Passive vs. Active Sampling: What a Danish Study Reveals

At the NORDROCS conference in Helsinki last month, Christina Voss Ernstsen of Central Denmark Region presented results from a Danish study evaluating passive sampling as an alternative to conventional active sampling for chlorinated volatile organic compounds (VOCs) in soil gas.

Active soil gas sampling is widely used for site characterization and VI assessment in Denmark but can be difficult or impossible with low-permeability soils and high groundwater levels.

To evaluate an alternative approach, passive soil gas samplers were tested at three contaminated sites representing different geological and contamination settings. At selected locations, Beacon Passive Samplers were compared with active charcoal tube sampling and another passive sampler.

Key findings included:

• Beacon Passive Samplers can be used to target chlorinated VOCs in soil vapor where active sampling may not be possible due to low permeability or high groundwater levels.
• Passive and active sampling showed similar spatial contamination patterns, although concentrations were generally higher with vacuum-induced active sampling.
• Beacon Passive Samplers successfully identified hotspots and defined the lateral distribution of VOCs.
• The other passive sampler tested, which has a higher uptake rate and is designed for indoor air sampling, showed comparatively inconsistent results.

Based on the study, it was found that a properly designed passive sampler is a valuable tool at sites where conventional active sampling is challenging or even impossible.

The results demonstrate that high-quality passive soil gas samplers are a robust, complementary tool for site characterization. By extending soil gas investigations into challenging geological and hydrogeological settings, or when vinyl chloride is in groundwater, passive sampling enables contaminant delineation and strengthens the conceptual site model used to guide further investigation and risk assessment.