1. Gradient CORPORATION Vapor Intrusion Attenuation Factors (AFs) – Measured vs. EPA Defaults A Case Study Presented by Manu Sharma and Jennifer DeAscentis Presented at EPA Vapor Intrusion Workshop Amherst, Massachusetts October 18, 2004

2. Gradient CORPORATION Outline  Site Setting  Environmental Data  Measured and Default AFs  Role of Background  Modifications to Default AF

3. Gradient CORPORATION Site Setting  Active equipment testing facility located in the northeast US  Urban setting – highway; other industrial/ commercial operations nearby  Long history of industrial activities – low volumes of chlorinated organics and hydrocarbon usage  Glacial outwash deposits – relatively high hydraulic conductivity  Depth to groundwater ~ 6 meters

4. Gradient CORPORATION Outline  Site Setting  Environmental Data  Measured and Default AFs  Role of Background  Modifications to Default AF

5. Gradient CORPORATION Sampling Locations

6. Gradient CORPORATION Environmental Data  Sources and groundwater plume reasonably defined  Low VOC concentrations present in groundwater – order of 1 to 100 ug/l  Indoor air sampling locations Known sources of VOCs (e.g., historical solvent usage areas) Small and large buildings with different ventilation characteristics

7. Gradient CORPORATION Indoor Air and Groundwater Data

8. Gradient CORPORATION Outline  Site Setting  Environmental Data  Measured and Default AFs  Role of Background  Modifications to Default AF

9. Gradient CORPORATION AF Calculation  Measured AFs Range: average air concentration and range of measured groundwater concentrations at associated wells Recommended Estimate: average air concentration and average groundwater concentration (temporal and spatial) at associated wells  Default AF from Figure 3b of EPA guidance Depth to contamination = 4 m Sandy soils

10. Gradient CORPORATION Measured vs. EPA Default AFs

11. Gradient CORPORATION Measured vs. EPA Default AFs (Cont’d)  Measured AF about an order of magnitude less than default AF (10 -4 vs. 10 -3 )  True AF may be even lower because: Air sampling locations near sources – air concentrations biased high Monitoring wells located downgradient of sources – groundwater concentrations biased low  No trend in measured AFs vs. building size or ventilation noted  Background may be masking indoor air concentrations attributable to subsurface contamination

12. Gradient CORPORATION Outline  Site Setting  Environmental Data  Measured and Default AFs  Role of Background  Modifications to Default AF

13. Gradient CORPORATION Role of Background Range of Measured Site Indoor Air Concentrations versus Range of Literature Background Concentrations Background concentrations from McHugh et al., 2004. Site indoor air concentration range Background indoor air concentration range

14. Gradient CORPORATION Role of Background  Measured indoor air concentrations at the Site consistent with literature background  Contributions from subsurface contamination vs. background at the Site cannot be conclusively apportioned Site-specific background data needed  Background indoor and outdoor air concentrations should be characterized to place risks in proper perspective  Challenges in background definition Variability in source and characteristics

15. Gradient CORPORATION Outline  Site Setting  Environmental Data  Measured and Default AFs  Role of Background  Modifications to Default AF

16. Gradient CORPORATION Modifications to Default AF  Building Area Current value (100 m 2 ) only applicable for small residential buildings Need to develop AF graphs for other building sizes (medium, large) to expand applicability  Air Exchange Rate (AER) Current value (0.25 hr -1 ) also applicable at small residential buildings Users should have flexibility to use higher AERs – commercial buildings, warmer setting