Vadose Zone Profiling g to Better Understand Processes Related to - - PowerPoint PPT Presentation

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 10 000 ,

More muted fluctuations near water‐table depth

1 000 10,000 n (µg/m3)

One‐half order of magnitude seasonal fluctuations in TCE concentrations in soil gas near foundation

100 1,000 Concentration

in TCE concentrations in soil gas near foundation depth with highs during the summer

10 100 Soil Vapor 10 Vapor Implant -Foundation Depth (7.5 ft bgs) Vapor Implant -Deep (32 ft bgs) Series2 Water table ranges between 37 to 38 ft bgs 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 Zone Profiling

PCE and TCE

Zone Profiling

• VOC mass sorbed onto

in Soil (μg/kg) 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 m3 (1700 kg dry weight) of vadose zone soil with a TCE concentration f C [ /

3] i

h h

• f C [µg/m3] in the vapor phase

TCE Mass in Vapor [in μg] TCE Mass in b d h TCE Mass in Vapor [in μg] = 0.35 x (1 – 0.3) x C x 1m3 ~ 0.25 C Sorbed Phase ~ 0.64 C

Moisture saturation of

TCE Mass in Sorbed Phase ~ 0.64 C TCE Mass in Vapor ~ 0.14 C

saturation of 30% Moisture saturation

TCE Mass in Aqueous Phase ~ 0.26 C TCE Mass in

saturation

• f 60%

TCE Mass in Aqueous Phase ~ 0.53 C

Illustration of Vapor Transport in the Vadose Zone Using SESOIL

Evapotranspiration Precipitation

in the Vadose Zone Using SESOIL

• SESOIL is a one‐dimensional model

Runoff

Ground Surface

Volatilization

used to simulate vertical transport in the unsaturated soil zone

Vadose

Moisture Vapor-Phase Diffusion

• SESOIL Combines:

H d l i C l

Zone (up to 4 x10 layers)

Infiltration

– Hydrologic Cycle precipitation, evapotranspiration, change in moisture storage

y )

Aqueous-Phase Advection Sorption/ Desorption

change in moisture storage – Contaminant Fate Cycle advection diffusion sorption

Groundwater Recharge p Moisture Storage

Water

advection, diffusion, sorption

Recharge Groundwater Contamination

Table

An Example of SESOIL Simulation

Problem formulation: 1. Consider a vapor intrusion site with historical groundwater sourcing

PCE Mass Sorbed from Vapor

groundwater sourcing… 2. At t = 0, sourcing from

Transport

groundwater is eliminated

• r substantially cleaned up

Vapor Intrusion Potential

3. How long does it take for PCE mass in the vadose zone to go away?

Clean(ed) GW Vapor Transport

zone to go away?

Clean(ed) GW

0 3 Initial PCE Concentration (mg/kg) 1 5 m Upper 0.3 Climate Properties Northeastern U S 1.5 m 0.15 m pp Layer 1 Northeastern U.S. 0.94 m (37 in.) of precipitation per year 1.5 m 0.15 m Layer 2 2 epth (m) Contaminant Prop. PCE H ~ 0.8 Vadose Zone 5 m Thick 0.15 m Layer 3 De Koc ~ 100 L/kg Da ~ 7 x 10-6 m2/s 1.5 m 0.15 m 3 4 Soil Properties n~ 0.25 f ~ 0 5% E i l t 0.5 m 0.05 m 10 m2

Lower Layer

5 foc ~ 0.5% 1 - Sand k ~ 10-8 cm2 2 - Silt k ~ 10-9 cm2 Equivalent to about 400 μg/L in groundwater

Model­Predicted Soil Vapor Concentration Profile – Sand

1.E‐01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 PCE Vapor Concentration (μg/m3) 0.5 1

Upward Diffusion and Volatilization

1.5 2 m) 2.5 3 Depth (m

PCE concentrations in vapor decrease by about 2 orders of it d 5

3.5 4 Year 1 Year 2 Year 3

magnitude every 5 years

4.5 5 Year 5 Year 10

Advection to GW

Model­Predicted Soil Vapor Concentration Profile – Sand vs. Silt

1.E‐01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 PCE Vapor Concentration (μg/m3) 0.5 1 1.5 2 m) 2.5 3 Depth (m Year 1 Year 2 SAND CLAY

SAND SILT

3.5 4 Year 2 Year 3 Year 5 4.5 5 Year 5 Year 10

PCE Concentration as a Function of Time – Prediction for Sand

1 E 06 1.E+05 1.E+06 Near Ground Surface (0 m) Near Foundation Depth (2.5 m) Near Water Table (5 m) 1.E+04 n (μg/m3) Near Water Table (5 m) 1.E+02 1.E+03 Concentratio 1.E+01 PCE Vapor C

Hypothetical Soil Gas

1 E 01 1.E+00

Soil Gas Screening Threshold for PCE

1.E‐01 ‐1 1 2 3 4 5 6 7 8 9 10 11 Year

1 E 06

PCE Concentration as a Function of Time – Sand vs. Silt

1.E+05 1.E+06 1.E+04 n (μg/m3) 1.E+02 1.E+03 Concentratio 1.E+01 PCE Vapor C Near Ground Surface (0 m) SAND CLAY

SAND SILT

1 E 01 1.E+00 Near Foundation Depth (2.5 m) Near Water Table (5 m) 1.E‐01 ‐1 1 2 3 4 5 6 7 8 9 10 11 Year

1 E 04

It’s not the concentration but the flux that matters…

1.E+03 1.E+04

Upward Diffusion Flux Near Foundation Depth (2.5 m) d l d

1.E+02

2/day)

Downward Flux to Groundwater at Water Table (5 m)

1.E+00 1.E+01 s Flux (μg/m

10 to 1000 μg/m2/day

1.E‐01 1.E+00 PCE Mas

Estimated range of diffusion flux that could drive PCE at indoor air

1.E‐02

concentrations observed in USEPA VI database (residential) Moisture cycling effect

1.E‐03 ‐1 1 2 3 4 5 6 7 8 9 10 11 Year

1 E 04

PCE Mass Flux – Sand vs. Silt

1.E+03 1.E+04 1.E+02

2/day)

1.E+00 1.E+01 s Flux (μg/m

Initial PCE mass in SAND: 90% volatilized (upward diffusion) 10% infiltration to the water table In SILT… the

• pposite

1.E‐01 1.E+00 PCE Mas SAND SILT

10% infiltration to the water table (downward advection) pp

1.E‐02

Upward Diffusion Flux Near Foundation Depth (2.5 m) Downward Flux to Groundwater at Water Table (5 m)

1.E‐03 ‐1 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

• f contemporaneous groundwater quality is flawed

Although we are often focused

• n concentration, it is the

flux that matters.