Ecosystems Research Division

Vapors from subsurface contamination may intrude (hence the name "vapor intrusion") into residences causing in some cases, acute health and safety problems or when concentrations are lower, chronic health problems. Assessment of potential vapor intrusion is plagued by difficulties that include:

• sometimes limited ability to sample within residences,
• distinguishing between subsurface contaminant sources and background ambient air contamination,
• sources and sinks of vapor contaminants within residences,
• temporaral variation of vapor entry due to variation in soil moisture, barometric pressure, and temperature,
• application of simplified models without site specific inputs or site specific validation

To address the last of these problems, research is being conducted on sensitivity and uncertainty of vapor intrusion models (primarily variants of the "Johnson-Ettinger" model). The online version of the model addresses uncertainties in two parameters: depth to water and soil type. These were chosen because they are the two site-specific inputs used in the draft guidance document developed by EPA.

In the guidance, the model is used at an early stage in site screening to determine which sites would not need further investigation or sampling. Notably the uncertainty associated with other parameters of the model is not assessed. Additional work is currently underway to provide the means to include uncertainty analysis for any parameter of the model.

To address some of the field problems of vapor intrusion, a field site has been instrumented for continuous measurement of moisture content, soil gas pressure, rainfall, and other climatic variables. The site consists of new construction; borings were made under the slab before the concrete was pored. Eight borings were instrumented with soil moisture probes, pressure sensors and sampling ports. The ninth and deepest boring was supplied with two injection ports for future tracer gas injection. Additional borings are to be made to instrument the surrounding area. This will allow for comparison between the native ground and the vadose zone below the building.

Construction Photos
Shallow holes were hand-augered for placement of moisture probes, pressure ports and sample tubes.
Screened intervals on sampling tubes.
The sampling tubes and moisture probe wires were pulled through conduits to protect them during construction.
Three completed borings/probe installations on the north edge of the building.
The nearly-finished installation with conduits run to all under-slab borings. Empty conduits were left for future installation of exterior borings.
Tubing and wires sealed for protection during cement pour.