Vapor Intrusion 2010 – Sept. 29-30 – Chicago Vadose Zone Profiling g to Better Understand Processes Related to Vapor Intrusion Related to Vapor Intrusion Daniel B. Carr, P.E., P.G., Laurent C. Levy, Ph.D., P.E., Allan H. Horneman, D.E.S.
Effect on Seasonal Variability – Field Observations 100,000 , More muted fluctuations near water‐table depth 10 000 10,000 n (µg/m 3 ) One‐half order of magnitude seasonal fluctuations in TCE concentrations in soil gas near foundation in TCE concentrations in soil gas near foundation 1 000 1,000 Concentration depth with highs during the summer 100 100 Soil Vapor 10 10 Vapor Implant -Foundation Depth (7.5 ft bgs) Vapor Implant -Deep (32 ft bgs) Water table ranges between 37 to 38 ft bgs Series2 1 Jan-05 Jul-05 Jan-06 Jul-06 Jan-07 Jul-07 Jan-08 Jul-08 Jan-09 Jul-09 Jan-10 Jul-10
Example of Vadose PCE and TCE Zone Profiling Zone Profiling in Soil ( μ g/kg) • VOC mass sorbed onto soil as a result of historical vapor transport from transport from groundwater • Wet soils likely control the magnitude of diffusive flux of across diffusive flux of across this profile
Phase Partitioning in the Vadose Zone the Vadose Zone Assume 1 m 3 (1700 kg dry weight) of vadose zone soil with a TCE concentration of C [µg/m 3 ] in the vapor phase f C [ / 3 ] i h h TCE Mass in TCE Mass in Vapor [in μ g] TCE Mass in Vapor [in μ g] Sorbed Phase b d h = 0.35 x (1 – 0.3) x C x 1m 3 ~ 0.64 C ~ 0.25 C TCE Mass in TCE Mass in Sorbed Phase Vapor Moisture ~ 0.64 C ~ 0.14 C saturation of saturation of TCE Mass in 30% Aqueous Phase ~ 0.26 C Moisture saturation saturation TCE Mass in TCE Mass in Aqueous Phase of 60% ~ 0.53 C
Precipitation Illustration of Vapor Transport in the Vadose Zone Using SESOIL in the Vadose Zone Using SESOIL Evapotranspiration Runoff Volatilization • SESOIL is a one‐dimensional model Ground Surface used to simulate vertical transport in Vapor-Phase the unsaturated soil zone Diffusion Vadose Moisture Infiltration Zone • SESOIL Combines: (up to 4 x10 layers) y ) – Hydrologic Cycle H d l i C l Aqueous-Phase Advection precipitation, evapotranspiration, Sorption/ change in moisture storage change in moisture storage Desorption p Moisture Storage – Contaminant Fate Cycle Groundwater advection diffusion sorption advection, diffusion, sorption Water Recharge Recharge Table Groundwater Contamination
An Example of SESOIL Simulation Problem formulation: 1. Consider a vapor intrusion PCE Mass site with historical Sorbed from Vapor groundwater sourcing groundwater sourcing… Transport 2. At t = 0, sourcing from groundwater is eliminated or substantially cleaned up Vapor Intrusion Potential 3. How long does it take for Vapor Transport PCE mass in the vadose zone to go away? zone to go away? Clean(ed) GW Clean(ed) GW
Initial PCE Concentration (mg/kg) 0 3 0.3 0 Climate Properties Upper pp Northeastern U S Northeastern U.S. 1 5 m 1.5 m Layer 1 0.94 m (37 in.) of 0.15 m precipitation per year Contaminant Prop. 2 Layer epth (m) PCE 1.5 m 2 H ~ 0.8 0.15 m 0.15 m De Vadose Zone K oc ~ 100 L/kg 3 5 m Thick D a ~ 7 x 10 -6 m 2 /s Layer 3 1.5 m Soil Properties 4 0.15 m n~ 0.25 f oc ~ 0.5% f ~ 0 5% E Equivalent i l t Lower 0.05 m 0.5 m to about 1 - Sand k ~ 10 -8 cm 2 Layer 5 400 μ g/L in 2 - Silt k ~ 10 -9 cm 2 groundwater 10 m 2
Model Predicted Soil Vapor Concentration Profile – Sand PCE Vapor Concentration ( μ g/m 3 ) 1.E ‐ 01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 0 0.5 Upward Diffusion and Volatilization 1 1.5 2 m) Depth (m 2.5 PCE concentrations in vapor decrease by 3 about 2 orders of magnitude every 5 it d 5 3.5 years Year 1 Year 2 4 Year 3 Advection to GW Year 5 4.5 Year 10 5
Model Predicted Soil Vapor Concentration Profile – Sand vs. Silt PCE Vapor Concentration ( μ g/m 3 ) 1.E ‐ 01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 0 0.5 1 1.5 2 m) Depth (m SAND CLAY SAND SILT 2.5 Year 1 3 Year 2 Year 2 3.5 Year 3 4 Year 5 Year 5 4.5 Year 10 5
PCE Concentration as a Function of Time – Prediction for Sand 1 E 06 1.E+06 Near Ground Surface (0 m) Near Foundation Depth (2.5 m) 1.E+05 Near Water Table (5 m) Near Water Table (5 m) n ( μ g/m 3 ) 1.E+04 Concentratio 1.E+03 1.E+02 PCE Vapor C 1.E+01 Hypothetical Soil Gas Soil Gas Screening 1.E+00 Threshold for PCE 1 E 01 1.E ‐ 01 ‐ 1 0 1 2 3 4 5 6 7 8 9 10 11 Year
PCE Concentration as a Function of Time – Sand vs. Silt 1 E 06 1.E+06 1.E+05 n ( μ g/m 3 ) 1.E+04 Concentratio 1.E+03 1.E+02 PCE Vapor C SAND CLAY 1.E+01 SAND SILT Near Ground Surface (0 m) 1.E+00 Near Foundation Depth (2.5 m) Near Water Table (5 m) 1 E 01 1.E ‐ 01 ‐ 1 0 1 2 3 4 5 6 7 8 9 10 11 Year
It’s not the concentration but the flux that matters… 1 E 04 1.E+04 Upward Diffusion Flux Near Foundation Depth (2.5 m) 1.E+03 Downward Flux to Groundwater at d l d Water Table (5 m) 1.E+02 2 /day) s Flux ( μ g/m 1.E+01 10 to 1000 μ g/m 2 /day 1.E+00 1.E+00 PCE Mas Estimated range of diffusion flux that could 1.E ‐ 01 drive PCE at indoor air concentrations observed Moisture cycling in USEPA VI database 1.E ‐ 02 effect (residential) 1.E ‐ 03 ‐ 1 0 1 2 3 4 5 6 7 8 9 10 11 Year
PCE Mass Flux – Sand vs. Silt 1.E+04 1 E 04 1.E+03 1.E+02 2 /day) s Flux ( μ g/m 1.E+01 Initial PCE mass in SAND: In SILT… the 90% volatilized (upward diffusion) opposite pp 1.E+00 1.E+00 10% infiltration to the water table 10% infiltration to the water table PCE Mas (downward advection) SAND SILT 1.E ‐ 01 Upward Diffusion Flux Near Foundation Depth (2.5 m) 1.E ‐ 02 Downward Flux to Groundwater at Water Table (5 m) 1.E ‐ 03 ‐ 1 0 1 2 3 4 5 6 7 8 9 10 11 Year
Recap � Vapor transport and vapor concentrations in the vadose zone can be influenced by moisture cycling and mass transfer between phases (soil solids, moisture or air‐filled porosity) p ( , p y) � VOC mass in soil moisture and sorbed onto the soil solids can substantially contribute to vapor intrusion potential � The common perception that VI potential is largely a function of contemporaneous groundwater quality is flawed � Although we are often focused on concentration, it is the flux that matters.
Recommend
More recommend
