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Case Study of Subsurface Vapor Intrusion at a Dry Cleaner Site Amy Goldberg Day AEHS Annual East Coast Conference on Soils, Sediments.

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Presentation on theme: "Case Study of Subsurface Vapor Intrusion at a Dry Cleaner Site Amy Goldberg Day AEHS Annual East Coast Conference on Soils, Sediments."— Presentation transcript:

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

2 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst Former Dry Cleaner Sewer Line Subject Building Source Area

7 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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/m 3 –95% UCL: 25,485,000  g/m non-source area PCE samples: –100 to 9,060,000  g/m 3 –95% UCL: 605,000  g/m 3

9 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst Air Data Summary Subject building vacant during first 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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/m 3

14 USEPA/AEHS Vapor Attenuation Workshop - October Amherst Results Comparison soil gas and air in  g/m 3 groundwater in  g/l DataSource Media (95% UCL) Modeled Indoor Air Measured Indoor Air (95% UCL) NS-GW1, S-GW48, NS-SGd605, S-SGd25,485, CK-IA6.95 (avg. flux in µg/min/ft) Bold indicates higher value

15 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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]*H c (used direct measured and J&E estimated indoor air concentrations)

16 USEPA/AEHS Vapor Attenuation Workshop - October Amherst Attenuation Factor Comparison DataEstimated Indoor Air Measured Indoor Air Figure 3 Values for Sandy Loam NS-GW S-GW NS-SGd3.3 x x x10 -3 S-SGd3.3 x x x10 -3 Crack flux data not useful for estimating attenuation factor 4.0 x x x x10 -6

17 USEPA/AEHS Vapor Attenuation Workshop - October Amherst Figure 3 Vapor Attenuation Factors Groundwater to Indoor Air (Sandy Loam)

18 USEPA/AEHS Vapor Attenuation Workshop - October Amherst Figure 3 Vapor Attenuation Factors Soil Gas to Indoor Air (Sandy Loam)

19 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 range

22 USEPA/AEHS Vapor Attenuation Workshop - October Amherst Attenuation Variance Possible 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst Attenuation Variance Possible 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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 USEPA/AEHS Vapor Attenuation Workshop - October Amherst 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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