case study of subsurface vapor intrusion at a dry … · case study of subsurface vapor intrusion...

Case Study of Subsurface Vapor Intrusion

at a Dry Cleaner Site

Amy Goldberg Day [email protected]

AEHS 14th Annual West Coast Conference on Soils, Sediments and Water

March 2004

Eric M. Nichols, [email protected]

USEPA/AEHS Vapor Attenuation Workshop - March 2004 - San Diego

Outline

• Background• Conceptual Site Model• Data Collection:

– Groundwater– Soil gas– Indoor air

• Vapor Intrusion Modeling Results• Comparison of Attenuation Factors• Observations and Conclusions


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

drinking water well• Dry cleaners implicated and ordered to

perform RI/FS type investigation


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


Background, Continued

• Existing buildings slab-on-grade• Some buildings had historical use of

PCE• All buildings have commercial use• Subject building 6000 ft2• Expected transport mechanisms:

– Diffusion from source zones– Advection and diffusion across foundation


Former Dry CleanerSewer LineSubject Building

• Source Area


Groundwater Data Summary

• 3 yrs of quarterly monitoring from 18 A-zone wells

• Analyzed using EPA Method 8260A• Source-area PCE detected in 13

of 13 samples:– 5,000 to 85,000 µg/l– 95% UCL 48,300 µg/l

• Non-source-area PCE detected in 118 of 124 samples:– 1.5 to 12,000 µg/l– 95% UCL 1,800 µg/l


Soil Gas Data Summary• Soil gas samples collected from

March 1997 through June 1998• Used “syringe” collection technique• Analyzed via on-site mobile lab using

EPA Method 8010 with Hall detector• 378 samples collected from 0 to 10 feet bgs• 77 source-area PCE samples:

– maximum detected 39,490 µg/l• 304 non-source area PCE samples:

– 1.0 to 9,060 µg/l– 95% UCL 605 µg/l


Flux Chamber Data Summary

• 13 indoor sample locations on observed floor seams and cracks

• 4 outdoor locations in planter boxes• Collection rate 1 L/min in 6-L evacuated

canisters• TO-14 SIM• PCE detected in all indoor

samples • Flux range 0.29 to 26 µg/min-ft


Air Data Summary• Indoor air samples collected in 6

buildings, 1 located over source area, 3 outdoor samples

• 15 samples collected over source area in 5 separate sampling events over 14 months

• 1 sample collected in each of the other buildings

• Samples collected in evacuated canisters over a 24-hour or 8 hour period


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


Air Data Summary, Continued• Method TO-14 SIM• PCE detected in all source-area indoor air

samples:– 0.150 to 0.380 µg/l– 95%UCL 0.260 µg/l

• PCE detected in all non-source area air samples:– 0.006 to 0.035 µg/l– 95%UCL0.029 µg/l

• PCE slightly analytical reporting limits in 2 outdoor samples (0.0014 & 0.0093 µg/l)


Vapor Intrusion Modeling Considerations

• Considered commercial setting• Estimated indoor air PCE

concentrations using Johnson & Ettinger model – from groundwater and soil gas data – with site-specific soil and building

parameters


Results of VI Modeling from Groundwater

• For J&E model, used measured building width, length, and volume

• Used ASTM default air exchange rate, foundation thickness, fraction open area in building foundation/walls

• Used 90th percentile of measured intrinsic permeability [8.6x10-12 m2]

• Estimated indoor air PCE concentration:– 0.181 µg/l over source area– 0.0068 µg/l over non-source area


Results of VI Modeling from Soil Gas

• Used same parameters as groundwater-source model

• Estimated indoor air PCE concentration:– 0.847 µg/l over source area – 0.020 µg/l over non-source area


Results of VI Modeling from Crack Flux Data

• Applied mean flux from 13 ft of sampled cracks to 425 ft of un-sampled cracks

• 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 0.00014 µg/l


Results Comparison (PCE in µg/l)

0.2600.84725,485S-SGd0.2600.000146.95

(avg. flux in µg/min/ft)CK-IA

0.0290.020605NS-SGd0.2600.18148,000S-GW0.0290.00681,832NS-GW

Measured Indoor Air

Modeled Indoor

Air

Source Media(95% UCL)

Data

Bold indicates higher value


Attenuation Factor Comparison

1.0x10-21.0x10-53.3x10-5S-SGd1.0x10-24.8x10-53.3x10-5NS-SGd1.0x10-35.4x10-63.8x10-6S-GW1.0x10-31.6x10-53.7x10-6NS-GW

Draft OSWER Default

Measured Indoor Air

Estimated Indoor Air

Data

Crack flux data not useful for estimating attenuation factor


Observations

• EPA Attenuation Factors 100 to 1000 times more conservative than empirical data

• Observed measured attenuation factors ranged from 1x10-5 to 1x10-6

• Johnson & Ettinger model with site-specific parameters reasonable predictor of indoor air concentrations and attenuation factors


Observations, Continued

• Flux chamber data was least accurate predictor of indoor air concentrations

• Sealing floor cracks and seams did not significantly reduce indoor air concentrations or apparent attenuation factor

• 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


Conclusions

• Reduction following SVE confirms origin of impact was from subsurface

• EPA generic attenuation factors are significantly more conservative than attenuation factors measured at this site

• Measured groundwater-indoor air or soil gas-indoor air attenuation factors were within one order of magnitude of modeled attenuation factors


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 likely influenced differences between measured and generic attenuation factors

case study of subsurface vapor intrusion at a dry … · case study of subsurface vapor intrusion...

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