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Soil Vapor Sampling SOP Page 8 of 11 Stantec Rev. 1.0 Apr 2019 • Assemble the aboveground sampling apparatus (valves, tubing and fittings). • Evacuate lines to a measured vacuum of 100 inches Water Column (in H2O) using a gas-tight syringe or sampling pump. If a pump is used, the valve is closed and the pump turned off. If constant vacuum pressure is maintained for 1 minute, the sampling equipment will pass the vacuum test. If there is observable loss of vacuum, fittings will be re-tightened and the test repeated. Results of the vacuum leak test will be recorded on the soil vapor sample collection data log. The second leak check procedure involves using a tracer gas to test for ambient air leakage around the surface seal and into the sampling system. Helium as the tracer gas is recommended because it has low toxicity, does not disrupt analytical measurements, is generally not found at contaminated sites, and has a high purity. Helium may be obtained from an analytical laboratory or directly from a helium supplier. • A sampling enclosure (shroud) will be constructed to facilitate leak testing during soil vapor sample collection. With the assumption that the surface sampling train has passed the shut-in test, the shroud should be of sufficient size to cover the surface seal. • Tubing from the subsurface probe is passed through a dedicated opening in the shroud and capped. The sampling enclosure is then filled and maintained to at least 10% laboratory-grade helium as measured using a helium detector (e.g., Dielectric° MGD thermal conductivity detector). Rapid depletion of helium indicates that there is an inadequate seal between the shroud and ground surface and corrective measures are necessary. • Helium present in the sample train may be measured in the field by collecting purge air in a Tedlar bag. A separate line and valve is connected to a Tedlar bag placed inside a lung box. The sample is collected using a small sampling pump. Alternatively, purge air may be directly collected into a Tedlar bag if a twin-port sampling pump is used. Helium is measured using the portable helium detector. If logistically feasible, these methods allow for corrective measures to be taken at the time of sample collection. However, in all cases, all samples will also be analyzed in the laboratory using U.S EPA Method TO- 3C or ASTM D-1946 (fixed gases). • The presence of helium in the bag of less than 5% of the helium concentration in the shroud generally is not indicative of ambient air intrusion (and subsequent dilution of the sample) into the sample train. Helium present at higher percentages will require corrective action including reapplication of materials used to establish the surface seal. Particular attention should be paid to ensuring that soil vapor implant tubing is immobilized to prevent annulus formation around it. In the event that helium is detected at greater than 5% of the shroud concentration after corrective measures, the boring must be abandoned and relocated. If a field detector is used that provides parts per million measurements these may be converted (for helium only) to percent helium as follows: THIS INFORMATION FOR AUTHORIZED COMPANY USE ONLY STANTEC CONSULTING