A Rational Approach to Vapor Intrusion Preferential Pathways

Transcription

1 A Rational Approach to Vapor Intrusion Preferential Pathways PRESENTED BY: David J. Folkes, PE Geosyntec Consultants, Inc. AEHS West Coast Conference EPA Vapor Intrusion Technical Workshop March 22, 2016

2 Preferential Pathway Definitions Natural or anthropogenic features that: EPA (2015) Depending on location and orientation: Enhance the migration of soil vapors or groundwater Enhance vapor entry into buildings ITRC (2007) Directly connect vapor source or vapor route to a building typical utility penetrations are not preferential Allow GW with high concentrations to migrate under building ASTM (2010) Have the least constraint on vapor migration May provide direct contact between vapor source and property Definitions are not always consistent (even within one document) and are generally vague

3 Examples of Natural Preferential Pathways in Guidance (EPA, ITRC, ASTM, DTSC) Karst terrain Faults Fractured rock Joints Bedding planes Weathered surfaces Preferential hydrogeological pathways High permeability layers Buried streambeds Gravel layers, lenses, and channels Sand channels Macropores Fractured sediments Facies change U.S. Geological Survey ITRC 2014

4 Examples of Anthropogenic Preferential Pathways in Guidance (EPA, ITRC, ASTM, DTSC) Exterior Features Tunnels Underground mines Utility corridors Sewer lines and manholes Sanitary sewers Storm drains Permeable bedding Subsurface drains Excavations Permeable fill Pavement Building Features Waste lines w/o functioning traps Uncapped dry well pipes HAVC ducts below building Elevator shafts Sumps and drainage pits Voids Non-typical utility penetrations Floor drains Pipes Unlined crawl spaces Earthen floors Foundation construction joints Foundation seams or cracks Rodent tracks All buildings will have one or more of these natural & anthropogenic preferential pathways

5 Effects of Preferential Pathway Lateral Inclusion Zone (LIZ) Distance? Plume edge LIZ ~100 ft* The LIZ distance is based on experience at real-world sites which have most of the preferential pathways listed Additional migration must require atypical pathways *Distance in most VI guidance documents What atypical pathways could extend vapor migration beyond the LIZ? Tom McHugh (GSI) ER Developing sewer pipe investigation protocol to evaluate these pathways NAPL or contaminated GW in sewer? Graphics courtesy of Tom McHugh Sewer in direct contact with NAPL impacted soil?

6 Effects of Preferential Pathway Attenuation Factor & Screening Levels? EPA 2012 VI Database study Groundwater AF of Atypical preferential pathways are likely required for higher AFs Typical preferential pathways have resulted in AFs within empirical range ITRC 2014 What atypical pathways could result in AFs exceeding empirical upper bounds? Diffusion-controlled pathways likely result in typical VI behavior Pressure-controlled pathways may be atypical if they connect vapor source and building

7 Diffusion-Controlled Preferential Pathways Attenuation factor distribution for groundwater (EPA 2012) SAND 95% 5% MF = - D eff A B ΔC SV /ΔZ JE model, EPA (2004) default slab on grade Qsoil = 5 L/min GW 1.5 m below slab 500 ug/l TCE, H of 0.21 at 10 o C, SAND

8 Diffusion-Controlled Preferential Pathways Attenuation factor distribution for groundwater (EPA 2012) 95% MF = - D eff A B ΔC SV /ΔZ CLAY 5% JE model, EPA (2004) default slab on grade Qsoil = 5 L/min GW 1.5 m below slab 500 ug/l TCE, H of 0.21 at 10 o C, SAND

9 Diffusion-Controlled Preferential Pathways Attenuation factor distribution for groundwater (EPA 2012) MF F = - D eff A F ΔC SV /ΔZ SAND A F = 10% A B CLAY 95% 5% JE model, EPA (2004) default slab on grade Qsoil = 5 L/min GW 1.5 m below slab 500 ug/l TCE, H of 0.21 at 10 o C, SAND Fractures filled with high D eff material may be preferential pathways with respect to the matrix, but do not result in high mass flux or attenuation factors Small fracture diffusion area A F limits mass flux Same factors apply to high D eff pipe bedding in low D eff matrix Diffusion-controlled preferential pathways are unlikely to cause vapor intrusion beyond normally observed ranges (not precluding factor)

10 Bedrock Fracture Case History N. Penn 12 Superfund Site TCE in groundwater below residential areas Siltstone/shale at 5 feet, fractured to 100 feet Upper 10 feet of bedrock weathered CSM predicted filled fractures in weathered zone to 100 µg/l TCE Indoor air, sub-slab vapor testing program Maximum sub-slab TCE was 4.5 ug/m3, most ND Consistent with negligible upward mass flux through fractures Ultimately no mitigation or further action required

11 Pressure Controlled Preferential Pathways JE model, EPA (2004) default basement scenario JE model, EPA (2004) default basement scenario 500 ug/l TCE, TCE, H H of 0.21 of 0.21 at 10at o C, 10 SAND o C, SAND Pipe air flow = Qsoil (open to sub-slab media) Pipe air flow min = Qsoil (open to sub-slab media) Pipe air flow max = 100 ft 4 pipe (open to building) C IA C SS MF PIPE? MF SOIL C O MF SOIL = - D eff A B ΔC SV /ΔZ Pressure-controlled preferential pathways connected to vapor source may result in atypical MF and temporal variability

12 A Rational Approach to Preferential Pathways We should distinguish between: Typical preferential pathways, where (by definition) vapor migration behavior is within normally observed ranges, and Atypical preferential pathways, where behavior is beyond normal ranges Typical pathways should not be precluding factors for the use of empirically based screening criteria e.g., LIZ, generic attenuation factors/screening levels Atypical pathways are likely to be pressure-driven open conduits connecting source materials to buildings, and: May be precluding factors for use of empirical screening criteria May increase spatial and temporal variability beyond typical ranges Pathways that include diffusion-controlled segments are unlikely to be atypical The potential for atypical preferential pathways may be indicated by: The known presence of open conduits connecting sources and buildings Odors, drafts, or abnormal spatial and temporal variability data Mass flux measurements during building depressurization tests