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Decontamination may consist of steam cleaning or a three-stage wash and rinse tracer compounds are applied to towels or clean rags and placed around all consisting of a wash with a non-phosphate detergent,a rinse with tap water,and connections in the sampling train in order to evaluate potential leaks of ambient a final rinse with distilled water. Collect one equipment blank at the beginning of air into the sampling train.The leak check compound selected should not be a sampling,and at least one each day after decontamination. Air dry equipment suspected site-specific contaminant. prior to re-use. Use new or unused sample tubing for each probe location. Reuse of tubing is not recommended. Seal integrity is confirmed by analyzing subsequent soil gas samples for the tracer compound. Alternatively,gaseous tracer compounds,such as helium, isobutene,propane,and butane,can be used along with a shroud or tent over Sub-Slab Investigation Methods the sampling equipment.Procedures for conducting a quantitative leak test using helium as a tracer compound are described in Appendix C. • The procedure for collecting sub-slab soil gas samples are the same as for collecting sub-surface soil gas samples except that a slower flaw rate and lower Leak check compounds(i.e.,liquid tracer compounds)should be included in the vacuum should be utilized.Using a flow rate of less than or equal to 50 milliliters method analyte list. The laboratory reports should quantify and annotate all per minute(ml/min)and maintaining a low vacuum of less than 100 inches of - detections of the leak check compound at the reporting limit of the target water should prevent ambient air breakthrough into samples(McAlary at al., analytes. If the concentration of the leak check compound is greater than or 2009).Methods and procedures for installing sub-slab vapor wells are described equal to ten times the reporting limit for the target analyte(s),.then corrective in the DTSC Vapor Intrusion Guidance(Cal/EPA,2005, Page G-1) action is necessary as discussed below. 2.4 Solt Gas Sampling If a leak check compound(i.e.,liquid tracer compounds)is detected in the sample,the cause of the leak should be determined,evaluated,and corrected Before obtaining soil gas samples,leak and purge volume tests are necessary. through retesting. If a leak is confirmed and the problem cannot be corrected, the ambient air leak may be quantified using a gaseous tracer with a shroud,as Leak Test described in Appendix C. An ambient air leak up to five(5)percent is acceptable if quantitative tracer testing is performed by shrouding. Otherwise,the soil gas A leak lest is used to evaluate whether a good seal was established in the vapor well should be decommissioned if the leak cannot be corrected. sampling train,ground surface,and probe interface. A leak test should be Replacement vapor wells:should be installed at least five(5)feet from location conducted at every vapor monitoring well each time a soil gas sample is where the original vapor well was decommissioned due to a confirmed leak. collected because a poor seal may result in soil gas samples that are diluted by Include the leak check compound concentrations detected in the soil gas ambient air.This may result in an underestimation of actual site contaminant samples in the laboratory report and discussed in the site characterization report. concentrations or,alternatively,introduce external contaminant into samples from ambient air. Note that if sampling is done exclusively from a SUMMe canister at a stationary laboratory and there is a significant leak,it will not be identified until after the Shut-In Test:Prior to purging or sampling soil gas,a shut-in test should be sample has been collected and analyzed. Therefore,field screening prior to conducted to check for leaks in the above ground fittings. The shut-in test laboratory confirmation is recommended. consist of assembling the above-ground apparatus(e.g.,valves,lines and fittings downstream from the top of the probe),and evacuating the lines to a measured vacuum of about 100 inches of water column(in-H2O),then shutting the vacuum 2.5 Purge Volume Test in with closed valves on opposite ends of the sampling train. The vacuum gauge connected to the line via'T"-fitting is observed for at least one minute,and if The purpose of a purge volume testis to ensure stagnant or ambient air is there is any observable loss of vacuum,the fittings are adjusted as needed until removed from the sampling system and to ensure samples collected are the vacuum in the above-ground portion of the sample train does not noticeably representative of subsurface conditions. The purge volume test should be the dissipate(McAlary at al.,2009). first soil gas sampling activity at the selected purge volume test point,and should be located as near as possible to the anticipated or confirmed contaminant Leak Check Compounds:Liquid tracer compounds,such as hexane,pentane, source,and in an area where soil gas concentrations are expected to be and n-propanol,can be used to evaluate the sample integrity. Typically,liquid elevated,based on lithology.The purge volume lest is conducted by collecting and analyzing a sample for target compounds after removing one,three,and ten 3/3/2010 -23- 3/3/2010 -24-