ......................................................................................6-16

6.3.10 Preparing the Building for Sampling and Conditions during Sampling ....................6-17

6.4 Indoor Air Analytical Methods..................................................................................6-186.4.1 Air Analysis Using USEPA Method TO-15..............................................................6-18

6.4.2 Air Analysis using Quantitative Passive Diffusive Samplers....................................6-19

6.5 Data Interpretation and Analysis ...............................................................................6-246.5.1 Data Organization and Reporting ..............................................................................6-24

6.5.2 Data Quality Evaluation.............................................................................................6-24

CHAPTER 7. METHODS FOR DISCERNING CONTRIBUTIONS OFBACKGROUND FROM INDOOR SOURCES

7.1 Constituent Ratios......................................................................................................7-2

7.2 Marker Chemicals......................................................................................................7-27.3 Spatial Trends ............................................................................................................7-3

7.4 Comparison of Indoor Air Data to Literature Background Concentrations ..............7-3

7.5 Modification of Building Pressurization....................................................................7-37.6 Emerging Methods.....................................................................................................7-4



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Table of Contents vi

7.7 Comparison of Measured to Predicted Indoor Air Concentrations ...........................7-4

CHAPTER 8. REPORTING AND DOCUMENTATION

CHAPTER 9. RFERENCES

CHAPTER 10. LIST OF ABBREVIATIONS AND ACRONYM

LIST OF APPENDICES

APPENDIX I. CONCEPTUAL SITE MODEL FOR VAPOUR INTRUSION AND

CONCEPTUAL SITE MODEL CHECKLIST ................................................. AI-1

APPENDIX II. IDENTIFICATION OF CONTAMINANTS OF POTENTIAL

CONCERN FOR VAPOUR INTRUSION.......................................................AII-1

APPENDIX II. SELECTED LABORATORY ANALYTICAL METHODS .......... AIII-1

APPENDIX VI. RECOMMENDED HEALTH-BASED INDOOR AIR

TARGET LEVELS FOR SELECTED VOC................................... AIV-1

LIST OF TABLES

TABLE 1 - Comparison of Different Media for Vapour Intrusion Investigations

TABLE 2 - Model Input Parameters for Screening Level Risk Assessment

TABLE 3 - Selection of Soil Texture Class

TABLE 4 - Decision Matrix for Recommended Actions

TABLE 5 - Comparison of Soil Vapour Measurement Locations

TABLE 6 - Evaluation of Factors Affecting Below Building Hydrocarbon VapourBioattenuation and Soil Vapour Data Representativeness

TABLE 7 - Soil Gas Sample Collection Containers and Devices

TABLE 8 - Summary of Common Soil Vapour Sampling and Analysis Methods

TABLE 9 - Dominant Sources of VOCs in Residential Indoor Air

TABLE 10 - Compilation of Indoor Air Quality Data from Canadian Studies



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Table of Contents vii

LIST OF FIGURES

FIGURE 1 - Flow Chart for Vapour Intrusion Guidance

FIGURE 2 - Conceptual Site Model for Vapour Intrusion Pathway

FIGURE 3 - Conceptual Model for Use of Soil Vapour Data

FIGURE 4 - Residential Groundwater to Indoor Air Attenuation Factors

FIGURE 5 - Residential Soil Vapour to Indoor Air Attenuation Factors

FIGURE 6 - Commercial Groundwater to Indoor Air Attenuation Factors

FIGURE 7 - Commercial Soil Vapour to Indoor Air Attenuation Factors

FIGURE 8 - Results of 3-D Oxygen-Limited Soil Vapour Transport Modeling forHigh Concentration Source (Cg = 100 mg/L) and ModerateConcentration Source (Cg = 20 mg/L) (from Abreu and Johnson,2005)

FIGURE 9 - Soil Vapour Sampling Locations and Vertical Profile Concept

FIGURE 10 - Lateral Transect Concept

FIGURE 11 - USEPA (2004) Recommended Design for Subslab Soil Gas Probes.

FIGURE 12 - Framework for IAQ Sampling and Analysis Program

LIST OF EXHIBITS

EXHIBIT 1 - Definition of Vapour Attenuation Factors

EXHIBIT 2 - Comparison of Generic Vapour Attenuation FactorsEXHIBIT 3 - Considerations for Cold Weather Sampling

EXHIBIT 4 - Preparation of Building for IAQ Sampling



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CHAPTER 1

INTRODUCTION

CHAPTER 1 INTRODUCTION.................................................................................................. 1-21.1 Background.......................................................................................................... 1-2

1.2 Objectives ............................................................................................................ 1-21.3 Relationship to Other Guidance Documents and Ontario regulations................. 1-3

1.3.1 Existing Soil Vapour Intrusion Guidance............................................................ 1-4

1.3.2 Relation of this Soil Vapour Intrusion Guidance Document and the Recordof Site Conditions (RSC), Ontario Regulation 153/04. ....................................... 1-6

1.4 Guidance Structure............................................................................................... 1-6



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DRAFT: DO NOT CITE OR DISTRIBUTE

Chapter 1. Introduction 1-2

CHAPTER 1INTRODUCTION

The Standards Development Branch (SDB) of the Ontario Ministry of Environment

(MOE) is currently involved in the development of a series of Technical Bulletins,Guidance Documents and Procedural Standards for contaminated sites. This Guidance

Document considers soil vapour behaviour, assessment and monitoring.

1.1 BACKGROUNDSoil vapour intrusion is the migration of volatile or semi-volatile chemicals

from contaminated groundwater and soil into overlying buildings. When

releases occur near buildings, volatilization of chemicals from the dissolved ornon-aqueous phases in the subsurface can result in the intrusion of vapour-

phase contaminants into indoor air. If the vapour intrusion pathway is viable

or complete, there may be the potential for unacceptable health risks tooccupants of buildings as a result of inhalation of these vapours. The focus of

this Guidance Document is an evaluation of the significance of vapour

intrusion with respect to potential chronic health risks due to long-term

exposure, as opposed to potential safety or acute risks which, in some cases,may arise from accumulation of vapours in buildings or confined spaces.

This Guidance Document is intended to provide the user with a basicunderstanding of soil vapour intrusion and the tools required to identify,

review and evaluate sites for vapour intrusion. This document is based on

current standards of practice and is generally consistent with recent guidance

from other sources (Health Canada, 2008, ITRC, 2007, EPRI, 2005). It shouldbe noted however, that the current standard methods of assessing vapour

intrusion are subject to considerable spatial and temporal variability, andchallenges resolving contributions of vapours from the subsurface compared

to background concentrations and analytical reporting limits for some

compounds. Therefore, it is reasonable to anticipate that new methods will

develop over time, and nothing in this document is intended to prevent thedevelopment and application of new technologies.

1.2 OBJECTIVESSpecific objectives of the Guidance Document are outlined below:

Promote the understanding of behaviour and migration of soil vapoursin the subsurface under various site conditions;

Assist in identifying sites where vapour intrusion may be viable, aswell as contaminants that may present a concern and when and wherevapour monitoring should be conducted;



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Specify the requirements and best practices for designing, conductingand assessing site conditions (i.e., soil vapour and subslab vapour

quality) that would enable accurate assessments of potential impacts toindoor air quality;

Provide a screening level assessment methodology for prediction ofpotential risks through inhalation of vapours migrating into indoor air,

with focus on the use of soil vapour data, and guidance on when

further detailed assessment may be warranted;

Provide guidance on the modeling of vapour transport from thesubsurface to indoor air;

Assist MOE staff in the review of site characterization and monitoringfor sites with volatile compounds present;

Assist MOE staff in identifying sites where soil vapours, subslabvapours and/or indoor air should be monitored and formulating

assessment requests and/or Environmental Protection Act (EPA)orders;

Provide assistance to proponents conducting soil vapour and subslabvapour investigations as part of assessment and/or remediation ofcontaminated and brownfield sites in Ontarios varied geological and

hydrogeological conditions; and,

Provide a general framework for relevant components of technicalreports including assessment and monitoring of soil vapours in the

subsurface where vapour intrusion is a viable pathway.

1.3 RELATIONSHIP TO OTHER GUIDANCE DOCUMENTS ANDONTARIO REGULATIONS

Over the past several years, there has been an increased recognition that soilvapour intrusion is an important potential exposure pathway. There have been

significant advances in understanding of vapour intrusion and in Canada,

regulatory guidance has been developed for Health Canada (HC, 2008) and bythe Canadian Council of Ministers of the Environment (CCME) (Golder 2008

and Geosyntec 2008), and by several provinces including British Columbia

(BC Ministry of Environment, 2009), and the Atlantic provinces (PIRI, 2006).

In the United States, regulatory guidance of note for vapour intrusion includes

those developed by the United States Environmental Protection Agency(USEPA, 2002), Interstate Technical and Regulatory Council (ITRC) (2007),

California Environmental Protection Agency Department of Toxic Substances

Control (2005), American Petroleum Agency (2005), Electric Power Research

Institute (2005), Tri-Services Environmental Risk Assessment Working Group(TSERAWG, 2008) and New Jersey Department of Environmental Protection

(2005).



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This document builds upon the above guidance and, in particular, incorporatesrelevant components of guidance developed for Health Canada, given the

significant effort that was undertaken to develop the approach, model and

supporting methods for this guidance. However, where warranted,

modifications have been made to reflect the policy of the Ontario MOE and

the conditions in Ontario.

This guidance reflects the current state of knowledge on vapour intrusion.Since vapour intrusion is a developing field of science, it is anticipated that the

MOE will update this document as needed to reflect refinements and advances

in the understanding of vapour intrusion.

1.3.1 Existing Soil Vapour Intrusion GuidanceA number of agencies in Canada, the US and Australia have recently

published guidance on evaluating soil vapour intrusion at sites contaminatedwith volatile compounds. The following section provides a brief overview of

key guidance. However, since it is a developing field, the reader should check

for updates of older guidance as well as guidance from other agencies, asrequired and appropriate.

In 2000, the CCME developed Canada-wide Standards for Petroleum

Hydrocarbon Compounds (CWS-PHC) that take into consideration the vapourintrusion pathway. The modeling approach used to estimate standards for

petroleum hydrocarbon fractions is based on the methodology developed by

Johnson and Ettinger (1991). The CWS-PHC was updated in 2008. Furtherdetails on the modeling approach can be found in the CCME (2008) technical

supporting document. The CCME soil standards for petroleum hydrocarbons

developed in 2000 were adopted by the Province of Ontario and are containedin Part X.V.1 of the EPA. Further details on the adoption of the CCME

standards are contained in the MOE document entitled Rationale for the

Development of Soil and Ground Water Standards for Use at ContaminatedSites in Ontario dated December 22, 2009.

The Atlantic Partnership in RBCA (Risk-Based Corrective Action)

Implementation (Atlantic PIRI) Committee has adopted a similar approach forthe assessment of petroleum hydrocarbons vapour intrusion that utilizes a

variation of the Johnson and Ettinger (J&E) Model. It is based on the ASTM

Standard E2081-00, Standard Guide for Risk-Based Corrective Action. In July2006, Atlantic PIRI published Guidance for Soil Vapour and Indoor Air

Monitoring Assessments under Atlantic RBCA Version 2.0 for Petroleum

Impacted Sites in Atlantic Canada User Guidance (Appendix 9).

The United States Environmental Protection released draft guidance in 2002

for evaluating vapour intrusion (US EPA, 2002). The guidance is intended for

use at RCRA Corrective Action, CERCLA (National Priorities List andSuperfund Alternative Sites) and Brownfields sites, but not at Underground



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Storage Tank (UST) sites. The document provides methods for assessingwhether sites are potential candidates for vapour intrusion and whether

vapours may be present at concentrations that pose an unacceptable exposure

risk. It follows a tiered approach to assessment with each tier having

increasing level of detail and complexity. The US EPA has publicly available

versions of the J&E model in spreadsheet format that are recommended forscreening level assessments. A detailed guidance document on use of the

models is also available from the US EPA website (US EPA, 2004).

The Interstate Technology and Regulatory Council (ITRC) released guidance

in January 2007 titled The Vapor Intrusion Pathway: A Practical Guide (VI-1). This Guide provides a generalized framework for evaluating the vapour

intrusion pathway and describes the various tools available for investigation,

data evaluation, and mitigation. The Vapor Intrusion Pathway: InvestigativeApproaches for Typical Scenarios (VI-2) (January 2007) is a supplement to

Vapor Intrusion Pathway: A Practical Guide. The supplement describes

applicable approaches for evaluating the vapour intrusion pathway in sixtypical scenarios.

http://www.itrcweb.org/Documents/VI-1.pdf

http://www.itrcweb.org/Documents/VI-1A.pdf

The California Environmental Protection Agency (Cal EPA) released guidance

in 2005 titled Interim Guidance for the Evaluation and Mitigation of

Subsurface Vapor Intrusion to Indoor Air on the evaluation and mitigation ofsubsurface vapour intrusion to indoor air (Cal EPA, 2005). It is a

comprehensive document that includes guidance on identifying candidate sites

for vapour intrusion, outlines data requirements for evaluating indoor air

exposures, describes sampling methods and provides an overview onapproaches for evaluating human health risks as well as mitigative measures

and monitoring requirements.

http://www.dtsc.ca.gov/AssessingRisk/upload/HERD_POL_Eval_Subsurface_

Vapor_Intrusion_interim_final.pdf

The New Jersey Department of Environmental Protections Vapour IntrusionGuidance (October, 2005) provides comprehensive guidance on site

characterization methods, including soil gas sampling and analysis.

http://www.state.nj.us/dep/srp/guidance/vaporintrusion/vig.htm

The American Petroleum Institute (API) developed a guidance document in1998 on assessing the significance of subsurface vapour migration to enclosed

spaces (API, 1998). They subsequently released bulletins including guidanceand modifications to the J&E model (API Bulletins 15 (2001); 16 (2002); and

17 (2002)). In November 2005, API released A Practical Strategy for

Assessing the Subsurface Vapor-to-Indoor Air Migration Pathway atPetroleum Hydrocarbon Sites, which includes guidance on soil gas sampling

approach, methods and analysis.

http://www.itrcweb.org/Documents/VI-1.pdfhttp://www.itrcweb.org/Documents/VI-1A.pdfhttp://www.dtsc.ca.gov/AssessingRisk/upload/HERD_POL_Eval_Subsurface_Vapor_Intrusion_interim_final.pdfhttp://www.dtsc.ca.gov/AssessingRisk/upload/HERD_POL_Eval_Subsurface_Vapor_Intrusion_interim_final.pdfhttp://www.state.nj.us/dep/srp/guidance/vaporintrusion/vig.htmhttp://www.state.nj.us/dep/srp/guidance/vaporintrusion/vig.htmhttp://www.dtsc.ca.gov/AssessingRisk/upload/HERD_POL_Eval_Subsurface_Vapor_Intrusion_interim_final.pdfhttp://www.dtsc.ca.gov/AssessingRisk/upload/HERD_POL_Eval_Subsurface_Vapor_Intrusion_interim_final.pdfhttp://www.itrcweb.org/Documents/VI-1A.pdfhttp://www.itrcweb.org/Documents/VI-1.pdf


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http://www.api.org/ehs/groundwater/lnapl/soilgas.cfm

The Western Australian Department of Environment (2004) released a

guidance document on quantifying behaviour, assessment and exposure of

petroleum and solvent vapours (WADE, 2004). The document provides an

overview of vapour fate and transport, sampling techniques, modelingapproaches and uncertainties, and data gaps.

In 1992, the American Society for Testing and Materials (ASTM) published

standards for soil gas monitoring in the vadose zone. The standards were

subsequently re-approved in 2001. In March 2008, ASTM published theirstandard E2600-08 Standard Practice for Assessment of Vapor Intrusion into

Structures on Property Involved in Real Estate Transactions.

1.3.2 Relation of this Soil Vapour Intrusion Guidance Document and the Record ofSite Conditions (RSC), Ontario Regulation 153/04.

This guidance was developed to promote the understanding of behaviour andmigration of soil vapours in the subsurface under various site conditions, and

to assist MOE staff in the review of site characterization and monitoring for

sites with volatile compounds present in the subsurface.

This Guidance Document can also assist qualified professionals conducting

RA under Ontario Regulation 153/04 to identify sites, or contaminant levels,

for which contaminant exposures through a vapor inhalation pathway maycause adverse health effects.

The soil vapour screening and assesment process outlined in this GuidanceDocument can also be used to identify the need for remediation of sources or

the implementation of risk management measures (RMM) as part of the

redeveleopment of brownfied sites.

Note that any submission of RA and RSC under the Ontario Regulation

153/04 must meet all the mandatory requirements specified in the regulation.

1.4 GUIDANCE STRUCTUREThe guidance is presented within the following structure:

Chapter 2 provides an overview of the site characterization and vapourintrusion assessment process.

Chapter 3 describes the conceptual site model for vapour intrusion andprovides an understanding of the processes governing soil vapour fate

and transport.

http://www.api.org/ehs/groundwater/lnapl/soilgas.cfmhttp://www.api.org/ehs/groundwater/lnapl/soilgas.cfm


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Chapter 4 describes the tiered screening process for evaluation of soilvapour intrusion and provides an overview of site characterization data

needed to support site screening and key issues for assessment ofvapour intrusion.

Chapter 5 provides detailed guidance on characterizing sites, withparticular emphasis on soil vapour sampling and monitoring.

Chapter 6 provides detailed guidance on indoor air quality (IAQ)monitoring.

Chapter 7 provides an overview of methods for determining whichconstituents are derived from background sources and which are likely

related to the contaminant release or spill into the environment.

Chapter 8 provides a brief description of the recommended minimumreporting and documentation requirements.



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CHAPTER 2

OVERVIEW OF THE SOIL VAPOUR INTRUSION ASSESSMENT PROCESS

CHAPTER 2 OVERVIEW OF THE SOIL VAPOUR INTRUSION ASSESSMENT PROCESS...................................................................................................................................................... 2-2

2.1 Screening process................................................................................................. 2-22.1.1 Development of Conceptual Site Model.............................................................. 2-2

2.1.2 Preliminary Screening.......................................................................................... 2-4

2.1.2.1 Comparison to the Generic Site Condition Standards ......................................... 2-42.1.3 Screening Level Vapour Intrusion Assessment ................................................... 2-4

2.1.4 Detailed Vapour Intrusion Assessment................................................................ 2-5

2.2 Guidance Application and Implementation Process............................................ 2-52.3 Vapour Intrusion Mitigation................................................................................ 2-7

2.4 Community Outreach........................................................................................... 2-7



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Chapter 2. Overview of the Soil Vapour Intrusion Assessment Process 2-2

CHAPTER 2

OVERVIEW OF THE SOIL VAPOUR INTRUSION ASSESSMENT PROCESS

This section provides a brief description of the soil vapour intrusion pathway and therecommended phased approach to assess and interpret data and multiple lines of

evidence, and interact with the various stakeholders.

2.1 SCREENING PROCESS

For sites impacted by volatile compounds, a tiered approach is recommendedwith increasing levels of complexity and information and data requirements.

The tiered approach consists of preliminary and generic screening, screening

level vapour intrusion assessment and detailed vapour intrusion assessment

(Figure 1). Depending on the results of an individual stage, additionalassessment may or may not be warranted. The following sections provide a

brief overview of the soil vapour intrusion assessment process. Additionaltechnical details and guidance for the completion of the various tiers areprovided in Chapter 4.

2.1.1 Development of Conceptual Site Model

A conceptual site model (CSM) should be developed for the site to facilitatean understanding of site conditions and to facilitate planning and completion

of supplementary site investigations and assessments, as required. The CSM

integrates available information relevant to the contaminant source anddistribution, mechanisms for volatilization, soil vapour fate and transport, soil

vapour intrusion into and movement within buildings, and as such involvesconsideration of subsurface processes, building characteristics and externalforces such as weather conditions.

The CSM should also reflect the risk paradigm for assessment of contaminated

sites, which often includes both a descriptive and diagrammatic presentation

of contaminant sources, release and transport mechanisms, exposure media,exposure pathways and receptors. The CSM should be continually updated as

new information becomes available for the site and will often be a useful

decision-making and communication tool for interested parties. Additional

details on CSM development along with a description of processes governingthe fate and transport of volatile compounds are provided in Chapter 3.



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Figure 1: Flowchart for Vapour Intrusion Assessment

Site CharacterizationAre there sufficient data to develop CSM?

1. Preliminary ScreeningDetermine if volatile/toxic chemicals are in

proximity of buildings; determine whether there isacute or safety risk;

Site of potential concern

NO

2. Screening levelAssessment

Soil, groundwater and soil vapour sampling and

testing

Obtain attenuation factorBased on soil type and depth to

contamination

Adjust attenuation factor1. Building mixing height2. Building air exchange rate3. Biodegradation

Calculate SiteSpecific Screening

Levels

Are groundwater and/or soilvapour concentrations below

screening levels?

3. Detailed AssessmentTypically includes modelling and sub-slab and

indoor air sampling

No

Are indoor airconcentrationsacceptable ?

CollectAdditionalData(e.g.soil,groundwater,soilvapour,sub-slabvapour,indoo

rair,outdoorair,

buildingdata)

ExposureC

ontrols(e.g.

institutionalcontrols,

barriers,venting

systems)

No

IncompleteExposure Pathway

Site not of concren Immediate action warranted

Vapour pathwaydoes not pose anunacceptable risk

Are data adequateto screen out site?

See Note

YesNo

Yes

Consider if No and well above SCS

Yes

Consider if No and well above SCS

No




See Note

YesNo

Yes


See Note

YesNo

Yes

Note: It is of critical importance that data adequacy, consistency with CSM and all lines of evidence (contaminant concentrations,geological conditions, biological factors) be carefully considered. Additional assessment may be warranted when within 10X ofstandards or screening levels. For future scenario, exposure controls implemented as part of construction may be required.

Comparison to MOE SCS- Identification of precluding conditions(earthen basements, gas underpressure, preferential pathways)- Additional considerations (shallowwater table, high permeability media)

Are siteconcentrations belowgeneric S-IA and/or

GW2?



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2.1.2 Preliminary Screening

The preliminary screening is a qualitative screening step that uses relatively

simple questions and criteria to categorize sites according to their potential forvapour intrusion and associated health effects. This step can be used to

identify sites that require immediate mitigative action to protect the health andsafety of occupants as well as candidate sites for risk assessments to assess

long-term (i.e., chronic) health concerns.

Key considerations include the type and location of contamination, presence

of non-aqueous-phase liquid (NAPL), building construction, presence ofodours linked to subsurface contamination and presence of hazardous vapour

concentrations in soil gas or indoor air. Additional details on this step are

provided in Chapter 4.

2.1.2.1 Comparison to the Generic Site Condition Standards

This screening process is a comparison of the soil and groundwater quality

information from the site and the MOE generic component values related tosoil vapour intrusion into indoor air (soil to indoor air S-IA and/or

groundwater to indoor air - GW2) (MOE, 2009a). The process begins with the

evaluation of precluding conditions to determine whether the genericcomponent values for the soil vapour intrusion pathways are applicable at the

site. If there are no precluding conditions, measured concentrations can be

compared to the MOE S-IA and/or GW2 (MOE, 2009a), and based on the

results, the need for further assessment should be evaluated.

2.1.3 Screening Level Vapour Intrusion Assessment

For sites identified as candidate sites for vapour intrusion assessment based on

the preliminary and generic screening steps, a screening level vapour intrusionassessment should be considered.

Screening level assessments typically rely on limited site-specific data,relatively simple predictive models or empirical factors, and conservative

assumptions. In many cases, the assumptions employed are similar to those

employed in the derivation of soil and groundwater standards, with minormodifications made where appropriate. For instance, the model developed by

Johnson and Ettinger (1991) (J&E model) is often used for screening level

assessments. Site screening is performed by comparing measured chemicalconcentrations in soil, groundwater, and/or soil vapour to the derived

objectives or screening levels.

This guidance provides an approach and recommended default inputparameters for screening level assessments using the J&E model. Since

available data are often limited at the screening level stage and to facilitate

appropriate model predictions, the use of the J&E model is constrained in that



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only certain input parameters may be varied within specified ranges. Inaddition, prior to conducting the screening process, potential precluding

conditions should be evaluated as described in Section 4.2.5.

If the results of a screening level assessment conclude that concentrations at

the site are below the derived screening levels for each contaminant ofconcern, then, a detailed vapour intrusion assessment would typically not be

required for the existing or prescribed land use and building configuration.However, additional monitoring of site conditions may be warranted to assess

whether conditions assumed for the screening level assessment continue to

apply, or if verification of estimates based on indoor air concentrations arerequired.

2.1.4 Detailed Vapour Intrusion Assessment

A detailed vapour intrusion assessment should be considered in situationswhere the screening level assessment is unable to rule out potential

unacceptable human health impacts or where site conditions are very complex

or differ considerably from the CSM assumed for models used to derivegeneric standards or used for screening level assessments. A detailed

assessment should also be considered in situations where technical or financial

constraints severely limit the options available for remediating a site.

A detailed vapour intrusion assessment includes all the key components of the

screening level assessment, but generally incorporates more detailed site-

specific data, which is often needed when more complex models are used. Inthe case of sites where vapour intrusion is a concern, measurements of soil gas

and/or indoor air quality are generally incorporated into the assessment.

Contaminant fate and transport modelling may remain an importantcomponent of a detailed assessment, but generally the assumptions employed

in the modelling exercise are less conservative and more representative of

actual site conditions than in a screening level assessment. The detailedassessment may also include the derivation of toxicity reference values for

compounds lacking published values, or where new toxicity data are available.

Similar to the screening level assessment, additional monitoring of site

conditions may be warranted to assess whether conditions assumed continue toapply, or if verification of risk estimates based on indoor air concentrations are

required.

2.2 GUIDANCE APPLICATION AND IMPLEMENTATION PROCESS

The vapour intrusion assessment approach described in this guidance applies

to contaminated sites where there is subsurface volatile or semi-volatile

contamination. However, the soil vapour assessment process described in thisdocument does not generally apply to landfills where there is generation of

gases through the anaerobic decomposition of organics and where there is

significant pressure-driven gas flow.



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The guidance is intended for application where there are currently occupied

buildings at existing residential and commercial sites, or where there is the

potential for the presence of occupied buildings in a future land use scenario.

The soil vapour intrusion assessment should typically be implemented in

sequential steps starting with characterization of the contamination sourcethrough soil and groundwater sampling together with data collection (e.g.,

hydrogeological and soil properties) needed to understand contaminantmigration within both the unsaturated and saturated soil zones. Where

concentrations in soil and groundwater are below the applicable MOE soil

condition standards (MOE 2009a), no further assessment of soil vapourintrusion may be necessary, although data adequacy and uncertainty should be

considered, particularly when measured concentrations are close to the

standards.

Given the limitations associated with the use of soil data for evaluating the

vapour intrusion pathway (see Section 4.4.1), it is recommended that soilvapour characterization should generally be conducted whenever there iscontamination within the unsaturated soil zone. The soil vapour assessment

should begin with a characterization of soil vapour concentrations near to the

contamination source and where warranted, continue closer toward thebuilding, and finally, if indicated by the results of previous steps,

characterization of indoor air quality.

The development of a CSM that is updated as new information is obtained and

consideration of data adequacy are especially important when screening a site

out of the assessment process. A comprehensive evaluation of the vapour

intrusion pathway that considers multiple lines-of-evidence is recommendedbefore drawing conclusions on risks posed by this pathway. The lines-of-

evidence include concentrations in different media and locations along the

anticipated migration pathway and information on site conditions and factorsthat could influence vapour migration such as varying soil properties (e.g.,

layered deposits), aerobic biodegradation (for petroleum hydrocarbons) and

environmental factors (e.g., weather conditions).

Although this guidance is structured to reflect a phased approach starting with

screening of sites through the use of subsurface data, it does not preclude the

testing of indoor air quality to evaluate this pathway at early stages of the

investigation process or pro-active implementation of exposure controls,which may be desirable and appropriate in some cases. An iterative approach

may also be warranted where additional testing of subsurface concentrations isconducted as follow-up to indoor air testing to assess whether there is a

complete vapour intrusion pathway. When indoor air quality results are

borderline or inconclusive, it may be appropriate to rely on a combination ofmeasurements from different media to decide on appropriate responses to

safeguard human health (EPRI, 2005, ITRC, 2007).



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It is also important to recognize that the approach to assessment of vapour

intrusion may depend on the site scenario. For example, at sites where there is

contamination below existing occupied buildings and where exposures are

potentially current and on-going, there will often be a focus on near building

soil vapour and indoor air characterization. At brownfield sites, where thereare no buildings, potential future exposures are predicted based on subsurface

concentrations measured near to contamination sources. This guidance appliesto both scenarios, although different approaches may be warranted.

The described process for soil vapour intrusion assessment can assist in caseswhere a risk assessment is conducted for the purpose of filing a record of site

condition (RSC) under the O.Reg. 153/04. However, it alone does not provide

all the information and evaluation required under the RSC regulation. In thesecases, the qualified person (QP) must follow the procedures and requirements

outlined in the O.Reg. 153/04 as amended and its related documents.

2.3 VAPOUR INTRUSION MITIGATION

Where site characterization information, such as indoor air quality data,

indicate that actions should be taken in response to levels of contaminants

entering buildings through vapour intrusion, mitigative measures should be

implemented. Vapour intrusion mitigation generally refers to measures takento prevent or control exposures. As such, these measures are typically

implemented near to the building (e.g., subslab depressurization or venting

systems) or within the building itself. Institutional controls or measuresdesigned to control use of buildings by receptors are a possible solution in the

short term. A longer term approach may be measures to remediate the

contamination that is the source of the vapours. Additional information onmitigation systems is provided in ITRC (2007).

2.4 COMMUNITY OUTREACH

Depending on the circumstances, community outreach may be an importantcomponent of a soil vapour assessment program. Where community outreach

is warranted, provincial and local regulatory agencies will, to varying degrees

be involved in the process. Although each site will tend to have its ownunique set of circumstances, there are several good practices that should be

followed for community outreach. Initial notification of residents and

business owners may be warranted when off-site characterization ofsubsurface contamination is planned. A more comprehensive communicationsstrategy should be implemented as part of an indoor air quality testing

program. Persons involved in indoor air quality program planning and

sampling should be tactful and communicate well with building occupantsgiven that indoor air quality programs may be disruptive.



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Several successful community outreach programs have involved contactingresidents door-to-door to discuss the program directly when requesting access

and providing information packets. Other communication tools include

community meetings, articles published in community newsletters, mailing of

project newsletters and development of a project website posting fact sheets

and other relevant information. A strong communications program is essentialto educate and reassure the local community in a meaningful and sensitive

manner. It is often helpful to communicate the potential risks from vapourintrusion in the context of risks from other chemicals, such as those commonly

associated with background sources in indoor air.



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CHAPTER 3

CONCEPTUAL SITE MODEL

CHAPTER 3 CONCEPTUAL SITE MODEL............................................................................. 3-2

3.1 What is a Conceptual Site Model?....................................................................... 3-2

3.2 Information Needed to Build the Conceptual Site Model ................................... 3-2

3.3 Fate and Transport Processes for Vapour Intrusion............................................. 3-33.3.1 Generation of Vapours and Gases........................................................................ 3-4

3.3.2 Fate and Transport in the Vadose Zone............................................................... 3-5

3.3.3 Soil Vapour Intrusion through the Building Envelope ........................................ 3-53.3.4 Mixing of Vapours inside the Building ............................................................... 3-6



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Chapter 3. Conceptual Site Model 3-2

CHAPTER 3CONCEPTUAL SITE MODEL

This section begins by describing the elements of a conceptual site model (CSM) andinformation requirements for building the CSM, followed by an overview of processes

and factors influencing vapour intrusion into buildings. Further guidance on conceptual

site models and fate and transport processes is provided in Appendix I.

3.1 WHAT IS A CONCEPTUAL SITE MODEL?A conceptual site model (CSM) is a visual representation and narrative

description of the physical, chemical, and biological processes occurring, orthat have occurred, at a contaminated site. The CSM should be able to

communicate how the site became contaminated, how the contamination was

and is transported, where the contamination will ultimately end up, and whomit may affect. To the extent possible, the CSM should provide information on

the three-dimensional nature of contamination and physical characteristics of

the site, as supported by maps, cross-sections and site diagrams.

A well developed CSM provides decision makers with an effective tool that

helps to organize, communicate and interpret existing data, while also

identifying areas where additional data are required. The CSM should beconsidered dynamic in nature and should be continuously updated as each

stage of the investigation program is completed (USEPA, 2002).

3.2 INFORMATION NEEDED TO BUILD THE CONCEPTUAL SITEMODEL

A CSM used for evaluating the risks associated with soil vapour at a

contaminated site should provide a summary of the following:

The source and distribution of contamination (history ofcontamination, present conditions, and potential future conditions);

The receptors that could be exposed to the contamination (under bothpresent and future land use scenarios); and,

The release mechanisms for contaminants and fate and transportpathways between the vapour source and the receptors (under bothpresent and future land use scenarios).

The CSM should include all relevant data from the site including:

Information on historical and current land uses, potential sources andtypes of volatile or semi-volatile contamination, and known andsuspected releases or spills that may have occurred at the site;



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Concentrations, distribution and extent of the chemicals of concern indifferent media (soil, groundwater, soil vapour and indoor air);

Form of contamination present (dissolved chemicals in groundwater,soil contamination and non-aqueous phase liquid (NAPL));

Approximate distance from the soil vapour sources to buildings(vertical and lateral); Hydrogeological information including depth to groundwater, the

groundwater flow direction, hydraulic conductivity, lateral and vertical

hydraulic gradients;

Geological information including soil units present at the site and typeor textural classification for each soil unit;

Vadose zone soil properties including water content, porosity, fractionof organic carbon, bulk density and soil-air permeability;

Location and characteristics of subsurface utilities; Information on buildings (present and future) including location, type

of building (residential, institutional, commercial, industrial), buildingsize and height, foundation type (e.g., crawlspace, basement, slab-at-

grade) and foundation characteristics (e.g., construction, utility

penetrations, sumps) and heating, ventilation and air conditioning(HVAC) system; and,

Information on background sources of volatile chemicals in indoor airincluding indoor sources, outdoor source, building materials and

consumer products.

It is recognised that not all of the above information may be collected or isneeded, to complete an assessment under this guidance.

3.3 FATE AND TRANSPORT PROCESSES FOR VAPOUR INTRUSIONThe fate and transport of a chemical from a subsurface source and ultimateconcentration inside a building is controlled by processes that occur in four

compartments, which consists of source partitioning or generation of soil

vapours and gases; the vadose zone; the subsurface building envelope(foundation, walls and nearby utility corridors) and the building. An example

of a CSM that addresses the above processes is shown on Figure 2.



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IndoorAir

Chemical

VapourTransportSoil Contamination(residual or mobile NAPL)

Groundwater Contamination

FIGURE 2: Conceptual Site Model for Vapour Intrusion Residential

Scenario

3.3.1 Generation of Vapours and GasesThere are two sources of subsurface vapours or gases encountered atcontaminated sites. The first source is partitioning of NAPL present above the

water table into soil gas (vaporisation) or partitioning of dissolved chemicals

in soil-water above the water table into soil gas (volatilization). The NAPL

is referred to as a primary source of vapours while a dissolved phase plume isreferred to as a secondary source. The second source of soil gases is

biologically-mediated generation of gases such as carbon dioxide, methane,

hydrogen sulphide and hydrogen under either aerobic or anaerobic conditions.

For a secondary source where chemicals are present only as a dissolved phase

in groundwater, their distribution below the water table will determine theirpotential to volatilize and eventually migrate to indoor air. If volatile

chemicals are present near the surface of the water table, volatilization will

readily occur. In contrast, if there is a layer of clean groundwater above



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contaminated water, then, the rate of volatilization will decrease since masstransport is controlled by diffusion and dispersion in groundwater.

3.3.2 Fate and Transport in the Vadose ZoneThe transport of chemicals in the vadose zone is controlled by chemicaldiffusion, gas-phase and water-phase advection, biological and chemical

transformation, and partitioning between the sorbed, soil gas, soil water and

NAPL phases (when present). Unless there is a constant replenishment of thechemical source, the processes are dynamic and transient since the chemical

will be depleted through biodegradation, volatilization (i.e., source depletion),

and dissolution through infiltration of surface water.

Diffusion occurs as a result of the movement of molecules as influenced by

their kinetic energy. The rate of diffusive transport is a function of the

concentration gradient and the temperature of the medium through whichdiffusion is occurring. Diffusion coefficients in air are about four orders-of-

magnitude higher than in water; therefore, diffusive flux tends to be much

higher through the air-filled than water-filled soil pores.

Driving forces for gas-phase advection are pressure gradients due to

barometric pressure variation, water movement, and density gradients due to

compositional and temperature variation. At most sites, diffusion tends to be amore important process for transport than advection, although in the shallow

vadose zone close to a building, soil gas advection due to building

underpressurization is often significant, as discussed below.

Environmental and seasonal factors that affect vadose zone transport processesinclude precipitation, barometric pressure, wind, water table levels,temperature, snow and frost cover.

3.3.3 Soil Vapour Intrusion through the Building EnvelopeWhen buildings are depressurized relative to subsurface soil, the primaryprocess for soil vapour intrusion tends to be soil gas advection through

openings in the building envelope, which may include untrapped drains,

perimeter cracks at the building wall and floor slab interface, utility servicepenetrations, expansion joints, and shrinkage cracks. Diffusion may also

occur through openings and cracks in the building envelope and may be

influenced by the properties of dust-filled cracks, the type of concrete

construction (i.e., poured concrete or concrete block), and subsurface moisturevapour barrier, if present.

The main driving forces for soil gas advection tend to be buildingdepressurization and barometric pressure fluctuations. The primary

mechanisms for building depressurization are (1) temperature differences



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between indoor and outdoor air, which create a stack effect within the building(i.e., warm air moves upward within the building drawing soil gas and outside

air from lower parts of the building); (2) the force of wind on buildings, and

(3) mechanical equipment that moves air into and out of the building (e.g.,

furnaces, fans). The influence of the stack effect and wind generally increases

with the height of the building (i.e., for tall buildings), although the effectfor multi-storey buildings will depend on construction and air leakage between

floors. For buildings where the stack effect is the most significant mechanismfor building depressurization, higher soil vapour intrusion rates would be

expected during winter months for Ontario climatic conditions.

The zone of influence for soil gas flow from building pressure differences is

usually less than a few metres. The rate of soil gas flow will be highly

dependent on site specific conditions such as soil permeability, foundation

backfill properties, potential preferential pathways such as utility corridors,and building foundation construction.

Soil vapour intrusion into a building with a crawlspace is largely affected by

the degree to which the crawlspace is ventilated by outside air, and whetherthe crawlspace is connected to the airspace in the rest of the building (e.g.,

through the heating system). In addition to migration through subsurface

components of the building envelope, volatile chemicals that have migrated

from the subsurface to ambient air could enter a building through windows,doors and other openings. Volatilization to outdoor air followed by transport

to indoor air is generally not considered to be a significant pathway due todilution that occurs in ambient air.

3.3.4 Mixing of Vapours inside the BuildingVapours inside enclosed spaces will diffuse as a result of chemical gradients

and disperse through air movement. Mixing between building floors will

depend on the HVAC system and possible unintentional air leakage between

floors. Most models used for screening level assessment assume uniform andinstantaneous mixing of vapour within the enclosed space.



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CHAPTER 4

SITE SCREENING PROCESS

CHAPTER 4 SITE SCREENING PROCESS.............................................................................. 4-2

4.1 Introduction.......................................................................................................... 4-2

4.2 Preliminary Screening.......................................................................................... 4-34.2.1 Site Characterization............................................................................................ 4-3

4.2.2 Are There Chemicals of Potential Concern for Vapour Intrusion? ..................... 4-44.2.3 Does the Site Represent a Safety or Acute Health Risk Concern? ...................... 4-4

4.2.4 Are Buildings Located in Sufficiently Close Proximity to Contamination? ....... 4-5

4.2.5 Comparison to the Generic Site Condition Standards (SCS)............................... 4-6

4.2.5.1 Precluding Conditions.......................................................................................... 4-64.2.5.2 Additional Considerations ................................................................................... 4-7

4.3 Screening Level Assessment................................................................................ 4-8

4.3.1 Site Characterization............................................................................................ 4-84.3.2 Screening Level Vapour Intrusion Assessment ................................................. 4-15

4.3.2.1 Determine Soil Textural Type ........................................................................... 4-16

4.3.2.2 Select Land Use ................................................................................................. 4-164.3.2.3 Estimate Distance to Vapour Contamination Source......................................... 4-17

4.3.2.4 Derivation of Vapour Attenuation Factor .......................................................... 4-17

4.3.2.5 Adjusted Attenuation Factor Based on Building Mixing Height....................... 4-21

4.3.2.6 Adjusted Attenuation Factor Based on Building Air Exchange Rate................ 4-214.3.2.7 Adjusted Attenuation Factor Based on Biodegradation..................................... 4-21

4.3.2.8 Back-calculate site specific screening levels for soil vapour and

groundwater ....................................................................................................... 4-244.3.2.9 Data Evaluation and Next Steps ........................................................................ 4-24

4.3.3 Soil Vapour Screening Using the Modified Generic Risk Assessment Model . 4-26

4.4 Detailed Vapour Intrusion Assessment.............................................................. 4-264.4.1 Estimation of Input Parameters.......................................................................... 4-27

4.4.2 Indoor Air Quality (IAQ) Testing...................................................................... 4-31

4.4.2.1 IAQ Study Design.............................................................................................. 4-314.4.2.2 Background Issues ............................................................................................. 4-32

4.4.3 Data Evaluation and Next Steps ........................................................................ 4-33

4.4.4 Exposure Controls.............................................................................................. 4-33



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Chapter 4. Site Screening Process 4-2

CHAPTER 4

SITE SCREENING PROCESS

This chapter addresses approaches and methods for the evaluation of the significance ofthe soil vapour intrusion pathway and describes the tiered framework for vapour intrusionassessment.

4.1 INTRODUCTION

The tiered framework for vapour intrusion assessment consists of thefollowing components:

Preliminary Screening, including comparison to applicable sitecondition standard (SCS);

Screening Level Assessment; and Detailed Assessment.

The above tiered process is consistent with recent developments in science and

regulatory policy for soil vapour intrusion assessment (e.g., Golder, 2007;

ITRC, 2007; Cal EPA, 2005; USEPA, 2002). The screening process issummarized in the flow chart shown in Figure 1.

The site screening process should begin with the development of a conceptualsite model (CSM) and site characterization. The site characterization process

for evaluation of soil vapour intrusion will often be completed in phases and,

therefore, it is essential that the CSM be updated as new information isobtained. This chapter provides a summary of the site characterization

process, while details are provided in Chapter 5. Key inputs to the screening

level and detailed vapour intrusion assessment process are vapour intrusion

attenuation factors, which are discussed in Section 4.3.2.

While the screening process is described below as a progression of steps

starting with obtaining subsurface data, the guidance does not precludeconcurrent implementation of the tiers below, which may be an appropriate

approach for some sites. The timelines of the decision points, for example

when to sample indoor air in addition to the subsurface media, should beconsistent with the relative urgency of obtaining the results. There may be

some situations (for example where initial testing in some residences hasdemonstrated vapour intrusion impacts) where indoor air sampling and

measurement should be conducted concurrently with subsurface sampling due

to a need to obtain the best possible estimate of exposure as soon as possible.It is also important to note that vapour intrusion investigations may follow an

iterative approach rather than simply proceeding through sequential,

independent steps.



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4.2 PRELIMINARY SCREENING

The preliminary screening step involves qualitative screening of sites to

evaluate the potential for vapour intrusion, and to identify site conditions that

warrant rapid assessment and possible mitigation. Preliminary screening can

be completed at early stages of the site assessment program but requires somesite characterization data, as described below. Following site characterization,

there are four components to preliminary screening: (a) determination ofwhether site contaminants of potential concern (COPCs) are of concern for

vapour intrusion either through reference to the generic standards (do site

chemicals have vapour pathway standards?) or through consideration ofchemical volatility and toxicity, (b) evaluation of whether the site represents a

safety or acute health risk concern, (c) evaluation of whether buildings are

located in sufficiently close proximity to the contamination to be of concern,and (d) whether the identified soil and/or groundwater concentrations are

above the generic MOE SCS. Although the steps are presented below as a

progression, they should be conducted concurrently in an integrated manner,and relevant aspects of the assessment should be updated when newinformation is obtained.

4.2.1 Site Characterization

Preliminary screening and comparison to generic standards requires certainsite characterization data. A Phase I Environmental Site Assessment (ESA)

should have been completed at the site, which should identify the areas of

potential environmental concern (APECs) and COPCs based on a historicalreview, site reconnaissance and other data collection activities.

From subsequent investigation phases (i.e., Phase II and III ESA), there shouldtypically be information on the presence and extent of COPCs in soil and

groundwater, hydrogeological conditions, soil properties and possible

preferential pathways. The contamination migration, plume stability andvertical and lateral extent of contamination should be delineated to enable

screening based on the distance criteria described under the preliminary

screening step below. Basic information on the existing and future receptors

and buildings at the site should be obtained to enable appropriate land usedesignation and identification of possible precluding conditions for generic

screening.

Under the preliminary screening, the maximum site-wide media (e.g. soil

and/or groundwater) concentrations should generally be used for screening

purposes to determine whether a higher tier assessment is required, therefore,it is important that the contamination source concentrations be properly

delineated. The delineation should be conducted by assuming that the lateral

and vertical extent of COPCs in soil or groundwater extends from the

sampling locations where volatile contaminants are present at concentrations



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equal to or greater than the applicable SCS to the next sampling location at

which the concentrations are below the applicable SCS.

Early intrusive phases of the investigation will often include soil vapour data

obtained using field screening methods (e.g., photoionization detector or

combustible gas detector) at existing wells screened above the water table,which can be useful for qualitative evaluation of potential vapour sources (soil

vapour testing using existing wells is further described in Section 5.4.3).

However, measurements taken by field screening equipment cannot be usedfor delineation or to demonstrate that the applicable SCS for a COPC has been

met.

4.2.2 Are There Chemicals of Potential Concern for Vapour Intrusion?

The chemicals with generic standards under Ontario Regulation 153/04

represent most COPCs for vapour intrusion. If there are volatile or semi-

volatile organic chemicals present at a site that do not have generic standards,a risk assessment should be conducted to further evaluate the significance of

these chemicals. In addition, there may be gases such as methane and

hydrogen sulphide that can represent a subsurface vapour threat and should be

investigated.

A chemical must be sufficiently volatile to cause vapour concentrations above

toxicologically-derived concentrations of concern in indoor air to pose a potential vapour intrusion concern. Screening approaches for evaluation of

COPCs should consider volatility, toxicity and mobility. Screening

approaches for evaluation of chemical volatility and toxicity are further

described in Appendix II.

If there are no COPCs for the inhalation pathway, further consideration of this

pathway is not warranted. Otherwise, the investigator moves to the next stepin the preliminary screening phase.

4.2.3 Does the Site Represent a Safety or Acute Health Risk Concern?

The first step under preliminary screening is to identify whether there areindications of safety or acute human health risks. The following factors

should be evaluated:

Potentially Explosive Conditions: Gas concentrations within a building,

sump, drain or other utility that exceed or are close to the explosive limit

represent a significant safety hazard through explosion or fire. The lowerexplosive limit for methane is 5% (50,000 ppm) in air, while the limit for

gasoline vapours is approximately 1.4% (14,000 ppm). The accumulation of

gases may also represent an asphyxiation hazard in rare circumstances.



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Odours: Reports by building occupants of chemical odours that do not appear

to be from indoor or ambient sources may be an indication of vapour intrusion

and should be investigated. Although odours may not necessary indicate thatthere is a safety or acute health risk, the odour thresholds of many chemicals

exceed toxicologically-based air concentrations based on chronic risk.

Physiological Effects: Exposure to higher levels of vapours could result in

physiological effects such as headaches, nausea, eye and respiratory irritation.

The sensitivity of individuals to these effects varies widely. The physiologicaleffects may or may not be attributable to vapour intrusion, but should be

investigated.

Direct Contact (Wet Basements): There is an increased risk for elevatedvapour concentrations inside buildings when there are buildings with wet

basements or sumps in direct contact with contamination. This is particularly

the case when there is non-aqueous phase liquid (NAPL) on the water table.

Site information indicating a safety or acute health risk may come to light in

various ways including reports from building occupants or owners.Depending on site conditions, a more pro-active approach such as a door-to-

door survey involving the use of questionnaires designed specifically to

evaluate potential vapour-related concerns (such as those described above),

may be warranted.

If any of the above conditions are present, immediate action should be taken to

investigate possible safety and acute health risks and where necessaryimplement mitigation measures. For reports of odours and physiological

effects, the outcome may depend on whether these effects could reasonably beexpected to be associated with subsurface contamination. Based on theinvestigation, the testing of indoor air quality, exposure controls, and/or re-

location of receptors may be warranted.

4.2.4 Are Buildings Located in Sufficiently Close Proximity to Contamination?

This screening step is designed to identify which buildings to consider for

more detailed pathway assessment. Vapour concentrations decrease with

increasing lateral distance from a subsurface vapour source, and eventuallydissipate to non detectable levels. The decrease in vapour concentrations is a

function of contamination source size and geometry, soil properties, physical-

chemical properties, and possible biological or chemical transformations

within the subsurface environment. Model predictions (Abreu and Johnson,2006, Lowell and Eklund, 2004, Mendoza, 1995) indicate that the vapour

intrusion pathway has sufficient natural attenuation to be of negligible concern

if the distance between the contamination and building is greater than:

Thirty (30) metres for recalcitrant chemicals; and



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Fifteen (15) metres for chemicals that readily biodegrade under aerobicconditions (biodegradable chemicals).

There is also empirical data from several Colorado sites (USEPA 2008) where

indoor air testing indicated no significant indoor vapour concentrations in

single family homes that were located more than one to two residential lots(i.e., roughly 30 m) away from the interpolated edge of the groundwater

plume.

The distance criterion of 30 m (or 15 m for biodegradable chemicals) should

not be applied when there are the following precluding conditions: (i) soil gasunder pressure, (ii) a subsurface utility conduit connecting the contamination

source and the building, (iii) an expanding contamination zone that is

migrating toward the building, and/or (iv) a continuous low permeability cover

between the contamination source and building (e.g., good quality concrete orasphaltic pavement), which prevents the normal dissipation of vapours and

that leads to enhanced lateral migration. For the above precluding conditions,professional judgement should be used to consider whether buildings should

be evaluated for possible vapour intrusion even if they are further than 30 m

(or 15 m for biodegradable chemicals) from the edge of the contamination.

Examples of situations where the 30 m lateral distance may not apply are

where a utility with high permeability backfill intersects a contamination zone(e.g., NAPL), when the chemical vapours are unlikely to degrade, and where

the site is covered with buildings or paved surfaces.

4.2.5 Comparison to the Generic Site Condition Standards (SCS)

The comparison to the generic standards begins with the evaluation of

precluding conditions to determine whether generic soil and groundwater

standards for the soil vapour intrusion pathway may be applied. If there are no precluding conditions, measured concentrations should be compared to the

applicable component values S-IA and/or GW2 (MOE 2009a), and based on

the results, the need for further assessment should be evaluated. While

Ontario MOE regulations allow sites to be assessed using soil andgroundwater data, there are potential advantages associated with the use of soil

vapour data, and potential disadvantages with use of soil data, as discussed in

subsequent sections of this Guidance Document. For details on the approachused by the MOE to develop the SCS and whether your site characteristics are

consistent with the MOE soil vapour conceptual site model and assumptions,

please refer to the Rationale for the Development of Soil and Ground WaterStandards for Use at Contaminated Sites in Ontario (MOE, 2009).

4.2.5.1 Precluding Conditions

The precluding factors described below were, in part, developed consideringthe CSM described by the J&E model, which was used in the development of



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the Ontario SCS. When site-specific conditions are not consistent with the

CSM used by the MOE, described in the Rationale for Development of

Generic Soil and Groundwater Standards for Use at Contaminated Sites inOntario (MOE 2009), the generic screening process should not be followed,

and instead the assessment should proceed to the screening level assessment or

detailed vapour intrusion assessment.

When conducting a generic