New Models to Support Vapor Intrusion Mitigation Design

Transcription

1 New Models to Support Vapor Intrusion Mitigation Design USEPA Work Shop, AEHS 27th Annual International Conference on Soil, Water, Energy and Air March 23-26, 2015, San Diego, CA Dr. Ian Hers, Dr. Parisa Jourabchi, Paul Hurst, Golder Associates Ltd. Presentation Outline Introduction to mitigation options Modified Johnson and Ettinger model Venting design tool Soil permeability estimation tool Steady State Hantush-Jacob Method Goal is improved design basis, expand tool box and hopefully lead to more sustainable and effective mitigations 1

2 Mitigation Options - Existing Buildings Subslab depressurization (SSD) Soil vapor extraction Can be effective approach for deeper vadose zones and coarse-grained soils Building HVAC modifications Install Heat Recovery Ventilator (if exhaust only ventilation) Modify HVAC to positively pressurize building and/or increase ventilation rate Energy cost associated with heating/cooling outdoor air Brings more moisture inside the building envelope (mold) Air purifying unit Not a permanent solution Mitigation Options Future Buildings Venting layer options Aerated subfloor (e.g., Cupolex) Conventional sand & gravel layer Barrier options Spray-applied Sheet membranes Venting options Active (fan or blower) Passive (stack effect, wind turbine) Heat absorbing materials March 26,

3 Modified J&E Model Conceptual Site Model Modified Johnson and Ettinger (J&E) Model Features slab on grade Sub-building garage 1-D soil gas diffusion through multiple (up to 5) soil layers Diffusion through bulk concrete Diffusion through geomembrane Benchmarked to the US EPA Superfund J & E Model Sub-building ventilated layer: Parking garage Crawl-space Aerated floor or gravel layer March 26, Casse Study #1: Modified J&E Model 10 Layers Mass balance: 9 algebraic equations and 9 unknown concentrations 1 _ 1 _ 2 9 & March 26,

4 Modified J&E Model Case Study Review of empirical data and modelling study performed for Ontario MOECC to better understand vapour attenuation factors for: High density residential building (condo) with underground parking structure (parkade) Commercial/industrial building with passive (wind turbine) or active venting system Goal was more efficient framework for evaluating risk management measures (RMMs) as part of Tier 2 Modified Generic Risk Assessment (MGRA) process and definition of multipliers or reduction factors for RMMs that could be reliably achieved and thus only warrant limited ongoing monitoring Modelling study involved calculation of Reduction Factors = Tier 1 generic attenuation factor (no mitigation)/ Tier 2 attenuation factor (mitigation) March 26, Modified J&E Case Study - Small Commercial Building J&E Parameters Parameter Unit Value Building width (m) 20 Building length (m) 15 Foundation thickness (-) 0.11 Crack width (mm) 1 Foundation crack ratio (-) 2.3E-04 Air change rate (hr-1) 1 Building height (m) 3.0 Distance building to (m) 0.3 vapor source Air-filled porosity (-) 0.39 Total porosity (gravel) (-) 0.40 Soil gas advection rate L/min 9.8 (Q soil ) Venting Layer Baseline Parameter Unit Value Air change rate 1 (hr-1) 1 Thickness (m) 0.3 Air-filled porosity (-) 0.39 Total porosity (gravel) (-) 0.40 Barrier Layer Baseline Parameter Unit Value Thickness of barrier (mm) 1.5 Permeation Coefficient 2 (m 2 /s) 2E-12 Defect ( crack ) Ratio 3 (-) 7.5E-08 1 Conservative estimate based on empirical data; 2 Based on published values for 60 mil Liquid Boot, 60 mil GeoSeal, 80 mil HDPE (McWatters & Rowe, 2010); 3 Based on landfill studies 4

5 Modified J&E Model Case Study March 26, E+03 1.E+02 1.E+01 1.E+00 1.E 01 1.E 02 1.E 03 1.E 04 1.E 05 1.E 06 Modeling Study Results Passive Venting, Geombrane Barrier Poorer performanc e 11 2 No mitigation: 6.1E /0 0.8/0 0/0 0/0.5 Q soil (L/min) / Vent ACH Passive venting performance variable, depends mostly on KEY POINT: whether Qsoil = 0, therefore venting efficiency and barrier quality control is important, barrier diffusion properties not as important AF = attenuation factor RF = No mitigation AF / mitigation AF Better performance AF RF Qsoil ( P) and Vent Layer Air Change Varied 5

6 Passive Venting Performance for New Buildings w\ Wind Turbines Pressure and Flow Study Building Performance Reinis et al. Commercial, Vent air flow rate out of stack = cfm, (2012) Oakland, N= 28 Negative pressure in venting layer CA Reinis et al. Commercial, Vent air flow rate out of stack = cfm, (2006) Oakland, N = 2 Negative pressure in venting layer CA Golder (2015) - unpublished Commercial, N = 2 Vent air flow rate = 1.2 cfm out of stack (wind speed = 1.4 m/s) KEY POINTS: Both convection from stack effect and wind turbine important mechanisms for passive venting; positive airflows out of vent stacks were measured, but performance decreased with increasing temperature (reverse stack effect) 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 1.E 01 1.E 02 1.E 03 1.E 04 1.E 05 1.E 06 1.E 07 Modeling Study Results Active Venting, No Barrier Vent Layer ACH 5159 AF RF Vent Layer Air Change Varied (Q soil = 0) KEY POINT: Approximate three orders of magnitude reduction in attenuation factor for conservative venting rate (1 hr -1 ) 6

7 Modeling Study Results Active Venting, No Barrier 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 1.E 01 1.E 02 1.E 03 1.E 04 1.E 05 1.E 06 1.E 07 1.E E E 4 2.3E AF RF Crack Ratio Varied (Vent ACH = 1 hr -1 ) Crack Ratio KEY POINT: Crack ratio is sensitive parameter therefore important to seal foundation Key Modeling Results Baseline simulations for industrial/commercial building according to MOECC defaults (vapour attenuation factor = 6.1x10-4 (indoor air/soil vapour concentration)) Mitigation attenuation factors expressed as reduction factor (RF) relative to baseline Model Results RF Range Representive RF Key Factors Passive 2 to 560 > 100 Building depressurization, liner quality (openings) and passive venting efficiency, if high quality liner with minimal openings then higher range of RF Active 100 to 9300 > 200 Depends on venting layer air change rate/foundation properties 7

8 Typical Leaks and Pathways Column penetrations Floor penetrations Inlet pipes Electrical conduits Damaged vapour barrier Plumbing fixtures 15 Venting Design Tool Subslab material options Aerated subfloor (e.g., Cupolex) Conventional sand and gravel layer Energy options Stack effect Wind turbine Solar-powered fan Electrical fan or blower KEY POINT: Range of options are available with variable cost and sustainability but quantitative design tools are limited March 26,

9 Venting Design Approaches Goal is to size conveyance piping, fans/blowers and address constructability issues 1. Sizing: can use existing design guidance for new buildings, e.g.: If there is clean crushed stone and 4- inch conveyance piping, a fan that can move 200 CFM at -1.0 inch w.c. can create a vacuum field of inch w.c. or greater over a 4,000 ft 2 area. 2. Alternatively, determine sizing based on site-specific venting design calculations Vapor Intrusion Mitigation in Construction of New Buildings Fact Sheet, Naval Facilities Engineering Command (NAVFAC) Venting Design Tool - Model Description Active systems use fan or wind turbine Components consist of venting layer (gravel or aerated floor) and risers, may or may not require barrier Air Exit Fan 1. Assume piping layout X-section 2. Choose flow rate (could be based on Modified J&E) 3. Calculate friction losses in 1) pipe and 2) venting layer Pipe lateral 4. Compare frictional losses and evaluate short-circuiting 5. Generate flow vs loss curve 6. Choose blower 7. Conduct pilot test 18 Air Inlet Plan 9

10 Venting Design Tool - Fan/Blower Sizing FAN BLOWER Modelling for selection of appropriate pipe material and diameter, fan/blower selection 19 Negative Pressure (in WC) HDPE Venting Design Tool - Comparison between Modeled to Measured Data Mid k=5e 8 m 2 6 " HDPE n = " PVC smooth n=7e 6 PB 10A 4 " HDPE n = " PVC smooth n=7e 6 6 HDPE measured Flow Rate (cfm) 8 Boss 1000 Big O HDPE 6" Lateral 7 Low k=5e 9 m2 Mid k=5e 8 m2 6 High k=5e 7 m2 5 PB 10A Measured Flow Rate (cfm) KEY For this example, pipe losses for 4 inch pipe were POINTS: predicted to be more important than losses in soil (varies depending on spacing laterals in soil) Negative Pressure (in w.c.) March 26, Jourabchi et al. Battelle

11 Venting Design Tool - Frictional Losses (Pressure Drop) in Soil q c = k c / * P l / L c Q c = W c H c k c / * P l / L c Pipe lateral Darcy Eq P l = (Q c * * L c ) / (W c * H c * k c ) m 3 /s N s/m 2 m = N =Pa m m m 2 m 2 Air flow Lc k c = soil-air permeability (m 2 ) dynamic viscosity of air (Ns/m 2 ) L c = distance in soil parallel to air flow (m) P l = pressure loss (Pa) W c = width of flow zone (m) H c = height of flow zone (m) Q c = flow rate (m 3 /sec) Wc Hc Venting Design Tool - Frictional Losses in Soil Example Calculation Pressure drop due to frictional loss estimated using Darcy s Law, 1-D horizontal laminar air is calculated based on eq. on previous slide Example calculation, assume 100 cfm flow, 0.2m thick venting layer, 20,000 sq.ft building Calculated pressure drop: Aerated subfloor (Cupolex) K = 1E8 darcies P = 0.01 Pa Gravel K = 5E4 Darcies P = 20 Pa P in gravel potentially significant for passive or wind turbine 22 11

12 Venting Design Tool - Frictional Loss in Pipe D Arcy-Weisbach equation for pipe loss Δ D Arcy-Weisbach friction coefficient, λ, calculated using Colebrook equation (if turbulent flow) See Engineering Toolbox Calculators l = Darcy-Weisbach friction coefficient, I l = length pipe, d hl = hydraulic diameter, q = density of air, V l = velocity air in pipe, Re = Reynolds # March 26, Venting Design Tool - Passive Methods - Wind Turbines Wind siphoning air flow for attic vents may be estimated from Australian Standard AS4740: For reasonable range of inputs, air flow rate of cfm is calculated for wind speed of 12 km/hr (moderate wind speed) However, this calculation does not account for reduction in flow rate due to pressure drop Air flow severely restricted when frictional losses October 4,

13 Venting Design Tool - Revel & Huynh (2004) Wind Turbine Testing October 4, Soil permeability estimation tool Steady State Hantush Jacob (H-J) Method Steady state solutions can be used to estimate soil air permeability of subslab fill for input into SSD design calculator H-J Method (ie removes Bessel function, (r/l <0.16 error<1%, r/l <= 0.22 error <2%, r/l <= 0.33 error < 5%, r/l <= 0.45 error < 10%) Analysis & Evaluation of Pumping Test Data, 2 nd Ed G.P Kruseman, N.A. de Ridder Pub 47 Int l Institute for Land Reclamation and Improvement K (m2) slab: 2.9E-15, K (m2) granular: 1.3 E log 1.12 DIFFERENTIAL PRESSURE (PASCALS) Analysis of Steady-State Flow in Semiconfined Aquifer: A New Approach S.K. March 26, Gupta & P. Sharma, 1985 Vol. 23, No.2- GROUND WATER K (m2) slab: 6.8E-14, K (m2) granular: 3.1 E y = 41.45ln(x) DISTANCE (M) Slope Intercept Leakage factor, L (m) Discharge m 3 /day 1264 K' (m 2 ) (slab) 6.8E-14 K i (m 2 ) (granulars) 3.1E-09 ROI (6 Pa), m 8.6 Grain Size Sample Results: Medium to fine-grained sand with 10% fines and 14% water conten Analyis using method in Analyisis of Steady-State Flow in Semiconfined Aquifiers: A New Approach S.K. Gupta & P. Sharma, 1985 Vol. 23, No.2- GROUND WATER 13

14 Conclusions Design tools are presented that potentially provide basis for optimized design. Modified J&E model enables incorporation of barrier and venting layer in calculations case study indicated how model could be used to support regulatory multipliers for risk management measures Active venting system performance predicted to be more robust than passive, but will depend on quality liner and efficiency of passive venting Venting design tool enables prediction of frictional losses in soil and piping systems, and provides for basis for optimized and more sustainable venting design Extra Slide - Modified J&E Model Upward chemical diffusion (vapor- and aqueous-phase) within the native soil layer(s); Instantaneous mixing of chemical vapors within a sub-building compartment and dilution through venting; Upward chemical diffusion through a liner placed above a venting layer (if present); Soil gas advection through foundation cracks due to sub-building depressurization; Chemical diffusion through dust-filled cracks in the concrete floor of the building and the sub-building; and optional consideration of chemical diffusion through the concrete foundation; Instantaneous mixing of vapors in the air space of the building through ventilation. March 26,