Part 3 Fundamentals. Most Common VI Bloopers. Handy Unit Conversions:

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1 Part 3 Fundamentals Units Contaminant Partitioning Vapor Migration Bioattenuation Site Conceptual Model Attenuation (alpha) Factors Risk/Screening Levels Most Common VI Bloopers Unit Confusion Assuming ug/l equivalent to ppbv Assuming ug/m3 equivalent to ppbv Screening Levels Comparing to generic screening levels Not calculating correct levels Sampling & Analysis Errors Program design: Which method? Wrong hardware, wrong analysis Handy Unit Conversions: 1 ug/l = 1000 ug/m3 1 ppmv = 1000 ppbv 1 ug/l = (24/mw) ppmv 1 ug/m3 = (24/mw) ppbv 1 ppmv = (mw/24) ug/l 1 ppbv = (mw/24) ug/m3 mw is the molecular weight of the compound 1

2 Contaminant Pathway into Structures Steps: Partitioning from gw Diffusion through vadose zone Advection near building Dilution in building Contaminant Partitioning Groundwater to Soil Gas (Henry s Constant): H = Csg/Cw, so, Csg = Cw * H Example: H benzene = 0.25 (dimensionless) For GW Conc = 10 ug/l Csg = 10 * 0.25 = 2.5 ug/l Assumes Equilibrium. Very Rarely Achieved (no mixers or blenders in the subsurface) 2

3 Measured Soil Gas Data vs. Predicted from Soil Phase Data Measured F1 in Soil Vapour (mg/m 3 ) 1.E+07 1.E+06 1.E+05 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 1.E-01 1.E-02 Difference depth soil gas & soil > 0.5 m Vm/Vp 50th = 2.1E-5, 90th = 3.6E-3 Difference depth soil gas & soil < 0.5 m Vm/Vp 50th = 9.3E-5, 90th = 7.4E-3 1:1 1:10 1:100 1.E-02 1.E+00 1.E+02 1.E+04 1.E+06 Predicted F1 in Soil Vapour (mg/m 3 ) CPPI Database.005 Measured vapor concentrations 10 to 1000x less than predicted How Do Contaminants Move in the Vadose Zone? Bus? Escalator? Train? How Do Contaminants Move? (Molecular Diffusion) Movement (Flux) = K d?/dx where: K is a proportionality constant d?/dx is a gradient Property Equation Constant Momentum: Flux = K dh/dx hydraulic cond Heat (Fourier s): Flux = Φ dt/dx thermal cond Mass (Fick s): Flux = D dc/dx diffusivity Momentum, heat, mass ALL move from High to Low 3

4 Common Vapor Profiles Concentration Depth Surface Source Depth Concentration Deep Source Depth Concentration Surface and Deep Sources How Fast Do Things Move? where: Distance = (2*D e *t) 1/2 D e is effective diffusivity, t is time Vapors through the Vadose Zone: D e ~ 0.01 cm 2 /sec Distance = (2*0.01*31,000,000) = 800 cm/yr Vapors through Liquid (into/out of GW): D e ~ cm 2 /sec Distance = (2* *31,000,000) = 8 cm/yr Transport in Vadose Zone 100 times faster than in GW How Do Contaminants Move? Advection - Air Itself Moves, Caused by: Pressure Gradients Wind speed (only if high) Barometric pressure changes not great Building effects (heating, ventilation, air conditioning, and fan operations) Methane Gas Generation Not Much Advection in Vadose Zone Except Close to Surface 4

5 Aerobic Biodegradation Basics Microorganisms (bugs) are Everywhere! Bioattenuation of HCs Occurs Reliably (hot & cold climates) Existing data suggest O 2 effective barrier 98% effective if O 2 >5% and 3-5 clean soil Document by vertical profiles of COC & O 2 Atmospheric air (21% Oxygen = 275 g/m 3 oxygen) provides the capacity to degrade 92 g/m 3 hydrocarbon vapors (92,000,000 ug/m3) Clean Soil Model for HC Vapors Bio-barrier Reaction Zone 5

6 Dirty Soil Model for HC Vapors Effect of Source Concentration = 0.18 h -1 ] Results suggest that there may be source vapor concentrations that are of little concern if soil gas beneath the foundation is welloxygenated (e.g., groundwater plume sources) = 7.1 x 10-5 = 7.2 x 10-8 = 5.6 x Note: 20 mg/l = 20,000 ug/l = 20,000,000 ug/m3 DEFINITION: Conceptual Site Model (or Site Conceptual Model) A Conceptual Site Model (CSM) is a simplified version (pictures and/or descriptions) of a complex real-world system that approximates its relationships 6

7 Components of a CSM Type of COC: Chlorinated or hydrocarbon Location: Underneath or to side of building Residential or Commercial Receptor Construction of buildings (slab, basement, cs) Underground utilities & pipes Existing & potential future buildings Type of HVAC system Soil stratigraphy Hydrogeology & depth to water table Vadose Zone characteristics Surface cover description in source and surrounding area ITRC VI Guidance Appendix B Chlorinated VOCs in GW Stack effect Diffusion through vadose zone Volatilization from water table Source Petroleum Hydrocarbons Gas Station Building UST Contaminated soil Strong vapor source from LNAPL & contaminated soil Clean aerobic soil Weak Vapor Source from Dissolved Plume LNAPL & contaminated soil Dissolved contamination HENRY S LAW CONSTANT 7

8 Differences Between PVI and CVI PVI Residual LNAPL Vapor LNAPL Plume Dissolved Plume O 2 Transport Aerobic Bio-begradation LNAPL CVI Residual DNAPL DNAPL Vapor Plume Dissolved Plume Variable PVI CVI Type of chemical petroleum hydrocarbon chlorinated hydrocarbon Example benzene tetrachloroethene (PCE) Source Type LNAPL DNAPL Aerobic biodegradation very rapid very limited Vapor intrusion potential low high Degradation products CO 2, H 2 O intermediates Attenuation Factors Indoor Air 10 μg/m 3 sg = Cindoor/Csg Alpha = 10/500 Alpha = 0.02 (shallow soil gas) 500 μg/m 3 Soil Gas (shallow) Attenuation (alpha) Factors Soil Gas: sg = C indoor /C sg Groundwater: gw = C indoor /(C gw *H) Lower alpha factor means higher attenuation EPA Guidance attenuation factors: 2002: Soil Gas for 5 bgs, 0.1 for sub-slab 2015: Soil Gas 0.03 for 5 bgs, 0.03 for sub-slab Groundwater = for 5 bgs Hydrocarbon sg <

9 Indoor Air & Sub-Slab Concentrations EPA OSWER Database 1000 Indoor Air Concentration (µg/m 3 ) Soil Gas Concentration (µg/m 3 ) Sub-Slab Attenuation Factors State Attenuation Factor California 0.05 Ohio 0.1 Hawaii Massachusetts New Jersey 0.02 Pennsylvania 0.01 Oregon EPA (2012, 95 th ) 0.03 EPA OUST ~1x10-7 9

10 RISK 101: Screening Level Acronyms RBSL: Risk Base Screening Level RBC (from ASTM): Risk Based Concentration RSL: EPA Regional Screening Levels PEL: OSHA Permissible Exposure Limits Need to Know When & How to Use 10

11 Cancer Risk Management Range Risk Management NFA Mitigate 1e-6 1e-4 For Some States, NFA at 1e-5 11

12 RISK 101: Why Are Indoor Air RSLs So Low? Benzene: EPA: 0.36 ug/m3 (1e-6) TCE: EPA: 0.48 ug/m3 (1e-6) Values Assume Exposure Times of: 24 hr, 350 days/yr, 26 years Ultra Conservative Assumptions Lower Allowed Levels and Bring in More Sites EPA RSLs Updated Every 6 months Factors Affecting Indoor Air Screening Levels Risk levels 10-4 to 10-6 cancer risk levels Non-Cancer risks (hazard quotients, HQ) Individual vs. cumulative risks Receptors Residential and non-residential receptors Exposure Times ITRC VI Guideline Appendix H 12

13 Determining Screening Levels From Lookup Tables (EPA Table 3) From Attenuation Factors From J-E Model/Spreadsheets Lower SLs Higher SLs Levels increase from top to bottom (less conservative) Screening Levels from Models Calculators Vapor Intrusion Screening Level Calculator (VISL) Johnson-Ettinger Some States Still Use Latest version: September 2017 Biovapor for hydrocarbons EPA VISL Calculator 13

14 Comparison: TCE in Soil Gas, Residential Receptor, 1-6 Risk 2002 Method Alpha RBSL (ug/m 3 ) EPA Q4 lookup EPA Q5 Att factor EPA Q6 Model EPA 2015 VISL EPA 2017 Model