Vapor Intrusion Modeling

1
Vapor Intrusion Modeling
Predicted Concentration
a
Measured Concentration
2
Alpha is the reciprocal of Attenuation
Predicted Concentration
a (e.g. 0.001) or Attenuation 1/a
(e.g. 1000)
Measured Concentration
3
Use of Alpha
Predicted Concentration
SG x a
GW x H x a
Soil Gas Concentration
Groundwater Concentration
4
Modeling Can Be Valuable

• For current exposures where actual data can not
  be obtained or is confounded by inside sources,
• For estimating future potential exposures.

5
Developing Alphas

• Develop site-specific alphas based upon JE Model
  (EPA On-line Tools )
• Develop generic alphas from empirical studies

6
Criteria for Alphas

• Per MCP, modeling must provide a reasonably
  conservative estimate of the indoor
  concentrations.
• Estimations of alphas should be consistent with
  empirical evidence.

7
Empirical Data U.S. EPA 2008 Database
MassDEP site data
8
Empirical Data U.S. EPA 2008 Database
MassDEP site data
9
Figure 7. Indoor air versus soil gas
concentrations.
Data Set 1 (2008) Subset of 2008 database
remaining after screening out data with
subsurface concentrations less than reporting
limits, field notes indicating the presence of
background sources, indoor air concentrations
greater than subsurface concentrations, or
inconsistent attenuation factors.
10
Figure 10a. Indoor air versus subslab
concentrations.
Data Set 1 (2008) Subset of 2008 database
remaining after screening out data with
subsurface concentrations less than reporting
limits, field notes indicating the presence of
background sources, indoor air concentrations
greater than subsurface concentrations, or
inconsistent attenuation factors.
11
Figure 10b. Indoor air versus subslab
concentrations.
Data Set 2 (2008) Subset of Data Set 1 (2008)
with indoor air concentrations greater than the
95th percentile background indoor air
concentrations (Table 4) or the reporting limits
(if higher than the 95th percentile).
12
Empirical Data U.S. EPA 2008 Database
MassDEP site data
13
MassDEPs Database

• Range of empirical alphas from 10 to 0.0001
• Most attenuation factors in the tens and hundreds