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• Purging of the sample probe and collection system before sampling <br /> • Leak-check of sampling equipment to reduce air infiltration <br /> • Keeping the length of all sample transfer lines as short as possible to minimize <br /> condensation of extracted gas in the lines. <br /> The choice of analytical methods for whole-air soil gas sampling depends on the <br /> contaminants of concern. Concentrations of volatile organic compounds (VOCs) in the soil gas are <br /> typically determined using EPA Method TO-14 or TO-15. In the case of semi-volatile compounds, <br /> an active sorbent sampling methodology can be used. In this case, a low-volume sampling pump is <br /> normally used to withdraw the soil gas, which is then routed to a polyurethane foam (PUF) plug. <br /> Vapor concentrations of semi-volatile contaminants sorbed to the PUF are then determined using <br /> EPA Method TO-10. The active soil gas sampling equipment can be assembled to allow for both <br /> canister sampling for volatiles and PUF sampling for semi-volatiles. <br /> Passive sorbent sampling involves burial of solid sorbent sampling devices called cartridges <br /> or cassettes to a depth of normally 5 feet or less. The cassettes may be configured with one or more <br /> sorbents depending on the list of target analytes, and are typically left in-ground for 72 to 120 hours <br /> or longer. During this time period, the vapor-phase soil gas contaminants pass through the cassette <br /> and are adsorbed as the soil gas moves toward the soil surface by diffusion and/or convection. <br /> Analytical methods for sorbent sampling depend on the target analytes and the sorbent used and may <br /> include EPA Method TO-10 or a modified EPA Method TO-1. Vapor-phase concentrations for <br /> some solid sorbent sampling systems are determined using the total mass of each contaminant <br /> recovered, the time in-ground, the cross-sectional area of the cassette, the diffusivity of the <br /> compound in air, and a quasi-empirical adsorption rate constant. <br /> Recent EPA technology verification reports produced by the EPA National Exposure <br /> Research Laboratory(EPA 1998, 1998a) concluded, at least for two such systems, that the sorbent <br /> methodologies accurately accounted for the presence of most of the soil gas contaminants in the <br /> studies. Further, the reports concluded that the sorbent systems showed detection of contaminants <br /> at low concentrations :not reported using an active whole-air sampling system. For one system, <br /> however, it was noted that as the vapor concentrations reported for the whole-air sampling system <br /> increased by 1 to 4 orders-of-magnitude, the associated concentrations reported for the sorbent <br /> system increased only marginally. Perhaps the best use of such passive sorbent sampling methods <br /> is to help confirm which contaminants are present in the soil gas and not necessarily contaminant <br /> concentrations. <br /> An excellent discussion of soil gas measurement methods and limitations can be found in the <br /> ASTM Standard Guide for Soil Gas Monitoring in the Vadose Zone D5314-92el. ASTM Standard <br /> Guides are available from the ASTM website at: <br /> bttp://www.astm.org. <br /> In addition, soil gas measurement method summaries can be found in the EPA Standard Operating <br /> Procedures for Soil Gas Sampling(SOP No. 2042) developed by the EPA Environmental Response <br /> 64 <br />