SOUTHWEST DIVISION Comparing Air Measurements and Modeling Results at a Residential Site Overlying a TCE Plume October 18, 2004 Presented by: Wilson Doctor, Remedial Project Manager Ronald J. Marnicio, Ph.D., P.E., TetraTech FW, Inc.
Acknowledgements Base Realignment and Closure Program Management Office West 1230 Columbia Street, Suite 1100 San Diego, California 92101 US Environmental Protection Agency Region 9 Co-Authors Dennis Goldman, Ph.D., RG Larry Dudus, RG 2 08/23/04 BCT
Presentation Outline Site Background Hydrogeologic Setting Original Vapor Migration and Intrusion Modeling Modeled and Measured Attenuation Coefficients Uncertainties Conclusions 3 08/23/04 BCT
Site Location Map 4 08/23/04 BCT
Site Background MCH part of the former NAS Moffett Field Orion Park Housing Area (OPHA) : 72 acres Previously agriculture or vacant land Housing construction from 1941-1982 Wescoat Housing Area (WHA) : 40 acres Previously vacant land Housing construction from 1933-1982 All but 1933 era housing demolished in 2004 Construction: slab-on-grade and crawl space 5 08/23/04 BCT
MCH Housing Area 6 08/23/04 BCT
Initial Investigations TCE detected in shallow groundwater in 1999/2000 at ~300 µg/l Investigated extent in 2002 taking 62 shallow groundwater samples Investigated vapor migration / intrusion potential in 2002 at 7 housing units Collected unit-specific data for modeling Collected indoor and outdoor air samples 7 08/23/04 BCT
Hydrogeologic Setting Coarse-grained sand and gravel channel deposits, incised in fine-grained floodplain and tidal deposits to 65 ft bgs Continuity of individual sand and and gravel layers limited Upper 3 to 5 layers comprise the A1 aquifer zone Upper 10 to 15 feet is unsaturated silts and clays First water encountered at depth ~15 ft bgs, confined 8 08/23/04 BCT
Simplified Geologic Cross Section 9 08/23/04 BCT
TCE Groundwater Contamination TCE Concentration Contours in the A1 Aquifer Zone 10 08/23/04 BCT
MCH VOC Plume 11 08/23/04 BCT
Regional Plume Map 12 08/23/04 BCT
Units Selected for Investigation Most locations over shallow groundwater plume Vacant, ground-floor units Building construction slab-on-grade and crawlspace 13 08/23/04 BCT
Building Construction 14 08/23/04 BCT
Original Vapor Intrusion Modeling J&E to model TCE attenuation coefficients at 7 units using 2000 guidance Site-specific parameters included: Groundwater Soil TCE Concentration Type Classification Soil/Groundwater Temperature Layer Thickness Depth to Water Table Dry Bulk Density Enclosed Space Total Porosity Floor/Foundation thickness Water-Filled Porosity Floor Length % Organic Carbon Floor Width/Height 15 08/23/04 BCT
Original Vapor Intrusion Modeling Recommended default parameters used were: physical- and chemical-specific properties linked to soil type soil-building pressure differential floor-wall seam crack width indoor air exchange rate 16 08/23/04 BCT
Additional Air Investigations Additional air samples collected in 11/03 and 5/04 to compare with original measurements and J&E model results 25 units, including original 7 units 24-hour air samples with Summa canisters Indoor air Breathing Zone Preferential Pathways Outdoor air (near indoor locations) TO-15 SIM, TCE MDL < 0.017 ug/m 3 2 sampling rounds per each event 17 08/23/04 BCT
Air Sampling Locations 11/2003 and 5/2004 18 08/23/04 BCT
Indoor Air TCE (ug/m 3 ) Location 18-Nov-03 25-Nov-03 07-May-04 14-May-04 619B 0.1 0.099 Not sampled Not sampled 620F 1 2 Not sampled Not sampled 703G 0.092 0.012 0.055 0.067 705F 0.07 0.19 0.054 0.071 720D 0.14 0.24 0.041 0.04 808A 0.18 0.11 0.036 0.058 819D 0.1 0.11 0.084 0.046 19 08/23/04 BCT
Comparison of Indoor and Outdoor TCE Concentrations 0.50 11/18/03 11/25/03 05/07/04 05/14/04 0.40 Outdoor Indoor TCE (ug/m 3 ) 0.30 0.20 0.10 0.00 Rank Order Distribution of Indoor-Outdoor Air Measurement Pairings 20 08/23/04 BCT
Comparison of Model Runs Original J&E run vs. 2002 Draft Subsurface Vapor Intrusion Guidance Updated 2002 defaults, full set of locationspecific inputs (Run A) Updated 2002 defaults, location-specific depth to water and soil parameters (Run B) Updated 2002 defaults, only locationspecific depth to water, slab-on-grade 21 08/23/04 BCT
Vapor Intrusion Modeling: Summary of Results TCE CONCENTRATIONS ATTENUATION COEFFICIENTS (unitless) Location Groundwater (µg/l) Source Vapor (µg/m3) Average Measured Indoor Air (µg/m3) Measured Original Run Run A [New Defaults] Run B [Min Location Specific Info] 2002 Vapor Intrusion Guidance (Slab-on- Grade) 619B 0.25 82 0.14 1.71E-03 3.45E-07 1.24E-06 1.88E-05 1.16E-05 620F 260 85,000 2.03 2.38E-05 3.02E-07 1.09E-06 1.88E-05 1.16E-05 703G 80 28,720 0.11 3.83E-06 3.45E-07 1.24E-06 1.88E-05 1.16E-05 705F 0.25 86 0.12 1.39E-03 5.87E-07 2.11E-06 1.89E-05 1.16E-05 720D 0.25 86 0.14 1.63E-03 2.80E-05 1.01E-04 1.88E-05 1.16E-05 808A 0.25 85 0.15 1.76E-03 1.08E-05 6.49E-06 1.97E-05 Crawl Space 819D 250 78,750 0.11 1.40E-06 3.61E-03 1.94E-05 7.67E-04 Crawl Space 22 08/23/04 BCT
Comparison of Modeled Attenuation Coefficients Run A ACs Comparisons: Order of magnitude larger than original J&E run (more TCE intrusion) Reflects greater net soil vapor flux and lower default building air exchange rate Order of magnitude smaller than 2002 Draft Guidance runs (less TCE intrusion) Reflects more soil vapor flow and less dilution in the space 23 08/23/04 BCT
Comparison of Modeled Attenuation Coefficients Run B ACs Comparisons: 2 orders of magnitude larger than original J&E run (more TCE intrusion) Reflects the effect of location-specific building parameters Similar to 2002 Draft Guidance runs 24 08/23/04 BCT
Measured Attenuation Coefficients Measured ACs Estimates: Source Vapor Concentration (SVC) (at soil/groundwater interface) calculated using unit-specific groundwater TCE concentration and temperature Used TCE groundwater MDL when no detection Measured indoor breathing zone TCE concentration divided by the SVC Assumes indoor air concentration due to vapor migration from groundwater 25 08/23/04 BCT
Uncertainties TCE detected in all air samples at similar concentrations (above-plume and off-plume) No significant vapor migration into indoor air from groundwater through preferential pathways TCE may be due to an urban society, regional TCE groundwater plumes, home products containing TCE, and/or construction materials 26 08/23/04 BCT
Uncertainties (cont d) Measured ACs may be related to ubiquitous nature of site TCE and not representative of vapor migration from underlying groundwater Comparisons of the measured ACs to modeled ACs may not be meaningful 27 08/23/04 BCT
Conclusions Vapor migration to indoor air pathway not dominant at MCH Explored relative impact of changing defaults (Run A) Explored relative impact of location-specific soil and building parameters (Run B and Guidance ) Illustrated limitations of using indoor air and groundwater measurement to AC construction 28 08/23/04 BCT
Conclusions (cont d) Comparison of measured and modeled results at MCH may not be meaningful Measurements and modeling important to drawing conclusions about the vapor intrusion pathways at a site Importance of gathering location-specific data to support modeling when site conditions differ from defaults 29 08/23/04 BCT