SABrownfields_VaporIntrusion-20110920-live

Vapor Intrusion AssessmentsGoing Beyond Basics

Kevin L. Long, ENVIRONPresented to the Brownfield Communities Network in associated with the National Association of

Local Government Environmental Professionals

September 20, 2011

Asheville, North Carolina

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Presentation Outline

Conceptual model of vapor intrusion

Basic approaches and pitfalls

Going beyond basics – 3 examples

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Conceptual ModelMain Aspects and Key Factors

Modified from USEPA (2004) User’s Guide for Evaluating Subsurface Vapor Intrusion Into Buildings

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Basic Approaches

Indoor air sampling– Collect indoor air data– Compare to generic risk-based indoor air screening levels

and/or calculate cumulative cancer and noncancer risk

Empirical attenuation factors– Collect soil gas data – Apply attenuation factor defined as:

– USEPA’s 2002 and 2008 databases of empirical attenuation factors

Mathematical modeling– Mechanistic model by Johnson and Ettinger (1991)– USEPA spreadsheets for Johnson and Ettinger model

vapor

bldg

C

C

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Basic ApproachesPitfalls With Indoor Air Sampling

From USEPA (2011) Background Indoor Air Concentrations of Volatile Organic Compounds in North American Residences (1990–2005): A Compilation of Statistics for Assessing Vapor Intrusion

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Indoor air data typically characterize short-term conditions, but assessments are supposed to be concerned with long-term exposures

From Hubbard and Others (1996) Studies On Temporal Variations Of Radon In Swedish Single-Family Houses. Environment International, vol. 22, Suppl. 1, pp. S715-S722.

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IA sampling is impractical for assessing sites before new buildings are built

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Generic IASLs are sometimes inappropriate for exposures in a particular building

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Basic ApproachesPitfalls of Using Empirical Attenuation Factors

USEPA generic attenuation factors: 0.1 for sub-slab; 0.01 for soil gas; 0.001 for groundwater

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Basic ApproachesPitfalls of Using Empirical Attenuation Factors

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Basic ApproachesPitfalls of Using Mathematical Modeling

From USEPA Johnson and Ettinger (1991) Model for Subsurface Vapor Intrusion into Buildings

(http://www.epa.gov/oswer/riskassessment/ airmodel/johnson_ettinger.htm)

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Beyond the BasicsExamples

1. Stop the vapor intrusion assessment from “creating” contaminant mass

2. Use knowledge about the building’s ventilation system to estimate α

3. Use a more realistic model of the soil moisture profile

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Beyond the BasicsMass Balance Check

Cair

(mg/m3)Csoil

(mg/kg)Depth

(m)MassRatio

Benzene 0.016 1 3 1.9Ethylbenzene 4.400 10 3 52.2Toluene 22.000 10 3 261.2Xylenes 0.440 10 3 5.2

Basis:Cair is for commercial/industrial; risk = 1E-5 or HQ = 1Air exchange rate 1 per hrBuilding height 2.44 mExposure duration 25 yrSoil bulk density 1.5 kg/L

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Beyond the BasicsVentilation System Knowledge

We can show (by using mass balance) that an upper-bound α is given by:

The outdoor air flow rate through the building is Qbldg

The soil vapor entry rate Qsoil can be estimated by incorporating knowledge about the indoor-outdoor pressure difference that the ventilation system is designed to maintain

bldg

soil

Q

Q

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Beyond the BasicsVentilation System Knowledge

Building TypeQsoil

(L/min)Ach

(1/hr)Lbldg

(m)Hbldg

(m)Qbldg

(L/min) EPA residential basement 5.00 0.25 10.0 3.66 1525 3.E-03EPA residential slab 5.00 0.25 10.0 2.44 1017 5.E-03MDEQ residential basemt 0.05 1.00 10.6 4.88 9070 5.E-06MDEQ commercial/ind 0.13 2.00 19.3 2.44 30264 4.E-06Positive pressure building 0.00 any reasonable value ~ 0E-00

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Beyond the BasicsSoil Moisture Profile

Soil moisture is the most important factor in determining a rate of diffusion

USEPA’s spreadsheet uses a simplified method that underestimates soil moisture

USEPA guidance allows the use of a more scientific method (the HYDRUS model)

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Beyond the BasicsSoil Moisture Profile

Moisture Profile

Effective Diffusion

Coarse Soil

Fine Soil

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ConclusionKey Points

Generic assessment methods incorporate many conservative, simplifying assumptions, which often grossly overestimate vapor intrusion risk in many situations

Some gross overestimation can be checked by applying fundamental scientific principles, such as the law of mass conservation

Some generic assumptions about building characteristics can be avoided by using site-specific knowledge, such as details of the building’s ventilation system design

Some simplifications are only for convenience (such as USEPA’s JEM spreadsheets), and can be avoided when necessary by using more robust and readily available computational tools

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Questions?

SABrownfields_VaporIntrusion-20110920-live

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