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6. Record helium reading for leak-check enclosure at least once every minute during <br /> purging and sampling. <br /> Sample Collection <br /> 1. Opening Sample Canister.Once a helium reading of at least 20%has been reached, <br /> open sample canister valve. Sampling takes approximately 5 minutes for a 1-liter <br /> Summa canister(at 150 ml/min sampling flow rate). <br /> 2. Close sampling canister valve when vacuum decreases to 5"mercury. Do not allow <br /> vacuum to fall below this range. <br /> 3. Post-Sample Vapor Screening.After sampling,open 3-way valve#1 so that the vapor <br /> screening assembly is connected to the probe,turn on the vacuum pump,and open 3-way <br /> valves#2 and#3 to partially fill the Tedlar bag within the vacuum chamber(iron lung). <br /> When Tedlar bag is sufficiently filled,return valves#2 and#3 to purging position. <br /> Check Tedlar bag for indication of sampling leakage using the helium gas analyzer.If <br /> helium concentration is below I%then sample is sufficiently representative.If helium <br /> concentration is above I%,then the sample may not be sufficiently representative;the <br /> probe may need to be repaired or re-installed and re-sampled.Additionally, check the <br /> Tedlar bag for contaminants using the PID for qualitative contaminant assessment <br /> (optional). <br /> 4. Shroud Sample.To confirm helium meter readings collect one shroud sample per day to <br /> analyze for percent helium.Connect the shroud sample summa canister and manifold to a <br /> port near the bottom of the shroud and open the canister valve at the beginning of <br /> sampling.Close sampling canister valve when vacuum decreases to 5"mercury. Do not <br /> allow vacuum to fall below this range. Disassemble sampling assembly,and cap(or <br /> remove and restore)vapor sampling point. <br /> 5. Analyses.Fill out chain-of-custody form for analysis for chemicals of concern(i.e. TO- • <br /> 15),and for leak-check compound for at least 10%of samples.Analyze all samples for <br /> percent oxygen by ASTM D1946-90.Additionally,samples may be analyzed for <br /> percent methane and carbon dioxide by ASTM D1946-90 when in support of sensitive <br /> human health risk assessments for regulatory review. Include final vacuum reading and <br /> serial numbers of canister and flow restrictor on chain-of-custody form. <br /> 6. For vapor sampling in support of sensitive human health risk assessments for regulatory <br /> review,collect at least one duplicate sample per site per sampling event from the <br /> sampling point with the anticipated highest vapor concentrations. The duplicate sample <br /> should be collected by attaching a fresh sample canister following collection of the initial <br /> sample.If a new manifold is used,follow the same purging and sampling procedures <br /> used for the original sample. If the same manifold is used,collect a sample without <br /> further purging,using the same sampling procedures used for the original sample. <br /> Decontamination and Decommissionina <br /> 1. Use a decontaminated sampling manifold and new tubing for each sample location. <br /> Return equipment to laboratory for decontamination. <br /> 2. Backfill any open soil vapor probe holes with bentonite slung or Portland cement and cap <br /> with concrete or other surface material to match the area. <br /> 3. To retain the subslab probe for future sampling,cap the Swagelock fitting and cover the <br /> probe with a small vault or other protective device. v <br /> 4 <br />