Soil gas sampling is increasing in frequency across the country as vapor intrusion continues to gain regulatory attention. When evaluating the potential for vapor intrusion at a particular site, it is useful to collect soil gas samples to find out how vapors and contaminants of concern are migrating in the subsurface, and whether or not those vapors are migrating indoors. Soil gas sampling, used in conjunction with state specific screening criteria and/or modeling, is often an intermediate step between screening based on groundwater concentration and collecting indoor air samples.
The goal of soil gas sampling is to collect a sample of the vapor that resides in the interstitial soil pores near a source of contamination and/or near a potential receptor structure. To sample soil gas, a temporary or permanent soil vapor probe is installed. If the well is installed incorrectly or is not sealed properly, leaks to the ambient air may occur. This can dilute or otherwise influence the concentrations seen, potentially leading to incorrect decision making.
Using a tracer gas can give quantitative proof that the sampling system was installed and sealed correctly. The tracer compound is placed around the soil gas probe at the ground surface, so that if the well is installed correctly and everything is sealed properly, no tracer compound will be seen in the sample. The soil gas sample is then collected. If the tracer compound is detected in the soil gas sample, it is an indicator that some amount of leaking has taken place and the sample may be deemed unrepresentative or even invalid.
Several state vapor intrusion guidance documents make recommendations about which soil gas tracers to use, but most states leave room for professional judgment by the environmental professional to use other compounds. Each potential tracer compound has its benefits and its drawbacks from a sampling and analytical viewpoint.
Many professionals have successfully used helium as a tracer compound for soil gas surveys. As opposed to other tracers, such as isopropyl alcohol, helium will not interfere with the TO-15 analysis even if there is a small leak. Another unique benefit is that helium may be monitored and evaluated onsite so leaks can be proactively fixed in the field prior to sampling.
In addition to choosing an appropriate tracer compound, when collecting a representative and defensible soil gas sample, it is also important to follow thorough quality control and quality assurance practices, and to have several lines of evidence to support your conceptual site model.
Measuring the levels of oxygen, carbon dioxide, and methane present in the soil gas (indicators of biological activity) can also prove useful. Measurement of these indicator compounds can be done onsite with a multigas meter and/or at the analytical laboratory via EPA Method 3C modified (GC/TCD). Interpretation of the fixed gas data can provide a secondary line of evidence to support the conceptual site model. For instance, if the oxygen profile is decreasing with depth and suddenly a deeper soil gas sample shows an increased concentration (or near ambient levels) of oxygen, it is possible that a leak occurred, letting in ambient air.
It is important to note that leak testing of the sampling train can also be done with the use of a vacuum pump and a magnahelic gauge. Evacuate the sampling line, close off the soil gas point and the canister with a gauge in line, and observe whether the gauge needle returns to zero. If the needle moves back to zero, a leak is present somewhere in the system.
The field of vapor intrusion is constantly evolving, and accepted sampling procedures may change. Always check for any applicable state or Federal regulatory guidance prior to conducting sampling.