The use of tracer gas is a common field quality assurance method for identifying atmospheric short-circuiting during the collection of soil vapor samples. It is widely used in several states to assess the integrity of soil vapor probes and determine whether a sample has been diluted and to what extent by ambient air. This paper reports on the results of a field validation study using helium as a tracer gas.

The study was conducted in two phases. Phase 1 was designed to obtain information regarding tracer gas detection. The primary Phase 1 result was that helium can be detected to soil depths of up to eight feet if administered according to published regulatory guidance.

Phase 2 of the study investigated the frequency and magnitude of short-circuiting under typical field conditions and standard probe installation protocols. Nineteen replicate soil vapor probes were installed in sand and twenty in glacial till during Phase 2. Although the helium never exceeded one percent in the samples collected from the sand, it was found to comprise up to 70 percent of the gas in the glacial till samples. It was also found that 20 percent of these samples were diluted by more than 10 percent.