Stylistic Modeling of Vadose Zone Transport Insight into Vapor Intrusion Processes

Transcription

1 March 15, 2011 Workshop, Addressing Regulatory Challenges In Vapor Intrusion, A State of the Science Update Stylistic Modeling of Vadose Zone Transport Insight into Vapor Intrusion Processes Daniel B. Carr, P.E., P.G., Laurent C. Levy, Ph.D., P.E., Allan H. Horneman, D.E.S. West Coast Conference on Soils Sediment, Water and Energy, San Diego California

2 Modeling of Transport Processes: Infiltration i Infiltration Advection Stylistic- generally illustrative of governing processes, and the style of transport acknowledging : model limitations and; greater complexity of field processes. Vapor, Aqueous, Sorbed Diffusion Partitioning

3 SESOIL Seasonal Soil Compartment Model (1984) Vertical transport in partially saturated soil Hydrologic Cycle precipitation, runoff, evapotranspiration, change in moisture storage, groundwater recharge Contaminant Fate Cycle Vadose Zone (up to 4 x10 layers) advection, diffusion, partitioning among gas, dissolved, and sorbed phases Precipitation Evapotranspiration Runoff Ground Surface Vapor-Phase Diffusion Moisture Infiltration Aqueous-Phase Advection Sorption/ Desorption Moisture Storage Groundwater Recharge Volatilization Water Table Mass Loading to Water Table

4 Concepts 1 D Vertical Model Diffusion 100 g/m 2 day C C indoor air C subslab 1 5m in H 2 0 2x10 6 g PCE 1 Meter 0 0 Infiltration 0 10 g/m 2 -day C GW-Vapor

5 Points of Interest 1. Relative Importance of VOC Mass in the Vadose Zone 2. Interplay between Diffusion and Infiltration 3. Implications of Transient Processes Graphics Courtesy of USEPA 2010 AEHS Conference

6 10,000 Correlation Groundwater & Vapor? Concentration (µg/l) 1, Foundation Depth Vapor Concentration (µg/m3) Groundwater 1 Near Water Table Depth Groundwater 0 6/23/04 10/23/05 2/23/07 6/23/08 10/23/09 Date

7 Illustrative Example: 1. Simple uniform sandy soil profile. 2. At t = 0, sourcing from groundwater at 400 μg/l. PCE Mass Aqueous and Sorbed from Vapor Transport 3. At t=3 years a ½ Oom improvement tin water quality source term to 80 μg/l. Vapor Intrusion Potential Vapor Transport

8 The Details Near Ground Surface Climate Northeastern U.S. 15m 1.5 Upper Layer 0.94 m (37 in.) of precipitation per year 0.15 m Soil Vadose Zone 5 m Thick Layer 2 Foundation 2.5 m Med well sorted Sand k ~ 10-8 cm 2 n~ 0.25 f oc ~ 0.5% Layer 3 Contaminant PCE C GW 400 to 80 g/l 0.5 m 0.05 m Lower Layer 1 m 2 Water Table 5 m K oc H ~ 0.8 ~ 100 L/kg D a ~ 7 x 10-6 m 2 /s

9 Vapor Concentrations Over Time 1.E+06 PC CE Vapor Co oncentratio on (μg/m 3 ) 1.E+05 1.E+04 1.E+03 Near Ground Surface (0 m) Near Foundation Depth (2.5 m) 1.E+02 Near Water Table (5 m) Year

10 Diffusive and Advective Infiltration Flux 1.E+04 PCE Mass s Flux (μg/m 2 /day) 1.E+03 1.E+02 Upward Diffusion Near Foundation Depth (2.5 m) Downward Infiltration to Water Table (5 m) 1.E Year

11 Mass Transfer Balance 6.E+06 PCE Cumulative Mass Tran nsfer (μg/m 2 ) 5.E+06 4.E+06 3.E+06 2.E+06 1.E+06 0.E E+06 1.E+06 Vadose Zone Storage 5.3E+06 Upward Diffusion 76% 4.3E+06 3.E Year

12 Groundwater Vapor to Indoor Air Attenuation GW Vap por to Indo oor Air Atte enuation Fa actor α 1.E 01 1.E 02 1.E 03 1E 1.E 04 1.E 05 1.E Year

13 Comparison USEPA 2010 AF Statistics After Step Constant Source 6 Months Later Shortly After Initial Arrival at Water Table

14 Summary Thoughts 1. VI Potential is a complex function of phase transfer and transport processes. Aqueous and sorbed mass in vadose zone are important. 2. Although we measure concentration, we should remember that it is the flux that matters. 3. What we observe in field sampling is probably always somewhat a reflection of transient processes. 4. The vadose zone is a sink and source for VOC mass originally transported from the water table. From Schuver, March 2010, AEHS Presentation

15 References: Questions? Daniel B. Carr, P.E. P.G. Vice President Sanborn, Head & Associates, Inc. 95 High Street, Portland Maine, direct 1. Conant, B.H, Gillham, R.W., and Mendoza, C.A., January 1996, Vapor Transport of TCE in the Unsaturated Zone: Field and Numerical Modeling Investigations, Water Resources Research, Vol. 32, No. 1 pgs Yu, S., Unger, A.J., and Parker, B., April 2009, Simulating the Fate and Transport of TCE from Groundwater to Indoor Air, Journal of Contaminant Hydrology 107 (2009) Yao, Y, Pennel, K.G., and Suuberg, E.M., October 2010, Influence of Transient Processes on Vapor Intrusion, Proceedings Air and Waste Management, Vapor Intrusion Specialty Conference, Chicago Illinois. 4. Carr, D.B., Levy, L.C., and Horneman, A.H., October 2010, Vadose Zone Profiling to Better Understand Vadose Zone Processes Related to Vapor Intrusion, Proceedings Air and Waste Management, Vapor Intrusion Specialty Conference, Chicago Illinois.

16 Question Response BONUS SLIDES

17 PCE and TCE in Soil (μg/kg) Example of Vadose Zone Profiling VOC mass in sorbed and aqueous phases as a result of historical vapor transport from groundwater Wet soils likely control the magnitude of diffusive flux of across this profile

18 Key Concepts Advective and diffusive vapor transport as f(water saturation S w ) Wet soils are present near the water table (capillary fringe) and may be found at contacts between finer and coarser soils Lithologic Layering Fine Presence of Capillary Barriers Coarse Limits Vapor Transport Influence Vapor Intrusion Potential Fine VOC mass transfer among vapor, aqueous, and sorbed phases may influence VI potential A. Corey, Mechanics of Immisc. Fluids in Porous Media, 1986

19 What the Model Can and Cannot Do Can Do: Model Vapor diffusion and infiltration flux under: Spatially variable VOC mass distribution Linear and Non Linear sorption Time variable climactic conditions Water and Chemical Balance Cannot Do: Spatially variable, non linear water saturation uses a harmonic mean water saturation across the profile Accurately simulate non linear soil moisture and vapor concentration profiles in layered soil with differing total porosity Does not fully account for capillary fringe effects Intragranular diffusion Desorption kinetics

20 Transient Numerical Modeling: Transient numerical modeling to analyze the relative importance of factors controlling fate and transport for VI. Key Findings: Diffusive flux across capillary fringe is the limiting process controlling indoor air impacts Sensitive to the thickness of capillary fringe and infiltration rate. Infiltration increases soil moisture saturation, and reduces advective transport. Bp fluctuations have a distinct impact on indoor air concentrations.

21 Brown University Yao, Pennel, Suuberg, d finite element modeling of diffusion and advection 8 meter thick vadose zone, no phase partitioning, no variable climate homogeneous air filled pore space 6 to 15 months to steady state 2 to 20 months for 1OOM reduced d after initial water table arrival concentration at foundation depth