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Vapor Intrusion: Investigation of Buildings Migration of VOCs through the building foundation and lessons learned from the detailed field investigation.

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Presentation on theme: "Vapor Intrusion: Investigation of Buildings Migration of VOCs through the building foundation and lessons learned from the detailed field investigation."— Presentation transcript:

1 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 (713) source area Air Exchange SITE BUILDING

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

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 Affected Soil Affected GW Overview of USEPA VI Guidance

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

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:

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)

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

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

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:

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:

11 11 Vapor Sampling: No Vapor Intrusion INDOOR AIR VOC Concentration (ug/m3) at Residence in Illinois S BELOW SLAB AMBIENT AIR

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

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

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

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

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

17 17 Study Design: Building Pressure Sample Event 1: Baseline Conditions Sample Event 2: Induced Negative Pressure soil subslab fill TIME Building Pressure TIME Building Pressure

18 18 Study Design: Test Site TEST SITE: Three single-family residences over a TCE plume near Hill AFB in Utah

19 19 Study Results: Impact of Depressurization on Air Flow soil 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:

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

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.

22 22 Study Results: Impact of Depressurization on VOC Concentration Res. #1Res. #2Res. #3 Location Concentration Ratio (Depressurization/ Baseline) Location Res. #1Res. #2Res. #3 Concentration Ratio (Depressurization/ Baseline) Res. #1Res. #2Res. #3 Location Concentration Ratio (Depressurization/ Baseline) 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

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)

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:

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

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

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. It’s 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

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 don’t increase knowledge: (e.g., multiple indoor samples; daily resamples.) KEY POINT:

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

30 30 Key Physical Processes at GW Interface Groundwater Interface Evapotranspiration

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.

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:

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

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:

35 35 Soil Gas Sample Collection: Scheme for Summa Canister

36 36 Soil Gas Sampling: Sample Collection Shallower Sample Point Pressure gauge Flow controller Deeper Sample Point

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

38 38 Sample Point Shroud Leak Tracer Gas Field Meter for Leak Tracer Soil Gas Sampling: Gas Phase Leak Tracer

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.

40 40 Results Comparison: Summa / Sorbent (ug/m 3 ) Summa vs Sorbent: Side-by-Side beacon-usa.com 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 TCE PCE SG-02 SG / / / ,200 / 5917 <2.7 / < / 225 SG-04 KEY POINT: Even skilled practitioners see up to 4x difference between Summa and sorbet tube results.

41 41 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

42 42 Sample Location Considerations Sample Location Considerations ■ Collect at least one outdoor sample ► Compare indoor and outdoor ■ Consider collection subslab samples (concurrent with indoor air samples) ► Compare indoor and subslab or near-slab ■ Recommend sampling in lowest level and consider sampling next highest level ► Investigate COC patterns ■ Consider sampling near potential indoor sources or preferential pathways ► Attached garage, industrial source ► Basement sump, bathroom pipes Indoor Sampling: Overview

43 43 ■ Placement of samplers NOTE: Little value to collect multiple samples in a single building zone (e.g. same room), unless collecting QA duplicates. ■ Place at breathing-level height ■ Avoid registers, drafts ■ Remember to sample for appropriate length of time ► Typically 24 hours for residential ► Typically 8-24 hours for occupational  Collect indoor and subslab samples concurrently  QA Samples: Collect greater of one duplicate per day or one per 20 samples. (Collect additional QA samples if required by regs.) Indoor Sampling: Sample Locations

44 44 Sub-Slab Sampling Sample Collection Measure VOC concentration below building foundation Outdoor Air Sampling Document ambient conditions

45 45 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

46 46 VI Investigation Methods: Non-VOC Measurements Non-VOC measurements can be used to evaluate vapor intrusion while avoiding background VOC issues. KEY POINT: Radon Building Pressure Naturally occurring tracer gas measures attenuation through building foundation. Magnitude and duration of building pressure fluctuations: negative vs. positive building pressure. Air Exchange Rate of ambient air entry into building. Supports mass flux evaluations.

47 47 Home Test Methods: Charcoal Canister, electret, alpha detector Air Samples: Radon concentration measured at off-site lab * Sub- Foundation Indoor Air Air Sample: Radon concentration measured at off-site lab * Electret: Placed over hole in foundation (questionable accuracy) * Off-site analysis provided by Dr. Doug Hammond, University of Southern California Radon: Measurement Options Cost/ Sample $10-50 $100 $25-50 Key Point: Radon analysis less expensive than VOC analysis ($ /sample for VOCs by TO-15).

48 48 BENEFITS: No common indoor sources of radon. Lower analytical costs compared to VOCs. Less bias caused by non-detect results indoors. Can be used for long-term testing (up to 6 months). Radon (Ra) as Tracer for Foundation Attenuation Indoor Ra = 0.9 pCi/L Sub-slab Ra = 833 pCi/L Test ResultsAF Calculation AF ss-ia = = Ambient Ra = 0.3 pCi/L

49 49 Rate at which indoor air is replaced by ambient (fresh) air. What ASHRAE Std SF 6 Air Exchange: What ‘n How WHY: Better understand observed VOC attenuation. Use value model or mass flux calculation. Recommended ventilation rates for commercial building. Ventilation Standards Tracer Gas Measure dilution of tracer gas to determine air exchange rate ESTIMATION METHODS J&E = Johnson and Ettinger model. Air Exchange BUILDING

50 50 Recommended Building Ventilation Rates KEY POINT: Buildings designed for high density use will have high air exchange rates. ANSI / ASHRAE Standard 62.1 – 2004 Ventilation for Acceptable Indoor Air Quality Building Type Air Exchanges (per day) USEPA Default (Residential) Office Space Supermarket Classroom Restaurant High Building Ventilation

51 51 KEY POINT: Site-specific measurement provides most accurate measure of air exchange under current operating conditions. Test Building How: Release tracer gas (SF 6 or helium) into building at constant rate. Measure steady-state concentration of gas in building. Calculate air exchange based on release rate, concentration, and building volume. Air Exchange: Measured Values

52 52 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

53 53 Residential Building Investigation: Recommended Sampling Program BUILDING PRESSURE: For more definitive results, conduct sampling program under induced negative pressure and positive pressure building conditions. 1.5 s s s s s s Radon VOCs, Radon Analyses Ambient Air Indoor Air Sub- slab Gas MEDIUM Samples per Building (lowest level) GAS MEASUREMENTS:

54 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 conc’s. > applicable limits. Subslab Vapors > Risk Limit Subslab vapors > applicable limits. >Std? Pressure gradient supports soil gas flow into building Building Pressure Supports VI S SG air

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 ! Indoor Air > Risk Limit? > Std? Indoor air conc’s. > applicable limits. Subslab Vapors > Risk Limit Subslab vapors > applicable limits. >Std? Pressure gradient supports soil gas flow into building Building Pressure Supports VI S SG air

56 Support provided by by the Environmental Security Technology Certification Program (ESTCP) Projects ER-0423 and ER-0707 Project Reports: (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)


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