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Case Study of Subsurface Vapor Intrusion at a Dry Cleaner Site

Case Study of Subsurface Vapor Intrusion at a Dry Cleaner Site. Amy Goldberg Day Amy.Goldberg.Day@lfr.com. Eric M. Nichols, PE Eric.Nichols@lfr.com. AEHS Annual East Coast Conference on Soils, Sediments and Water October 2004. Outline. Background Conceptual Site Model Data Collection:

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Case Study of Subsurface Vapor Intrusion at a Dry Cleaner Site

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  1. Case Study of Subsurface Vapor Intrusion at a Dry Cleaner Site Amy Goldberg Day Amy.Goldberg.Day@lfr.com Eric M. Nichols, PEEric.Nichols@lfr.com AEHS Annual East Coast Conference on Soils, Sediments and Water October 2004

  2. Outline • Background • Conceptual Site Model • Data Collection: • Groundwater • Soil gas • Indoor air • Comparison of Attenuation Factors • Variance from EPA Default Attenuation Factors • Observations and Conclusions

  3. Background • Shopping center in Central California with 3 dry cleaners • Routine disposal of dry cleaning fluids into sanitary sewer • Sewer line leaks resulted in PCE releases • PCE identified in downgradient municipal water well • Dry cleaners implicated and ordered to perform RI/FS type investigation

  4. Background, Continued • Interbedded fine-grained sediments to ~25 ft bgs • Discontinuous coarse-grained sediments from ~25 to 50 feet bgs • Depth to groundwater ~50 feet bgs • Human health risk assessment performed using applicable data considering source and non-source areas

  5. Background, Continued • Existing buildings slab-on-grade • Some buildings had historical use of PCE • All buildings have commercial use • Expected transport mechanisms: • Diffusion from source zones • Advection and diffusion across foundation

  6. Source Area Former Dry Cleaner Sewer Line Subject Building

  7. Groundwater Data Summary • 3 yrs of quarterly monitoring from 18 A-zone wells-EPA (Level IV Data Validation) • Analyzed using EPA Method 8260A • Source-area PCE detected in 13 of 13 samples: • 5,000 to 85,000 g/l • 95% UCL: 48,300 g/l • Non-source-area PCE detected in 118 of 124 samples: • 1.5 to 12,000 g/l • 95% UCL: 1,800 g/l

  8. Soil Gas Data Summary • Soil gas samples collected from March 1997 through June 1998 • Analyzed via on-site mobile lab using EPA Method 8010 (Level III DV) • 381 samples collected from 0 to 10 feet bgs • 77 source-area PCE samples: • maximum detected 39,490,000 g/m3 • 95% UCL: 25,485,000 g/m3 • 304 non-source area PCE samples: • 100 to 9,060,000 g/m3 • 95% UCL: 605,000 g/m3

  9. Flux Chamber Data Summary • 13 indoor sample locations on observed floor seams and cracks • 4 outdoor locations in planter boxes • TO-14 SIM • PCE detected in all indoor samples • Flux range: 0.29 to 26 g/min-ft

  10. Air Data Summary • Indoor air samples collected in 6 buildings, 1 located close to source area; 3 outdoor sample locations • 15 samples collected over source area in 5 separate sampling events over 14 months • 1 sample collected in each of the other buildings • Level III Data Validation

  11. Air Data Summary • Subject building vacant duringfirst air sampling event • Doors closed; HVAC on • Cracks and seams were sealed before third sampling event • Similar results • Building was reoccupied and floor covering added before fourth sampling event • Fourth and fifth sampling events were during normal business hours, with doors opening and closing throughout day

  12. Vapor Intrusion Modeling • Estimated indoor air concentration using Johnson & Ettinger model with site-specific soil and building parameters • Used J&E for both soil gas and groundwater results (95% UCLs) • Compared estimated indoor air concentration to measured indoor air concentration

  13. Results of VI Modeling from Crack Flux Data • Assumes cracks are only significant route of vapor entry (BIG assumption!) • Applied box mixing model with building volume and air exchange rate • Estimated indoor PCE concentration: 14 g/m3

  14. Results Comparisonsoil gas and air in g/m3 groundwater in g/l Bold indicates higher value

  15. Attenuation Factors • Following the guidance in Appendix F •  = [indoor air]/[soil gas] (used direct measured and J&E estimated indoor air concentrations) •  = [indoor air]/[groundwater]*Hc (used direct measured and J&E estimated indoor air concentrations)

  16. Attenuation Factor Comparison 1.2 x10-5 2.8x10-6 4.0 x10-3 2.8x10-6 4.0 x10-3 4.0 x10-6 Crack flux data not useful for estimating attenuation factor

  17. Figure 3 Vapor Attenuation Factors Groundwater to Indoor Air (Sandy Loam)

  18. Figure 3 Vapor Attenuation Factors Soil Gas to Indoor Air (Sandy Loam)

  19. Observations • Estimated attenuation factors ranged from 1x10-5 to 4x10-6 • Figure 3 attenuation factors range from 2x10-3 to 4x10-3 • Johnson & Ettinger model with site-specific parameters was reasonable predictor of indoor air concentrations and attenuation factors using soil gas data

  20. Observations, Continued • Sealing floor cracks and seams did not significantly reduce indoor air concentrations or apparent attenuation factor • Flux chamber data was least accurate predictor of indoor air concentrations (possibly used incorrect assumption) • HVAC on or off did not significantly reduce indoor air concentrations or apparent attenuation factor • Installation and operation of SVE system reduced measured indoor air concentrations to below reporting limits

  21. Observations, Continued • EPA Figure 3 attenuation factors are significantly more conservative than attenuation factors estimated at this site • Indoor air concentrations likely not influenced by background concentrations • Other cases with very high PCE soil gas concentrations had ’s in the 10-5 range

  22. Attenuation VariancePossible Reasons for Variance from EPA Figure 3 • Complex geologic subsurface conditions – shallow fine-grained material may have restricted vapor intrusion • Sampling biased towards areas of higher concentrations – possible biases in data set

  23. Attenuation VariancePossible Reasons for Variance from EPA Figure 3 • Highest detected concentrations of PCE in both soil gas and groundwater were in the parking lot--- no indoor air samples were collected directly over this “hottest” area • Extremely high source media concentrations Sub-slab soil gas data could have resolved some of these issues

  24. Conclusions • Reduction following SVE confirms origin of impact was from subsurface • Measured groundwater-indoor air or soil gas-indoor air attenuation factors were within one order of magnitude of modeled attenuation factors

  25. Conclusions • For this well-characterized site, use of soil gas or groundwater data were appropriate to predict attenuation factors • Site-specific subsurface and building conditions and extremely high source concentrations likely influenced differences between measured and EPA Figure 3 attenuation factors

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