case study of subsurface vapor intrusion at a dry cleaner site amy goldberg day...

Case Study of Subsurface Vapor Intrusion at a

Dry Cleaner Site

Amy Goldberg Day [email protected]

AEHS Annual East Coast Conference on Soils, Sediments and Water

October 2004

Eric M. Nichols, [email protected]

USEPA/AEHS Vapor Attenuation Workshop - October 2004 - 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


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


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• Expected transport mechanisms:

– Diffusion from source zones– Advection and diffusion across foundation


Former Dry CleanerSewer Line

Subject Building

• Source Area


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


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


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


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


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


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


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


Results Comparisonsoil gas and air in g/m3 groundwater in g/l

Data Source Media(95% UCL)

Modeled Indoor

Air

Measured Indoor Air

(95% UCL)

NS-GW 1,832 6.8 29

S-GW 48,000 181 260

NS-SGd 605,000 20 29

S-SGd 25,485,000 847 260

CK-IA 6.95 (avg. flux in µg/min/ft)

14 260

Bold indicates higher value


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)


Attenuation Factor Comparison

Data Estimated Indoor Air

Measured Indoor Air

Figure 3 Values for

Sandy Loam

NS-GW

S-GW

NS-SGd 3.3 x10-5 4.8 x10-5 2.0 x10-3

S-SGd 3.3 x10-5 1.0 x10-5 2.0 x10-3

Crack flux data not useful for estimating attenuation factor

4.0 x10-3

4.0 x10-3

2.8 x10-6

2.8 x10-6

1.2 x10-5

4.0 x10-6


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

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

0 5 10 15 20 25 30 35 40

Depth to Groundwater (m)

Vap

or

Att

en

uati

on

Facto

r


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

1.00E-05

1.00E-04

1.00E-03

1.00E-02

0 5 10 15 20 25 30 35 40

Depth to Soil Gas (m)

Vap

or

Att

en

uati

on

Facto

r


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


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


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


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


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 werecollected directly over this “hottest” area

• Extremely high source media concentrations

Sub-slab soil gas data could have resolved some of these issues


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


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

case study of subsurface vapor intrusion at a dry cleaner site amy goldberg day...

Documents