Field Validation of Helium as a Tracer Gas During Soil Vapor Sample Collection

This paper reports the results of a field study to evaluate the use of helium as a tracer gas during soil vapor sampling. The authors found that the helium tracer method recommended in regulatory guidance can detect atmospheric leakage. However, the degree of leakage can be underestimated due to: 1) losses of helium from the tracer reservoir during sampling; 2) the entry of atmospheric air from an area outside the tracer reservoir; 3) temporal variability of leakage; and 4) flow-related reduction in helium detector response. Frequent leakage was observed in soil vapor probes installed using conventional direct push techniques. Probes installed at a depth of 8 ft exhibited less leakage than those installed at 4 ft. Probes installed in glacial till exhibited greater and more frequent leakage than those installed in sandy loam. Based on the frequency of observed leakage, refinement and standardization of tracer methods, with routine incorporation of tracers and in-line pressure and flow monitoring, is recommended.