1. Sean Anderson, P.Eng., QPESA Steve Russell, B.Sc., QPRA
Comparison of Measured vs Modeled Soil Vapour Concentrations and Effects on Decision Making
Maxxam Spring 2018 Science Summit: Soil Vapour Intrusion, Site Assessment and Remediation
April 27, 2018
Sean Anderson, P.Eng., QPESA
Steve Russell, B.Sc., QPRA

2. Overview
Early Steps in Vapour Intrusion Assessment
Johnson and Ettinger Vapour Transport Model
Vapour Intrusion Conceptual Site Model
Case Study 1 – Soil Source
Case Study 2 – Groundwater Source
Case Study 3 – Multiple Sources
Implications on Decision Making

3. Early Steps in Vapour Intrusion Assessment

4. Early Steps in Vapour Intrusion Assessment
Screening against risk-based component values
S-IA (Soil to Indoor Air)
GW2 (Groundwater to Indoor Air)
MOE, 2011
What if concentrations exceed the component values?

5. Johnson and Ettinger (J&E) Vapour Transport Model
Screening level model
Recently revised in 2017 (can enter a groundwater concentration or soil vapour concentration)
Considers advective and diffusive mechanisms to estimate subsurface vapour transport into buildings
No biodegradation assumed
One-dimensional model
Estimates attenuation coefficient
Relationship between vapour concentration in indoor air and vapour concentration at source

6. J&E Vapour Transport Model
User can enter site-specific information:
Groundwater concentration and depth (or soil vapour concentration and depth)
Vadose zone characteristics
Building characteristics
Chemical properties of contaminant(s)
3 m
12 m
SAND
SILTY CLAY
LOAM
50 µg/L
(Trichloroethylene)

7. J&E Vapour Transport Model
Model outputs:
Estimated source vapour concentration (if groundwater concentration entered)
Estimated attenuation factor
Estimated indoor air concentrations
Risk estimates (HQ or ILCR)
0.23 µg/m3
SAND
α = 1.80E-05
SILTY CLAY
LOAM
12,700 µg/m3
50 µg/L
(Trichloroethylene)

8. J&E Vapour Transport Model
Generally considered to be conservative and appropriate for screening purposes.
Why only for screening purposes?

9. Vapour Intrusion is complex
VI Processes
Vapour Intrusion is complex
Typically requires multiple lines of evidence approach
MOE, 2013; USEPA, 2002

10. VI Conceptual Site Model (CSM)
Simplified representation of environmental conditions
Primary communication vehicle for VI
Basis for decision-making
Should be iterative and evolve with new data

11. VI CSM – Building Blocks
CSM should incorporate:
Vapour contamination sources in soil
Vapour contamination sources in groundwater
Geology & hydrogeology
Building construction details
Sampling results:
indoor air
sub-slab vapour
soil vapour

12. VI CSM – Complicating Factors
CSM should also consider:
Temporal variability
Spatial variability
Soil heterogeneity
Nature of contaminants
Preferential pathways
Background sources in indoor air
Acknowledging and communicating uncertainty
Client needs

13. CASE STUDY 1 – SOIL SOURCE
Proposed brownfield redevelopment
0.4 acre site
Characterization began in 2009
Soil impacts redistributed across site following characterization
Poor Quality Fill
Former Retail Fuel Outlet A’

14. A A’ CASE STUDY 1 0m ? ? 1m Non-Impacted Sand Fill 2m 3m ?
Heterogeneous Impacted Fill ? 4m
Bedrock 5m

15. CASE STUDY 1 – SOIL SOURCE
Client had no desire to re-characterize soil
Therefore, proceeded with existing (limited) dataset
Derived Soil Vapour Concentrations using J&E Model:
Assumed pre-grading soil maximum concentrations
Assumed impacts directly beneath future building
Predicted unacceptable risks for vapour intrusion
As a result of uncertainty, recommended vapour mitigation and collection system

16. CASE STUDY 1 – SOIL SOURCE
Several years later, property still vacant
Sold to another developer
Firm redevelopment plans
Asked MTE to re-evaluate original recommendations
New client authorized soil vapour sampling
CSM was updated based on new information

17. A A’ CASE STUDY 1 Proposed Building (slab-on-grade, no basement) 0m 1m
Non-Impacted Sand Fill 2m
Measured
Soil Vapour 3m
Heterogeneous Impacted Fill 4m
Bedrock 5m

18. CASE STUDY 1 – SOIL SOURCE
Understanding of site was enhanced by incorporated soil vapour results & proposed building construction details into CSM
Other compounds either not detected or measured more than 10 times below the soil vapour screening criteria
Vapour mitigation no longer warranted or recommended
Benzene Soil Vapour Concentrations (μg/m3)
Predicted
Measured
10,500
12.9

19. TAKEAWAYS
Update CSM as our understanding of site evolves and new information becomes available
Soil vapour sampling likely more appropriate than J&E Model when addressing potential vapour contamination sources in soil
Additional sampling effort may allow clients to avoid unnecessary mitigation and other costly measures

20. Soil Vapour Concentrations (μg/m3)
CASE STUDY 2 – GROUNDWATER
SOURCE A
Soil Vapour Concentrations (μg/m3)
SVP-1 50
Predicted
Measured
50,700
7,000
200
400
SVP-2
Predicted
Measured
101,000 5 75
SVP-3
Big Difference! Why?
180
Predicted
Measured
45,600
13,000
100 A’
TCE in groundwater exceeds SCS (concentration in μg/L)

21. A A’ CASE STUDY 2 – GROUNDWATER SOURCE SVP-1 SVP-2 SVP-3 7,000 5
13,000
5m-
Screened interval where maximum groundwater concentration was measured is not at the water table.
180
200
170
400
SILTY SAND
TCE concentration measured in soil vapour (concentration in μg/m3)
TCE in groundwater exceeds SCS (concentration in μg/L)
TCE in groundwater meets SCS

22. TAKEAWAYS
Ensure assumptions of J&E model are appropriate, including maximum groundwater concentrations used as inputs
Consider soil vapour sampling results in context of CSM and overall understanding of site
Once again, soil vapour sampling appears to provide more reasonable assessment of potential for vapour intrusion

23. Soil Vapour Concentrations (μg/m3)
CASE STUDY 3 – MULTIPLE SOURCES
Soil Vapour Concentrations (μg/m3)
First installed deep soil vapour probes to investigate (~ 15 mbgs)
Predicted
Measured
1,300
2,000
Existing Building 3 12
Predicted
Measured
3,000 A 5 A’
However – soil vapour screening level = 1,000 µg/m3 4
TCE in groundwater exceeds SCS (concentration in μg/L)
TCE in groundwater meets SCS

24. A A’ CASE STUDY 3 – MULTIPLE SOURCES 14,000 μg/m3 4m- 3,000 μg/m3
SAND FILL
SAND
14,000 μg/m3
4m-
SILT SEAM
3,000 μg/m3
Measured soil vapour concentrations and silt seam observation helps to better develop VI conceptual site model
SAND
2,000 μg/m3
3,000 μg/m3
15m-
5 μg/L
12 μg/L

25. TAKEAWAYS
In this case, the model accurately calculated the source vapour concentrations based on groundwater data
Consider soil vapour sampling at multiple depths when dealing with chlorinated solvents
By measuring soil vapour at different depth intervals, the VI conceptual site model was improved and a better understanding of actual conditions was realized

26. CONCLUSIONS
Models can be useful for screening, but results are only as good as the quality of information that is available and entered
Measured soil vapour can improve your understanding of VI conceptual site model
Measured soil vapour can inform better decisions regarding potential impacts or health risk

27. THANK YOU!