14. Appendix D: Risk-Based Soil Gas Cleanup Levels, Schlage OU

Appendix D, Schlage OU, San Francisco, California November 4, 2009 

Feasibility Study/Remedial Action Plan, Schlage OU/UPC OU 

MACTEC Project No. 4096088522 

KB63190 FS-RAP Appendix D Text-Schlage 

TABLE OF CONTENTS 

ACRONYMS AND ABBREVIATIONS ................................................................................................. Div 

D1.0 INTRODUCTION ..................................................................................................................... D1-1 

D2.0 BACKGROUND ....................................................................................................................... D2-1 

D2.1 Site Physical Setting ..................................................................................................... D2-1 

D2.2 Site Redevelopment Plans............................................................................................. D2-1 

D2.3 Previous Investigations ................................................................................................. D2-1 

D3.0 EXPOSURE ASSESSMENT .................................................................................................... D3-1 

D3.1 Potentially Exposed Populations ................................................................................... D3-1 

D3.2 Potential Exposure Pathways ........................................................................................ D3-1 

D3.2.1 Soil Gas Exposure Pathways ........................................................................ D3-1 

D3.3 Exposure Assumptions ................................................................................................. D3-2 

D4.0 TOXICITY ASSESSMENT ...................................................................................................... D4-1 

D4.1 Noncancer Toxicity Assessment ................................................................................... D4-1 

D4.1.1 Noncancer Reference Dose .......................................................................... D4-1 

D4.1.2 Target Hazard Index ..................................................................................... D4-1 

D4.2 Cancer Slope Factors and Target Risk Level ................................................................ D4-2 

D4.2.1 Cancer Slope Factors .................................................................................... D4-2 

D4.2.2 Target Risk Level ......................................................................................... D4-2 

D5.0 INDOOR AIR TARGET CONCENTRATIONS ...................................................................... D5-1 

D5.1 Indoor Air Target Concentrations ................................................................................. D5-1 

D5.2 Indoor Air Concentrations - Residential Living Space ................................................. D5-2 

D6.0 CLEANUP LEVELS ................................................................................................................. D6-1 

D6.1 Soil Gas Cleanup Levels ............................................................................................... D6-1 

D6.2 Redevelopment Zone Cleanup Levels .......................................................................... D6-1 

D6.2.1 Residential Over Commercial (Zone 2) and Residential Over Podium 

Garage (Zone 3) Cleanup Levels .................................................................. D6-2 

D7.0 APPLICATION OF CLEANUP LEVELS ................................................................................ D7-1 

D8.0 CONCLUSION .......................................................................................................................... D8-1 

D9.0 REFERENCES .......................................................................................................................... D9-1 

Dii





TABLES 

D3-1 Exposure Factors to Calculate Risk Based Human Health Cleanup Levels for Chlorinated 

Volatile Organic Compounds 

D4-1 Toxicity Values for Chlorinated Volatile Organic Compounds 

D5-1 Target Indoor Air Concentration 

D6-1 Soil Gas Cleanup Levels Protective of Indoor Air 

D6-2 Final Soil Gas Cleanup Levels 

ATTACHMENT 

D-1 INTERSTATE TECHNOLOGY AND REGULATORY COUNCIL SCENARIO FOR 

MULTI-FAMILY DWELLING 

Diii





ACRONYMS AND ABBREVIATIONS 

µg/L 

microgram per liter 

µg/m 3 

microgram per cubic meter 

1,1-DCE 

1,1-dichloroethene 

AF 

Attenuation factor (unitless) 

AMSL 

above mean sea level 

AT 

averaging time 

ATSDR 

Agency for Toxic Substances and Disease Registry 

B&M 

Burns and McDonnell Engineering Company, Inc. 

BAAQMD 

Bay Area Air Quality Management District 

BP 

BP PLT-1, LLC 

BW 

body weight 

Cµg/m 3 Ambient air background concentration (µg/m 3 ) 

CalEPA 

California Environmental Protection Agency 

cfm/ft 2 

cubic feet per minute per square foot 

CHHSLs 

California Human Health Screening Levels 

cis-1,2-DCE 

cis-1,2-dichloroethene 

COCs 

chemicals of concern 

C ppb 

Ambient air background concentration (ppb) 

CULs 

cleanup levels 

CVOCs 

chlorinated volatile organic compounds 

days/year 

days per year 

DTSC 

California Department of Toxic Substances Control 

ED 

exposure duration 

EF 

exposure frequency 

EIR 

environmental impact report 

EPA 

United States Environmental Protection Agency 

EPC 

exposure point concentration 

ET 

exposure time 

FS/RAP 

Feasibility Study/Remedial Action Plan 

IA Indoor air target concentration (µg/m 3 ) 

IA c Carcinogen indoor air concentration (µg/m 3 ) 

IA nc Noncarcinogen indoor air concentration (µg/m 3 ) 

IRIS 

Integrated Risk Information System 

ITRC 

Interstate Technology and Regulatory Council 

IUR 

inhalation unit risk 

kg 

kilograms 

LOAEL 

lowest-observed-adverse-effect level 

MACTEC 

MACTEC Engineering and Consulting, Inc. 

mg/m 3 

milligrams per cubic meter 

MW 

Molecular weight of compound 

MW PCE Molecular weight of PCE = 165.8 

MW TCE Molecular weight of TCE – 131.4 

MW VC Molecular weight of VC = 62.5 

NOAEL 

no-observed-adverse-effect level 

OEHHA 

Office of Environmental Health Hazard Assessment 

PCE 

tetrachloroethene 

Div





PPRTV 

provisional peer review toxicity values 

RAP 

remedial action plan 

RfC 

reference concentration 

RSL 

Regional Screening Levels 

Schlage OU 

Schlage Lock Operable Unit 

SG Soil gas concentration (µg/m 3 ) 

Site 

Schlage OU and San Francisco County portion of UPC OU 

SP OU-1 Southern Pacific Rail Yard Operable Unit 1 

SP 

Southern Pacific 

T&R 

Treadwell and Rollo 

TCE 

trichloroethene 

THQ 

target hazard quotient 

TR 

Target risk (unitless) 

trans-1,2-DCE 

trans-1,2-dichloroethene 

TRL 

target risk level 

UCL 

upper confidence limit 

UPC OU 

Universal Paragon Corporation Operable Unit 

UPC 

Universal Paragon Corporation 

VC 

Vinyl Chloride 

VOCs 

volatile organic compounds 

Dv





D1.0 INTRODUCTION 

On behalf of BP PLT-1, LLC (BP), MACTEC Engineering and Consulting, Inc. (MACTEC) is pleased to 

submit to the California Department of Toxic Substances Control (DTSC), this report that presents our 

risk analysis for the Schlage Lock Operable Unit (Schlage OU), San Francisco, California. The Schlage 

OU is defined as the former Schlage Lock site, together with soil and groundwater on the northern portion 

of the former Southern Pacific Brisbane Rail Yard – Operable Unit 1 (SP OU-1) that is impacted with 

volatile organic compounds (VOCs). The Universal Paragon Corporation (UPC) OU is defined as soil 

and groundwater on SP OU-1 that is impacted with chemicals other than VOCs. Together, the Schlage 

OU and the San Francisco portion of the UPC OU constitute the Site. 

For the Schlage OU, this risk analysis document presents the site physical setting, UPC’s planned 

redevelopment, a summary of previous investigations, and estimates of cleanup levels (CULs) for 

chemicals of concern (COCs) in soil gas at the Site. 

D1-1





D2.0 BACKGROUND 

D2.1 Site Physical Setting 

The Schlage OU consists of approximately 12.66 acres. The Site consists of approximately 20 acres in 

San Francisco, located north of Sunnyvale Avenue, between Bayshore Boulevard on the west, and the 

Union Pacific/Joint Powers Board railroad tracks on the east, and Blanken Avenue to the north. The 

surface elevation of the Site ranges from approximately 50 feet above mean sea level (AMSL) in the 

north to approximately 10 feet AMSL in the south. 

D2.2 Site Redevelopment Plans 

UPC plans to redevelop the Site in accordance with an Environmental Impact Report (EIR) certified by 

the San Francisco Redevelopment Agency on December 16, 2008 and the Planning Commission on 

December 18, 2008. The redevelopment plan consists of the following three redevelopment zones, which 

are consistent with the plans approved by the San Francisco Planning Department: 

• Zone 1 – Public open space on grade. 

• Zone 2 – Residential over commercial podium construction. 

• Zone 3 – Residential over podium parking construction. 

D2.3 Previous Investigations 

Investigations to assess the nature and extent of contaminants have been conducted on the Site, with 

regulatory oversight, beginning in 1982. Documentation of the field investigations are included in reports 

submitted to the DTSC (Treadwell and Rollo (T&R), 2001; Burns & McDonnell Engineering Company, 

Inc. [B&M], 2006a and 2006b). Historical soil analytical data is presented in Appendix A of the 

Feasibility Study/Remedial Action Plan (FS/RAP) for the Site. A compilation of cumulative water level 

and water quality data from groundwater monitoring wells at the Site is presented in the draft third quarter 

2008 groundwater monitoring report (MACTEC, 2008). A description of the nature and extent of soil and 

groundwater contamination and the COCs identified in soil at the Schlage OU and UPC OUs, and 

groundwater at the Schlage OU are presented in the Feasibility Study/Remedial Action Plan (FS/RAP) for 

the Site. 

D2-1





D3.0 EXPOSURE ASSESSMENT 

In the exposure assessment for the development of soil gas CULs, potential exposed populations and 

potential pathways of exposure are identified. The exposure assessment considers the current and future 

land use in order to identify pathways and potentially exposed populations. The exposure assessments 

from the Human Health Risk Assessment of Joint Groundwater Operable Unit (B&M, 2003), Human 

Health Risk Assessment for the San Francisco County Parcel of Operable Unit – 1 (B&M, 2006c), and 

Human Health Risk Assessment for the San Mateo County Parcel of Operable Unit – 1 (B&M, 2006d) 

were used in developing the populations and pathways in this CUL document. 

D3.1 Potentially Exposed Populations 

Potentially exposed populations include those persons whose locations and activities create an 

opportunity for contact with the COCs. As described in Section D2.2, the UPC OU redevelopment zones 

for the Site correspond to three human health exposure scenarios. Zone 1 corresponds to “Recreational” 

exposure scenario, and Zones 2 and 3 correspond to both “Mixed Use Multi-Family Residential” and 

“Commercial” exposure scenarios. A “Mixed Use Multi-Family Residential” (hereafter referred to as 

“residential”) exposure scenario was also evaluated for comparison purposes; however, although Zones 2 

and 3 both provide for residential development, the podium type of building construction precludes 

residential occupation of the ground level. The “Construction/Excavation Worker” exposure scenario 

was considered for the entire Site. 

The receptors selected are consistent with the receptors that were quantitatively evaluated in the risk 

assessments prepared by B&M (B&M, 2003 and 2006c, d), with the exception of recreational receptor. 

The previous risk assessments qualitatively evaluated Visitors/Trespassers, which are now quantitatively 

evaluated as recreational receptors. Soil gas CULs will be developed for commercial worker receptors, 

and residential receptors for comparison purposes only, as described below in Section D3.2. 

D3.2 Potential Exposure Pathways 

Health risks may occur when there is contact with a chemical by a receptor population. Exposed 

populations must then either ingest, inhale, or dermally absorb the chemical to complete an exposure 

pathway and experience a possible health risk. The following is a discussion of the pathways with which 

CULs were developed for the primary media of concern, which is soil gas. Soil, groundwater, and soil 

gas at Schlage OU have shown to contain chlorinated volatile organic compounds (CVOCs), which 

include 1,1-dichloroethene (1,1-DCE), cis-1,2-dichloroethene (cis-1,2-DCE), trans-1,2-dichloroethene 

(trans-1,2-DCE), tetrachloroethene (PCE), trichloroethene (TCE), and vinyl chloride (VC). CULs for the 

CVOCs will be developed for soil gas. 

D3.2.1 

Soil Gas Exposure Pathways 

Conceptually, CVOCs in groundwater can partition to soil gas. Soil gas can then passively migrate or be 

actively drawn into the enclosed space of structures that overlie the groundwater containing CVOCs. At 

the Schlage OU, soil gas could migrate to the air within the on-grade commercial buildings (podium 

construction) at redevelopment Zone 2, or to the air within the on-grade parking garages at redevelopment 

Zone 3. At each of these redevelopment zones, air within the on-grade buildings or parking garages could 

then migrate (or be drawn) into the residential spaces that are constructed on top of the buildings/garages, 

which is further discussed in Section D5.2. CULs will be developed that are protective of commercial 

workers expected to occupy the on-grade structures (full time workers in commercial buildings, or full 

D3-1





time parking attendant in garages) and be exposed via inhalation of indoor air. These CULs will also be 

protective of the residents living in the residential spaces constructed on top of the buildings/garages as 

described in Section D5.2. Under a residential exposure scenario, soil gas could migrate to the air within 

the multi-family residential on-grade building. CULs will be developed for comparison purposes only 

that are protective of residential receptors who maybe exposed via inhalation of indoor air. The 

commercial CULs can be compared with CVOC concentrations in soil gas collected prior to initiation of 

Site redevelopment activities. Construction/excavation workers and recreational receptors were 

considered to have incomplete pathways to soil gas because they were assumed to spend all of their time 

outdoors. 

D3.3 Exposure Assumptions 

The exposure assumptions used to evaluate the exposure pathways in B&M 2003 and 2006c, and d risk 

assessments are the same exposure assumptions used to develop soil gas CULs. Exposure assumptions 

are provided in Table D3-1. 

The adult receptors were assumed to weigh 70 kilograms (kg) (United States Environmental Protection 

Agency [EPA], 1991; DTSC, 1996) and child receptors were assumed to weigh 15 kg (EPA, 1991; DTSC, 

1996). 

The exposure frequency (EF) parameter of 350 days per year (days/year) is used for residential receptors 

(EPA, 1991; DTSC, 1996). The EF parameters for commercial worker receptors are based on a standard 

work schedule of 250 days/year. For the commercial worker, the exposure duration (ED) of 25 years is 

assumed (EPA, 1991; DTSC, 1996). 

The averaging time (AT) is the period over which exposure is averaged in days. The noncancer AT for 

all receptors is the ED multiplied by 365 days/year (EPA, 1989), which is provided on Table D3-1. The 

cancer AT is 70 year lifetime risk multiplied by 365 days/year for all receptors (EPA, 1989). 

The exposure time (ET) for the commercial worker receptor is 8 hours/day, which is based on the average 

work day (B&M, 2006c, d) in a garage and commercial building. 

D3-2





D4.0 TOXICITY ASSESSMENT 

Toxicity assessment is the process of using the existing toxicity information from human and/or animal 

studies to identify potential health risks at various dose levels in exposure populations (EPA, 1989). To 

estimate these potential health risks, the relationship between exposure to a chemical (in terms of chronic 

daily intake for individuals) and an adverse effect (in terms of bodily response to a specific intake dose 

level) must be quantified. The methodologies used to develop toxicity factors differ, depending on 

whether the CVOC is a potential carcinogen (i.e., has the potential to cause cancer) and/or has noncancer 

adverse effects. 

Both California and EPA-derived toxicity values were compiled for the CUL development. For 

California, the California Environmental Protection Agency’s (CalEPA) Office of Environmental Health 

Hazard Assessment (OEHHA) online toxicity database (OEHHA, 2008) was consulted. The EPA values 

were compiled from EPA’s Integrated Risk Information System (IRIS), an online database (EPA, 2008c), 

the Agency for Toxic Substances and Disease Registry (ATSDR), provided in EPA’s Regional Screening 

Levels (RSLs) tables (EPA, 2008a), and the provisional peer review toxicity values (PPRTVs), provided 

in EPA’s RSLs tables (EPA, 2008a). These sources are updated regularly based on toxicity and exposure 

studies. The toxicity values for the CVOCs are discussed below and presented in Table D4-1. 

D4.1 Noncancer Toxicity Assessment 

D4.1.1 

Noncancer Reference Dose 

In deriving dose-response criteria for assessing the potential for noncancer health effects from exposure to 

chemicals, it is assumed by regulatory agencies that noncancer health effects occur only after a threshold 

dose is reached. This threshold dose is usually estimated by regulatory agencies from the no-observed 

adverse effect level (NOAEL) or the lowest-observed adverse effect level (LOAEL) determined from 

chronic (i.e., long-term) animal studies or human epidemiological studies. The NOAEL is defined as the 

highest dose at which no adverse effects are observed, while the LOAEL is defined as the lowest dose at 

which adverse effects are observed. 

Uncertainty factors or safety factors are applied to the NOAEL or LOAEL observed in animal studies or 

human epidemiologic studies to establish inhalation “reference concentrations” (RfCs) in units of 

milligrams per cubic meter (mg/m 3 ). Chronic RfC are an estimate of a dose level that is not expected to 

result in adverse health effects in persons exposed for a lifetime, even among the most sensitive members 

of the population (e.g., children and the aged). Use of these uncertainty and modifying factors add 

conservatism into the derivation of the RfC. The chronic inhalation RfCs are presented in Table D4-1. 

D4.1.2 

Target Hazard Index 

Noncarcinogenic effects are health effects pertaining to the function of various organ systems due to 

chemically caused toxic endpoints other than cancer and gene mutations. Therefore, EPA (1991) 

established a target hazard quotient (THQ) to correspond to a hazard index of one or unity, which is 

equivalent to the degree of chemical exposure from all significant exposure pathways in a given medium 

below which it is unlikely for even sensitive populations to experience adverse health effects. 

D4-1





D4.2 Cancer Slope Factors and Target Risk Level 

D4.2.1 

Cancer Slope Factors 

Some chemicals have been shown, and many more are assumed to be, potential human carcinogens. To 

be health protective, the EPA (1989) assumes that a relatively small number of molecular events can elicit 

changes in a cell, ultimately resulting in uncontrolled cell proliferation and cancer. Based on this theory, 

the EPA uses a two-part process in evaluating the potential cancer risk of contaminants: (1) assigning a 

weight-of-evidence classification, and (2) calculating an inhalation unit risk (IUR) for inhalation 

exposures. 

The EPA (2005) weight-of-evidence classification system for carcinogenicity is as follows: 

• A Known human carcinogen; 

• B1 or B2 Probable human carcinogen; 

• C Possible human carcinogen; 

• D Not classifiable as to human carcinogenicity; and 

• E Evidence of noncarcinogenicity in humans. 

The weight-of-evidence classification is based on the source of the data (human epidemiology study or 

animal bioassay) and whether cancer has been observed in more than one animal species. These 

alphanumeric classifications are currently being phased out by EPA as toxicity data are reviewed and 

revised under the Guidelines for Carcinogenic Risk Assessment (EPA, 2005). Under the revised 

guidelines, a greater emphasis is placed on the conditions under which the observed effects may be 

expressed, such as whether the potential for carcinogenicity appears limited to a specific route of 

exposure, or whether carcinogenic activity may be secondary to another toxic effect. The current weightof-evidence 

system is a narrative classification, as follows (EPA, 2005): 

• Carcinogenic to humans; 

• Likely to be carcinogenic to humans; 

• Suggestive evidence of carcinogenicity, but not sufficient to assess human carcinogenic potential; 

• Data are inadequate for an assessment of human carcinogenic potential; and 

• Not likely to be carcinogenic to humans. 

In general, IURs have been calculated and are available for potential carcinogens in Groups A, B1, and 

B2, but are calculated only on a case-by-case basis for Group C (EPA, 1989). The IUR is used to develop 

inhalation CULs and is considered an upper-bound excess lifetime cancer risk estimated to result from 

continuous exposure to an agent at a concentration of 1 micrograms per cubic meter (µg/m 3 ) in air. IURs 

have been developed for known, likely, or suggestive evidence carcinogens for which inhalation 

assessments have been conducted and reviewed by EPA. The IURs for the development of CULs are 

presented in Table D4-1. 

D4.2.2 

Target Risk Level 

Since EPA (1989) assumes that a relatively small number of molecular events can elicit changes in a cell, 

EPA uses a two-part process in evaluating the potential cancer risk of contaminants. One part of the 

D4-2





process is calculating an IUR for inhalation exposures. The IUR directly relates to the incremental cancer 

risk over a lifetime, which is assumed to be 70 years. EPA (1991) considers a target risk range of one-inone 

million (1E -06 ) –to one in-ten thousand (1E -04 ) to be safe and protective of public health. Therefore, a 

target risk level (TRL) that corresponds to a 1E -06 was selected to evaluate the incremental risk of an 

individual developing cancer over a lifetime as a result of exposure to the potential carcinogen from all 

significant exposure pathways for a given medium. The TRL is presented on Table D3-1. 

D4-3





D5.0 INDOOR AIR TARGET CONCENTRATIONS 

This section describes the development and selection of residential and commercial indoor air target 

concentrations, and describes the approach that is used to evaluate potential exposures to residential 

receptors that live in residential units located over the garages or commercial space. 

D5.1 Indoor Air Target Concentrations 

CULs for soil gas and groundwater that are a vapor source to indoor air are developed by back-calculating 

a soil gas or groundwater CVOC concentration that would not result in unsafe levels of the CVOC in 

indoor air. Therefore, indoor air target concentrations were selected from published indoor air screening 

levels or calculated to represent the levels of CVOC in indoor air that would be safe (protective) for 

residents or commercial receptors. The residential and commercial indoor air screening levels from the 

California Human Health Screening Levels (CHHSLs) (CalEPA, 2005) were compared to the residential 

and industrial air RSLs (EPA, 2008a), respectively. The residential CHHSLs and RSLs for trans-1,2- 

DCE, PCE, and TCE were similar in value, but VC was not similar because RSL was derived using a less 

conservative IUR factor than the CHHSL value. A CHHSL value was not available for 1,1-DCE, and a 

RSL value was not available for cis-1,2-DCE. Therefore, the indoor air target concentrations for 

residential receptors were identified as the residential CHHSLs (CalEPA, 2005), with the exception of 

1,1-DCE, for which the residential air RSL (EPA, 2008a) was used. The commercial indoor air screening 

levels from the CHHSLs were not selected in that the CHHSL values are inconsistent with calculated 

indoor air commercial receptors. The calculated indoor air values are consistent with the RSLs, which are 

more current than the CHHSL values, with the exception of VC because a more conservative CalEPA 

IUR was selected than was used for the RSL. Therefore, the indoor air target concentrations for 

commercial receptors were calculated as risk-based values using the RSLs User Guide (EPA, 2008b). The 

noncancer and cancer indoor air concentrations were calculated to correspond to a hazard index of one or 

unity or a cancer risk of 1E -06 , respectively. The following equations were used to develop commercial 

target indoor air noncancer and cancer concentrations. 

Noncarcinogenic 

Where: 

µ g 

THQ × AT × 1000 

mg 

IA nc 

= 

1 

EF × ED × ET × 

RfC 

IA nc = Noncarcinogen indoor air concentration (µg/m 3 ); 

THQ = Target hazard quotient (unitless); 

AT = Averaging time (days); 

EF = Exposure frequency (days/year); 

ED = Exposure duration (years); 

ET = Exposure time (hours/hour); and 

RfC = Reference concentration (mg/m 3 ). 

Carcinogenic 

IA TR × AT 

= c EF × ED × ET × IUR 

D5-1





Where: 

IA c = Carcinogen indoor air concentration (µg/m 3 ); 

TR = Target risk (unitless); 

AT = Averaging time (days); 

EF = Exposure frequency (days/year); 

ED = Exposure duration (years); 

ET = Exposure time (hours/hour); and 

IUR = Inhalation unit risk (µg/m 3 ) -1 . 

The exposure assumptions and toxicity values are provided on Tables D3-1 and D4-1, respectively. 

Calculated commercial indoor air concentrations and selected residential indoor air concentrations are 

provided on Table D5-1. For the CVOCs that have noncancer and cancer indoor air concentrations, the 

more conservative value was selected as the target indoor air concentration for the commercial worker 

receptors. 

D5.2 Indoor Air Concentrations - Residential Living Space 

For comparison purposes, the potential for airflow from on-grade commercial buildings or parking 

garages into residential living spaces that are located on top of the buildings/parking garages is evaluated 

by determining the amount of air leaking into the residential units from the garage or the commercial 

spaces below. The air leakage factor between the garage or commercial spaces and the residential units 

above was established based on published studies of airflow distribution in multifamily buildings (Feustel 

and Diamond, 1996; and Interstate Technology and Regulatory Council [ITRC], 2007). The ITRC 

scenario for multi-family dwelling that is located over a former gas station is provided in Attachment D-1. 

In these studies, the air leakage between the garage and the residential units above ranged from less than 

4%, to 5%. An air leakage value of 5% from ITRC was used to estimate the indoor air concentrations 

within the residential units. Specifically, the commercial indoor air target concentrations, which represent 

the maximum level of CVOCs that would exist in on-grade commercial buildings or parking garages, 

were multiplied by a 5% leakage factor to estimate the potential CVOC concentrations that could exist in 

overlying residential spaces. 

The estimated air concentrations within the residential units were compared to the residential CHHSLs 

(CalEPA, 2005) and ambient air background concentrations from the Bay Area Air Quality Management 

District (BAAQMD). A CHHSL value was not available for 1,1-DCE, so the residential air concentration 

from the RSLs (EPA, 2008a) was used. The ambient air background concentrations provided by 

BAAQMD are provided in parts per billion. The concentrations were converted to µg/m 3 using the 

following equation: 

Where: 

C 3 = 0. 0409 × C 

ppb 

× MW 

µ g / m 

C µg/m3 = Ambient air background concentration (µg/m 3 ); 

C ppb = Ambient air background concentration (ppb); 

MW = Molecular weight of compound: 

MW PCE = Molecular weight of PCE = 165.8; 

MW TCE = Molecular weight of TCE = 131.4; and 

MW VC = Molecular weight of VC = 62.5. 

D5-2





The table below presents the calculated commercial indoor air target concentration, the estimated indoor 

air concentrations in overlying residential space (represented as 5% of the commercial indoor air 

concentration), residential CHHSLs, and ambient air background concentrations. 

Chemicals of Concern 

Commercial 

Indoor Air 

Target 

Concentration 

(µg/m 3 ) 

Indoor Air Concentrations 

Estimated 

Residential 

Indoor Air 

Concentration b 

(µg/m 3 ) 

Residential 

California 

Human Health 

Screening Levels 

(CHHSLs) 

(µg/m 3 ) 

Ambient Air 

Background 

Concentrations c 

(µg/m 3 ) 

1,1-Dichloroethene a 876 44 210 -- 

cis-1,2-Dichloroethene 153 7.7 36.5 -- 

trans-1,2-Dichloroethene 263 13 73 -- 

Tetrachloroethene 2.08 0.10 0.41 0.54 

Trichloroethene 6.13 0.31 1.22 0.16 

Vinyl chloride 0.16 0.008 0.03 0.38 

a The residential value is from EPA, 2008a because there is not a CHHSL value for 1,1-DCE. 

b Calculated as commercial indoor air target concentration x 5%. 

c Ambient air background concentrations from BAAQMD, 2007 and DTSC, 2009. 

-- = Not available. 

As indicated in this table, estimated indoor air concentrations in residential living space located above ongrade 

commercial buildings or parking garages would be below the residential CHHSLs. The estimated 

residential indoor air concentrations for PCE and VC are below the ambient air background 

concentrations. The estimated residential indoor air concentration of TCE is marginally higher than the 

ambient air background concentration. However, the soil gas or groundwater CULs are protective for 

vapor migration to on-grade commercial space occupied by full time workers (commercial buildings or 

parking garages) would also be protective for the indoor air in residential space located over (on top of) 

the commercial buildings/parking garages. An air leakage value of approximately 20% would be needed 

to meet the residential CHHSL values. 

D5-3





D6.0 CLEANUP LEVELS 

This section describes the development and selection of soil gas CULs by receptor. This section also 

provides the equations used to develop soil gas CULs. 

D6.1 Soil Gas Cleanup Levels 

Soil gas CULs were developed to evaluate soil gas samples that will be collected when the soil and 

groundwater remediation is completed to ensure residual concentrations in soil and volatilization from 

groundwater is not a significant source to indoor air. The best method of evaluating potential vapor 

intrusion is via soil gas, and not soil or groundwater measurements. The soil gas CULs were developed 

using the Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air 

(DTSC, 2005) for commercial receptors for Zones 2 and 3, which assume there is a full-time commercial 

worker in the on-grade building for Zone 2 and a full-time parking attendant in the on-grade garage for 

Zone 3, and for residential receptors for comparison purposes, which assume that a resident lives in ongrade 

residential structures, using the following equation. 

Where: 

SG = 

IA 

AF 

SG = Soil gas concentration (µg/m 3 ); 

IA = Indoor air target concentration (µg/m 3 ); and 

AF = Attenuation factor (unitless). 

The attenuation factors that were used to calculate the soil gas concentrations for residential and 

commercial receptors are default attenuation factors from the Guidance for the Evaluation and Mitigation 

of Subsurface Vapor Intrusion to Indoor Air (DTSC, 2005). The attenuation factor for future residential 

slab-on-grade was used to develop residential soil gas concentrations for comparison purposes only, and 

the future commercial slab-on-grade construction was used to develop commercial soil gas concentrations 

for the Site. The residential and commercial soil gas concentrations are provided on Table D6-1. The 

CHHSL values were not selected due to the inconsistencies with indoor air concentrations as stated in 

Section D5.1 for commercial workers, and the CHHSL soil gas concentrations for both residential and 

commercial workers were not developed for new construction. Also, the soil gas CULs are conservative 

as they do not take into consideration an air exchange rate that would further enhance dilution of CVOCs 

present in indoor air. In accordance with the 2007 California Mechanical Code Section 406.4.2 and 

Chapter 4 Table 4-4, a mechanical ventilation system shall be provided for an enclosed parking garage 

where the air exchange rate needs to meet a minimum exhaust rate of 0.75 cubic feet per minute per 

square foot (cfm/ft 2 ). 

D6.2 Redevelopment Zone Cleanup Levels 

This section describes the selection of soil gas CULs for CVOCs at redevelopment zones 2 and 3 at the 

Schlage OU. The soil gas CULs for redevelopment zones 2 and 3 are provided below and on Table D6-2. 

D6-1





D6.2.1 

Residential Over Commercial (Zone 2) and Residential Over 

Podium Garage (Zone 3) Cleanup Levels 

Redevelopment Zone 2 is designated as residential over commercial buildings, and Zone 3 as residential 

over podium parking construction. Both zones assume commercial receptors on the ground floor with 

residential receptors above. Soil gas CULs are protective of vapor intrusion to indoor air for commercial 

receptors, but are also protective for residential receptors that may be exposed to air that may leak from 

on-grade commercial buildings or parking garages to the overlying residential space. 


Soil Gas Cleanup Levels 

(g/m 3 ) 

(g/L) 

1,1-Dichloroethene 2,190,000 2,190 

cis-1,2-Dichloroethene 383,250 383 

trans-1,2-Dichloroethene 657,000 657 

Tetrachloroethene 5,197 5.19 

Trichloroethene 15,330 15 

Vinyl chloride 393 0.39 

µg/L: micrograms per liter 

D6-2





D7.0 APPLICATION OF CLEANUP LEVELS 

The CULs will be compared to an exposure point concentration (EPC). The EPA defines EPCs as the 

representative chemical concentration a receptor may contact in an exposure area over the exposure 

period (EPA, 1989). The typical concept of human exposure at a Site or within a defined exposure area is 

an individual’s contact with the contaminated medium on a periodic and random basis. Because of the 

repeated nature of such contact, the human exposure does not really occur at a fixed point but rather at a 

variety of points with equal likelihood that any given point within the exposure area will be the contact 

location on any given day. Thus, the EPCs should be the arithmetic averages of chemical concentrations 

within the exposure area. To account for uncertainty in estimating the arithmetic mean concentration, the 

EPA recommends that an upper confidence limit (UCL) be used to represent the EPC. The EPCs for the 

CVOCs for each media of concern will be calculated using EPA’s ProUCL Version 4.0. The maximum 

concentration will be used for CVOCs that have insufficient number of samples to calculate a UCL. 

If the chemical specific ratio of the EPC to the CUL is less than 1, this indicates that a 1x10 -06 health risk 

for carcinogens and a hazard index of 1 for non-carcinogens has been met and that substantial health risks 

will not likely to be associated with that chemical. If the ratio of the EPC to the CUL is greater than 1, 

this indicates that the concentration of the chemical may exceed the value protective of public health. To 

evaluate possible exposure to multiple chemicals, the effects of multiple chemicals will be assumed to be 

additive and the ratios will be added together to calculate a ratio sum. A ratio sum of 1 or less indicates 

that substantial health risks are not likely to be associated with exposure to the multiple chemicals 

evaluated; whereas a ratio sum greater than 1 indicates further action may be necessary. 

D7-1





D8.0 CONCLUSION 

The soil gas CULs were developed using the previous risk assessments prepared by B&M as a guide for 

population selection, pathway selection, and exposure assumptions. Indoor air target concentrations were 

developed in order to calculate CULs for vapor intrusion from soil gas, soil, and groundwater. The soil 

gas CULs are presented on Table D6-1. The selected CULs for each redevelopment zone are provided on 

Table D6-2 and Section D6.2. 

D8-1





D9.0 REFERENCES 

Bay Area Air Quality Management District (BAAQMD), 2007. Annual Report 2003. Toxic Air 

Contaminant Control Program. August. 

Burns & McDonnell Engineering Company, Inc. (B&M), 2003. Human Health Risk Assessment of Joint 

Groundwater Operable Unit. March. 

_____, 2006a. Groundwater Monitoring & Sampling Results, Schlage Lock Company Site, San 

Francisco, California. January 11. 

_____, 2006b. Soil Sampling Summary Report, Schlage Lock Company Site, San Francisco, California. 

January 25. 

_____, 2006c. Human Health Risk Assessment for the San Francisco County Parcel of Operable Unit-1. 

April. 

_____, 2006d. Human Health Risk Assessment for the San Mateo County Parcel of Operable Unit-1. 

May. 

California Environmental Protection Agency (CalEPA), 2005. Use of California Human Health 

Screening Levels (CHHSLs) in Evaluation of Contaminated Properties. January. 

Department of Toxic Substances Control (DTSC), 1996. Supplemental Guidance for Human Health 

Multimedia Risk Assessments of Hazardous Waste Sites and Permitted Facilities. August. 

_____, 2005. Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air. 

California Environmental Protection Agency. Revised. February 7. 

_____, 2008. Consent Order, dated May 22, 2008, Docket Number HAS-CO 07/08-185. 

_____, 2009. Fwd: FW: Toxics Monitoring Data. Electronic mail correspondence between Calvin 

Willhite, DTSC, and Nyree Melancon, MACTEC. July 13. 

Feustel, H.E. and R.C. Diamond (Feustel and Diamond), 1996. Air Flow Distribution in a High-Rise 

Residential Building. Lawrence Berkeley National Laboratory, Berkeley, USA. 

Interstate Technology and Regulatory Council (ITRC), 2007. Technical and Regulatory Guidance 

Supplement; Vapor Intrusion Pathway: Investigative Approaches for Typical Scenarios. A Supplement to 

Vapor Intrusion Pathway: A Practical Guideline. Vapor Intrusion Team. January. 

MACTEC Engineering and Consulting, Inc. (MACTEC), 2008. Quarterly Groundwater Monitoring 

Report, Third Quarter 2008, Schlage OU, San Francisco and Brisbane, California. December 18. 

Office of Environmental Health Hazard Assessment (OEHHA), 2008. Toxicity Criteria Database. 

California Environmental Protection Agency (CalEPA). 

http://www.oehha.ca.gov/risk/ChemicalDB/index.asp. December. 

Treadwell and Rollo (T&R), 2001. Soil Operable Unit Remedial Investigation Report. Schlage Lock 

Company Site, 2401-2555 Bayshore Boulevard, San Francisco, California. June. 

United States Environmental Protection Agency (EPA), 1989. Risk Assessment Guidance for Superfund 

Volume I - Human Health Evaluation Manual, Part A (RAGS). EPA/540/1-89/002. 

_____, 1991. Risk Assessment Guidance for Superfund, Volume 1: Human Health Evaluation Manual, 

Supplemental Guidance, Standard Default Exposure Factors. Interim Final. OSWER Directive 

9285.6-03. 

D9-1





_____, 2005. Guidelines for Carcinogen Risk Assessment. EPA/630/P-03/001F. March. 

_____, 2008a. Regional Screening Levels. Region 9. September. 

_____, 2008b. Regional Screening Levels User Guide. September 18. 

_____, 2008c. Integrated Risk Information System (IRIS). Http://www.epa.gov/iris. December. 

D9-2

TABLES

Appendix D 

Feasibility Study/Remedial Action Plan, Schlage OU, 

San Francisco, California 

MACTEC Project 4096088522 

November 4, 2009 

KB63190 FS-RAP Appendix D Tables 

Table D3-1. Exposure Factors to Calculate Risk Based Human Health 

Cleanup Levels for Chlorinated Volatile Organic Compounds 

Exposure Parameters Units a Default or Assumed Value Reference b 

Target Noncancer Hazard Quotient (THQ) unitless 1 EPA, 1991 

Target Cancer Risk (TR) unitless 1.E-06 EPA, 1991 

Body Weight (BW) 

Child kg 15 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Adult kg 70 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Averaging Time (AT) 

Noncarcinogenic 

Residential 

Child days 2,190 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Adult days 8,760 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Commercial days 9,125 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Carcinogenic days 25,550 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Exposure Frequency (EF) 

Residential days/year 350 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Commercial days/year 250 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Exposure Duration (ED) 

Residential 

Child years 6 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Adult years 24 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Commercial years 25 EPA, 1991; DTSC, 1996; B&M, 2006c,d 

Conversion Factor (CF) 

µg/mg 1,000 -- 

day/hour 0.042 -- 

Exposure Time (ET) 

Residential hour/day 24 B&M, 2006c,d 

Commercial hour/day 8 B&M, 2006c,d 

Footnotes: 

a kg = kilogram; days/year = days per year; µg/mg = microgram per milligram; day/hour = day per hour; and hour/day = hours per day. 

b References: 

Burns & McDonnell (B&M), 2006c. Human Health Risk Assessment for the San Francisco County Parcel of Operable Unit - 1. Universal Paragon 

Corporation. April. 

Burns & McDonnell (B&M), 2006d. Human Health Risk Assessment for the San Mateo County Parcel of Operable Unit - 1. Universal Paragon 

Corporation. May. 

Department of Toxic Substances Control (DTSC), 1996. Supplemental Guidance for Human Health Multimedia Risk Assessments of Hazardous 

Waste Sites and Permitted Facilities. August 

United States Environmental Protection Agency (EPA), 1991. Risk Assessment Guidance for Superfund, Volume 1: Human Health Evaluation Manual, 

Supplemental Guidance, Standard Default Exposure Factors. Interim Final. OSWER Directive 9285.6-03. 

Checked: NAM 

Approved: MS 

Page 1 of 5







Analyte 

Table D4-1. Toxicity Values for Chlorinated Volatile Organic Compounds 

Noncancer Chronic Toxicity Values 

Reference Conc. 

(mg/m 3 ) 

Source a 

RfC 

Cancer Toxicity Values 

Unit Risk 

(µg/m 3 ) -1 

Source a 

IUR 

1,1-Dichloroethene 2.00E-01 (i) -- -- C 

cis-1,2-Dichloroethene 3.50E-02 (oral) -- -- D 

trans-1,2-Dichloroethene 6.00E-02 (p) -- -- -- 

Tetrachloroethene 2.70E-01 (a) 5.90E-06 (o) B 

Trichloroethene 6.00E-01 (o) 2.00E-06 (o) B 

Vinyl chloride 1.00E-01 (i) 7.80E-05 (o) A 

Abbreviations: 

mg/m 3 = Milligrams per cubic meter. 

µg/m 3 = Micrograms per cubic meter. 

-- = Not available. 

EPA = United States Environmental Protection Agency. 

CalEPA = California Environmental Protection Agency. 

Footnotes: 

a Values compiled from the following sources: 

(a) - Agency for Toxic Substances and Disease Registry (ATSDR), provided in EPA, 2008a . 

(i) - Integrated Risk Information System (IRIS) online database (EPA, 2008c ). 

(p) - Provisional Peer Review Toxicity Values (PPRTVs), provided in EPA, 2008a . 

(o) - Office of Environmental Health Hazard Assessment (OEHHA) Toxicity Values, provided in CalEPA, 2008 . 

(oral) - Oral value used for inhalation by route-to-route extrapolation. 

b Weight of Evidence classification (EPA, 2005 ) as listed in OEHHA and IRIS: 

A - Known human carcinogen. 

B - Probable Human Carcinogen 

B1 - Limited evidence of carcinogenicity in humans 

B2 - Sufficient evidence of carcinogenicity in animals with inadequate or lack of evidence in humans. 

C - Possible human carcinogen. 

D - Not classifiable as to human carcinogenicity. 

E - Evidence of noncarcinogenicity for humans. 

Weight-of- 

Evidence b 

References: 

Office of Environmental Health Hazard Assessment (OEHHA), 2008. Toxicity Criteria Database . California Environmental Protection Agency 

(CalEPA). http://www.oehha.ca.gov/risk/ChemicalDB/index.asp. December. 

United States Environmental Protection Agency (EPA), 2005. Guidelines for Carcinogen Risk Assessment . EPA/630/P-03/001F. March. 

United States Environmental Protection Agency (EPA), 2008a. Regional Screening Levels . San Francisco, California. September. 

United States Environmental Protection Agency (EPA), 2008c. Integrated Risk Information System (IRIS) . Http://www.epa.gov/iris. December. 

Checked: NAM 

Approved: MS 

Page 2 of 5







Table D5-1. Target Indoor Air Concentration 

Calculated Indoor Air Concentrations 

Commercial 

Target Indoor Air Concentration 

Chemicals of Concern Noncancer a Cancer b Commercial c Residential d 

(µg/m 3 ) (µg/m 3 ) (µg/m 3 ) (µg/m 3 ) 

C nc C c C c/i_ia C r_ia 

1,1-Dichloroethene 876 -- 876 210 

cis-1,2-Dichloroethene 153 -- 153 37 

trans-1,2-Dichloroethene 263 -- 263 73 

Tetrachloroethene 1,183 2.08 2.08 0.41 

Trichloroethene 2,628 6.13 6.13 1.22 

Vinyl chloride 438 0.16 0.16 0.03 



-- = Not available / not applicable. 

Footnotes: 

a From Tables D3-1 and D4-1. 

C nc = (THQ x AT nc x CF µ/m ) / (EF x ED x ET x CF d/h x (1/RfC)) 

b From Tables D3-1 and D4-1. 

C c = (TR x ATc) / (EF x ED x ET x CF d/h x IUR) 

c The lower value of the noncancer and cancer indoor air concentration selected as the target indoor air concentration. 

d The target residential indoor air concentration are indoor air California Human Health Screening Levels (CHHSLs) 

(CalEPA, 2005 ). A CHHSL value was not available for 1,1-DCE, so the residential air concentration from the RSLs 

(EPA, 2008a ) was used. 

References: 

California Environmental Protection Agency (CalEPA), 2005. Use of California Human Health Screening Levels 

(CHHSLs) in Evaluation of Contaminated Properties . January. 

United States Environmental Protection Agency (EPA), 2008a. Regional Screening Levels . San Francisco, California. 

September. 

Checked: NAM 

Approved: MS 

Page 3 of 5







Table D6-1. Soil Gas Cleanup Levels Protective of Indoor Air 

Attenuation Factor a Target Indoor Air Concentration Soil Gas Cleanup Levels e 

Chemicals of Concern Residential a Commercial b Residential c Commercial d Residential Commercial 

(unitless) (unitless) (µg/m 3 ) (µg/m 3 ) (µg/m 3 ) (µg/L) (µg/m 3 ) (µg/L) 

1,1-Dichloroethene 0.0009 0.0004 210 876 233,333 233 2,190,000 2,190 

cis-1,2-Dichloroethene 0.0009 0.0004 37 153 40,556 41 383,250 383 

trans-1,2-Dichloroethene 0.0009 0.0004 73 263 81,111 81 657,000 657 

Tetrachloroethene 0.0009 0.0004 0.41 2.08 456 0.46 5,197 5.20 

Trichloroethene 0.0009 0.0004 1.22 6.13 1,356 1.36 15,330 15.33 

Vinyl chloride 0.0009 0.0004 0.03 0.16 33 0.033 393 0.393 



µg/L = Micrograms per liter. 

Footnotes: 

a The attenuation factor is based on future residential slab-on-grade construction (DTSC, 2005 ). 

b The attenuation factor is based on future commercial slab-on-grade construction (DTSC, 2005 ). 

c The target residential indoor air concentration are indoor air California Human Health Screening Levels (CHHSLs) 

(CalEPA, 2005 ). A CHHSL value was not available for 1,1-DCE, so the residential air concentration from the RSLs 

(EPA, 2008a ) was used. 

d From Table D5-1. 

e Soil Gas Concentration = Indoor Air Concentration / Attenuation Factor 

References: 

California Environmental Protection Agency (CalEPA), 2005. Use of California Human Health Screening Levels 

(CHHSLs) in Evaluation of Contaminated Properties . January. 

Department of Toxic Substances Control (DTSC), 2005. Guidance for the Evaluation and Mitigation of Subsurface 

Vapor Intrusion to Indoor Air . December 15, 2004. Revised February 7. 

United States Environmental Protection Agency (EPA), 2008a. Regional Screening Levels . San Francisco, California. 

September. 

Page 4 of 5 

Checked: NAM 

Approved: MS







Table D6-2. Final Soil Gas Cleanup Levels 

Soil Gas Cleanup Level 


Commercial a 

(Zone 2 and 3) 

(µg/m 3 ) 

(µg/L) 

1,1-Dichloroethene 2,190,000 2,190 

cis-1,2-Dichloroethene 383,250 383 

trans-1,2-Dichloroethene 657,000 657 

Tetrachloroethene 5,197 5.20 

Trichloroethene 15,330 15 

Vinyl chloride 393 0.39 



µg/L = Micrograms per liter. 

Zone 2 = Residential over commercial construction. 

Zone 3 = Residential over podium parking construction. 

Checked: NAM 

Approved: MS 

Footnotes: 

a The soil gas cleanup level is protective of vapor intrusion to indoor air for 

commercial receptors. 

Page 5 of 5

ATTACHMENT D-1 

INTERSTATE TECHNOLOGY AND REGULATORY COUNCIL SCENARIO 

FOR MULTI-FAMILY DWELLING

Technical and Regulatory Guidance 

Supplement 

Vapor Intrusion Pathway: 

Investigative Approaches for Typical Scenarios 

A Supplement to Vapor Intrusion Pathway: A Practical Guideline 

January 2007 

Prepared by 

The Interstate Technology & Regulatory Council 

Vapor Intrusion Team

ABOUT ITRC 

Established in 1995, the Interstate Technology & Regulatory Council (ITRC) is a state-led, national 

coalition of personnel from the environmental regulatory agencies of some 46 states and the District of 

Columbia, three federal agencies, tribes, and public and industry stakeholders. The organization is 

devoted to reducing barriers to, and speeding interstate deployment of better, more cost-effective, 

innovative environmental techniques. ITRC operates as a committee of the Environmental Research 

Institute of the States (ERIS), a Section 501(c)(3) public charity that supports the Environmental Council 

of the States (ECOS) through its educational and research activities aimed at improving the environment 

in the United States and providing a forum for state environmental policy makers. More information 

about ITRC and its available products and services can be found on the Internet at www.itrcweb.org. 

DISCLAIMER 

ITRC documents and training are products designed to help regulators and others develop a consistent 

approach to their evaluation, regulatory approval, and deployment of specific technologies at specific 

sites. Although the information in all ITRC products is believed to be reliable and accurate, the product 

and all material set forth within are provided without warranties of any kind, either express or implied, 

including but not limited to warranties of the accuracy or completeness of information contained in the 

product or the suitability of the information contained in the product for any particular purpose. The 

technical implications of any information or guidance contained in ITRC products may vary widely based 

on the specific facts involved and should not be used as a substitute for consultation with professional and 

competent advisors. Although ITRC products attempt to address what the authors believe to be all 

relevant points, they are not intended to be an exhaustive treatise on the subject. Interested parties should 

do their own research, and a list of references may be provided as a starting point. ITRC products do not 

necessarily address all applicable health and safety risks and precautions with respect to particular 

materials, conditions, or procedures in specific applications of any technology. Consequently, ITRC 

recommends also consulting applicable standards, laws, regulations, suppliers of materials, and material 

safety data sheets for information concerning safety and health risks and precautions and compliance with 

then-applicable laws and regulations. The use of ITRC products and the materials set forth herein is at the 

user’s own risk. ECOS, ERIS, and ITRC shall not be liable for any direct, indirect, incidental, special, 

consequential, or punitive damages arising out of the use of any information, apparatus, method, or 

process discussed in ITRC products. ITRC product content may be revised or withdrawn at any time 

without prior notice. 

ECOS, ERIS, and ITRC do not endorse or recommend the use of, nor do they attempt to determine the 

merits of, any specific technology or technology provider through ITRC training or publication of 

guidance documents or any other ITRC document. The type of work described in any ITRC training or 

document should be performed by trained professionals, and federal, state, and municipal laws should be 

consulted. ECOS, ERIS, and ITRC shall not be liable in the event of any conflict between ITRC training 

or guidance documents and such laws, regulations, and/or ordinances. Mention of trade names or 

commercial products does not constitute endorsement or recommendation of use by ECOS, ERIS, or 

ITRC. The names, trademarks, and logos of ECOS, ERIS, and ITRC appearing in ITRC products may not 

be used in any advertising or publicity, or otherwise indicate the sponsorship or affiliation of ECOS, 

ERIS, and ITRC with any product or service, without the express written permission of ECOS, ERIS, and 

ITRC.

Vapor Intrusion Pathway: 

Investigative Approaches for Typical Scenarios 

A Supplement to Vapor Intrusion Pathway: A Practical Guideline 

January 2007 

Prepared by 

The Interstate Technology & Regulatory Council 

Vapor Intrusion Team 

Copyright 2007 Interstate Technology & Regulatory Council 

444 North Capitol Street, NW, Suite 445, Washington, DC 20001

Permission is granted to refer to or quote from this publication with the customary 

acknowledgment of the source. The suggested citation for this document is as follows: 

ITRC (Interstate Technology & Regulatory Council). 2007. Vapor Intrusion Pathway: 

Investigative Approaches for Typical Scenarios. VI-1A. Washington, D.C.: Interstate 

Technology & Regulatory Council, Vapor Intrusion Team. www.itrcweb.org.

ACKNOWLEDGEMENTS 

The members of the Interstate Technology & Regulatory Council (ITRC) Vapor Intrusion Team 

wish to acknowledge the individuals, organizations, and agencies that contributed to this 

supplement to the technical and regulatory guidance document Vapor Intrusion Pathway: A 

Practical Guideline. 

As part of the broader ITRC effort, the Vapor Intrusion Team effort is funded primarily by the 

U.S. Department of Energy. Additional funding and support have been provided by the U.S. 

Department of Defense and the U.S. Environmental Protection Agency. ITRC operates as a 

committee of the Environmental Research Institute of the States (ERIS), a Section 501(c)(3) 

public charity that supports the Environmental Council of the States (ECOS) through its 

educational and research activities aimed at improving the environment in the United States and 

providing a forum for state environmental policy makers. 

The team co-leaders, Bill Morris (Kansas Department of Health and Environment) and John 

Boyer (New Jersey Department of Environmental Protection), also wish to recognize the 

individual efforts of the following state team members: 

• Delonda Alexander, North Carolina Department of Environment and Natural Resources 

• Tonia R. Burk, Georgia Environmental Protection Division 

• Mary Camarata, Oregon Department of Environmental Quality 

• Craig Dukes, South Carolina Department of Health and Environmental Control 

• Peter Eremita, Maine Department of Environmental Protection 

• Richard Galloway, Delaware Department of Natural Resources and Environmental Control 

• Jerry Grimes, Virginia Department of Environmental Quality 

• Marilyn Hajicek, Colorado Department of Labor and Employment 

• Jeanene Hanley, Arizona Department of Environmental Quality 

• Jim Harrington, New York Department of Environmental Conservation 

• Tom Higgins, Minnesota Pollution Control Agency 

• Greg Johnson, Colorado Department of Labor and Employment 

• Allan V. Jones, Utah Department of Environmental Quality 

• Bheem R. Kothur, Florida Department of Environmental Protection 

• Diedre Lloyd, Florida Department of Environmental Protection 

• William McKercher, Mississippi Department of Environmental Quality 

• John S. Mellow, Pennsylvania Department of Environmental Protection 

• Robin Mongeon, New Hampshire Department of Environmental Services 

• Evelina Morales, Oklahoma Department of Environmental Quality 

• Susan Newton, Colorado Department of Public Health and Environment 

• Richard Olm, Arizona Department of Environmental Quality 

• Nelly F. Smith, Alabama Department of Environmental Management 

• Neil Taylor, Utah Department of Environmental Quality 

• Rod Thompson, Indiana Department of Environmental Management 

i

The co-leaders also wish to thank the industry and federal agency team members, contributing in 

various forms, to the guidance: Leah Alejo, Naval Facilities Engineering Service Center; Harry 

Anderson, Andre Brown, Jay Hodny, and Jim Whetzel, W. L. Gore & Associates, Inc.; Vanessa 

J. Bauders, U.S. Army Corps of Engineers; Tom Biksey, Environmental Strategies Consulting, 

LLC; Anita Broughton, Haley & Aldrich, Inc; Richard Burns, Connestoga Associates; Douglas 

Cox, Mitretek Systems; Dianne Easly, U.S. Environmental Protection Agency Region 7; Diana 

Marquez, Burns and McDonnell Engineering Company, Inc; Amy L. Edwards, Holland & 

Knight, LLP; Bart Eklund, URS Corporation; Rachel Farnum, GE Global Research; Mark J. 

Fisher, U.S. Army Corps of Engineers; Douglas M. Fitton and Eric M. Nichols, LFR Inc.; David 

J. Folkes, EnviroGroup Limited; Kimberly Gates, Naval Facilities Engineering Service Center; 

Ken Gilland, Buck Engineering; Sandra Gaurin, BEM Systems, Inc.; Jonathan Gledhill, Policy 

Navigation Group; Annette Guiseppi-Elie and Jenny Liu, DuPont Engineering; Blayne Hartman, 

H&P Mobile Geochemistry; Stephen Hoffine, Burns & McDonnell Engineering Company, Inc.; 

Harley Hopkins, American Petroleum Institute; Alana Lee, U.S. Environmental Protection 

Agency Region 9; Ronald J. Marnicio, Tetra Tech, EC; Todd McAlary, GeoSyntec Consultants; 

Denise Miller, ARCADIS; Ian T. Osgerby, U.S. Army Corps of Engineers New England 

District; Gina M. Plantz, Newfields; Henry Schuver, U.S. Environmental Protection Agency 

Office of Solid Waste; Fred Tillman, U.S. Environmental Protection Agency Ecosystems 

Research Division; Matthew Traister, O’Brien & Gere; Robert S. Truesdale, RTI; and Yvonne 

Walker, Navy Environmental Health Center. 

Special thanks to the community stakeholders, Lenny Siegel (Center for Public Environmental 

Oversight) and Peter Strauss (PM Strauss and Associates) for their insightful contribution to the 

value of the community and residents of facilities being investigated for the vapor intrusion 

pathway. And finally thanks to Andrea Futrell, Stacey Kingsbury, Gleness Knauer, and Steve 

Hill for their support and guidance in the preparation of the guidance. 

ii

EXECUTIVE SUMMARY 

This document is not meant to be a stand-alone document—it should be used in conjunction with 

Vapor Intrusion Pathway: A Practical Guideline (ITRC 2007). Appendix A of this document 

also contains Figure 3-3, “Site Investigation Flowchart,” from that technical and regulatory 

guidance document. 

The ITRC Vapor Intrusion Team comprises of a wide variety of state regulators, federal partners, 

industry representatives and stakeholders. It was apparent during team discussions that many 

vapor intrusion scenarios exist, but several seemed to continuously engage the conversation. 

Since these reoccurring discussions evolved around the same types of sites, the team determined 

that walking the reader through these common scenarios may assist in the decision-making 

process for a vapor intrusion investigation. 

The scenarios are based on the assumption that these sites start with a “yes” answer to the 

question in Step 7 of the practical guideline: Does the site require further investigation based on 

a preliminary assessment All emergency (acute) exposures, nuisance conditions, and 

preliminary screening have been completed, and the site has not exited from the vapor intrusion 

assessment process. For the purposes of the following discussion, the need for further 

investigation was warranted, though the reasons for the additional investigation may have been 

different for each scenario. 

Innumerable variations of vapor intrusion scenarios are possible, based on the multitude of 

sources and contaminants of concern, geologic and groundwater conditions, and potentially 

impacted properties and buildings. Differences in these conditions can lead to numerous 

investigation issues, constraints, and options, all of which affect the investigation work plan and 

its implementation. While it is impossible to describe every scenario that could result from 

varying circumstances, experience has shown that a few situations tend to occur more frequently 

than others. This document describes six different, yet common, hypothetical vapor intrusion 

scenarios and the investigation approaches that might be followed. Key decision points and the 

technical rationale for these decisions are identified in the scenarios. These key points are bolded 

in the text to assist the reader. Alternative approaches and investigative tools that may be chosen 

during the various stages are also identified. 

Vapor intrusion investigations can be very complex, and the scenarios are tools in themselves. 

The main theme of each of the scenarios is to highlight the decision process and the reasoning 

behind the decision, the selection of a specific tool vs. an alternative investigative strategy, and 

how the tool is used in the hypothetical scenarios. 

Review of these hypothetical case histories may help users better understand the nuances of 

various investigative procedures, particularly if their site is similar to one of the six scenarios, 

which are as follows. 

1. Gas station in residential neighborhood 

2. Dry cleaner in strip mall adjacent to neighborhood 

iii

3. Large industrial facility with long plume under several hundred buildings 

4. Vacant lot with proposed brownfield development over a groundwater plume 

5. Vacant large commercial building with warehouse space and office space 

6. Apartment building with parking garage over a groundwater plume 

iv

TABLE OF CONTENTS 

ACKNOWLEDGEMENTS............................................................................................................. i 

EXECUTIVE SUMMARY ........................................................................................................... iii 

SCENARIO 1. Residential and Commercial Receptors Located near an Active Service 

Station ...................................................................................................................1 

SCENARIO 2. Dry-Cleaning Operations in a Strip Mall near a Residential 

Neighborhood .......................................................................................................9 

SCENARIO 3. Degreasing Solvent Contamination from a Small Industrial Site on an 

Adjoining Mixed-Use Neighborhood .................................................................15 

SCENARIO 4. Brownfield Redevelopment Site (Vacant Land).................................................25 

SCENARIO 5. Large Industrial Building Undergoing Redevelopment......................................33 

SCENARIO 6. Multifamily Dwelling Located over a Former Gas Station ................................41 

APPENDIX Site Investigation Flowchart ...............................................................................47 

v

ITRC – Vapor Intrusion Pathway: Investigative Approaches for Typical Scenarios January 2007 

Scenario 1 

Residential and Commercial Receptors 

Located Near an Active Service Station 

This scenario illustrates a typical case in the assessment of the vapor intrusion (VI) due to 

petroleum hydrocarbon vapors. 

1.A DESCRIPTION 

The Situation 

As part of a corporate divesting program, a site investigation was completed on an operating gas 

station located in a dense residential neighborhood and submitted to the state due to the 

discovery of petroleum contamination. The site investigation encountered contaminated soils in 

the vicinity of the existing underground storage tanks (USTs)—a petroleum hydrocarbon 

dissolved-phase plume and a small light, nonaqueous-phase liquid (LNAPL) plume at the site. 

The contaminant levels in soil and groundwater and the presence of LNAPL prompted the state 

UST regulatory agency to require a vapor intrusion investigation to determine whether receptors 

were at risk. 

Conceptual Site Model 

A site visit and review of the state UST regulatory agency files allowed the new facility owner’s 

consultant to develop an initial conceptual site model (CSM). 

The site, located in a dense residential neighborhood, is mostly paved and includes a 

convenience store and operating gas station. Groundwater flow direction is generally to the south 

towards a residence that has a dirt floor crawl space. Another residential property abutting the 

site to the south has a full concrete basement. During a site visit, odors were noted in the on-site 

convenience store but not in the residences that abut the site to the south. 

The previous site investigation found the top of groundwater at 20 feet below ground surface 

(bgs). Soil borings consistently indicated sandy soil from 

ground surface to 30 feet below grade. The previous site 

investigation delineated an LNAPL plume approximately 

70 feet long extending downgradient from the existing 

USTs. A dissolved plume extended an additional 110 feet 

downgradient of the LNAPL plume onto a residential 

property to the south. 

The downgradient edge of the LNAPL plume has not left 

the site property and is approximately 50 feet upgradient 

from a residential structure (House 1) with the dirt floor 

crawl space (see Figure 1-1). A monitoring well located at 

Site Summary 

• Active service station 

• Groundwater @ 20 feet bgs 

• Soil contamination, LNAPL, 

and dissolved plume 

• Sandy soils present at site 

• Two residences downgradient 

of site, one with crawl space, 

the other with basement 

• High benzene concentrations 

upgradient of residences 

• LNAPL contained on site 

1


the edge of the LNAPL 

plume has a benzene 

concentration 

of 

10,000 μg/L. A monitoring 

well located near House 1 

has a benzene concentration 

of 500 μg/L. The most 

downgradient well— 

located between House 1 

and House 2, which has a 

full concrete basement— 

has a benzene concentration 

of 5 μg/L. 

Analysis of soil 

immediately adjacent to the 

existing USTs detected 

benzene at 100 mg/kg 

approximately 10 feet from 

the on-site convenience 

store. 

Figure 1-1. Residential neighborhood gas station. 

1.B VI INVESTIGATION PROCESS 

Is vapor intrusion occurring at this site (Use any existing data to assess whether the pathway is 

potentially complete). 

Benzene is 500 μg/L in the well closest to the residence, exceeding state risk-based groundwater 

screening levels by several orders of magnitude. The state oversight agency does not have any 

screening levels for soil phase data, so a soil gas concentration was calculated from the soil 

phase data using the U.S. Environmental Protection Agency (EPA) Office of Solid Waste and 

Emergency Response (OSWER) spreadsheet. The calculated soil gas values exceed state riskbased 

soil gas screening levels by several orders of magnitude. 

Conclusion: There is a need to collect additional data. 

Step 8. Choose Investigative Strategy (see Site Investigation Flowchart, Figure 3-1 of the 

Practical Guideline (ITRC 2007), reproduced in the appendix of this document) 

There are two known sources of contamination: the groundwater (free-phase product and 

dissolved) and contaminated soil in the tank area. 

2


Table 1-1 summarizes the pros and cons of the various investigation methods. 

Table 1-1. Pros and cons of investigative methods for Scenario 1 

Alternative Pros Cons 

Indoor air 

sampling 

Direct confirmation if 

benzene in building 

Likely to have contributions from numerous 

sources, especially store 

Can’t differentiate source 

Legal complications at residences 

Groundwater 

or soil phase 

data 

Passive soil 

gas 

investigation 

External soil 

gas 

investigation 

Subslab soil 

gas 

Can search and 

delineate extent of 

contamination sources 

Less invasive 

More coverage for cost 

Gives actual vaporphase 

values and 

reflects bioattenuation 


Closest to receptor 

Preferred by many 

agencies 

Decision: External soil gas investigation was chosen. 

Public awareness required 

Sources already delineated 

Vapor intrusion risk often overestimated from 

these matrices 

Sources already delineated; values are qualitative 

Depth of sampling is typically 3 feet bgs, and 

basements are ~8 feet bgs 

Attenuation factor unknown 

Conservative screening levels 

Values may not be same as subslab 


Very intrusive; legal issues 

Public awareness required 

Rationale: Since hydrocarbons are the contaminant of concern (COC) and bioattenuation 

in the vadose zone may be reducing the soil gas concentrations, exterior soil gas data will be 

most representative of the subsurface contamination and be less invasive than subslab 

sampling. Indoor air was considered not to be a good alternative because of the likely 

presence of vapors from the active service station. Subslab sampling will be considered 

depending on the results from the exterior sampling. 

Steps 9 and 10. Design and Implement VI Investigative Work Plan 

There are three primary receptors: House #1, House #2, and the on-site convenience store. The 

initial sampling plan was designed to assess the risk to each of these receptors but not to go onto 

the residents’ properties to minimize legal complications. 

• For House #1 (located over the dissolved plume), soil gas samples from three locations at 5 

feet bgs along the property line adjacent to the house are planned. If values exceeding the 

risk-based levels are detected, a vertical profile of the soil gas from 5 feet bgs to the surface 

at the location showing the highest concentration will be conducted to determine whether the 

contamination is making it to the surface, keeping in mind that this building has a dirt crawl 

space instead of a slab construction. The logic for this approach is that if the 

contamination is not making it to the surface over a higher concentration portion of the 

plume, then the same will likely be true further downgradient where other houses 

3


reside. The vertical profile will also test for the presence of any hydrocarbons moving 

laterally from the station at shallower depths, which can happen if the site is completely 

paved over. 

• For House #2 (not over the groundwater plume), soil gas samples were collected at depth 

intervals of 3, 8 and 13 feet bgs (5 feet below the basement floor). Three locations along the 

property line adjacent to the residence toward the source were selected to get the 

concentration profile at depths corresponding to the basement walls, basement floor, and 

below the basement floor. The logic for this approach is that if the contamination levels 

are not above risk-based levels at depths corresponding to the basement walls and 

basement floor closer to the source, then the same will likely be true at the house that is 

farther away from the source. 

• For the on-site convenience store, four soil gas samples are 

planned on 10-foot spacing on the side of the store towards 

the tank pit at a depth just below the surface cover (asphalt 

or cement). The logic for this approach is that the 

ground surface at most service stations from the tank 

pit to the store is typically covered by impervious 

material, so near-slab soil gas data should reflect subslab soil gas data and be 

obtainable less intrusively. In addition, the samples will be closer to the source, so they 

likely will be higher concentrations than subslab samples located farther from the 

source. If the near-slab data exceed risk-based levels, then either additional samples will be 

collected around the store to get a concentration profile around the store’s footprint or, if 

allowed, interior samples below the slab will be collected. 

Having the ability to add sampling locations both spatially and vertically in real time will 

optimize the field effort, so on-site analysis is planned. 

Continuous soil cores may be collected at several locations at the property line and near each 

residence to get soil physical properties for later use in vapor intrusion modeling. Depending on 

its acceptance by the oversight agency, modeling might be used to determine site-specific 

screening values. 

Step 11. Evaluate Data 

Note: Some states may 

require different exposures 

pathways (future use, 

commercial, etc.) regarding 

the on-site convenience store. 

The state oversight agency follows the EPA OSWER guidance for soil gas samples 5 feet below 

the receptor but uses a different default attenuation factor of 0.01 for subslab samples. 

• For the residences, risk-based screening levels for benzene in soil gas at a 1-in-1-million risk 

level are 150 μg/m 3 for soil gas samples collected 5 feet below a receptor and 30 μg/m 3 for 

subslab soil gas samples. 

• For the convenience store, screening levels were calculated using the state-allowed subslab 

attenuation factor for residences of 0.01 adjusted for default exposure times and ventilation 

rates for commercial settings. For benzene at a 1-in-1-million risk level, the residential risk- 

4


based screening level is 30 μg/m 3 using an attenuation factor of 0.01. For commercial 

settings, assuming an exposure time of 12 hours/day, 250 days a year, and a indoor air 

exchange rate of 1 per hour, the calculated risk-based screening level is 11 times higher, so a 

value of 330 μg/m 3 will be used as the not to exceed level. 

Both EPA test methods 8021 and 8260 can be used for soil gas samples conducted on site and 

reach detection levels of 30 μg/m 3 and 100 μg/m 3 , respectively. Method 8021 was selected to 

reach the screening level required at the site. 

Since the COC is a hydrocarbon, oxygen and carbon dioxide data will be collected to show 

bioattenuation and document the presence of highly aerobic soils. These data can be collected 

using gas chromatography or with a portable field meter such as a Land Tec GEM-2000. 

House #1 (above the groundwater plume): The benzene values at the 5-foot collection depth 

ranged 1000–2000 μg/m 3 , which exceeded screening levels by >10 times. To test whether 

bioattenuation was reducing concentrations in the shallow vadose zone, it was decided to collect 

additional samples at shallower depths even though more influences from the surface would be 

expected. A vertical profile in the upper 5 feet gave values 

of 500 μg/m 3 at 3 feet bgs, 150 μg/m 3 at 2 feet bgs, and 

below detection (detection level of 30 μg/m 3 ) at 1 foot bgs. 

Oxygen levels were 12% at 3’ bgs reaching 20% at 1’ bgs. 

The vertical profile indicated that bioattenuation was 

occurring and that at least 3’ of highly aerobic (>10%) soils 

existed. A sample from the 1’ depth was collected for offsite 

TO-15 analysis to confirm field results. 

House #2 (not over the groundwater plume): The vertical profiles at the property line adjacent to 

this residence showed a rapid decrease in the benzene concentration and increase in the oxygen 

concentration from depth towards the surface due to bioattenuation. Benzene values 3–8 feet bgs 

(depth of the basement floor) were below detection (30 μg/m 3 detection level), and oxygen levels 

exceeded 15%. Benzene values at 13 feet bgs (5 feet below the basement) ranged from below 

detection to 110 μg/m 3 , which is below the risk-based screening level. All samples with 

nondetect benzene values were collected for off-site TO-15 analysis to reach the subslab riskbased 

detection limit of 30 μg/m 3 , which would apply if contamination were immediately against 

the basement walls. 

Convenience Store: The benzene concentrations in the four samples ranged 500–2000 μg/m 3 , 

exceeding the calculated risk-based level of 330 μg/m 3 . Additional samples around the store 

showed values ranging from 500 μg/m 3 closer to the source to below detection on the side away 

from the source. 

Step 12. Is Additional Investigation Warranted 

Note: Groundwater 

concentrations should be stable 

prior to making final decisions 

regarding vapor intrusion. If the 

plume is expanding, then soil 

gas concentrations can 

increase with time. 

House #1: Possible risk exists although bioattenuation is apparent in the upper 5 feet of the soil 

column. Off-site TO-15 (of 1-foot bgs soil gas sample) will document whether benzene is below 

subslab risk level of 30 μg/m 3 . The investigator will have to convince the oversight agency that 

5


the shallow (


Although the samples were not collected immediately below the store footprint, there is no 

reason to expect there would be any significant difference under the slab, so subslab sampling is 

not likely to yield any benefit. Two other types of data might prove more useful for this situation: 

• Determination of store ventilation rate—Retail stores such as these have an enormous 

amount of foot traffic, which increases the ventilation rate of the store with every opening of 

the door. The risk-based screening levels assume a default ventilation rate of 1 room 

exchange per hour. The actual rate is likely to be significantly higher. The risk-based 

screening level increases linearly with the ventilation rate. Ventilation rates are often 

available from architectural drawings or are easy to determine using tracer gases. 

• Determination of a slab-specific attenuation factor—The risk-based screening levels assume 

a conservative value of the attenuation factor of 0.01. However, many slabs, especially those 

with floor coverings, have a higher attenuation. A slab-specific attenuation factor can be 

determined using natural conservative tracers, most commonly radon analyses. The riskbased 

screening level is inversely proportional to the attenuation factor. 

Step 13. Is Mitigation Warranted 

The determination of whether mitigation is warranted at either the residential properties or the 

convenience store will depend on the investigative results, regulatory agency preferences, the 

time frame, and numerous legal issues. If time is not a factor, legal complications could hinder 

sampling on the residential property. The regulatory agency may not be comfortable that the 

demonstration of bioattenuation on the site applies to the residence based on the SCM. In these 

situations, mitigation would be a suitable choice. 

1.C WHAT WAS UNIQUE ABOUT THIS SCENARIO 

• Vertical profiles were used to document bioattenuation. 

• Different sampling strategies were used for different buildings/receptors. 

• Supplemental approaches such as flux chambers, radon measurements, and ventilation rates 

were used. 

1.D LESSONS LEARNED 

• Near-slab data can be used to make a decision on the need for subslab data. 

• Supplemental tools can be extremely useful, especially for commercial settings. 

1.E NEXT STEPS 

• If flux chambers show flux into crawl space, how do we mitigate 

• If increased ventilation rate and slab-specific attenuation from radon still do not lower the 

risk levels above measured levels, then mitigation is required. 

• Refer to oversight agency regarding other remediation that may be necessary at the site (e.g., 

soil removal, groundwater remedy, etc.) 

7


Scenario 2 

Dry-Cleaning Operations in a Strip Mall 

Near a Residential Neighborhood 

This scenario illustrates a typical assessment of the vapor intrusion risk to adjoining businesses 

due to tetrachloroethene contamination emanating from a dry cleaner. 

2.A Description 

The Situation 

This is a typical strip mall site, with a day care center, candy store, dry cleaner, hardware store, 

and fast food restaurant located in one commercial building with slab-on-grade construction. An 

odor complaint in the day care center was submitted to the local department of health, which 

collected an air sample. The department of health determined that the odor was likely due to 

tetrachloroethene (PCE) and that the levels of PCE were unacceptable but did not exceed acute 

levels to warrant immediate action. The investigation was addressed ultimately under the state’s 

voluntary cleanup program. 


A preliminary assessment (PA) of the site revealed that the only viable source of PCE was the 

dry cleaner located several doors down from the day care center. The current owner of the dry 

cleaner had recently installed new equipment; however, the PA found that the dry cleaner’s 

historical operations likely resulted in a discharge of PCE to the back of the building via a 

subsurface floor drain. The new dry-cleaning equipment apparently eliminated any waste 

discharge, so there is no current release of PCE to the subsurface soil environment. The dry 

cleaner does have an air permit for the equipment. 

Preliminary subsurface data developed during geotechnical 

investigations for the construction of the strip mall indicate 

that groundwater is approximately 40 feet bgs and that the 

surficial soil consists primarily of a thick clay layer to the 

groundwater interface. Construction plans indicate a 6- to 8- 

inch-thick gravel layer directly under the slab of the building. 

An environmental consultant is contracted by the owners of 

the dry cleaner to determine the extent and severity of the 

potential exposure of the strip mall tenants to the historical 

release of PCE. 

Site Summary 

• Historic release of PCE to 

subsurface 

• No current release 

• Air permit for PCE at 

current drycleaner 

• Groundwater >40 feet bgs 

• Lithology is 5 feet silty clay 

with >35-foot-thick clay 

9



Data must be developed to sufficiently characterize the fate and transport of the PCE release, 

including whether the release has generated vapors that potentially could affect the other tenants 

in the strip mall. 

Step 8. Choose Investigative Strategy 

There are no preexisting data for this site to use in determining the optimal investigation method; 

however, the location of the source—the dry cleaner—is known. Table 2-1 summarizes the 

alternatives. 


Alternatives Pros Cons 

Perform passive soil 

gas investigation 

around perimeter of 

building 

Easy to perform; costeffective 

to identify areas 

of additional investigation 

Works well in tight soils 

Data reported in mass, not 

concentration 

Two- to three-week delay in results 

Sample groundwater 

underneath the strip 

mall 

Investigate the 

subslab soil gas 

under the entire strip 

mall area 

External soil gas 

investigation. 

Sample indoor air 

quality in the tenant 

buildings 

Determine whether 

secondary source exists 

that may affect strip mall 

and surrounding properties 

Determine whether PCE 

may be present at 

concentrations that could 

affect indoor air quality 

Gives actual vapor phase 

values 

Less invasive 


Direct measurement of 

potential exposure point 

concentrations 

Decision: External soil gas investigation was chosen. 

Surface source of PCE likely to be in 

soil and soil vapor before groundwater 

The initial geotechnical data suggest 

that the clay lenses would inhibit the 

vertical migration of the PCE to the 

groundwater 

More intrusive than initial 

characterization 

May be unnecessary if determined that 

only portions of subsurface are affected 

by the release 



Results may be confounded by other 

sources of PCE 

Rationale: This strategy was considered to offer the most advantages and fewest 

disadvantages for locating the areas of contamination both spatially and vertically and 

initially assessing the vapor intrusion pathway. 

10


Steps 9 and 10. Design and Implement VI Investigative Work Plan 

Decision: The initial VI 

investigation was to focus on 

the characterizing of the 

subsurface conditions along 

the perimeter of the strip mall 

using direct push and 

sampling for volatile organic 

compounds (VOCs) in soil 

gas at depths of 2, 6, and 10 

feet. Direct-push borings 

were located along the front 

and back of the strip mall 

(Figure 2-1) approximately 

5–15 feet from the building 

and included a utility location 

survey. 

Rationale: There were no 

characterization data for 

the reported release, and 

the air permit was already 

issued to control an outdoor 

air emission source. 

Step 11. Evaluate the Data 

Figure 2-1. Neighborhood strip mall. 

Results of Sampling: The initial investigations determined the following environmental 

conditions of the site: 

Table 2-2. Soil gas sampling results, μg/m 3 

• The source area was identified by soil gas Sample 2 feet 6 feet 10 feet 

sampling, only in the back area of the strip SG-1 100,000 2,000 ND* (50) 

mall. Table 2-2 shows results of soil gas SG-2 1,000 75 ND (50) 

sampling. 

SG-3 100 ND (50) ND (50) 

SG-4 350 ND (50) ND (50) 

• The vapor plume extends radially from SG-5 ND (50) ND (50) ND (50) 

zone of release in back of dry cleaner to SG-6 250 50 ND (50) 

the surrounding area in a decreasing SG-7 ND (50) ND (50) ND (50) 

gradient to the day care center and fast SG-8 8,000 100 ND (50) 

food tenants. 

SG-9 100 ND (50) ND (50) 

SG-10 ND (50) ND (50) ND (50) 

• Confirmed soil geology in the affected SG-11 75 ND (50) ND (50) 

subsurface area is gravel base immediately SG-12 ND (50) ND (50) ND (50) 

beneath the strip mall slab to silty gravel *ND = nondetect with detection limit in parenthesis. 

with a thick clay layer to the groundwater 

11


interface. 

• The presence of the sewer line may be an additional transport mechanism for the PCE to 

areas away from the site via the sewer line trench. Additional soil gas sampling of the sewer 

line to determine the concentration and lateral dispersion of PCE may be needed. Initial data 

(SG-6, SG-9) indicate some contamination along the trench, but show concentrations rapidly 

decreasing with distance from the building. 


Determine whether PCE from normal dry-cleaner air emissions is being recirculated into 

the adjoining businesses. Check design of the exhaust of the dry cleaner to determine whether 

the discharge permitted from the dry cleaner could affect air intakes of the other tenants or of any 

adjacent buildings downwind of the dry cleaner that may have an air intake. 

Establish whether the air discharging from the dry cleaner may be contributing to PCE 

concentrations in the day care and the other tenants of the building at levels below an odor 

threshold. 

Based on the inspection of the dry-cleaner air discharge and the other tenants’ air intakes, it is 

unlikely, based on wind direction and location of intake, that PCE-affected air from dry cleaner 

is getting to the day care center; however, other tenants may be affected. 

Determine whether the concentrations of PCE detected along the back perimeter of the 

strip mall (landscaped area) are potentially migrating under the slab and potentially 

affecting indoor air quality. The vertical profiles indicate that the PCE contamination is a 

surface release, so movement laterally under the slab is a likely mechanism. The 2-foot samples 

closest to the candy store and day care are below commercial risk exposures but exceed allowed 

subslab values for a 1-in-1-million residential risk exposure (40 μg/m 3 ). These values are 

considered close enough to allowable levels that further assessment is deemed necessary. 

Conduct subslab and indoor air sampling in following buildings: fast food restaurant, hardware 

store, dry cleaner, candy store, and day care center. (Note, at the end of strip mall, there is a play 

area located outside and immediately adjacent to the day care center). Outdoor ambient sampling 

should be included during this phase of the investigation, and some focus on the outdoor play 

area may be appropriate. Community outreach with affected parties will be done at this stage, if 

not already instigated. 

Indoor air and subslab samples were taken from the 

occupants of the strip mall with the exception of the dry 

cleaner. Indoor air and subslab samples are often coupled 

together to aid in the determination of vapor intrusion and 

to enable determination of background. Results (Table 2- 

3) showed measurable levels in all but the fast food 

restaurant (indoor) location. 

Table 2-3. Subslab and indoor 

air sampling results, μg/m 3 

Location Subslab Indoor 

Day care 100 5 

Candy 1,500 15 

Dry cleaner 10,000 NA* 

Hardware 1,500 20 

Fast food 75 ND (0.2) 

*NA = not applicable. ND = nondetect with 

detection limit in parenthesis. 

12


The dry cleaner had the highest subslab soil gas concentration of PCE at 10,000 μg/m 3 , the 

hardware and candy stores had PCE concentrations at 1,500 μg/m 3 , the day care at 100 μg/m 3 , 

and the fast food restaurant at 75 μg/m 3 . 

The state oversight agency’s acceptable indoor air level for PCE for a 1-in-1-million risk 

exposure for a residential setting is 0.4 μg/m 3 . For commercial settings, exposure times are 

approximately 5 times lower, and the state uses a 1-in-100,000 risk level, so the allowable level 

is 20 μg/m 3 . 


Although the measured indoor air concentrations appear to be acceptable, the levels in the 

subslab soil gas data clearly indicate that PCE from the dry cleaner has migrated underneath the 

slab and could impact the indoor air of the neighboring businesses. In addition, the day care 

center is considered a sensitive receptor, which could lead to legal ramifications if the problem is 

ignored. The choices here are as follows: 

• Do not mitigate, but continue to monitor the situation on a regular basis. 

• Remove the source of vadose zone contamination. Implement subslab depressurization 

mitigation measures, and monitor PCE concentration in depressurization discharge on a 

quarterly basis to determine whether source removal has mitigated the presence of PCE 

subslab gas and concentrations are decreasing with time. 


The first issue facing investigators at dry-cleaning sites is to determine whether PCE is currently 

being used in the process and whether there are any permitted waste streams that may contain 

PCE. Permitted waste streams that contain PCE must be considered in the relative overall 

assessment of potential risks from unpermitted releases that may have occurred historically or 

that may be continual release to the environment. 

The second major issue affecting dry-cleaning vapor intrusion sites is the presence of other 

tenants that may be affected by dry cleaners located in multiuse buildings, such as strip malls or 

office buildings with commercial use on the first floor. 

Another issue to keep in mind in a strip mall scenario is that some buildings and tenants may 

share heating, ventilating, and air conditioning (HVAC) systems, which can further complicate 

any VI investigations. There may be contributions to indoor air through the shared HVAC rather 

than from vapor intrusion. 

Finally, dense vapors of chlorinated solvents leaking from surface sources such as washing units 

can create vapor contamination underneath the slab that moves laterally underneath adjacent 

businesses. In such cases, groundwater data are likely to have little correlation with soil gas 

samples collected in the vadose zone. 

13



• Permitted discharges from dry-cleaning operations must be considered as a potential 

secondary source that may affect adjacent buildings via a pathway independent of vapor 

intrusion through the subsurface. 

• Soil gas may follow preferential pathways that lead to confounding sampling results, (e.g., 

the results from SG-6 seem to indicate that vapors have migrated away from the building into 

the disturbed soils of the utility trench and may lead to a potential impact to receptors outside 

the CSM initially developed. 


Future activities at the site will include monitoring of the depressurization discharge to determine 

whether the removal action of the vadose zone source area was successful. 

Refer to the oversight agency regarding other remediation that may be necessary at the site (e.g., 

soil removal, groundwater remedy, etc.). 

14


Scenario 3 

Degreasing Solvent Contamination 

from a Small Industrial Site 

on an Adjoining Mixed-Use Neighborhood 

This scenario illustrates a typical investigation of a large vapor intrusion site due to chlorinated 

VOCs (CVOCs) contamination of groundwater beneath occupied structures. 


The Situation 

An industrial facility with a small degreaser contaminated groundwater off site more than 20 

years ago, impacting the adjoining mixed-use community. The degreaser created a small zone of 

high-concentration soil contamination of multiple CVOCs, resulting in a large (several-mile) 

groundwater plume beneath several hundred structures. The community is primarily residential, 

with mixed zoning of commercial properties, schools, home day cares, etc., in the area. A legacy 

evaluation showed that ineffective source control was installed 20 years ago on the facility 

property. The evaluation has been reopened specifically to evaluate vapor intrusion into 

structures that overlie the plume. This is a state-led site, and project costs are an issue. 


Reports from the original assessment of this site were reviewed. Nothing in the reports indicated 

conditions likely to result in vapor impacts to the surrounding community from the soil itself or 

laterally from the plume. Existing state vapor intrusion guidance follows the EPA OSWER 

guidance and recommends that all structures within 100 feet laterally of the contamination plume 

be assessed. Depth to groundwater in the community over the area of the plume varies 15–30 

feet. Groundwater is not used as a potable source of water. The report also classified the 

lithology/geology of the site as alluvial sands with a clay layer at 3–5 feet bgs. 

Existing groundwater data and trends were reexamined 

with the vapor intrusion pathway in mind. Such data 

were compared to state-approved vapor intrusion 

screening levels. A definite hot spot in the 

groundwater was identified where concentrations 

exceeded screening levels by a factor of 100. 

Most of the buildings in the adjacent community are 

30–50 years old. Such structures are known to have 

basements or crawl spaces or were built on slabs on 

grade. Special populations include grade school, home 

day care, and several homebound individuals. 

Site Summary 

• Known CVOC contamination in 

groundwater 

• Groundwater @ 15–30 feet bgs 

• Lithology: alluvial soil with clay 

layer 3–5 feet bgs 

• Plume several miles long 

• Hundreds of occupied structures 

above plume 

• Structures have basements and 

crawl spaces; some slab on grade 

• Hot-spot concentration 100 times 

screening levels 

15




Should the investigation encompass the entire area of groundwater contamination or proceed 

with a more focused approach There are many factors that go into this decision. It is important 

to develop a comprehensive work plan and CSM. Table 3-1 summarizes the alternatives. 



Ability to evaluate an 

entire site ensures that all 

areas and conditions are 

considered (most 

conservative approach) 

Investigate entire area 

where groundwater 

exceeds screening 

levels to reduce area 

of VI concern 

Statistical selection of 

structures within 

contamination area 

Model groundwater 

data to limit area of VI 

concern (regulatory 

agency may not allow 

modeling) 

Focus area on hottest 

part of plume 

Gives a representative mix 

of sampling locations 

Provides broader coverage 

than just hot spots 

Inexpensive 

Can be done with existing 

data if of sufficient quality 

and detail 

Saves cost 

Minimizes disturbance to 

residents 

Decision: Focus the investigation on the hottest part of the plume. 

Very costly 

May be unnecessary if it is 

determined there is no VI in hot spot 

Can be costly if sample size needs to 

meet data quality objectives (large 

sampling size) 

Although costs can be reduced, the 

size of the investigation is not 

necessarily reduced 

Conservative assumptions should be 

used due to model imprecision and 

uncertainty 

May miss some impacted receptors 

Not-included residences may get 

concerned 

Rationale: There are sufficient groundwater concentration data to identify the hottest part 

of plume. Geologic conditions are relatively uniform across the site, such that groundwater 

concentrations are likely to define the area with the greatest VI potential (in some cases, 

depth to groundwater, soil type, and building conditions may be as or more important than 

groundwater concentrations, making selection of a hot spot more difficult). So it is 

concluded that it is safe to limit the initial VI investigation on this area in the plume. 

The hot spot covers approximately five square blocks (~50 structures). Two sensitive 

populations are located in the hot spot, and one is directly adjacent to the hot spot. 

Table 3-2 summarizes the several methods that can determine the presence and/or concentrations 

of contaminants at various points along the vapor intrusion pathway. 

16


Table 3-2. Pros and cons of investigative methods for Scenario 3 along identified pathway 


Additional 

groundwater 

sampling 

Groundwater concentrations 

are likely to define the area 

with the greatest VI potential 

More expensive and slower than soil gas 

sampling 

May or may not reflect soil gas 

Passive soil gas Can be used to focus areas of 

investigation 

Less invasive, easily installed, 

greater number of sampling 

locations for lower costs 

External soil 

gas 

investigation 

Indoor air 

sampling 

Investigate the 

subslab soil gas 

under receptors 

over hot spot 

Less invasive than subslab 

sampling 

Faster, less expensive than 

groundwater sampling 

Can obtain actual exposure 

data; can be cheaper if there is 

a signature compound (e.g., 

1,1-dichloroethene, carbon 

tetrachloride) 

If geology doesn’t warrant soil 

gas investigation 

Gives actual value under 

receptors 

concentrations under or near receptors 

Quantitative testing will still be required 

to evaluate vapor intrusion potential and 

risks, if any 

Location of hot spot already known 

External soil gas results may not be 

representative of subslab data in lowpermeability, 

heterogeneous, or 

fractured materials 

Background contaminant issues make it 

difficult to interpret indoor air results 

Has the potential to be more costly, 

(multiple sampling events, hardware 

problems, etc.) 

Must start community relations/ 

communications at this point 

Very intrusive and more expensive 

Fewer data points 

Decision: The alternative of an external soil gas investigation in the hot spot area was chosen. 

Rationale: This is the most practical approach based upon consistent geology and the fact 

that the preexisting data enable identification of the worst case area of the plume. Soil gas 

sampling allows evaluation of a large area with relatively low impacts to occupants of the 

structures. 

Step 9. Design VI Investigative Work Plan 

Determine Target/Screening Levels: Risk-based screening levels for COCs in soil gas at a 

1-in-1-million risk level are ~250 μg/m 3 for soil gas samples collected 5 feet bgs. Commercial 

receptors are regulated at 1-in-100,000 risk level, so target levels are at least 10 times higher 

(>2,500 μg/m 3 ) 

To perform the initial soil gas investigation, samples are to be taken in the rights-of-way along 

the axis of the plume with multiple transects across the focus area (Figure 3-1). Soil gas samples 

will be collected at 5-foot intervals, vertical spacing may be different if implants are to be 

installed, and several vertical soil gas profiles will be taken. 

17


Figure 3-1. Potential locations for soil gas sampling. 

Initial samples will be analyzed on site by EPA Methods 8021/8260 if a mobile lab is available. 

(Quality assurance note: ~10%–20% samples collected in canisters for TO-15 off-site 

analysis.) Once work starts in any neighborhood, community relations/communication may 

be necessary. 

Steps 10 and 11. Collect and Evaluate Data 

(These two steps are actually combined for this type of 

scenario. Since the sampling program is designed to collect 

data, review data, and make additional decisions regarding step 

out sampling, it highlights the iterative process of vapor 

intrusion investigations.) 

Soil gas data indicated about 15–20 structures are of higher 

concern and need additional investigation (the shallow soil gas 

concentrations in the area are approximately two orders of 

magnitude over screening levels, the rest of the focus area 

about one order of magnitude higher). The data suggest 

additional investigation is needed. 

Note: The COC(s) at the site 

may change the investigative 

methodology chosen. Certain 

chemicals are good tracers 

and do not have the same 

issues with background (e.g., 

1,2–dichloroethene, carbon 

tetrachloride). Indoor air 

sampling may be the most 

appropriate investigative tool 

in these cases. The oversight 

agency may also require 

indoor air sampling as the 

primary investigative tool. 

18


Alternatives: Conduct additional exterior near-slab sampling around structure or subslab 

sampling in all structures near high soil gas concentrations. 

Decision: Conduct subslab sampling. Community participation/communications are necessary at 

this point. 

Rationale: Exterior soil gas concentrations are very high at shallow depths, and access 

limitations are not a problem; otherwise, additional exterior near-slab data might have 

been chosen. 

The subslab data exceeded allowable risk-based screening levels at eight structures by a factor of 

at least 10. 

Step 12. Additional Investigation for the Affected Structures 

Alternatives: Measure slab-specific attenuation factors using radon, measure indoor ventilation 

rate, measure pressure gradients, use flux chambers in crawl spaces, measure indoor air. 

Decision: Indoor air sampling is selected as next step. 

Rationale: COCs were not common household chemicals, so potential for background 

sources is not considered, and all houses were slab-on-grade with floor coverings. 

Six of the eight structures failed indoor air levels by up to factor of 10. 

Alternatives: Monitor indoor air or mitigate structures. 

Decision: The six structures with high indoor levels were mitigated. At the other two structures, 

resampling was proposed. 

Rationale: Due to the costs associated with indoor air sampling and issues related with 

sampling, it was determined that installing subslab depressurization systems was more 

cost-effective than additional sampling at the six homes having high indoor air 

concentrations. 

Since some of the structures over the highest concentration part of the plume clearly had indoor 

air impacts, additional investigation to the nearby homes is warranted. Alternatives include the 

following: 

• Additional exterior soil gas sampling near and around foundations 

• Additional subslab sampling 

• Additional indoor air sampling 

Decision: Collect additional subslab samples by stepping out two additional structures in all 

directions from mitigated houses. Continue this sampling protocol until lines of evidence and the 

two structures show no vapor intrusion impacts. 

19


Rationale: This decision point would depend on the results of the first indoor air sampling 

event. (That is, for this scenario the levels found within the structures were within an order 

of magnitude of the allowed risk-based level. Had they been several orders of magnitude, it 

may have been prudent to step out more than two houses.) This point should be covered in 

the CSM and work plan, and the regulatory authority will have input. Any community 

action groups should be notified of this strategy up front as a practical approach. 

Once this process is stated, it is practical to continue until there are several clean structures since 

there has been a continual community involvement effort. 

These decision points may be different depending on original course of the investigation. 

Step 13. Mitigation to Receptors and Remediation of Groundwater 

Decision: Verify clean structures are “clean” and mitigation systems are working properly. 

Rationale: An additional indoor sampling event when buildings were closed was performed 

and verified “clean.” (Community participation/communications have been ongoing at 

site.) 

Depending on the approach selected, it is important to remember that anytime during this process 

modeling with current data collected at the various phases may be performed to determine 

whether additional sampling, closure, etc. is/are warranted. 

Decision: Institute groundwater—long-term monitoring and closure. 

Rationale: Since vapor intrusion has been identified, source control should be considered 

unless it can be documented that source concentrations will decline within the time frame 

of the operation and maintenance of the mitigation systems. Monitoring and closure 

activities will be different from state to state and possibly from structure to structure. 


Numerous issues are unique to large vapor intrusion sites, including the extensive resources and 

time required to address them, the need for public communications and outreach, and the 

logistical challenges associated with investigating and potentially mitigating large numbers of 

buildings. Other issues faced by most vapor intrusion sites are often exacerbated by the size and 

complexity of large sites, including variable site conditions, seasonal factors, future land use, and 

separating the contributions of vapor intrusion from those of background sources. 

The first issue facing investigators at large vapor intrusion sites is simply the resources, time, and 

money required to determine the extent of vapor intrusion impacts, if any, and to address these 

impacts through mitigation or other actions. As a result, site screening becomes a more critical 

step to ensure that the costly and involved process triggered by intrusive testing is warranted. 

Unfortunately, generic screening levels are often set at (or, in some jurisdictions, below) 

20


maximum contaminant levels so that the entire plume area is, by definition, above the generic 

screening level. Semi-site-specific screening levels may be difficult to apply to large plume areas 

due to variable depths to groundwater and soil types and may not raise screening levels high 

enough to eliminate an entire plume. Therefore, selection of worst-case buildings often becomes 

a critical step for large sites, allowing site-specific testing of a more manageably sized area. 

Worst-case building selection can be challenging on its own because of the large number of 

factors that contribute to the potential for vapor intrusion, including concentrations in 

groundwater, depth to groundwater, geologic conditions, buildings conditions, and building use. 

As a result, more than one “worst-case” area may warrant investigation, and more than one 

building should be investigated in each worst-case area. To be effective, a worst-case area must 

serve as a surrogate for the entire site; i.e., if no vapor intrusion impacts are found in the worstcase 

area(s), then no further investigations should be required in the other areas of the site. 

Additional groundwater and/or soil vapor data may be warranted to better define worst-case 

areas and buildings. 

Delineation of the extent of vapor intrusion impacts, if detected in worst-case buildings, is 

another issue often faced at large sites. Once again, because of the costs and time associated with 

testing a large number of buildings, large sites require efficient decision-making strategies to 

limit unnecessary tests. Delineation is commonly accomplished by a “step-out” process, with a 

methodical set of rules for selecting buildings for testing (and for stopping testing) around worstcase 

or other buildings found to have vapor intrusion impacts. Issues that investigators face when 

designing step-out testing programs include the following: 

• How many buildings do you test beyond a building with impacts, and in what directions 

• Where do you not test (e.g., when requests for testing are received) 

• When do you stop testing What are the criteria A certain distance (e.g., 100 feet) beyond 

the edge of the groundwater plume A certain number of unimpacted buildings 

• What kind of tests do you conduct The same tests in all buildings or different types of test 

depending on concentrations, proximity to the edge of the plume, etc. 

• Can you make reliable decisions based on fewer tests per building (compared to a single 

building case), based on the knowledge gained from testing many buildings in the same area 

• Do you make preemptive risk-management decisions without testing all buildings (e.g., 

blanket mitigation) 

The second major issue affecting large vapor intrusion sites is public communications and 

outreach. Smaller sites may involve only the responsible party’s own site, their lessees, or a 

small number of adjacent landowners. Large sites may involve hundreds of different landowners 

with different types of buildings and issues, including single-family homes, multifamily 

buildings, commercial operations with employees and customers, schools, churches, day care 

operations, and public institutions such as libraries. A comprehensive public communications 

program will be essential to educate the public and put risks into perspective, gain access to 

properties and/or buildings for testing, schedule tests, report results, and install mitigation 

systems, if necessary. In addition, community leaders, government representatives, other 

regulatory agencies, realtors, and other members of the public may be indirectly affected by the 

investigation and require timely information. Finally, the media will need to be informed to 

21


minimize miscommunication of information to the public. A key issue facing investigators and 

regulators of large sites is to determine when public outreach should begin so as to provide 

timely notice of potential concerns and investigation activities without creating unnecessary 

alarm. 

The third major issue affecting both investigators and regulators at large sites is the logistical 

challenge associated with investigating and potentially mitigating a large number of buildings in 

a relatively short period of time. Investigators must be prepared, often with little advanced 

warning, to coordinate access and schedule tests with a large number of property owners; to 

obtain, store, and ship large numbers of samples (potentially bulky Summa canisters); to manage 

and report large quantities of data to both agencies and property owners on a continual and 

relatively rapid turnaround basis; and to coordinate the installation, monitoring, and operation 

and maintenance of a large number of mitigation systems. With most of the contamination being 

off site, institutional controls may not applicable in this scenario, so long-term monitoring of the 

systems needs to be considered. Regulators may be contacted by a large number of interested 

parties asking for information over an extended time period and may be under a great deal of 

pressure to make a large number of risk-management decisions in short time frames. 

Finally, issues potentially affecting all vapor intrusion sites regardless of size are often 

exacerbated at large sites. Geologic, groundwater, and building conditions are likely to vary to 

greater degrees across larger sites. Each building at a large site may also have unique materials 

and occupant activities creating potential background sources. At the same time, it is impractical 

to study each building and property at a large site to the degree of detail feasible at smaller sites. 

Therefore, building-specific decisions have to be made using less information than typically 

available at smaller sites. On the other hand, the database of information provided by testing at 

numerous buildings may provide 

different tools for evaluating vapor 

intrusion impacts that are not available 

at smaller sites. For example, spatial 

patterns and correlations with other site 

factors based on testing at other 

properties may aid interpretation of 

individual test results. 


• There are an infinite number of 

investigative strategies to handle 

large vapor intrusion sites. There is 

no cookie-cutter approach to their 

investigation. The process must to 

start with the CSM and proceed 

from there. 

• Expect surprises, especially 

additional sources when dealing 

with large plumes. 

Lessons Learned from Redfields 

• Very low levels of groundwater contamination can 

cause vapor intrusion. 

• If using soil gas sampling for screening large site, 

make sure to use state-of-the-art techniques, 

including the use of vapor implants and tracer 

compounds to ensure that the results are the 

subsurface soil gas and not ambient air. 

• Subslab sampling is likely to be a better indicator of 

vapor intrusion potential than soil gas sampling 

remote from the building (e.g., in public areas) or 

even adjacent to the structure. 

• There can be many complications with sampling 

indoors due to background chemicals in the 

structure. Indoor product inventory is essential to 

demonstrate that contributions to indoor air 

concentrations are not due to vapor intrusion. 

However, inventories may not identify all sources, 

particularly if the compound is not indicated on the 

container or is present in building materials 

• Large sites will be very costly no matter what type of 

investigation strategy is used. 

• External drivers that dramatically affect the cost of 

mitigation include asbestos, dry-layered foundation 

walls, and multiple foundations (grade beams, etc.). 

22


• Soil gas investigations may be the preferred method of screening a large vapor intrusion site 

if subsurface conditions are suitable and conservative assumptions (i.e., attenuation factors or 

modeling assumptions) are applied, but they may not be adequate to close out a site. 

• Community involvement is paramount at all sites, more so at large sites. It can greatly 

influence how well the investigation proceeds. Significant resources are required for gaining 

access to buildings, scheduling tests, and communicating with building owners and 

occupants. 

• While mitigation of typical residences is straightforward and in many cases less expensive 

than resampling multiple times, there will be exceptions. 

• Ongoing monitoring of both mitigated and unmitigated buildings can be very expensive, 

depending on the number of homes selected for monitoring and the testing frequency. 


• Future activities at the site will include groundwater and soil gas monitoring, maintenance, 

and performance testing of the vapor intrusion mitigation systems. 

• Groundwater, soil gas, and indoor air monitoring will occur for the foreseeable future. 

• Groundwater movement may create vapor intrusion issues within structures that are not 

currently impacted. An effective groundwater/soil gas monitoring network will be 

established and sampled at appropriate intervals. 

• Existing mitigation systems will be inspected on a regular basis. A work plan should be 

created that details the inspection procedures, frequency, and termination procedure. 

• A groundwater or vadose zone source control remedy may be implemented to reduce 

subsurface vapor concentrations. The remedy will need to be coordinated with the vapor 

intrusion investigation. 

• Data collected from the subsurface and indoor air sampling will be analyzed to develop sitespecific 

attenuation factors. The attenuation factors may be useful as a screening tool. When 

used in conjunction with the soil gas data from the monitoring wells, vapor intrusion problem 

spots may be identified. However, it should be noted that even site-specific attenuation 

factors may span several orders of magnitude; therefore, application of the most conservative 

attenuation factor (generally necessary unless attenuation factors can be correlated with other 

parameters, such as depth to groundwater or soil type) will still result in a significant number 

of false positives. 

• Community involvement will be an ongoing activity at the site. The vapor intrusion 

mitigation systems and source control are long-term actions that will require community 

input. A community involvement plan will be developed. 

• Refer to the oversight agency regarding other remediation that may be necessary at the site 

(e.g., soil removal, groundwater remedy, etc.). 

23


Scenario 4 

Brownfield Redevelopment Site (Vacant Land) 

This scenario illustrates a typical vapor intrusion investigation at a vacant brownfield site slated 

for future development. 


The Situation 

A 20-acre brownfield site is being sold for redevelopment. The proposed redevelopment plan 

consists of converting an old abandoned factory into apartments and the remaining 15 acres into 

mixed commercial and residential uses. A Phase 1 evaluation indicated that the old factory 

contained a degreaser and parts-washing operation using chlorinated solvents and that the 

undeveloped area was used for fire training by the local fire departments. Records indicate that 

the fire training operation used pits for fuel oil, but historical aerial photographs did not show 

evidence of the locations of the pits. 


The site consists of uniform stratigraphy, primarily silty sand with some fill material near the old 

factory. Depth to groundwater is approximately 20 feet across the site. No groundwater wells 

exist on the property, so the gradient is unknown. From neighboring properties, it is expected 

that the factory is downgradient of the area containing the training pits. 

A limited Phase 2 investigation found soil contamination in defined areas to 10-feet depths 

within the fire training areas believed to be the former pits. Free product was detected on the 

groundwater in the vicinity of the former pits and lies about 500 feet from the factory. A limited 

number of discrete water samples were collected near the factory, and no evidence of chlorinated 

solvent contamination was detected in the groundwater. Discrete groundwater samples were also 

collected at one location on each border of the property, and no contamination was found. Since 

there are no occupied buildings currently on the property, no further work was done pending 

future development. 

Since there are no receptors currently of concern, the 

issue is whether there might be potential vapor 

intrusion risk to future buildings. Fuel oil does not 

contain high concentrations of benzene but does 

contain naphthalene and a mix of alkanes that are of 

concern to some regulatory agencies. The factory 

used chlorinated solvents that might have left 

contamination, regardless of the previous 

groundwater data showing little contamination. In 

this case, the available data are too limited to answer 

Site Summary 

• Phase 2 found soil contamination in 

fire pits 

• Free product found in pit area 

• Limited water samples found no 

groundwater contamination 

• No additional work done, awaiting 

property redevelopment 

• No current receptors—“Future Use” 

• Very little data to work with 

25


whether vapor intrusion may be a concern to future buildings. Since little is known about the site 

and proposed redevelopment covers most of the site, the entire parcel needs to be investigated. 

Conclusion: Sufficient data do not exist to close VI pathway. Need to collect additional data. 


Step 8. Choose Investigatory Strategy 

There are two known sources of contamination: 

• The fuel oil in the training area 

• Chlorinated hydrocarbons near the factory building 

Table 4-1 summarizes the pros and cons of the various applicable methods. 



Groundwater 

or soil phase 

data 

Can search and delineate extent of 

contamination sources 

Vapor intrusion risk is often 

overestimated from these matrices 

Expensive for a large site 

Soil phase data unreliable for 

Active soil 

gas 

investigation 

Passive soil 

gas 

investigation 

Will reflect soil and groundwater 

contamination 

Less expensive than soil and 

groundwater data 

Gives quantitative vapor-phase values 

Identifies areas that may need further 

investigation 

Easy to install 

Gives best coverage for least cost 

vapor intrusion 

Not as easy or inexpensive for a 

large site 

Data may require follow-up 

quantitative sampling 

Decision: A combined passive soil gas and active soil gas program was considered to offer the 

most advantages for this site. 

Rationale: The passive soil gas survey would be used to determine areas where VOC 

contamination exists on the site to be followed by an active soil gas program in the 

identified contamination areas. 

Step 9. Design VI Invesigative Work Plan 

To determine target/screening levels, the state oversight agency has its own guidance, which 

allows risk-based screening values to be determined from modeling. Version 3.1 of the Johnson- 

Ettinger soil gas screening spreadsheet was used. The scenario modeled was slab-on-grade 

construction, silty vadose zone, and default values for all other parameters. Since residences are 

26


planned in several portions of the site, residential screening levels apply. Subslab screening 

levels were obtained from the same spreadsheet using a 2-foot depth below the slab. The 

allowable indoor air concentrations at a 1-in-1-million risk level or a hazard index of 1 from the 

state guidance for TCE, 1,1,1-TCA, and naphthalene are 0.022 μg/m 3 , 2,200 μg/m 3 , and 3.0 

μg/m 3 , respectively. Table 4-2 shows the risk-based screening levels derived from the model 

compared to generic screening values in the EPA OSWER guidance using default attenuation 

factors. Also included are the dilution factors for soil gas to indoor air defined as the inverse of 

the attenuation factor. Comparison of the generic screening values to the site-specific values 

derived from the model show that the generic values are about 30 times more conservative for 

the subslab values, but very similar for the 5-foot depth. 

Table 4-2. Risk-based screening levels derived from J&E Model versus generic values from 

EPA OSWER guidance 

Location 

TCE 1,1,1-TCA Naphthalene 

Generic Modeled Generic Modeled Generic Modeled 

Subslab, μg/m 3 0.22 6.7 22,000 680,000 30 1,000 

Dilution factor 10 306 10 308 10 341 

5 feet, μg/m 3 11.0 11.2 1,100,000 1,100,000 1,500 2,000 

Dilution factor 500 510 500 510 500 613 

Since fuel hydrocarbons are present, EPA test method 8021 may be subject to interferences and 

false positives. Method 8260 (GC/MS) is not subject to interferences, can be conducted on site, 

and reaches detection levels except for subslab naphthalene detection levels. Method 8260 will 

be used with a subset of subslab samples collected for off-site analysis for naphthalene by 

method TO-15 or TO-17 if the 8260 analysis is nondetect. 

On-site analysis for total alkanes will be done by method 8015 modified (GC–flame ionization 

detector). This method gives a detection limit of approximately 50 μg/L for C10 to C20 

hydrocarbons. If nondetect, a sample will be collected for off-site analysis by method TO-15 or 

TO-3. 

Methane will be measured by EPA method 8015 modified, and oxygen and carbon dioxide with 

a portable field meter (e.g., Land Tec GEM-2000, etc.). 

Step 10. Collect Data 

Passive Soil Gas Program: Based on the Phase 2 assessment, passive soil gas samples were 

collected on a regularly spaced grid of 100-foot centers in the undeveloped areas. The suspected 

fire pit areas, degreaser location, and the old factory were subjected to a more dense sampling 

grid of 50-foot centers. The sorbent-based samplers were placed 2–3 feet bgs and were analyzed 

by gas chromatography (GC)/mass spectroscopy (MS) (8260/8270 or TO-15) for fuel-related and 

chlorinated compounds. 

27


Results from the Passive Soil Gas Survey: 

Factory: High levels of trichloroethene (TCE) were 

found in the area containing the degreaser and 

adjacent to this location beneath the factory slab 

(Figure 4-1). 

Fire Training Area: Numerous detections of 

hydrocarbons out into the C20 range and naphthalene, 

were detected in the area measuring roughly 100 feet 

in diameter (fire pits, Figure 4-2). 

Figure 4-1 

The remainder of the site was expected to be 

contaminant-free; however, an area of 

1,1,1-trichloroethane (TCA) was detected northwest 

of the factory (Figure 4-3). 

Property Borders: No VOCs were detected in the 

samples collected along the property borders, 

indicating that the VOC contamination is primarily 

limited to the interior of the site. 

Figure 4-2 

Active Soil Gas Program: Based on the passive soil 

gas data, 35 initial soil gas sample locations were 

selected as follows (Figure 4-4): 

• Five vertical profiles (3-, 5- and 10-foot sample 

depths) were taken in the fire pit training area, one 

from the 1,1,1-TCA hot spot, and another near the 

former degreaser. 

• Soil gas samples at a 5-foot sample depth were 

located around the factory and along and across 

Figure 4-3 

the axes of the naphthalene and 1,1,1-TCA soil 

gas plumes. 

• Five subslab soil gas samples were collected from underneath the factory slab. 

• Samples were collected across the remainder of the site at 5-foot sample depths to test for 

background concentrations of VOCs and for the possible presence of methane gas that could 

not be detected by the passive survey). 

Soil gas samples were analyzed for VOCs including naphthalene, hydrocarbons ranging from 

C10 to C20, methane, and fixed gases (oxygen and carbon dioxide). 

28


Having the ability to add sampling locations both spatially and vertically in real time was 

considered useful to optimize the field effort, so on-site analysis was planned. 

Continuous soil cores were collected from 10 locations across the site to get soil physical 

properties for later use in vapor intrusion modeling. 


Fire Training Area: 

Figure 4-4. Initial soil gas sample locations. 

• The vertical profiles showed a consistent trend in all three locations. Total hydrocarbons 

were high at the deepest depth (>500 μg/L), decreasing to below detection at the 3 feet. 

Oxygen was near atmospheric levels to 3-foot depths and then decreased rapidly to only a 

few percent at depth. Carbon dioxide also increased with depth to percent levels. 

• Methane values exceeded 10% in all locations at 5- to 10-foot sampling depths but decreased 

to 500–1000 parts per million by volume (ppmv) at the 3-foot depths. 

Area Around and Underneath Factory: TCE values at the 5-foot collection depth ranged from 

nondetect to 10,000 μg/m 3 , which exceeded screening levels by 20 times. Subslab samples had 

TCE at concentrations ranging 50,000–100,000 μg/m 3 , exceeding screening levels by many 

29


orders of magnitude. The highest values were in the northwest portion of the factory building 

coincident with the location of the former degreasers. 

Background Areas 

• 1,1,1-TCA values were determined to be below the regulatory action level for residential 

scenarios. 

• No VOCs were detected, and only modest quantities of methane (


• Use of vertical profiles to document bioattenuation 

• Unexpected presence of methane gas and an approach to see if it represents a risk 

• Finding unexpected source areas 

• Institutional controls and the problems with implementing them long term 


• Passive soil gas is an effective screening tool (located additional unknown source area). It 

reduced overall investigation costs substantially (fewer active soil gas samples taken). 

• Bioattenuation of hydrocarbons is likely when oxygen is present. 

• Vapor phase contamination (vapor clouds) can exist with no associated soil phase or 

groundwater contamination. 


• Validation tests (flux chambers) can ensure shallow methane is not a problem. 

• Soil removal in the fire training area should be completed to remove contamination, and any 

buildings erected in this area should be have integral mitigation systems built in. 

• Mitigation of the TCE plume under the factory should be done prior to any redevelopment 

and subsequent sampling performed to ensure that vapor intrusion is not occurring after 

redevelopment. 

• Groundwater monitoring of the plume should continue to document contaminate migration 

and/or plume stability. 



31


Scenario 5 

Large Industrial Building 

Undergoing Redevelopment 

This scenario illustrates a typical vapor intrusion investigation of an existing structure located at 

a former industrial site. 


The Situation 

A large industrial facility with warehouse and offices under one roof is being sold. An 

environmental assessment is conducted at the site and contamination is found. Previous shallow 

soil data from hand borings indicated a suite of CVOCs directly below a portion of the building. 

Building construction is slab on grade and divided into three distinct ventilation areas—open 

warehouse, warehouse rooms, and attached offices. Low-level contamination is found outside the 

building footprint in one area, but it is suspected that higher concentrations still exist directly 

under the building. Soil type in the area is likely to be a tight clay soil matrix, and it is assumed 

there is a coarse-grain material directly beneath the slab. The seller (Principal Responsible Party) 

applies to state voluntary cleanup program for assistance and is trying to lease building in the 

interim. The regulatory agency determines that VI needs to be investigated. 


The building plans, historical uses of the facility, and the environmental assessment of this site 

were reviewed. Nothing in the review indicated conditions 

likely to result in vapor impacts to the surrounding 

community. Minimal contamination was detected outside the 

building footprint. Therefore, the only potential vapor 

impacts would be the existing building above the 

contamination. Groundwater at the site varies 15–30 feet bgs 

and is not impacted. 

After the review, several areas of additional investigation are 

identified, including an old degreaser, former drum storage 

area, and several floor drains and sump pits. All of these are 

identified as potential pathways of contaminants to the 

subsurface. 

Site Summary 

• Former industrial facility 

• CVOCs found directly below 

building 

• Contamination contained in 

building footprint 

• Several areas of concern 

identified 

• Site owner applies to state 

Voluntary Cleanup Program 

• Regulatory agency requests 

VI assessment 

33


5.B VI INVESTIGATIVE PROCESS 

Step 8. Choose Investigatory Strategy 

See Table 5-1. 



Collect indoor air samples Direct measure of 

VI 

High chances of background sources 

from materials in warehouse 

Collect soil samples around 

or under building 

Familiar method Soil data generally considered not 

representative of vapor concentrations 

External soil gas 

investigation around the 

perimeter of the building 

Do not have to drill 

through the slab 

inside building 

External soil gas results may not be 

representative of subslab data 

Perform an investigation 

under the building (subslab 

and vertical soil gas samples) 

Closest point to 

receptor 

Decision: Collect internal soil gas and subslab gas samples. 

More intrusive 


Rationale: Access is not a problem, and contaminant source zone lies directly underneath 

structure. 


Subslab soil gas samples are to be collected at the following locations (Figure 5-1): 

• Near the location of the former degreaser 

• Near the floor drains and sumps 

• Near the former drum storage area 

• Along the edges of the building slab 

Vertical profiles of the soil gas are proposed in the hottest locations to ensure no threat to 

groundwater. 

A second subslab sample will be collected from several points for radon analysis to determine an 

average building-specific attenuation factor. 

Determine Screening Levels and Test Method: The state oversight agency has developed its 

own screening levels for vapor intrusion investigations. The state agency has developed riskbased 

screening levels and applies conservative attenuation factors for subslab, soil gas, and 

groundwater of 0.1, 0.01, 0.001, respectively. Commercial (non–Occupational Safety and Health 

Administration) screening levels apply. Risk-based screening levels for PCE, TCE, and 

1,1,1-TCA in subslab soil gas at a 1-in-100,000 risk level are 700, 225, and 14,500 μg/m 3 , 

respectively. 

34


EPA test methods 8021, 8260, and TO-15 can be used for soil gas samples and reach detection 

levels suitable for these screening levels. On-site analysis is not available, high concentrations 

are anticipated, and a mixture of compounds are present, so method 8260 is chosen, with 

approximately 10 of the samples to be confirmed by TO-15. 

Step 10. Implement Work Plan 

The results of the investigation show various levels of soil gas contamination under the structure 

footprint. One hot spot near the former degreaser and the offices was identified with subslab 

concentrations (for PCE and TCE) approximately 100 times higher than the screening levels. The 

hot spot was contained to a very small area; soil gas concentrations decrease laterally away from 

the hot spot. A vertical profile at the highest subslab location indicates the contamination 

decreases with depth and is below detection at 10 feet bgs. 


Figure 5-1. Subsurface soil gas sampling sites. 

Warehouse Area: The radon data show a slab-specific attenuation of 0.005, which is 20 times 

lower than default values used to determine screening levels. Using this attenuation factor, 

detected subslab values are now slightly above screening levels. Since large warehouse areas will 

generally have large doors and usually have more air exchanges per hour, it is concluded that 

there is little risk to the warehouse workers. No more data required. 

35


Office Area: Since ventilation rates are lower in the offices, it is possible that the subslab values 

might represent a risk to the office workers. Additional data are needed. 

Step 12. Additional Investigation Warranted 

See Table 5-2. 



Collect indoor air Direct measure in room Background sources can 

complicate interpretation 

Model indoor air 

concentrations based on soil 

gas and subslab concentrations 

Pressure measurements 

Continuous monitoring 

Can be inexpensive 

Eliminates any background 

interferences 

Provide evidence if subslab 

contaminants entering building 

Decision: Collect indoor air samples in offices and warehouse. 

Regulatory agency may 

not allow just modeling 

the data 

Data may be variable 

Data may be variable 

Rationale: Sampling indoors was chosen for a couple of reasons. First, it is believed that 

background interferences will be negligent. Second, we want to see actual indoor air 

concentrations. Main focus is the offices; however, indoor air sampling in the warehouse 

space is conducted at the request of the oversight agency to ensure site specific attenuation 

factors are applicable. 

Take representative samples from various area of the building, including 8-hour summa cans 

collected during normal business hours. 

Office results: Office #1, nearest the hot spot, is below commercial risk levels; however, Offices 

#2 and #3, away from the hot spot, show elevated levels (Table 5-3). Office #3 is above 

commercial standards. Office #2 is below the commercial risk level but above residential 

standards. It is determined from a former employee that Office #2 was a printing supply 

room, so background contamination may be responsible for elevated levels. Remove 

background contamination and resample. Blueprints were used to identify a sewer line 

under the offices, and additional investigation confirms leaking sewer line below the Office 

#3 accounting for the indoor air impacts. [Matching current data with historic blueprints 

and past uses enabled identifying background contributions and additional source(s) 

missed during original investigation.] 

Warehouse results: Main open spaces 

were below commercial risk levels; 

several smaller warehouse rooms were 

moderately above commercial risk level. 

Table 5-3. Office sampling results, μg/m 3 

Chemical Office #1 Office #2 Office #3 

PCE ND (10) ND (50) 75.0 

TCE ND (2.5) 17.0 30.0 

1,1,1-TCA ND (10) 500 850 

36


Decision: Retest after the source remedy. 

Rationale: Since a remedy is going to be installed to remediate soils under the building, waiting 

until after the remedy seemed to be appropriate. 

Table 5-4. Pros and cons of investigative methods the Scenario 5 warehouse 


Perform mitigation steps to 

eliminate risks to smaller 

warehouse rooms, then 

resample 

If source remedy not a 

priority, then this would 

be the only route to take 

May not be needed if source 

control measures are adequate to 

reduce subsurface concentrations 


Remedy #1: Use soil vapor extraction (SVE) in the source area to meet soil cleanup goals, 

with an additional SVE leg to the subslab of Office #3 above the leaky sewer pipe. It is 

hoped that levels in warehouse spaces will be reduced with the installation of the SVE 

system. Once installation of remedial system is completed, resampling should occur in the 

smaller warehouse rooms and offices. 

Results from the samples show the two warehouse rooms are still slightly above commercial risk 

levels. All other samples are nondetect for the COCs. It is determined that a second remedy is 

warranted. 

Remedy #2: Adjust that HVAC system for the smaller warehouse rooms until source 

remedies are more effective. An operation, maintenance, and monitoring plan for the SVE 

system and HVAC system adjustments is put into place. Confirmation sampling at a later 

date is requested by regulatory agency to obtain clean closure. 


This scenario is different from the others because the potential source of vapor intrusion impacts 

is in the vadose zone soil directly beneath the building. Therefore, investigations had to focus on 

the vadose zone rather than groundwater. In addition, investigations had to be conducted inside 

the building and had to consider how foundation infrastructure might impact both source and 

vapor distributions. This scenario also demonstrates how multiple sources of vapor intrusion 

impacts may be located under the same building. 

Another issue associated with this scenario was the need for a rapid solution so that the building 

could be put back into productive use and generate revenue, even while remediation was under 

way. 

Although this scenario has only one building, variations in room size, ventilation rates, and 

location with respect to the source zone resulted in varying potentials for vapor intrusion impacts 

across the building. Therefore, testing strategies had to account for each of the subregions within 

the building. For example, areas closer to the source area may have been subject to higher 

37


subslab vapor concentrations. On the other hand, the floor slabs were likely thicker and more 

robust in former manufacturing and warehouse areas, compared to the office space. In addition, 

the large warehouse rooms had larger volumes for mixing of vapors and may have had higher air 

exchange rates than the office space. Ventilation systems may also have distributed air from 

impacted areas to otherwise unimpacted areas of the building. 

Another issue was operation of the HVAC system during indoor air testing. Because HVAC 

operations can control vapor intrusion impacts in large buildings, the decision was made to test 

with the HVAC system operating normally. In other situations—for example, where future 

operation of the HVAC system cannot be relied on—it may be better to turn off the HVAC 

system before conducting indoor air tests. 

Finally, this scenario presented the need and opportunity for flexible approaches to mitigation. 

The nature of the impacted space, the proximity to the source zone, and the opportunity to use 

existing remediation and HVAC equipment varied in different portions of the building. No single 

approach was best for the entire building, and the vapor intrusion mitigation strategy also varied 

over time as the source zone soils were remediated. 


• In a step-wise, structured assessment protocol, the extent and concentration of the 

contaminant(s) under the building would normally be delineated before collecting soil vapor 

samples, which would then focus primarily on the hot spots. However, in this instance it was 

relatively easy and more economical to collect the subslab vapor samples while the directpush 

sampling equipment was at the site and holes were cored through the slab. The extra 

expense of analyzing the extra soil vapor samples (from areas away from the hot spots) is 

easily justified in light of the cost of remobilizing the equipment to the site and personnel 

labor rates. 

• There may be considerable spatial distribution and variation in the contaminant 

concentrations under large buildings. While some of the individual concentrations may 

exceed threshold action levels, the overall average concentration under the building is a more 

reliable indicator of whether remedial action may be required. (Note: Unless the sampling 

has been conducted on a statistical grid, it may be more prudent to use the third quartile as 

the concentration on which to base decisions, thus biasing the data “high” without being as 

overly conservative as using strictly the highest concentration.) 

• Indoor air sampling in the two offices near the subslab hot spot (degreasing area) did not 

detect significant levels. This finding illustrates that vapor migration into interior spaces is 

not an absolute “given” because of the tremendous variations that can occur in building 

characteristics and interior ventilation patterns. Vapor migration tends to follow preferential 

cracks through the foundation slabs and only poorly penetrates through intact concrete. If the 

carpeting in these offices were removed, further investigation would probably show that the 

underlying concrete slab had no cracks or voids that facilitate vapor migration. Alternatively, 

the air exchange rate in the offices may be high enough to dilute out any vapor that does 

intrude through the floor. 

38


• Significant levels were found in two other offices farther away from the hot spot, which were 

sampled more as “control samples” (instead of collecting ambient outdoor air). At first 

glance, it would seem that these results suggest that vapor intrusion is not predictably 

connected to “hot spots”; however, this is not the case. In each instance, there is a logical, 

adequate explanation why something was detected where nothing was expected. In one of the 

offices, the concentrations were detected because of residual consumer products desorbing 

from the building materials. In the second office, the contamination was due to vapor 

intrusion from a previously unrecognized subslab source (the leaking sewer line). 

• Finding significant levels in the office that was formerly used for the printing supply office 

illustrates the problem one encounters because of volatiles sorbing and desorbing off of 

building materials, carpet, furniture surfaces, etc. Even though no consumer products were 

present during the time of sampling, their residues can continue to desorb from surfaces long 

after the cans and bottles containing the consumer products have been removed from the 

premises. Obtaining a thorough history of the use of an area can reduce additional, 

unnecessary assessment work. 

• Finding the significant levels in the second office illustrates what may occur if all subslab 

source areas are not adequately determined. If further work had not found the broken sewer 

pipe underlying this area of the building, the results would have suggested that significant 

vapor intrusion was occurring from the hot spot under the building and that somehow these 

vapors were concentrating in this office area. This erroneous conclusion would undoubtedly 

lead to additional investigation of the HVAC system, more subslab sampling, modeling, 

scrutiny of the attenuation factors, etc. 

• The SVE system can be modified to address the sewer line break by running a vacuum line 

over the roof of the building from the hot spot area. This is an easy way to remediate the 

subslab concentration at that spot if the distance between the locations and the aesthetics of 

running a branch line across the roof are not major factors in the design. If a branch line from 

the main SVE system cannot address the sewer line break, a small dedicated SVE system can 

be installed directly over that area; however, the cost may be significantly higher because of 

the need for two SVE pumps, filter collection systems, etc. 

• Modification of the HVAC system is a practical and economical means to reduce the 

exposure dose for small areas that near borderline indoor concentrations. 


• Monitor the SVE system; sample to ensure that the system is working sufficiently. 

• Resample the warehouse rooms; determine whether risks are below commercial levels now 

that HVAC and SVE systems are operational. If so, it may not be necessary to resample as 

long as SVE and HVAC systems are monitored. 



39


Scenario 6 

Multifamily Dwelling 

Located Over a Former Gas Station 

This scenario illustrates special issues in determining the significance of background 

concentrations of COCs and assessment for petroleum hydrocarbon vapors. 


The Situation 

As part of an urban land revitalization project, a former service station site and an adjacent 

vacant parcel were redeveloped into a high-end condominium complex. Five years after the 

complex was completed, a resident notified the complex’s management company of 

petroleumlike fumes in his unit. After an inadequate response by the management company, the 

resident notified the public health department. A single air sample collected by the health 

department detected benzene at 4 µg/m 3 . Because the site was previously occupied by a service 

station that had experienced a UST release, the state UST regulatory agency was brought in to 

conduct an investigation to determine the source of the vapors. 


A site visit and review of the state UST regulatory agency files allowed investigators to develop 

an initial CSM. The condo complex consists of two four-story buildings separated by a 

courtyard. The entire complex overlies a two-level parking garage with elevators to higher 

levels. Maps submitted to the agency by the former property owner indicate that the previously 

delineated and remediated LNAPL footprint lies beneath the building containing the unit of the 

resident who had complained about petroleum odors. During the site visit, investigators noticed 

that the sewer line to the building intersected the zone of contamination. 

Previous remedial site investigations found the top of groundwater at 20 feet BGS. Soil borings 

consistently indicated the sandy soil from ground 

surface to 40 feet bgs. A previous consultant’s 

report had delineated a petroleum hydrocarbon 

plume extending beneath the former UST pit to 

groundwater and spreading radially and slightly 

downgradient, creating a contaminated zone at the 

water table approximately 50 feet in diameter. A 

dissolved plume extended an additional 100 feet 

downgradient of the LNAPL plume. 

Agency records of the remedial action showed that 

the gasoline-contaminated zone was remediated by 

soil vapor extraction. Concentration-based 

Site Summary 

• Former service station site 

• Currently two condominium complexes 

separated by a courtyard 

• Parking garage below condominiums 

• Groundwater at 20 feet bgs 

• Previous soil removal and closure by 

regulatory agency 

• Sandy soils 

• Sewer line intersects site 

• Single odor complaint led to air sample 

by health department that revealed 

slightly elevated benzene levels 

41


remediation goals for soil were met in the area of known contamination and a “No Further 

Action” letter was issued by the agency five years prior to construction of the condos. 

Vapors emanating from the vehicles in the parking structure were thought to be a likely source of 

vapors. However, ongoing drought conditions and dewatering operations at a nearby 

construction site led investigators to consider the possibility that a previously unidentified and 

recently exposed LNAPL zone from the former service station was the possible vapor source. 


Use any existing data to assess whether the pathway is potentially complete. In this case, the only 

recent data that exists is one indoor air measurement. No subsurface recent data exist to allow 

this determination, so additional data must be collected. 


Based on the initial site CSM, five potential sources exist for the detected benzene: 

• Upward migration of vapors from petroleum-contaminated soil and groundwater remaining 

from the former gas station 

• Vapors transmitted along the utility corridor created by the sewer line that transects the 

former zone of petroleum contamination 

• Gasoline vapors from the parking structure 

• Ambient air 

• Background sources 

Table 6-1 summarizes the pros and cons of the various investigatory methods. 



Indoor air 

sampling 

Direct confirmation of 

previous measured result 

Likely to have contributions from 

numerous sources 

Cannot differentiate source 

Groundwater 

or soil phase 

data 

Perform soil 

gas 

investigation 

Can search for and delineate 

extent of contamination 

source 

Can be used to locate source 

spatially and vertically 

Less invasive, easily installed, 

greater number of sampling 

locations for lower cost 

Legal complications 

Logistically difficult at this site due to 

parking garage 

No history of these matrices as likely 

sources 

Vapor intrusion risk often overestimated 

from these matrices 



42


Decision: The condominium management company, its environmental consultant, and agency 

investigators agreed that a soil gas investigation was the best initial approach to determine 

whether previous contamination was a vapor source. The management company’s newly hired 

consultant began preparation of a work plan, which included soil gas sampling and an 

investigation of the garage ventilation system. 


Since the location of the source is unknown, the first goal is to determine and delineate the 

source spatially by collecting soil gas at 5 feet below the parking garage on an even (50-foot 

center) spacing in all directions, starting below the location of the unit with the indoor air 

detection. If contamination is found, a depth profile of the soil gas in the location(s) of the 

highest concentration will be performed to assess whether the source is from the vadose zone or 

from the groundwater. In addition, at locations where contamination exceeding the risk-based 

screening levels is found at 5 feet bgs, soil gas samples will be collected immediately under the 

parking garage slab to see whether the contamination exists under the garage floor. 

Since the source is unknown, having the ability to add sampling locations both spatially and 

vertically in real time will optimize the field effort, so on-site analysis is planned. 

Determine Target/Screening Levels: The state oversight agency follows the EPA OSWER 

guidance. For benzene in soil gas at a 1-in-1-million risk level, the risk-based screening levels 

are 150 μg/m 3 for soil gas samples collected 5 feet below a receptor and 3 μg/m 3 for soil gas 

samples collected immediately below the slab. 

Both EPA test methods 8021 and 8260 can be used for soil gas samples conducted on site and 

reach detection levels of 10–100 μg/m 3 . For the subslab samples, if the on-site analysis gives 

nondetect values, then samples will need to be collected for off-site analysis with a method (such 

as TO-15) offering a lower detection level. 

Since the COC is a hydrocarbon, oxygen and carbon dioxide data will be useful to show 

bioattenuation and document the presence of highly aerobic soils. These data can be collected 

using GC or with a portable field meter such as a Landtec GEM-2000. 

Steps 10 and 11. Collect and Interpret Data 

The 5-foot soil gas data showed only one contamination zone located near the sewer line but not 

immediately beneath the condo unit. Maximum values were approximately 1500 μg/m 3 , which 

exceed screening levels by more than 10 times. 

What is the source Conduct vertical sampling for concentration profile. A vertical profile 

of the soil gas at the same location showed that concentrations did not increase with depth, so 

groundwater is not the source. 

43


Is the sewer line the source Collect additional 5-foot soil gas along sewer line. Additional 

soil gas data showed no correlation with sewer line. Only one localized subsurface contamination 

source of moderate strength appears to exist on the site at depths of 5 or more feet. 

Is there a shallower source Are high values underneath the parking garage slab Collect 

subslab soil gas below garage at contamination zone. The maximum subslab value is about 50 

μg/m 3 , which exceeds the EPA OSWER acceptable level of 3 μg/m 3 for a slab-on-grade 

structure. However, this complex has a parking garage between the 50 μg/m 3 value and the 

residences, so it is not likely that the benzene source is from the subsurface. 

Could the garage be the source Inspection of the ventilation system in the garage shows that 

the ventilation system is the type that operates intermittently when carbon monoxide levels build 

up to a value that turns the system on. Is the system operating enough to ventilate the garage 

effectively And is there communication between the parking garage and the overlying condos 

What other tools or data can we collect to test this possibility A tracer can be used to 

determine whether there is communication between the garage and condo. SF6 was injected into 

the garage, and air samples were collected from the condo. The ratio of SF6 in the condo to the 

garage showed about 5% leakage into the condo. Benzene measurements were taken again in the 

garage and condo unit to see how they compare. Benzene was 40 μg/m 3 in the garage and 

4 μg/m 3 in the condo unit. The tracer study showing a 5% leakage rate into the condo unit 

implies a ~2 μg/m 3 benzene contribution to the condo from garage. 


• The primary sources of benzene in the condo are from background sources and the garage. 

• The source of benzene in the garage is likely from cars since subsurface soil gas values 

would be expected to be at least 10 to 100 times higher than the garage, but they are nearly 

equivalent. 


• Detected indoor benzene was not from a subsurface source. This scenario gives a sequence of 

steps and questions that were taken to reach that conclusion. 

• The expected contributions from background and ambient sources precluded simple indoor 

air sampling and necessitated the sequence of steps. 


• Vertical profiles of soil gas are helpful to determine subsurface sources. 

• Supplemental tools—in this case, tracers—can be useful in atypical situations. 

44



• The contribution from the garage to the overlying residences can be reduced if the garage 

benzene value is reduced. This effect can readily be accomplished by changing the 

ventilation system in the garage to operate continuously. 

• After the ventilation system is changed, it might be appropriate to remeasure garage and 

condo concentrations to see whether they decrease. 

• Refer to oversight agency regarding other remediation that may be necessary at the site (e.g., 

soil removal, groundwater remedy, etc.). 

45


APPENDIX A 

Site Investigation Flowchart 

From Chapter 3, Vapor Intrusion Pathway: A Practical Guideline 

(ITRC 2007) 

47


49

14. Appendix D: Risk-Based Soil Gas Cleanup Levels, Schlage OU

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?