Use of Crawl Space Sampling Data and Other Lines of Evidence

for Evaluating Vapor Intrusion

C.Y. Jeng and William BosanHuman and Ecological Risk Office

Cypress, CA

CALIFORNIA ENVIRONMENTAL PROTECTION AGENCYDEPARTMENT OF TOXIC SUBSTANCES CONTROL

Common Building Configurations

• Basements• Slab-on-grade• Crawl spaces• Row houses• Tall buildings• Parking structures• Mobile homes/Portable buildings

Source: ITRC VI Guidance (2007)

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Pictures of crawl space types

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Cal/EPA DTSC Vapor Intrusion Guidance (2011)

Crawl Space Sampling

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Considerations for crawl space sampling

• Similar to indoor air sampling, but less disruptive:– Access agreement still required.

• Require no/minimum leak check test.

• Apply tubing and extension pole/stand to secure sampling point if access is limited:– Purge tubing to remove

stagnant air.• Collect outdoor air samples.

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Placement of crawl space samples

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#3

#2

#4

#1

Case Studies

Case Study 1: Potential VI of CVOCs in a Mobile Home Park

Case Study 2: Potential VI of Petroleum HCs in a Daycare Center

Case Study 3: VI Investigation/Mitigation of an Apartment Building

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Case Study 1: Mobile Home Park

• Near a former refuse disposal facility • Elevated chlorinated VOCs detected in

shallow soil vapor probes installed at street• J&E model predicted > 1x10-5 cancer risk

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Sampling Locations

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Air Sampling Results for VOCs Detected in Soil Gas

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PCE Levels in Mobile Homes

Unit 3/5: Crawl Space > Outdoor Air (~ Indoor Air)Unit 4: Crawl Space < Outdoor Air

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Case Study 1: Summary

• Multiple lines of evidence for VI evaluation:

• Default crawl space-to-indoor air attenuation factor ( =1) in USEPA VI database may be conservative.

Line of Evidence Unit 3 Unit 4 Unit 5

Present in Soil Gas? COCs for VI evaluation

Crawl Space > Outdoor Air?

Yes(Possible VI)

No(Incomplete VI)

Yes(Possible VI)

Indoor Air > Outdoor Air or IA CHHSL?

No(Insignificant VI) --- No

(Insignificant VI)

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Case Study 2: Daycare Center

• Former petroleum tank farm converted to park and daycare center

• Site characterization:– Soil data down to 100’– Multi-depth soil-vapor

points down to 32’– GW monitoring wells

(DTW ~ 40’)• Chemicals of concern:

– BTEX– Methane

2 modular buildings housing daycare

Soil Gas Concentrations at 5 feet bgs

Benzene:> 100x CHHSL

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Methane:> 50,000 ppm

Step-Wise Process in DTSC VI Guidance

Step 4 => Imminent hazard?• Methane as immediate explosion hazard ruled out by local FD.

Step 5 => Pass screening evaluation?• No, based on CHHSL exceedance.

Steps 6 & 7 => Additional data and site-specific evaluation?• Crawl space/utility screening• Crawl space & outdoor air sampling• Indoor air monitoring

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LOE #1: Crawl Space and Utility Field Screening Sample Locations

• Field screening of crawl space & utility locations all less than methane action levels. 16

• Benzene & methane levels in crawl space similar to those in outdoor air and local air monitor station.

• Anomaly due to dirty canisters and/or parking lot traffic.

LOE #2: Crawl Space & Outdoor Air Time-Integrated (8-Hour) Sampling Results

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LOE #3: Daycare Indoor Air Monitoring

• Bi-weekly indoor air monitoring showed no detectable methane level (< 1 ppmv). 18

Case Study 2: Summary• No VI concern based on multiple lines of evidence:

– Field screening of crawl space & utility less than methane action levels;

– Time-integrated BTEX/methane concentrations in crawl space and outdoor air similar; and

– No detectable methane level in all indoor air screening points.

• Evaluation of temporal trend is important to identify interference from other VOC sources.

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Case Study 3: VI Investigation & Mitigation of Apartment Building

• Apartment building & single residence adjacent to a former waste solvent recycling facility

• PCE and TCE detected in shallow soil vapor and GW (DTW ~ 30’)

• J&E model predicted > 1x10-4 cancer risk at site boundary

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Proposed Sampling Locations

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Ratios of Crawl Space‐to‐Outdoor Air Concentrations

PCE

TCE

VC

TolueneBenzene

1,1-DCEChloroethane

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Ratios of Crawl Space‐to‐Soil Vapor Concentrations 

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Case Study 3: Summary• Out of Seven VOCs retained for VI evaluation

based on crawl space-to-outdoor air ratios:– Four were eliminated by examining crawl space-to-

soil vapor ratios.– Concentrations of PCE and TCE in crawl space >

indoor air CHHSLs at apartment building.• Preventive mitigation measures planned to

eliminate the need for future sampling:– Sealing of ground surface & interior floor underside.– Applying mitigation fan to provide active ventilation.

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Final Thoughts

• Crawl space data can be used to provide multiple lines of evidence for making VI decisions: – Crawl Space vs. Outdoor Air– Crawl Space vs. Soil Vapor– Crawl Space vs. Outdoor Air vs. Indoor Air

• USEPA’s crawl space-to-indoor air attenuation factor ( =1) may be conservative for exterior crawl spaces common in Southern California.

• Crawl space ventilation can be used as required or preventive measures to mitigate vapor intrusion.

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Disclaimer

The analyses and findings in this presentation are solely those of the authors, and do not represent official policy of the California Department of Toxic Substances Control or California EPA.

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POLICY

Questions/Comments

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Contact InformationC.Y. Jeng – cjeng@dtsc.ca.govBill Bosan – bbosan@dtsc.ca.gov