Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.12



Figure 7-41: Installing a Passive Soil Vapor Sample Collector by Hand The hole is drilled using a roto-hammer. The soil vapor sample collector is installed using an insertion rod. The hole is then covered.


Figure 7-42: Example Plume Map from Grid-based Passive Soil Vapor Survey.

Passive sampling can be applied to either soil vapor or indoor air. Although the principle is the same in application to these media, the techniques for adsorbent placement, retrieval and analysis are different.

7.12.1 Passive Sampling of Soil Vapor

Implementing a passive soil vapor sampling strategy in the field requires careful consideration of the pertinent sampling variables, as described below.

Sample Spacing: The selection of sampling locations for passive sampling is based upon the same considerations as active soil vapor methods: project objectives and the need for adequate coverage. Predetermined and widely spaced grid patterns are most commonly used for reconnaissance work, while closely spaced, irregularly situated locations are commonly used for covering specific source areas.

Collection Depth: Although passive sample collectors have often been installed close to the surface (6 inches to 3 feet), that burial depth has been for convenience in deploying and retrieving the collector and not for technical reasons. In general, passive sample collectors should be deployed as near to the suspected soil or groundwater source as possible in order to reduce the chance of placing the sampler in a pocket of vapor-free soil in an otherwise contaminated area. In addition, collectors buried close to the surface will be very susceptible to air infiltration due to changes in barometric pressure and surface temperature. Therefore, at locations with uncovered soil, it is advisable to bury the collector to a depth of at least three feet. Placement of samplers at shallower depths may be acceptable for paved areas, depending on the objectives of the investigation. Check with the vendor for additional guidance.

Exposure Period: As with collection depth, the exposure period for passive collectors has often been generally selected more for convenience factors than for technical reasons. Typical exposure times have been a few days to a month.

In practice, the exposure period for a passive collector should depend upon the concentration of the contaminant of interest and desired detection levels. In areas of suspected high concentration, collectors can be left in the ground for shorter periods (1 to 5 days). In areas of suspected low concentrations, collectors are often left in the ground for two or more weeks. For areas of unknown concentration, the optimum approach is to determine the deployment time by burying a number of collectors in the same location and measuring them over a period of time.

The key assumption that is made when interpreting passive soil vapor data is that each collector is exposed to the same quantity of soil vapor. Therefore, it is most important that passive collectors within a sampling program be deployed for the same period of time in order for the data to be comparable.

QA/QC: The most important factors affecting the quality of passive soil vapor data is consistency of deployment and potential contamination of the samplers. Sampling teams should be trained in the deployment of the passive sample collectors to ensure consistent methodology is employed for each sample collector that is installed. To assess field contamination, analysis of field blanks and/or trip blanks is extremely important to verify that detected contamination was not from another source, such as the passive collector itself, or from handling and storage during transport to the laboratory.

There are several vendors of passive soil vapor sample collectors and those vendors should be consulted regarding specific installation procedures for their sample collectors and for any adsorbent-specific information, such as uptake rates. A time-sequence series of photographs that illustrates the installation of one vendor-supplied sample collector is shown in Figure 7-41. An example plume map from a grid-based passive soil vapor survey is shown in Figure 7-42.

7.12.2 Passive Sampling of Indoor Air

Passive sampling of indoor air is an evolving technique for evaluating potential indoor air impacts. VOCs are the most common target compounds but the technique is applicable to SVOCs as well. The primary advantage in comparison to active sampling is that passive sampling can be done over longer time periods and thereby reflect a longer-term average concentration. It may also be possible to deploy multiple passive samplers within a targeted floor or room and then combine them for extraction and analysis. This helps provide better coverage of targeted areas without increasing lab costs.

There are several vendors of passive indoor air sample collectors and those vendors should be consulted regarding specific applications and instructions for use. Data from passive samplers can be used to help focus the collection of whole air samples or sorbent tube samples.