Discerning Background Sources from Vapor Intrusion

1. Discerning Background Sources from Vapor Intrusion Jeffrey Kurtz, Ph.D. and David Folkes, PE EnviroGroup Limited Denver Boston Albuquerque Seattle Colorado Bar Association – October 26, 2005

2. Proliferation of Vapor Intrusion Guidance • At least 14 states, as well as the EPA, have developed Vapor Intrusion Guidance in the past few years. • These documents vary widely in their approach to, and discussion of, background indoor air.

3. New Risk Levels for 1,1-DCE and TCE • Recently the EPA, and several states, have implemented new risk levels for 1,1-DCE and TCE. • TCE is now the risk driver at many sites. • TCE is a trace to major component of many common consumer products.

4. Consumer Product Examples • correction fluids • paints & varnishes & removers • glues, adhesives and sealants • spot removers & laundry aids • rug cleaning fluids • metal cleaners • lubricants • pesticides

5. Approach for Separating Indoor and Subsurface Sources • Experience at several large sites led to “lines of evidence” approach for separating indoor & subsurface sources. • Accepted by CDPHE and applied at several Colorado sites.

6. Application Lines of Evidence approach can be used to: • Identify false positives • Limit unnecessary mitigation • Limit indoor air sampling • Identify indoor air COCs • Limit continued sampling & mitigation

7. VOC Ratio Method • Principal line of evidence relies on basic chemical properties of the chlorinated volatile organic compounds (VOCs). • This line of evidence requires at least 2 chlorinated VOCs in the subsurface.

8. VOC Chemical Properties • Relative volatility (expressed as Henry’s Law Constant). Factor of 50 range for common chlorinated VOCs. • Relative soil sorption (Koc) – similar for most chlorinated VOCs. • Relative degradability – similar for most chlorinated VOCs.

9. Groundwater Sources • Calculate VOC ratios • Evaluate ratio trends over time • Evaluate spatial variation of ratios • Adjust for relative volatility of VOCs (Henry’s Law Constants) • Map predicted soil vapor ratios • Predict indoor air ratios and TCE concentrations

11. Soil Vapor Sources • Calculate VOC ratios • Evaluate ratio trends over time • Evaluate spatial variation of ratios • Map measured soil vapor ratios • Predict indoor air ratios and TCE concentrations • High quality, reproducible soil vapor data essential

12. TCE (VI) = 0.5 x DCE TCE 80 ug/m3 DCE 20 ug/m3 TCE (BG) = TCE (OBS) – TCE (VI) TCE 1000 ug/m3 DCE 2000 ug/m3 TCE = 0.5 x DCE COC Ratios (Soil Gas)

13. Ideal Case • Denominator is a VOC with no, or very low, indoor air background (e.g. 1,1-DCE). • Indoor air concentration of denominator VOC is direct measure of vapor intrusion. • Ratio directly predicts vapor intrusion concentration of other COCs.

14. Typical Case • Use a VOC with the lowest indoor air background as the denominator in the ratio (e.g. TCE). • Indoor air concentration of the denominator VOC is an upper limit measure of vapor intrusion. • Can estimate predicted upper limit vapor intrusion concentration of other COCs from the ratio.

15. Case Study • A Colorado site with a large chlorinated solvent groundwater plume. • Groundwater COCs are TCE; 1,1-DCE; PCE and 1,1,1-TCA. • Hundreds of single family residences overlying the plume. • Documented vapor intrusion based on indoor air 1,1-DCE.

16. Case Study • Change in 1,1-DCE and TCE action levels required re-evaluation of indoor air data. • Decision needed on new extent of vapor intrusion exceeding action levels. • Indoor air background TCE caused numerous “false” exceedances of action level.

17. Case Study • Background varies on a “house-by-house” basis. • Statistics from homes outside plume and from post-mitigation indicate 15% of homes in area would exceed action level due to background. • Household chemical surveys generally fail to identify all indoor sources.

18. Case Study • Groundwater COCs present in relatively consistent proportions spatially. • Little variation (or predictable trend) over time in TCE/DCE in groundwater. • Adjust groundwater ratios for relative volatility (Henry’s Law Constants). • Predict soil vapor TCE/DCE ratio.

19. Predicted TCE/DCE Ratio in Soil Vapor Estimated DCE Plume Boundary (7 ug/L) Interpolation Boundary TCE/DCE > 0.5 (Henry’s Law Corrected) TCE/DCE 0.4 – 0.5 TCE/DCE 0.3 – 0.4 TCE/DCE 0.2 – 0.3 TCE/DCE 0.1 – 0.2 TCE/DCE 0.01 – 0.1

20. Case Study • Map predicted soil vapor TCE/DCE ratio. • Compare to measured indoor air TCE/DCE ratio. • Generally excellent agreement, with some prominent exceptions due to background. • Edge of groundwater plume clearly marked.

21. TCE / DCE Ratios in Pre Mitigation and Unmitigated Indoor Air Estimated TCE 5 µg/L Contour in Groundwater 0.01 – 0.3 TCE NOT DETECTED / DCE DETECTED 0.31 – 1.0 TCE DETECTED/ DCE NOT DETECTED 1.01 – 5.0 > 5.0 TCE AND DCE NOT DETECTED

22. Spatial Patterns • General correlation with plume • Absolute concentrations within plume can be more variable and hard to correlate • Indoor air COC ratios often indicate anomalies

23. Spatial Patterns (IA Ratios)

24. TCE Source Attribution from Multi-media Ratio Comparison Predominately Vapor Intrusion Derived TCE Predominately Indoor Source (background) TCE Estimated TCE 5 µg/L Contour in Groundwater

25. DCE > 7 ug/L DCE > 0.49 ug/m3 Correlation with GW Plume

26. Case Study Results • TCE/DCE ratio pattern distinctly marks edge of vapor intrusion – limits indoor air sampling to define “extent”. • TCE/DCE ratio for indoor air compared to groundwater clearly shows locations with “anomalously high” background TCE. • TCE/DCE ratio allows direct determination of maximum potential vapor intrusion derived TCE.

27. Implications • COC ratios for chlorinated VOCs can provide an accurate method to separate background from vapor intrusion. • Can use ratios from groundwater, soil vapor, or mitigation system emissions. • Useful when at least two chlorinated VOCs are present in the subsurface source.

28. Line of EvidenceSecondary Factors • Building survey • Indoor air background databases

29. Building Survey • Identify potential background sources • Household products • Resident activities • Options • Delay testing • Remove and test • Residual impacts?

30. Residual Background Impacts

31. Residual Background Impacts

32. Background Databases • Compare IA concentrations to “typical” levels in published surveys • Concentrations within typical ranges may support other background LOE’s

34. Databases Issues • Data sparse for many compounds • Comparability issues • Building type and use differences • Regional differences • Time period differences • Extremes often removed from databases

35. Site-specific background • Control and/or mitigated buildings may not be available • Background study may be impractical, especially for small sites • Large number of samples required to achieve required statistical confidence

36. Other Lines of Evidence • Radon system emission levels/ratios • Temporal patterns

37. Radon System Emission Ratios

38. Temporal Patterns • Requires indoor air tests over time • Post versus pre-mitigation concentrations • Change in resident • Correlation with activities

39. Pre vs Post Mitigation

40. Impact of Resident 33

41. Impact of Resident 34

42. Summary • Several lines of evidence may be needed to separate background from vapor intrusion sources of indoor air VOCs • Comparison of COC ratios in various media is often the most compelling LOE • If available, temporal and spatial patterns are also useful lines of evidence

43. Conclusions • VOC ratios can provide more definitive answers than assumed sub-slab to indoor air attenuation factors or the JE Model. • VOC ratios can discriminate background on a “house-by-house” basis. • VOC ratios can prevent the need for “background sampling”.

44. Information Resources • www.envirogroup.com • Vapor Intrusion Newsletter signup • Links by state and by topic • jkurtz@envirogroup.com • Questions