Soil Vapor Intrusion in Occupied Spaces

April 8, 2019

 

Volatile organic chemicals (VOCs) are common in many products, but are especially seen in solvents and gasoline and a special risk if they are present in vapors within soil of a property.  Such “soil vapor” can penetrate floors and accumulate in occupied space, creating an adverse indoor air quality (IAQ) issue.

This is especially notable for vapors deriving prior use of a property beneath or near a dry-cleaning operation.  Most of these have used chlorinated solvents such as perchloroethylene (“perc”), with vapors showing extreme capacity for penetrating intact concrete and other barriers.

When a Phase I report reveals historical site/vicinity usage involving dry-cleaning or other soil vapor sources, a judgement must be made as to the likelihood for movement of vapor as could affect IAQ.

 

Soil vapor; bases for judgement, means of assessment:

A Phase I environmental assessment is usually the basis for assessing risk from soil vapor; in it, the likeliest rationale for assessing risk from soil vapor is distance.  Generally, 1/3-mile is taken as a minimal distance for discounting this threat, although this varies with other site features such as soil type, with dense, clayey soils usually credited for reductions in the minimal distance before a known source is deemed a “recognized environmental condition” (REC), per the ASTM guidelines.  Also considered are the depth to groundwater and the type of source, with mere use of solvents obviously less of a threat than actual release.  If onsite or offsite threats are deemed sufficient to warrant it and an REC is declared, further assessment usually takes the form of soil vapor sampling. This is done with a relatively narrow, shallow boring; approximately 2-inch diameter and 1 to 5 feet in depth.

 

Expect future blog entries for issues involving soil-vapor, such as:

  • The special technologies that provide rationales that may increase or decrease risk compared with simpler assessments based on a single point in time and place.

  • Other sampling that can assess impact from soil vapor.

  • Use of special technologies, such as those provided by Groundswell Technology, Inc.

  • Options for reduction of soil vapor risk.

 

Shown is a graphic found on Vapor Intrusion from the California Water Boards Website. For further information of California State Vapor Intrusion Information please go to:

https://www.waterboards.ca.gov/water_issues/programs/site_cleanup_program/vapor_intrusion/ 

 

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