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May. 26, 2009 - Merrillville, Indiana

Vapor Intrusion

Authored by: Microbac Laboratories

Vapor Intrusion refers to the process by which volatile chemicals move from a subsurface source into the indoor air of overlying or adjacent buildings. The class of chemicals that is most likely to be of issue for vapor intrusion is VOCs (Volatile Organic Chemicals). These chemicals volatilize at ambient and low temperatures. Therefore, these organic vapors are more likely to migrate from contaminated soil or groundwater to building structures above. These vapors can migrate directly through concrete floors and enter the occupied indoor air of a building and cause potential health hazards.

Two of the most common types of VOC contamination in soil and groundwater are petroleum and chlorinated solvents. Petroleum contamination often occurs as the result of leaking underground storage tanks, pipelines or spills. Chlorinated solvent contamination often results from dry-cleaning facilities or industries that have used degreasing agents in their processes.

Vapor intrusion studies often involve analysis for specific indicator compounds. For gasoline contaminated sites, benzene is the predominant contaminant of concern. Benzene is a carcinogen and the indoor air health protective level for this compound is among the lowest for all the gasoline-related compounds. The contaminants of concern for chlorinated sites are Tetrachloroethylene (PERC), Trichloroethene (TCE), 1,2-Dichloroethane (1,2-DCA), and Vinyl Chloride (VC). These compounds have low indoor air health protective levels.

When performing a vapor intrusion study, there are several different methods of sampling that can be performed. These involve sub-slab sampling, soil gas sampling, and indoor air testing.

Sub-slab sampling is a process in which air samples are collected at a point immediately below the basement or slab of a building. This process involves drilling one or more holes through the concrete floor, placing a probe through the concrete and then collecting the air sample into a stainless steel canister (summa canister).

The sub-slab sampling procedure mentioned above is the preferred technique for evaluating the potential for vapor intrusion. However, this method is difficult and intrusive because drilling holes through the concrete is required. Therefore, soil gas sampling is another option. Soil gas sampling involves sampling the soil gas adjacent to the building of concern. The soil gas sampling points are in up-gradient and down-gradient areas of the groundwater flow. This method involves creating a borehole in the soil and then sampling the soil gas within this space. As with sub-slab sampling, this is performed using summa canisters to collect the sample.

The sub-slab and soil gas sampling methods described above only determine if a potential for vapor intrusion exists. If the potential for vapor intrusion does exist within a building, indoor air sampling may be required. Sampling indoor air involves using a summa canister to take an air sample inside the occupied space of a building.

Using the procedures described above, vapor intrusion or the potential for vapor intrusion can be determined.

Summa canisters are stainless steel canisters that typically come in 1L and 6L volumes. These canisters are prepared at the laboratory and are sent to the field under vacuum. Once the air sample is ready to be collected, the valve is opened and the evacuated canister draws in an air sample. These summa canisters are then returned to the laboratory for analysis. Results are typically reported in ppbv or ug/m3.

Many state programs are now implementing guidance documents and regulations on vapor intrusion in addition to their soil and groundwater remediation objectives.

The Chicagoland Division of Microbac Laboratories, Inc. performs air analysis for vapor intrusion studies and is familiar with the guidance and regulations of several state programs. Microbac Laboratories can provide all sampling media needed, including summa canisters, flow controllers, tubing, etc.

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