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Vapor Intrusion Study at Dry Cleaner Site: Case Analysis 2004

This case study investigates subsurface vapor intrusion at a dry cleaner site in Central California. The study delves into the conceptual site model, groundwater and soil gas data collection, indoor air analysis, and the comparison of attenuation factors. By examining the variance from EPA default factors, the study offers insights and observations essential for environmental assessments. The research highlights the influence of various factors on vapor intrusion pathways and the efficacy of mitigation measures. The comprehensive analysis provides valuable data for understanding vapor intrusion risks and formulating effective remediation strategies.

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Vapor Intrusion Study at Dry Cleaner Site: Case Analysis 2004

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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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