GeoSyntec Future Directions for Assessing Vapor Intrusion by Todd McAlary, GeoSyntec Consultants, Inc. AEHS VI Workshop October 19, 2004

GeoSyntec Outline Improved Protocols – What’s Coming Improved Protocols – What’s Coming Qsoil Qsoil Perimeter crack model vs 0.05>Q soil /Q bldg > Perimeter crack model vs 0.05>Q soil /Q bldg > Barometric Pumping Barometric Pumping Implications for Data Variability Implications for Data Variability Alternative Sampling Options Alternative Sampling Options More discrete in time and space More discrete in time and space More integrated More integrated

GeoSyntec API - API - Collecting and Interpreting Soil Gas Samples from the Vadose Zone: A Practical Strategy for Assessing the Subsurface Vapor-to-Indoor-Air Migration Pathway at Petroleum Hydrocarbon Sites – Final Draft, July 2004 Health Canada – Soil Vapour Intrusion Guidance for Health Canada Screening Level Risk Assessment (SLRA) – Final Draft, October 2004 EPRI - Best Practices Manual For Evaluating Subsurface Vapor Intrusion to Indoor Air – Preliminary Draft, Sept 2004 ITRC – Workshop next week to develop an outline Others? Improved Protocols

GeoSyntec Conceptual Models Conceptual Models More tools in the toolbox More tools in the toolbox Standardization Standardization QA/QC checks QA/QC checks Interpretive Tools Interpretive Tools Areas for Advancement

GeoSyntec Q soil : Perimeter Crack Model 0.8 L/min > Qsoil > 0.05 L/min

GeoSyntec Q soil : Current Recommendation 0.05 > Qsoil/Qbuilding > (Johnson, 2002) (perhaps 0.01 to is more common) For a typical residence: V ~ 600m3, AER ~0.5 hr-1 Qbuilding ~ 300 m3/hr or 5,000 L/min 250 L/min > Qsoil > 0.5 L/min (perhaps 50 to 0.5) Generally higher than perimeter crack model OSWER Guidance uses 5 L/min, strictly empirical

GeoSyntec Barometric Pumping Increasing B.P. Decreasing B.P. Gas is compressible, so changes in B.P. either compress soil gas or allow it to expand. No net flow in the long-term, but short term... Ideal Gas Law: P 1 V 1 = P 2 V 2 Ground Surface

GeoSyntec Transient Effect of Barometric Pumping Q soil-BP = transient soil gas flow rate induced by B.P. change Q soil-BP = transient soil gas flow rate induced by B.P. change A = area of the footprint of the building A = area of the footprint of the building  a = air-filled porosity  a = air-filled porosity x bp = depth of barometric pressure propagation x bp = depth of barometric pressure propagation ΔP = barometric pressure change over time period “t bp ” ΔP = barometric pressure change over time period “t bp ” P o = mean air pressure P o = mean air pressure 0.02 > ΔP/P o t bp > (Parker, 2002)

GeoSyntec Q soil : Perimeter Crack Model vs Delta BP Magnitude agrees well with empirical estimate of 5 L/min But in the short-term, Qsoil can be -5 to 5 L/min!

GeoSyntec Barometric Pressure Changes Arrows represent approximately 24 hours, during which BP may change a lot (>200Pa in this example), or hardly at all Compare this to 4Pa stack effect! (dP can be up to 5,000 Pa)

GeoSyntec Temporal Variability

GeoSyntec Implications for Data Variability How much of this is due to short-term barometric pressure fluctuations?

GeoSyntec Alternative Sampling Options Option 1: More Integrated Data Time-Integrated: sample over several barometric cycles (e.g. 7 days) to smooth temporal variability Volume-Integrated: to smooth spatial variability Option 2:More Discrete Data over time Real-time monitoring Vertical Profiling Monitor BP and delta P to establish correlation

GeoSyntec Time-Integrated Samples Summa Canisters Summa Canisters Smaller critical orifice Smaller critical orifice Solenoid valve “timer” Solenoid valve “timer” ATD Tubes ATD Tubes Lower Flow Rate over Longer Time Lower Flow Rate over Longer Time VOST Samplers – borrow stack testing tools? VOST Samplers – borrow stack testing tools? PUF: TO-13 designed for 300m 3 samples PUF: TO-13 designed for 300m 3 samples 10m by 10m house, 10 m vadose zone, 30% air-porosity… 10m by 10m house, 10 m vadose zone, 30% air-porosity… Or Qsoil for 24 hours… Or Qsoil for 24 hours… Radon analogy: passive electret samplers

GeoSyntec Volume Integrated Samples (DiGiulio, 2004) Representative Purge Volume

GeoSyntec High Purge Volume Sampling

GeoSyntec PID readings ~ 1,000 ppmv over 1,700,000 L (3 days) indicates concentrations are spatially uniform Total mass removed ~ 10 kg (10 -6 risk needs only kg!) Combine with pneumatic testing, assess gas K Q = 15 scfm

GeoSyntec Is Bigger Always Better?

Huntington Beach Soil Gas

GeoSyntec Summary & Conclusions New and Improved Protocols are coming – data quality New and Improved Protocols are coming – data quality Qsoil from BP changes is probably not negligible Qsoil from BP changes is probably not negligible What is the contribution to data variability? What is the contribution to data variability? $$ to find out? $$ to find out? Risk assessment requires long-term average exposures Risk assessment requires long-term average exposures Time- or volume-integrated samples may help Time- or volume-integrated samples may help Vertical profiling needed for assessing biodegradation Vertical profiling needed for assessing biodegradation