Building Trust. Engineering Success.

Vapor Intrusion into Large BuildingsDavid Shea, PEDaniel B. Carr, PE, PGSanborn, Head & Associates, Inc.

The 22nd Annual International Conference on Soil, Water, Energy, and AirSan Diego California

USEPA Workshop: Recent Advances to VI Application and Implementation-A State of the Science Update

HVAC Basics• Fan: moves air, creates +/- pressure• Dampers: adjusts air flow through ducts• Coils: heat or cool air

Mass rate = Qbldg x Cindoor = AER x V x Cindoor [g/day]

Qbldg

V, Cindoor

Large bldg AER: typically 1 to 4/hr

Air Handling Units (AHUs) Exhaust Fans

Things to Look For: Bldgs w AHUs AHU/Airflow Balance

HVAC equipment/components in contact with the floor slab

Building Wide Plenum w return and outside air (often above ceilings or beneath raised flooring).

Areas of “dead” low AER/ACH

Areas of potential low air pressure (mechanical rooms, fan rooms, laboratories, kitchens)

Variability of HVAC operations (nightly and weekend turndown, outside air damper position –economizers, operator over-rides)

AHU Balance = Outdoor Air-Relief Air–Exhaust = + or -

4

Positive Pressure SpaceMore air actively supplied (in) than exhausted (out);Pressure is greater in relation to abutting space

Air supply

+ + + +

“Positive” or “negative” are defined in relation to an adjacent space, e.g. outdoors, subslab, abutting room, etc.

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Negative Pressure SpaceMore air actively exiting (out) than entering (in);Pressure is lower in relation to an abutting space

Air exhaust

- - - -

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Positive AND negative pressure space can exist side-by-side

+ + +- - - -

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8

HVAC Operations Variability

When system is active, bldg is “positive pressure”When system is in sleep mode, bldg is “neutral”

Indoor sampling should account for HVAC variability

Air flow Airflow

Fan

Stand-alone AC units

Neg P area

Active “positive pressure” clean room

Vapor intrusion pathway and driver

Mitigation by vapor barrier and increase in room pressure

Cindoor decreased by a factor of 10

Mitigation by subslab depressurization

Cindoor decreased to ND

12

Using mass flux to link subsurface conditions with indoor air

Mass flux estimation methods:

• Bldg/space mass balance using AER (previous slides)

• Diffusion across slab

• Diffusion across vadose zone

• SSD – measure flow and concentration

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Example: Assessment and mitigation of a 330,000 SF industrial bldg

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Soil profiling to estimate mass flux through vadose zone

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Using mass flux as an indicator of likely indoor air levels

Adjustments to existing HVAC operations:Indoor VOCs decreased to levels consistent with expectations based on increased ACH

Expected Reduction Factor = 1 – (ACHbefore / ACHafter)

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5

% P

CER

edu

ctio

n

HVAC Zone

Expected

Actual

HVAC Zone

ACH Before HVAC Mods[hr-1]

ACH After HVAC Mods[hr-1]

1 0.01 13

2 1.0 6.8

3 0.31 2.5

4 0.01 3.1

5 1.2 2.3

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Summary:• VI in large buildings is influenced by HVAC design and operation

Need to understand the air flow/pressure balance HVAC can have favorable or unfavorable effects

• Mass flux is a better measure of VI potential than empirical attenuation factor α is unreliable (and likely ultra-conservative) given the wide range

of subsurface and bldg conditions Typical regulatory benchmarks for α are not relevant for buildings

with AHUs J (µg/m2/day) reflects actual mass transfer processes that can be

measured/estimated

• It’s the flux that matters - not only concentration – in assessing and mitigating VI – let’s reframe our evaluations in terms of mass flux

Questions?David Shea, PE

Sanborn, Head & Associates, Inc.dshea@sanbornhead.com

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Extra slides

Diffusion

+

= 2

33.3

2

33.3

φθ

φθ w

H

waae K

DDD

« Millington Relationship » USEPA, From McWhorter, 1987

Da >>Dw = free air and free water diffusion coefficients [L2/t]

θa, θw = volumetric air and water-content (unitless fraction)

Φ = soil porosity (unitless fraction);

θa +θw = Φ

KH = Henry’s Constant (unitless ratio).

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At steady-state,Mass in from subsurface = mass out through HVAC

J * A = Cindoor * Qbldg= Cindoor * V * (Qbldg / V)

J = Cindoor * H * ACH

Predicting Cindoor from mass flux:

Cindoor = J/(H * ACH)

Predicting Csubslab from mass flux:

Csubslab = Cindoor * H * ACH * L / Deff