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

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

Early Steps in Vapour Intrusion Assessment

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?

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

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)

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)

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

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

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

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

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

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’

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

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

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

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

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

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

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)

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

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

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

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

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

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

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