Q: What flow rate should I use when sampling soil gas?
A: The recommended flow rate for collecting a soil gas sample is 200 cc/min or less. If a sample probe has good flow and no vacuum the flow rate can be up to 200cc/min. If a probe has a high vacuum the sampling flow rate should be decreased to lower the vacuum on the sample probe.
Q: How long after rain do I need to wait to sample soil gas probes?
A: Soil gas sampling should not occur during a significant rain event and should only occur after five days without a significant rain event. A significant rain event is defined as ½ inch or greater of rainfall during a 24-hour period.
Q: How long can soil gas probes last in the ground?
A: If properly set in the ground, with the above ground tubing and fittings well protected, a soil gas probe can last for many many years. Usually it is the above ground fittings and tubing becoming brittle, cracked, or damaged that will require a soil gas probe to be decommissioned.
Q: What is a tracer gas?
A: Tracer gas is used to detect leaks. A leak test is used to evaluate whether ambient air is introduced into the soil gas sample during the collection process. Atmospheric leakage occurs in three ways:
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- Advection voids in the probe packing material and along the borehole sidewall
- Advection directly through the soil column
- Advection through the fittings in the sampling train at the surface.
Q: What is the purpose of tracer gases?
A: If there is a leak a tracer gas will show up during the sample run indicating that ambient air was introduced into the sample.
Q: Does Helium make a good tracer gas?
A: Helium is not a very good tracer for many sites when you are attempting to measure its presence in the field. This is especially true for sites where hydrocarbons are suspected, vegetative degradation is occurring, high water vapor is present or where natural gas is present. . If you are collecting soil gas samples via passivated stainless steel canisters (e.g., SUMMA® canisters) or PVF bag and sending them to a stationary lab, helium can be used as a tracer compound although one should be careful with samples deeper than 5 feet because of helium’s tendency to rise. Unfortunately, there are many situations where helium cannot be used as a tracer because of its penchant for false positives
For a more in-depth look at Helium as a tracer gas please see: Helium Tracer Gas Myths
Q: What is the Dodge Method while soil gas testing?
A: The Dodge Method can be used to determine if there are leaks between different levels of a nested set of samples in a given boring. This technique involves putting an additional Magnehelic gauge on the end of the sample tube at an adjacent depth in a nested soil gas boring. (Figure:Dodge Method) Leaks are determined by pulling a vacuum at one depth and looking for the formation of a vacuum on an adjacent depth. For instance, if you are sampling at a 5 foot depth, the second Magnehelic is placed at the end of the sample tube at the 15 foot depth. (Note: The second Magnehelic should be more sensitive. We recommend no more than 5 inches/H2O as a maximum). A vacuum is pulled at the depth that the sample is being taken and watching the more sensitive Magnehelic at the adjacent depth. If there is movement, it strongly suggests that there is a leak or a pathway between the two boring depths and the possibility of improper hydration between depths has occurred. There are situations where a pathway can exist other than through the boring, for instance in a sandy soil, so each situation should be considered on its own merit.
Q: Can I collect a soil gas sample from an SVE system?
A: Yes, samples can be collected from SVE systems. Usually samples are collected from the influent, mid, and effluent to monitor carbon capture. Samples can also be collected from the individual SVE wells before the confluence to monitor individual wells for VOCs and to maximize the efficiency of the SVE system. A ball valve with a ¼ inch barb fitting is usually required to attached sampling equipment to a SVE system.
Q: Can I collect soil gas samples from an SVE well?
A: If an SVE system has been decommissioned or turned off, the SVE well can still be used to collect soil gas samples. A PVC cap with a ¼ barb needs to be put on the top of the SVE well for a pump to be connected to it. Note that SVE wells require a large purge time because of the borehole diameter, SVE pipe diameter, and screened section.
Q: What do you recommend for a purge volume on my site?
A: The purpose of purging is to remove stagnant air from the sampling system so that representative samples can be collected from subsurface. A default of three purge volumes should be used. Purge volume testing is no longer recommended.
Q: What flow rate should I use when purging?
A: The flow rate while purging is dependent on several factors: purge volume, tubing size, and vacuum. A flow rate greater than 200cc/min may be used when purging times are excessive, such as for deep wells with larger-diameter tubing. However, a vacuum of 100 inches of water or less must be maintained during sampling whenever a higher flow rate is used.
