Case Study of Modeled and Observed TCE Attenuation from Groundwater to Indoor Air Christopher G. Lawless Johnson Wright, Inc. October 18, 2004
Presentation Contents SITE BACKGROUND STUDY METHODS RESULTS DISCUSSION CONCLUSIONS
Study Questions How well does site-specific J&E modeling predict indoor air concentrations? What are the measured and site-specific modeled vapor attenuation factors? How does the EPA attenuation factor compare to measured and modeled attenuation factors for the site?
Site Background In 2003, groundwater to indoor air vapor intrusion study was conducted Superfund Site (US EPA lead agency) Contamination caused by subsurface release of solvents
Site Background 1985, Soil cleanup completed GW cleanup since 1986 using extraction and treatment Two aquifers impacted with TCE Groundwater Treatment System
Building Conditions Commercial facility Constructed 1965 Slab on Grade - Sub-slab material unknown 2 sects w/ separate HVAC systems Section 1 vacant Section 2 occupied Site Building
Site Conditions Average depth to water = 14 ft Soil type = silty clay TCE GW concentration = 10-270 ppb - Stable over last 5 years
Technical Approach J & E MODELING SITE SCREENING BASELINE SAMPLING HVAC REPAIR POST-HVAC REPAIR SAMPLING MODEL VERIFICATION
STUDY METHODS
STUDY METHODS Johnson & Ettinger (J&E) Modeling J&E Modeling to predict probable-case (PC) & worst-case (WC) conditions Both model runs used site-specific data when available Most conservative J&E practical range values used to represent WC site conditions Default values used to represent PC site conditions
STUDY METHODS J&E Modeling Key Assumptions INPUT PARAMETER INITIAL GW CONCENTRATION (ug/l) SOIL TYPE SOIL TOTAL POROSITY BUILDING PRESSURE DIFFERENTIAL (g/cm-s2) INDOOR EXCHANGE RATE (1/h) PC 147 SIC 0.43 40 1.2 WC 270 SI 0.53 200 0.18
STUDY METHODS Field Activities BASELINE SAMPLING POST HVAC REPAIR SAMPLING DATE ACTIVITY APRIL 2003 SITE SCREENING MAY 6, 2003 SAMPLING EVENT 1 MAY 13, 2003 SAMPLING EVENT 2 JUNE 2003 DATA EVALUATION JULY-AUG 2003 HVAC REPAIR/TESTING SEPT 4, 2003 SAMPLING EVENT 3 SEPT 11, 2003 SAMPLING EVENT 4 NOV 2003 DATA EVALUATION DEC 23, 2003 SAMPLING EVENT 5
STUDY METHODS Site Screening Review HVAC & building construction drawings Search for outdoor TCE emission sources within 1 mi. Indoor air quality survey PID screening of potential sampling locations NO INDOORS OR OUTDOOR TCE SOURCES OR PID DETECTIONS
STUDY METHODS Air Sampling & Analysis Methods Sample duration = 12-hour integrated 6-L pre-cleaned Summa canisters Analyze for GW chemicals of concern (e.g. TCE) Analyze using EPA Method TO-15 Select Ion Monitoring (SIM)
STUDY METHODS Air Sample Setup
STUDY METHODS Air Sampling Schematic REPRESENTATIVE SAMPLE PATHWAY SAMPLE
STUDY METHODS Air Sampling Locations LEGEND SECT 2 TCE Iso-conc. contour (ppb) Indoor breathing zone sample SECT 1 Indoor pathway sample Outdoor HVAC intake sample
STUDY METHODS Pathway Samples 1/4 TEFLON TUBE 1/4 TEFLON TUBE FLOOR DRAIN FLOOR CRACK BUILDING SECTION 1 BUILDING SECTION 2
STUDY METHODS Representative Samples FLOOR CRACK BUILDING SECTION 1 BUILDING SECTION 2
STUDY METHODS Building Sect 2 HVAC Repairs HVAC REPAIR Replace parts, fluids, filters Conduct pressure/flow testing Add outdoor air intake to one HVAC unit Conduct air balancing AIR EXCHANGE RATE AFTER REPAIRS = 1.2 ACH INDOOR-TO-OUTDOOR PRESSURE DIFFERENCE= 0.01 H 2 O
RESULTS
RESULTS Building Section 1 Samples TCE Concentration (ug/m3) 6 5 4 3 2 1 0 5.63 0.01 0.19 0.18 J 0.16 J 0.35 0.37 0.5 0.210.15 J <0.19 <0.19 <0.19 J&E PC J&E WC RS RS RS RS RS RS RS RS RS PS PS J&E WC ALL REPRESENTATIVE & PATHWAY SAMPLES
RESULTS Building Section 2 Samples (Pre-HVAC Repair) TCE Concentration (ug/m3) 60 50 40 30 20 10 0 5.63 0.01 0.9 0.84 1.4 0.61 0.9 0.53 J&E PC J&E WC RS RS RS RS RS RS J&E WC 17 PS 49 PS PATHWAY SAMPLES
RESULTS Section 2 Samples (Post-HVAC Repair) 6 5.63 TCE Concentration (ug/m3) 5 4 3 2 1 0 0.01 J&E PC 0.62 0.3 <0.2 <0.19 0.27 0.23 <0.19 <0.19 <0.19 <0.19 J&E WC RS RS RS RS RS RS RS RS PS PS J&E WC ALL REPRESENTATIVE & PATHWAY SAMPLES
DISCUSSION
DISCUSSION Draft EPA Guidance SITE DEFAULT FACTOR FLOOR CRACK
DISCUSSION Representative Sample Vapor Attenuation Factors 1.0E-02 Building Section 1 Building Section 2 Vapor Attenuation Factor 1.0E-03 1.0E-04 1.0E-05 1.0E-06 EPA AF J&E Worst Case 1.0E-07 J&E Probable Case
DISCUSSION Pathway Sample Vapor Attenuation Factors 1.0E-02 Building Section 1 Building Section 2 Vapor Attenuation Factor 1.0E-03 1.0E-04 1.0E-05 1.0E-06 EPA AF HVAC Malfunctioning J&E Worst Case 1.0E-07 J&E Probable Case
DISCUSSION Ventilation & Attenuation Malfunctioning HVAC = elevated TCE air concentrations / less attenuation Normal HVAC operation - Conc./ attenuation within the J&E predicted range - Below EPA default
DISCUSSION Pathway Sample Attenuation 0.90 µg/m 3 EPA Sample @ 1 ft ags=1 µg/m 3 3.5 ft 49 µg/m 3 REPRESENTATIVE SAMPLE PATHWAY SAMPLE
Conclusions During normal HVAC operation: EPA default attenuation factor is conservative; and, measured attenuation factors are within modeled predictions.
Conclusions Ventilation & Building Pressurization effectively minimize vapor intrusion J&E Modeling is a reasonable method for initially predicting indoor air concentrations at commercial facilities
QUESTIONS?