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10 6. Prior to collecting the sample in the canister, collect a sample in a 1-liter Tedlar bag. Analyze this Tedlar bag sample with the handheld detector for the tracer gas and with the helium detector and record the concentrations detected. If the concentration of the tracer is >10% of the concentration which was measured from the shroud, the probe should be re-sealed. The tracer test will be performed again, and sample collection will proceed when the tracer concentration is<10%. 7. Following collection of the sample in the Summa canister, an additional soil gas sample will be collected in a Tedlar bag and monitored for helium. If both the pre- and post-sampling Tedlar bag samples contain helium at <10% of the shroud concentration, then the sample will have passed the leak test. If not, additional corrective measures will be taken in the field (e.g., tightening fittings) and the test will be repeated. If the<10%criteria cannot be met at a given probe, that location and depth will be eliminated from the validation program. Soil Gas Ambient Air Leak Detection Protocol using Duster The real time leak testing procedures using the "duster" as the tracer compound will be accomplished as follows: 1. Place a shroud over the annulus of the soil gas sampling probe. The sampling tube should extend out of the opening in the top of the shroud. Press the top of the shroud into the soil to achieve a seal. Alternatively if sampling through a hard surface, prepare a seal of hydrated bentonite around the bottom edge of the shroud (see diagram on following page). 2. Attach a standard handheld refrigerant leak detector used in the automotive industry to the soil gas sampling tube using appropriate fittings. Automotive Freon leak detectors will not respond to non-halogenated VOCs such as benzene. Tum on the refrigerant leak detector (knowing the pump rate of the detector) and time the detector to purge approximately three casings volume of soil gas. High levels of halogenated VOCs present in the soil gas may trip the detector. If a response to the soil gas is noted, reset the instrument for background concentrations. 3. Spray into the open shroud a "Duster", "Dust-off' or "Computer Duster" compound. Most are either difluoroethane or tetrafluoroethane and both will respond to the refrigerant detector. A second detector can be used to insure adequate "Duster" is added to the shroud (the instrument should give the maximum reading). Record the concentration of the tracer in the shroud. The Freon in "Duster" is heavier than air and can persist up to 30 minutes in the shroud in calm air. Under windy conditions the former bottom of the shroud can be used to shield the duster from the wind. Collect a soil gas sample in a Tedlar bag and screen it for the tracer. A failed well annulus will be noted by a response from the detector either above background readings or above detection levels. A failed well annulus may be corrected by replacement/rehydration of the bentonite seal of the soil gas probe or by checking for a possible "short circuit" of ambient air through the soil or sub-grade (such as asphaltic concrete sub-grade). Soil gas probes that continue to leak at the annulus should be abandoned and moved to an adjacent location. Record all screened concentrations.