Vapor Intrusion: Investigation of Buildings Migration of VOCs
through the building foundation and lessons learned from the
detailed field investigation of the vapour intrusion process at
Altus and Hill Air Force Bases Vingsted Center Monday, March 9,
2009 GSI ENVIRONMENTAL INC. Houston, Texas www.gsi-net.com (713)
522-6300 [email protected] source area Air Exchange SITE
BUILDING Slide 2 2 Vapor Intrusion: Investigation of Buildings
United States Regulatory Framework Spatial and Temporal Variability
Impact of Indoor Sources on VI Investigations Air Flow and VOC
Migration Around Buildings Controlled Investigation of Vapor
Intrusion in Buildings Conclusions and Recommendations United
States Regulatory Framework Spatial and Temporal Variability Impact
of Indoor Sources on VI Investigations Air Flow and VOC Migration
Around Buildings Controlled Investigation of Vapor Intrusion in
Buildings Conclusions and Recommendations Slide 3 3 Conceptual
Model for Vapor Intrusion: KEY POINT: Standard conceptual model for
vapor intrusion does not account for variable air flow in
buildings. Building Attenuation Due to Exchange with Ambient Air
Advection and Diffusion Through Unsaturated Soil and Building
Foundation Partitioning Between Source and Soil Vapor Groundwater
-Bearing Unit Air Exchange BUILDING Unsaturated Soil 3 2 1 Affected
Soil Affected GW Overview of USEPA VI Guidance Slide 4 4 Effect of
Building Pressure on VOC Transport Lower building pressure
Residence in winter (chimney effect); bathroom, kitchen vents Flow
in EXAMPLES Gas flow from subsurface into building High Pressure
Low Pressure DOWNWARD VOC TRANSPORT Low Pressure High Pressure
UPWARD VOC TRANSPORT Higher building pressure Building HVAC
designed to maintain positive pressure Flow out EXAMPLES Gas flow
from building into subsurface Variable building pressure Barometric
pumping; variable wind effects Reversible flow EXAMPLES
Bi-directional flow between building and subsurface Slide 5 5
Effect of Weather on Building Pressure COLD WEATHER Temperature and
wind create pressure gradients that influence air movement in and
around buildings. Stack Effect: Warm air leaks through roof
creating negative building pressure Stack Effect: Warm air leaks
through roof creating negative building pressure soil subslab fill
++ -- WIND Wind on Building creates pressure gradient that results
in air flow. soil wind + + + + subslab fill KEY POINT: Slide 6 6
Effect of Mechanical Ventilation Mechanical ventilation can create
localized or building-wide pressure differences that drive air
flow. KEY POINT: MECHANICAL VENTILATION Examples in Houses: - HVAC
system - Exhaust fans (kitchen, bath) - Furnace - Other combustion
appliances (water heater, cloths dryer, etc) Slide 7 7 Pressure
Gradient Measurements: School Building, Houston, Texas Differential
Pressure (Pascals) Time (July 14-15, 2005) Neg. Pressure Pos.
Pressure Pressure gradient frequently switches between positive and
negative within a single day. KEY POINT: Pressure Transducer Slide
8 8 Pressure gradients potentially influenced by wide variety of
factors. Measurements document non- representative sampling
conditions. Pressure Gradient Measurements: Tropical Storm Cindy
KEY POINT: Pressure Transducer Differential Pressure (Pascasl) Time
(July 5-6, 2005) High south wind High north wind & low
atmospheric pressure Positive pressure: HVAC Neg. Pressure Pos.
Pressure Test Site Storm Track: TS Cindy Slide 9 9 Negative
Pressure Positive Pressure Worst Case VI conditions. No current VOC
transport from subsurface. Indoor VOCs due to background sources.
Bi-directional VOC transport. Carefully consider potential sources
of measured indoor and sub-slab VOCs. Pressure Reversal
Interpretation of VOC Measurements PRESSURE CONDITION
INTERPRETATION OF VOC DATA Pressure gradients drive VOC transport.
Multiple indoor VOC sampling events may be needed to measure VI.
KEY POINT: Slide 10 10 Typical Building VI Investigation: Outdoor,
Indoor, and Sub-Slab Sampling Sub-Slab Sampling Data at Apartment
Complex Concurrent sampling of sub-slab, indoor air, and outdoor
air. KEY POINT: Slide 11 11 Vapor Sampling: No Vapor Intrusion
INDOOR AIR VOC Concentration (ug/m3) at Residence in Illinois S
BELOW SLAB AMBIENT AIR Slide 12 12 KEY POINT: Common indoor sources
of VOCs p-Dichloro- benzene Used as air freshener and indoor
pesticide for moths and carpet beetles. Petroleum-based solvents,
paints, glues, gasoline from attached garages. BTEX Even at sites
with no subsurface source, these chemicals will commonly be
detected in indoor air and sub-slab samples. Emitted from molded
plastic objects (e.g., toys, Christmas decorations). 1,2-DCA
1,2-DCA = 1,2-dichloroethane Slide 13 13 VOC Transport Model:
Bidirectional Flow Model simulates advective transport of chemicals
between building air and subsurface soil through building slab.
Positive Pressure Negative Pressure Slide 14 14 Model Results:
Transient Indoor VOC Source VOCs from building can be trapped below
slab. KEY POINT: VOC Conc. vs. Time: Transient Source Indoor
Sub-Slab BIDIRECTIONAL VOC TRANSPORT Vapors trapped below slab
PRESSURE Slide 15 15 Vapor Intrusion: Investigation of Buildings
United States Regulatory Framework Spatial and Temporal Variability
Impact of Indoor Sources on VI Investigations Air Flow and VOC
Migration Around Buildings Controlled Investigation of Vapor
Intrusion in Buildings Conclusions and Recommendations United
States Regulatory Framework Spatial and Temporal Variability Impact
of Indoor Sources on VI Investigations Air Flow and VOC Migration
Around Buildings Controlled Investigation of Vapor Intrusion in
Buildings Conclusions and Recommendations Slide 16 16 Study Design:
Sampling Program MEASUREMENT PROGRAM: Measure VOC concentrations in
and around building under baseline and induced negative pressure
conditions. 1.5 s s s s s s SF 6 Radon VOCs, Radon VOCs, Radon, SF
6 Analyses Ambient Air Indoor Air Sub- slab MEDIUM Samples per
Building 1 - 3 3 - 5 Slide 17 17 Study Design: Building Pressure
Sample Event 1: Baseline Conditions Sample Event 2: Induced
Negative Pressure soil subslab fill -2.50.5 TIME Building Pressure
TIME Building Pressure Slide 18 18 Study Design: Test Site TEST
SITE: Three single-family residences over a TCE plume near Hill AFB
in Utah Slide 19 19 Study Results: Impact of Depressurization on
Air Flow soil 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 Res. #1 Res.
#2 Res. #3 AER Ratio (Depressure/ Baseline) subslab fill
Cross-Foundation Pressure Gradient Gradient (Pa) Baseline
Depressure Change in Air Exchange Rate (AER) Induction of negative
building pressure resulted in 3 to 6-fold increase in air exchange
rate. KEY POINT: Slide 20 20 Study Results: Chemical Concentration
Ratios Sub-slab to indoor air concentration ratio provides an
indication of the likely source of the chemical. However, multiple
sources may contribute to indoor air impact. KEY POINT:
Concentration Ratio (Sub-slab/Indoor air) Baseline
SamplesDepressurization Samples Residence #1Residence #2 SS
SourceIndoor Source Concentration Ratio (Sub-slab/Indoor air) SS
SourceIndoor Source Residence #3 Slide 21 21 Study Results:
Volatile Chemical Detection Frequency All chemicals commonly
detected in indoor air samples. Chemicals w/ subsurface sources
(Radon and TCE) more commonly detected in sub-slab samples. KEY
POINT: Detection Frequency Indoor Air SamplesSub-slab Gas Samples
Baseline SamplesDepressurization Samples Note: Detection frequency
is for combined sample set from all three residences. Slide 22 22
Study Results: Impact of Depressurization on VOC Concentration Res.
#1Res. #2Res. #3 Location 0.1 1 10 Concentration Ratio
(Depressurization/ Baseline) Location 0.1 1 10 Res. #1Res. #2Res.
#3 Concentration Ratio (Depressurization/ Baseline) 0.1 1 10 Res.
#1Res. #2Res. #3 Location Concentration Ratio (Depressurization/
Baseline) 0.1 1 10 Res. #1Res. #2Res. #3 Location Concentration
Ratio (Depressurization/ Baseline) 1,2-DCA PCE SF 6 Benzene Radon
TCE Radon TCE Subsurface Source Indoor Source VOC Conc. in sub-
slab gas VOC Conc. in indoor air Slide 23 23 BUILDING Air Exchange
Study Results: Impact on VOC Conc. VOC conc. in sub-slab gas VOC
conc. in indoor air VOCs from indoor source VOCs from subsurface
source (DCA, PCE, SF 6, Benzene) (TCE, Radon) Slide 24 24 Building
depressurization does NOT appear to increase the magnitude of vapor
intrusion. Building depressurization improves ability to detect
vapor intrusion by increasing the contrast between VOCs from indoor
vs. subsurface sources. Impact of Building Pressure on Evaluation
of Vapor Intrusion Building Depressurization: Project Findings
Worst Case Vapor Intrusion C ia Low Pressure High Pressure Use
building depressurization to increase contrast between indoor and
subsurface sources of VOCs. KEY POINT: Slide 25 25 Vapor Intrusion:
Investigation of Buildings United States Regulatory Framework
Spatial and Temporal Variability Impact of Indoor Sources on VI
Investigations Air Flow and VOC Migration Around Buildings
Controlled Investigation of Vapor Intrusion in Buildings
Recommendations United States Regulatory Framework Spatial and
Temporal Variability Impact of Indoor Sources on VI Investigations
Air Flow and VOC Migration Around Buildings Controlled
Investigation of Vapor Intrusion in Buildings Recommendations Slide
26 26 Vapor Intrusion: Recommendations General Strategy Groundwater
Sampling Soil Gas Sampling Indoor Air Sampling Non-VOC Measurements
Typical Building Sampling Program General Strategy Groundwater
Sampling Soil Gas Sampling Indoor Air Sampling Non-VOC Measurements
Typical Building Sampling Program Slide 27 27 VOCs: Practical Tips
from the Field VOCs are pervasive. You will always find hits in
indoor air. Use radon as a tracer to control for background. Its
Background, Stupid Cartridges are Funky, Summas are Re-Used Run
full Method T0-15 scan to be able to distinguish petroleum
hydrocarbon composition of soil vapor vs. indoor air. For
Petroleum, Run Full VOC Scan Sorbent cartridges affected by
moisture, less repeatable. Summa canister preferable, but have
individually-certified clean. Summa Canister Slide 28 Understand
variability in VOC concentration: 1) Indoor Air: 2) Subsurface:
Single sample can accurately characterize well-mixed space.
Consider multiple measurement locations and sample events:
-Separate sample events by months -Evaluate uncertainly based on
observed variability Accounting for Variability Skip samples to
dont increase knowledge: (e.g., multiple indoor samples; daily
resamples.) KEY POINT: Slide 29 29 Vapor Intrusion: Recommendations
General Strategy Groundwater Sampling Soil Gas Sampling Indoor Air
Sampling Non-VOC Measurements Typical Building Sampling Program
General Strategy Groundwater Sampling Soil Gas Sampling Indoor Air
Sampling Non-VOC Measurements Typical Building Sampling Program
Slide 30 30 Key Physical Processes at GW Interface Groundwater
Interface Evapotranspiration Slide 31 31 Distribution of TCE in
Shallow Groundwater Based on >150 water table samples VOC
distribution at water table is difficult to predict and may be very
different from deeper GW plume. KEY POINT: Graphic from
presentation by Bill Wertz (NYSDEC) made at ESTCP-SERDP Conference,
December 2008. Slide 32 32 Groundwater Sampling: Key Considerations
- Understand physical processes at water table. - For vapor
intrusion, collect water samples from top of water table. KEY
POINT: Slide 33 33 Vapor Intrusion: Recommendations General
Strategy Groundwater Sampling Soil Gas Sampling Indoor Air Sampling
Non-VOC Measurements Typical Building Sampling Program General
Strategy Groundwater Sampling Soil Gas Sampling Indoor Air Sampling
Non-VOC Measurements Typical Building Sampling Program Slide 34 34
Soil Gas Sampling: Considerations Sample Volume: Lab often needs
only 50 mL of sample. Use 1L sample vessel (not 6L Summa), if
available. Purge Volume: Use small diameter sample lines to
minimize purge volume. Sample Rate: Use lower flow rate in fine
grain soils to minimize induced vacuum. Goal: Minimize the flow of
gas in subsurface due to sample collection Where Does Your Sample
Come From? Flexibility required to allow use of newly validated
sample collection and analysis methods. KEY POINT: Slide 35 35 Soil
Gas Sample Collection: Scheme for Summa Canister Slide 36 36 Soil
Gas Sampling: Sample Collection Shallower Sample Point Pressure
gauge Flow controller Deeper Sample Point Slide 37 37 Liquid Tracer
Apply to towel and place in enclosure or wrap around fittings.
Examples: DFA, isopropyl alcohol, pentane High concentrations in
samples may cause elevated detection limits for target analytes
(Check w/ lab before using) Gas Tracer Inject periodically or
continuously into enclosure around fittings and sample point:
Examples: Helium, SF 6 On-site analysis (helium) Potentially more
quantitative DFA = 1,1-difluoroethane, SF 6 = sulfur hexafluoride
Photo from Todd McAlary Photo from Blayne Hartman Soil Gas
Sampling: Leak Tracers Slide 38 38 Sample Point Shroud Leak Tracer
Gas Field Meter for Leak Tracer Soil Gas Sampling: Gas Phase Leak
Tracer Slide 39 39 Summa Canisters Soil Gas Sampling: Summas vs.
Sorbent Tubes Sorbent Tubes Most accepted in U.S. Simple to use
Less available outside U.S. Canisters are re-used, subject to
carry-over contamination More available world wide Better for
SVOCs* Use is more complex - pump calibration - backpressure -
breakthrough of COC - selection of sorbent * = Analysis for SVOCs
not typically required, but sometimes requested by regulators.
Slide 40 40 Results Comparison: Summa / Sorbent (ug/m 3 ) Summa vs
Sorbent: Side-by-Side beacon-usa.com 1-800-878-5510 PHOTO PROVIDED
BY: Reference: Odencrantz et al., 2008, Canister v. Sorbent Tubes:
Vapor Intrusion Test Method Comparison, Proceedings of the Sixth
International Conference on Remediation of Chlorinated and
Recalcitrant Compounds, Monterey, California, May 2008. TCE PCE
SG-02 SG-03 20.5 / 10.5292 / 149 3070 / 135722,200 / 5917 54
Guidelines for Vapor Intrusion Evaluation Identifying Sites Needing
VI Mitigation KEY POINT: Step-wise approach can help distinguish VI
sources from indoor sources. Swell ! Indoor Air > Risk Limit?
> Std? Indoor air concs. > applicable limits. Subslab Vapors
> Risk Limit Subslab vapors > applicable limits. >Std?
Pressure gradient supports soil gas flow into building Building
Pressure Supports VI S 3 2 1 SG air Slide 55 55 Guidelines for
Vapor Intrusion Evaluation Identifying Sites Needing VI Mitigation
KEY POINT: Step-wise approach can help distinguish VI sources from
indoor sources. Cause = Indoor/Ambient Source? Data set shows clear
indoor/ambient source. Radon Data Suggest Actual VI? Rn attenuation
factor suggests VOCs may enter house, too. Pressurization and
depressurization of bldg. show VI through slab. Pressurization
shows Actual VI ? S Rn air P Rn Swell ! 6 5 4 Indoor Air > Risk
Limit? > Std? Indoor air concs. > applicable limits. Subslab
Vapors > Risk Limit Subslab vapors > applicable limits.
>Std? Pressure gradient supports soil gas flow into building
Building Pressure Supports VI S 3 2 1 SG air Slide 56 Support
provided by by the Environmental Security Technology Certification
Program (ESTCP) Projects ER-0423 and ER-0707 Project Reports:
www.estcp.org (Search 0423 & 0707) Special Thanks to:
Acknowledgements Tim Nickels and Danny Bailey (GSI) Sam Brock
(AFCEE) Kyle Gorder (Hill AFB) Blayne Hartman David Folks
(Envirogroup), Todd McAlary (Geosyntec)
