What is a Soil Gas Sample?

Soil gas sample set up“What is a Soil Gas Sample?”

It’s a good question. It’s part of a string of questions most of us are familiar with -  the what, why, who, when, where, and how of something. Let’s check the list.

What is a Soil Gas Sample?

A soil gas sample is what you get when you remove a small portion of gas inside soil and hold it in a container for testing. Underground soil is like a sponge in that it has empty spaces. Between the surface of the ground and underground water (groundwater), these soil spaces are filled with gas and water. The gas is mostly air, but it can also have gaseous contamination from underground spills of chemicals such as gasoline and cleaning solvents (tetrachloroethylene or PCE). The soil gas may also have by-products of chemical spills like the vinyl chloride that comes from the breakdown of PCE.

Why get a Soil Gas Sample?

Soil gas is sampled to see if there are any harmful gasses that are either naturally occurring or the result of a spill. The samples are usually analyzed at a State-certified lab to find out what chemicals are in the soil gas sample and what the levels are. There are several reasons to collect a soil gas sample.

One reason is to find out if there is a risk to human health and the environment. To check for risk, chemical levels detected at the lab are typically compared to levels judged to be safe by the regulatory community. These levels, sometimes called environmental screening levels (ESLs), are considered safe when they do not pose a potential risk to public health and the environment.

Another reason to collect soil gas samples is to find out where contamination is and where chemical levels are highest. A map showing where chemicals are found and at what levels they are found can provide a good idea of the area of contamination and highlight areas with higher levels. This can help to steer property development plans and keep people away from potentially harmful areas. This information can also significantly lower investigation costs by showing where soil and groundwater sampling should be targeted.

In the case of known underground contamination, soil gas samples may be collected over a period of time to monitor the change in chemical levels. A change that shows a decrease in chemical levels is necessary for environmental case closure.

Sometimes the reason for soil gas sampling is to just find out whether there is underground contamination. For example, a property owner may have soil gas samples collected along a property boundary shared with a dry cleaning business to find out if contamination from the dry cleaner moved beyond the dry cleaner’s property line.

Who is the Soil Gas Sample for and Who Collects the Sample?

Usually, an environmental consultant or environmental oversight agency proposes collection of soil gas samples. As mentioned above, the reasons are to find out what chemicals are in the soil gas, what the levels are, and where the chemicals are located.

In most cases, personnel trained in soil gas sampling collect soil gas samples. Sampling goals and how the sampling will be done are typically spelled out in a work plan that is approved by a regulatory agency prior to samping.

Where are Soil Gas Samples Collected and Where Do They Go?

Soil gas samples are collected in areas of known or suspected underground contamination. Once the soil gas samples are collected, they are taken to a laboratory for chemical testing. In some cases, an on-site field laboratory is used to analyze samples.

In the field, soil gas sample containers are labeled with the sampling location, time the sample was collected, and a sample identification number. After labeling, samples are placed in a sealed container and transported to a state-certified laboratory for analyses. 

A chain of custody is prepared at the point of sample collection and it travels with the samples to the laboratory. The chain of custody contains contact information, a list of the samples collected, and other important information. It is a clear line of responsibility from the sample location to the laboratory.

When are Soil Gas Samples Collected and When do the Results Arrive?

Soil gas samples can be collected at any time, but it is suggested that samples not be collected within 5 days of a good rainfall. Policy makers suggest that after you know where the soil  gas contamination is, you should take samples twice per year to show levels during rainy and dry seasons. Once at the lab, it usually takes about two weeks to get the results. Of course, if you are willing to pay more, sample testing can be “rushed”.

How are Soil Gas Samples Collected?

Soil gas sampling equipment and wellSoil gas samples are collected from wells installed to at least 5 feet underground or from sample points that allow access to soil just underneath a concrete floor. In most cases samples are collected using a stainless steel canister that is supplied by a laboratory. The canister is delivered sealed and under a vacuum. During sampling, the canister is attached to a manifold that connects the canister to the well or sample point. The manifold is also equipped with pressure gauges, a filter, and a flow limiter that controls the flow of soil gas into the sample container to maintain sample quality. Sample collection stops when the canister pressure gauge shows the canister is nearly filled.


We answered the five Ws and a H for a soil gas sample. There are plenty of questions to ask about soil gas, contamination, and exposure at our homes and workplaces. We are here to help answer those questions and provide related services such as soil gas sampling and vapor intrusion mitigation system design,  installation, and monitoring.

If you have any other questions about soil gas, or need help with another environmental question, please call us or click the button below for a free consultation. We’ve been helping folks for 35 years and we know what we are doing.