Vapor Intrusion from Subsurface to Indoor Air: Biodegradable d Petroleum Vapors versus Recalcitrant t Chemicals Vapor Intrusion 2010: Air & Waste Management Association (A&WMA) Conference Chicago, IL. Dates: September 29-30, 2010 Use this area for cover image (Maximum height 6.5cm & width 8cm) George DeVaull george.devaull@shell.com 1
To Cover Conceptual Models Compartments: Building; Foundation; Soil Layers; Vapor Source What are conditions for potential worst-case case indoor air impacts? For non-degrading chemicals For aerobically degrading g petroleum eu hydrocarbons Worst-case occurs at extremes of parameters not averages Helps reconcile models and observed behavior trends Footer: Title may be placed here or disclaimer if required. May sit up to two lines in depth. 2 2
Conceptual Model: Non-degradable chemicals What are potential worst case conditions? Building: High vapor resistance Closed up. Low air exchange rate. Holds vapors indoors. Foundation Low vapor resistance High vapor migration through floor. Dirt floors, cobble foundations, open cracks, etc. Worst-case is NOT poured concrete foundation Soils and Source Air-filled soils. Low vapor resistance smaller separation, higher h concentrations ti 3
Foundation Resistance Advective flow: Pressure gradients across foundation drives airflow. Diffusive flow: Chemical flow driven by concentration gradients total t flux = diffusion i + advection 10 Pa = 1.02 mmh 2 O ~10-4 atm Plot: Patterson & Davis, Environ. Sci. Technol., 2009, 43, 650 656. Flow both ways Cyclic fluctuating flow through a foundation causes advective mass transfer, even if time-averaged airflow is zero. The foundation breathes. Below foundation air moves into indoors, and Indoor air moves below the foundation One-way pressure-driven flow from soil through h foundation to indoors not confirmed by measurement 4
Foundation Resistance Could be either (or both) Advection or Diffusion Advective Flow Diffusive Flow Q = Equivalent airflow Foundation Area, A f D eff = Effective Diffusion coefficient Foundation Thickness, L f Either inferred from concentration measurement Either way, chemicals move across the building foundation: 5 5
Equivalent Airflow Through Foundation Empirical Field Data Summary Total range: 0.23 to 60 L/min bare and dirt floors, cobble foundations, highly permeable foundations, depressurized buildings 0.23 to 15 1 to 10 (avg.) 0.9 to 28 37 to 60 concrete slab on grade, concrete basement, enclosed crawl space with vapor barriers, concrete pavement diffusion through air dry concrete (10 to 20 cm thickness) 0.1 1 10 100 Equivalent Air Flow Rate (L air/minute) for 100 m 2 foundation Broad foundation range reconciles indoor/sub-slab data variability. Potential Worst case is at extremes, not at the average. Average Value Average Indoor/Subslab Attenuation Factor Worst-case Value Worst-Case Indoor/Subslab Attenuation Factor 6
Biodegradable Hydrocarbons: Evidence for Degradation z 0 = profile knee, below which: (low) zero oxygen, water table, residual oil, or high organic soil Exponential concentration decrease above z 0: C (z) / C 0 = exp (- z / L R ) L R = log slope: 0.6 ft ( 0.16 to 1.6 ft ) Median [range] L R = Reaction Length Distance to exp(-1) ~ 0.368 Key: Vadose Biodegradation Rates are Large (with O 2 Present) Greater aerobic depth, more attenuation 7 7
Conceptual Model: Aerobically Biodegradable chemicals What are worst case conditions? Compared to no-biodegradation Aerobic Biodegradation reduces indoor air concentration levels. Less Biodegradation Less oxygen in shallow subsurface Shorter aerobic depths Occurs for: Greater oxygen demand. Greater petroleum impacts Higher foundation resistance (if O 2 limited). i Key Ideas: Worst Case conditions i for Building, Soils and Source, same as non-degrading di case Worst Case Foundation Type is the Opposite Extreme 8
Trends Effect of Equivalent Foundation Airflow Base Case Exclusion Distance : 5 ft depth, water-dissolved source 1 mg/l benzene, 10 mg/l BTEX R. Davis (2010) or Air Benz zene Concen ntration (ug g/m 3 ) 100 10 1 Without Biodegradation Higher foundation airflow, Higher indoor air concentration With Aerobic Biodegradation Higher foundation airflow, Lower indoor air concentration Indo 0.1 0.01 0.001 (if oxygen limited) 0.0001 Equivalent Foundation Airflow (L/min) 0.1 1 10 100 Biodegradation d i neglected Biodegradation included Below nominal indoor air background Soil Layer Entirely Aerobic Model Estimates (BioVapor, www.api.org/vi) Residential default parameters 9
Summary Potential worst case indoor air conditions? Building: Low Air Exchange. Smaller Mixing Height Foundation: Effective Airflow Soils Non-degrading d Chemicals: 37 to 60 L/min (High) Aerobically Degrading: 0.23 to 1 L/min (How) For 100 m 2 foundation Air-filled soils. Sand Soils. Smaller Separation Distance. Source Higher Source Vapor Concentrations. 10
Summary: Key Ideas Foundation: Simpler description Effective Airflow per Unit Area: 0.23 to 60 L/min for 100 m 2 foundation Advection or Diffusion not differentiated Bounded Range Compartment Level Evaluation: Building, Foundation, Soils, Source Sensitivity Checks: Permutation from Screening Criteria (example) Which compartment has the greatest variability? Choose assessment or mitigation choices for most sensitive compartments t 11
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