This blogpost is a summary based on a webinar hosted by TestAmerica, presented by Taryn McKnight on January 24, 2017 entitled, Vapor Intrusion Assessments Part One: Improving Data Quality Using Today’s Best Practices for Sample Collection.
Vapor Intrusion is the term given to the migration of hazardous materials (especially Volatile Organic Compounds, VOCs) from the subsurface into overlying buildings. Other possible vapor intrusion materials may include certain semi-volatile organic compounds and inorganic chemicals, such as elemental mercury, naturally occurring radon and hydrogen sulfide. Sources from the subsurface may include contaminated groundwater or impacted soils. Varying hazards may occur from the vapor intrusion of differing chemicals such as a safety hazard (potentially related to flammable vapors intruding a building or enclosed area) or a health hazard both acute (headaches, nausea, etc) and chronic (carcinogenic, etc).
Vapor intrusion science has progressed from the Johnson and Ettinger model in 1991 to the EPA establishing the OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air in 2015.
Variability (factors) in vapor intrusion studies include but are not limited to, barometric pressure, surface cover, preferential pathways, soil moisture & permeability, building depressurization, biodegradation, background air, etc…
Pros of indoor air sampling include, actual indoor air concentration, no modeling, no attenuation factors, relatively quick with no drilling or heavy equipment, and less spatial variability than soil gas. Cons associated with indoor air sampling include, planning time with the home/property-owner to perform sampling events, the removal of potential interior or lifestyle sources and contribution from unknown indoor sources and ambient air. The collection of outdoor air in conjunction with indoor air is a way to check the contribution of ambient air to the indoor air sample. It is important to conduct a building survey prior to the collection of an indoor sample. The building should include the deduction of potential background contamination sources such as common household products, building materials, etc… Sub-slab sampling can help resolve the issue of background contamination by bypassing the contamination potential from the room and sampling closest to the source of vapor intrusion.
Pros of soil gas sampling include, near source data and may provide an estimate of source vapor concentration. Additionally, it can be performed without entering the structure as to eliminate the need for coordination with the buildings occupants. Cons of soil gas sampling include, significant lateral and vertical variability and may not be representative of vapor concentrations under the buildings. The EPA suggests several rounds of sampling are generally recommended. Soil gas sampling protocols include, purging the tube using a syringe, bag or canister, dialing in a flow rate for purging/ collecting using a flow controller, measure the vacuum being applied on the subsurface using a vacuum gauge, rotometer or syringe, measure for biodegradation at sites containing petroleum by testing for CH4, O2, and CO2 and application of a tracer gas such as helium, Freon or isopropyl alcohol (IPA) using a shroud and a field detector.
A leak check using a tracer gas is to be performed to ensure no leaks from ambient air into the sample collection system is found. The shroud is placed over the sample probe with a hydrated bentonite seal where an inert tracer gas is added to the shrouded environment. A test sample is then drawn from the sampling probe and if detection of the inert gas is observed, then ambient air is capable of reaching the subsurface and contamination of the sample collected is likely. Refer to the figure below to view a basic “in field” leak test.
A second leak test referred to as the shut-in test is to be performed on the sample collection system. This test is to ensure there is an air tight seal between the sample probe/tube and sample collection system such as a Summa canister. The figure below highlights the steps to perform an air tight seal.
Finally, canisters have no preservation required and can be shipped via air with few caveats. The hold time on canisters is specified in TO-15 as 30 days so it is generally dependent on the consultant’s necessary turn-around-time. The quality of data collected is dependent on the entire process of data collection from beginning to end. A careful process is necessary for thorough, accurate and precise data.