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When practical,the probes may be left for several days or more between installation Low-Flow Probe Screening and Sampling:A modified purging and sampling and the initial sampling event to facilitate re-equilibration. Otherwise,the probes should procedure should be used for low-flow probes.If the pore volume of the sand pack plus be developed by removing a volume of soil gas equal to the dead-volume of the tubing the dead volume of the tubing up to the surface is about 1 L,then approximately 0.3L of plus the pore volume of the sand pack as soon as feasible after installation. This should soil gas can be extracted under an applied vacuum of about 100 inches water,even if be performed using the purging and field screening protocols described below, there is no significant influx of gas from the surrounding soil.This yield is essentially the volume of gas removed from the pore space of the filter pack and tubing in response to Shut-in-Test: Prior to soil-gas permeability testing,purging or sampling,a shut-in test applying a vacuum of 100 inches of water,according to the Ideal Gas Law.At a flow should be conducted to check for leaks in the above ground fittings(see:Page 22). rate of 100 mUmin,this will take about three minutes,after which,the flow may diminish Solt-Gas Permeability Testing:Prior to sampling,asoil-gas permeability test should to a negligible amount. be conducted on all soil-gas probes by measuring the flow rate and corresponding If a flow of about 100 mL/mM is not sustainable,then purging should be suspended . vacuum.A vacuum gauge should be connected as near as practically possible to the when the vacuum reaches about 100 inches of water by closing the ball valve on top of ball valve on top of the probe using a"T"-fitting. A pump and flow-meter can then be the probe. Over time,as soil gas flows into the sand pack,the vacuum will gradually used to withdraw soil gas at a measured rate(See Figure D-1). When practical,flow dissipate. The rate of vacuum dissipation can be monitored using a vacuum gauge with and vacuum measurements should be made at more than one flow rate(e.g„50,100, either continuous or intermittent measurements. Each application of this technique 200 and 500 mUmin)to assess the consistency of the flow/vacuum.ratio. These data yields an aliquot of 0.3 L of soil gas over a period of a few minutes,followed by a period can used to calculate the gas permeability of the soils surrounding the probe tip using of time over which the vacuum dissipates. This length of time will depend on the gas the method described by Johnson at al. (1990). During this pneumatic test,the applied permeability of the soils surrounding the sand pack. The sampling crew may proceed vacuum should not exceed 100-inches of water column to minimize the risk of drawing with other activities while waiting for vacuum to dissipate, water up the probe or upsetting the partitioning of vapors from pore water to soil gas (API 2005). If the probe will be sampled within the first few days after installation,the volume of gas purged prior to sample collection should include at a minimum the volume of atmospheric air entrained during probe installation,which will be approximately equal to the pore volume of the sand-pack plus the dead-volume of the tubing. The volume of sample collected after this should be sufficient for field screening and laboratory eolomslee amvmetm oaomumn analysis. Depending on the field screening instruments and laboratory methods, several aliquots of soil gas samples may be required for each low-flow probe. wJ l° mwn�c- nomol°oleo Helium Tracer Leak Testing:On-site leak testing with helium can be performed using ah�covpe, an apparatus similar to the one shown in Appendix C(Figure C-1)or see McAlary et al. oconxo vocwmF m (2009; Figure 5). If the Flow controller to SUMMA canister is calibrated to 100 mUmin, Wow an aliquot of 0.3-L.of soil gas can be collected in three minutes into the Tartar'ba V-1 ��o inside the lung box. In this example,the contents of the Tedlar°bag is then screened using a portable helium meter(e.g.,MGD-2002Tn to assess whether there are any a va ° unacceptable leaks in real-time. If the helium concentration in the Tedlar°bag is less IPIVAc than.about 0.05%(if initial helium concentration is 10%),the sample consists of>95% vawer9.�°°IY soil,gas,which is considered acceptable. Figure D-1:An example of soil-gas permeability testing equipment(McAlary at al., 2009) An alternative method using laboratory analysis of helium is also acceptable as described in Appendix C. In this example,a SUMMA°canister is connected to the probe through a dedicated flow controller calibrated to 100 mUmin. The probe value is If a probe cannot sustain a flow rate of 100 ml/min for more than three minutes with an opened,and the canister and probe is quickly covered with a shroud,which is filled with applied vacuum less than 100 inches of water,the low-flow probe screening and at least 10%helium for about 3 minutes,after which the canister and the probe valve sampling procedure should be used. are both closed,the canister is disconnected and stored and the probe vacuum is allowed to dissipate,typically overnight. This process is repeated until the volume of sample is sufficient to meet analytical requirements. Typically,laboratories request a 12/10/2009 -61 - 12/10/2009 -62-